Changeset 29 for draft-ietf-httpbis

11/12/07 09:53:42 (15 years ago)

Partition RFC 2616 into seven (mostly) independent documents.
No semantic changes. Some meaningless crossreferences to prior
editorial decisions have been removed from appendices.

Structural changes minimized to simplify diff versus rfc2616.
This was a lot harder than it looks.

Part 8 (Cookies) is for future specification based on RFC 2965.

8 edited


  • draft-ietf-httpbis/latest/p1-messaging.xml

    r18 r29  
    1 <?xml version="1.0" encoding="UTF-8"?>
     1<?xml version="1.0" encoding="utf-8"?>
    22<!DOCTYPE rfc [
    33  <!ENTITY MAY "<bcp14 xmlns=''>MAY</bcp14>">
    1111  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
    1212  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
     13  <!ENTITY ID-VERSION "latest">
     14  <!ENTITY caching                "[Part 6]">
     15  <!ENTITY payload                "[Part 3]">
     16  <!ENTITY CONNECT                "[Part 2]">
     17  <!ENTITY content.negotiation    "[Part 3]">
     18  <!ENTITY diff2045entity         "[Part 3]">
     19  <!ENTITY entity                 "[Part 3]">
     20  <!ENTITY entity-header-fields   "[Part 3]">
     21  <!ENTITY header-cache-control   "[Part 6]">
     22  <!ENTITY header-expect          "[Part 2]">
     23  <!ENTITY header-pragma          "[Part 6]">
     24  <!ENTITY header-warning         "[Part 6]">
     25  <!ENTITY idempotent-methods     "[Part 2]">
     26  <!ENTITY qvalue                 "[Part 3]">
     27  <!ENTITY request-header-fields  "[Part 2]">
     28  <!ENTITY response-header-fields "[Part 2]">
     29  <!ENTITY method                 "[Part 2]">
     30  <!ENTITY status-codes           "[Part 2]">
     31  <!ENTITY status-100             "[Part 2]">
     32  <!ENTITY status-1xx             "[Part 2]">
     33  <!ENTITY status-414             "[Part 2]">
    1435<?rfc toc="yes" ?>
    15 <?rfc symrefs="no" ?>
     36<?rfc symrefs="yes" ?>
     37<?rfc sortrefs="yes" ?>
    1638<?rfc compact="yes"?>
    1739<?rfc subcompact="no" ?>
    2042<?rfc-ext allow-markup-in-artwork="yes" ?>
    2143<?rfc-ext include-references-in-index="yes" ?>
    22 <rfc number="2616" category="std" obsoletes="2068" xmlns:x='' xmlns:ed="" >
    23 <x:assign-section-number number="18" builtin-target="authors"/>
     44<rfc obsoletes="2068, 2616, 2617" category="std"
     45     ipr="full3978" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
     46     xmlns:x='' xmlns:ed="">
    26   <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
    28   <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
    29     <organization abbrev="UC Irvine">Department of Information and Computer Science</organization>
     49  <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
     51  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
     52    <organization abbrev="Day Software">Day Software</organization>
    3053    <address>
    3154      <postal>
    32         <street>University of California, Irvine</street>
    33         <city>Irvine</city>
     55        <street>23 Corporate Plaza DR, Suite 280</street>
     56        <city>Newport Beach</city>
    3457        <region>CA</region>
    35         <code>92697-3425</code>
     58        <code>92660</code>
     59        <country>USA</country>
    3660      </postal>
    37       <facsimile>+1(949)824-1715</facsimile>
    38       <email></email>
     61      <phone>+1-949-706-5300</phone>
     62      <facsimile>+1-949-706-5305</facsimile>
     63      <email></email>
     64      <uri></uri>
    3965    </address>
    4066  </author>
    42   <author initials="J." surname="Gettys" fullname="James Gettys">
    43     <organization abbrev="Compaq/W3C">World Wide Web Consortium</organization>
     68  <author initials="J." surname="Gettys" fullname="Jim Gettys">
     69    <organization>One Laptop per Child</organization>
    4470    <address>
    4571      <postal>
    46         <street>MIT Laboratory for Computer Science, NE43-356</street>
    47         <street>545 Technology Square</street>
    48         <city>Cambridge</city>
     72        <street>21 Oak Knoll Road</street>
     73        <city>Carlisle</city>
    4974        <region>MA</region>
    50         <code>02139</code>
     75        <code>01741</code>
     76        <country>USA</country>
    5177      </postal>
    52       <facsimile>+1(617)258-8682</facsimile>
    53       <email></email>
     78      <email></email>
     79      <uri></uri>
    5480    </address>
    5581  </author>
    5783  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    58     <organization abbrev="Compaq">Compaq Computer Corporation</organization>
     84    <organization abbrev="HP">Hewlett-Packard Company</organization>
    5985    <address>
    6086      <postal>
    61         <street>Western Research Laboratory</street>
    62         <street>250 University Avenue</street>
     87        <street>HP Labs, Large Scale Systems Group</street>
     88        <street>1501 Page Mill Road, MS 1177</street>
    6389        <city>Palo Alto</city>
    6490        <region>CA</region>
    65         <code>94305</code>
     91        <code>94304</code>
     92        <country>USA</country>
    6693      </postal>
    67       <email></email>
     94      <email></email>
    6895    </address>
    6996  </author>
    7198  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    72     <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     99    <organization abbrev="Microsoft">Microsoft Corporation</organization>
    73100    <address>
    74101      <postal>
    75         <street>MIT Laboratory for Computer Science, NE43-356</street>
    76         <street>545 Technology Square</street>
    77         <city>Cambridge</city>
    78         <region>MA</region>
    79         <code>02139</code>
     102        <street>1 Microsoft Way</street>
     103        <city>Redmond</city>
     104        <region>WA</region>
     105        <code>98052</code>
     106        <country>USA</country>
    80107      </postal>
    81       <facsimile>+1(617)258-8682</facsimile>
    82       <email></email>
     108      <email></email>
    83109    </address>
    84110  </author>
    86112  <author initials="L." surname="Masinter" fullname="Larry Masinter">
    87     <organization abbrev="Xerox">Xerox Corporation</organization>
     113    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    88114    <address>
    89115      <postal>
    90         <street>MIT Laboratory for Computer Science, NE43-356</street>
    91         <street>3333 Coyote Hill Road</street>
    92         <city>Palo Alto</city>
     116        <street>345 Park Ave</street>
     117        <city>San Jose</city>
    93118        <region>CA</region>
    94         <code>94034</code>
     119        <code>95110</code>
     120        <country>USA</country>
    95121      </postal>
    96       <email></email>
     122      <email></email>
     123      <uri></uri>
    97124    </address>
    98125  </author>
    115142    <address>
    116143      <postal>
    117         <street>MIT Laboratory for Computer Science, NE43-356</street>
     144        <street>MIT Laboratory for Computer Science</street>
    118145        <street>545 Technology Square</street>
    119146        <city>Cambridge</city>
    120147        <region>MA</region>
    121148        <code>02139</code>
     149        <country>USA</country>
    122150      </postal>
    123       <facsimile>+1(617)258-8682</facsimile>
     151      <facsimile>+1 (617) 258 8682</facsimile>
    124152      <email></email>
    125153    </address>
    126154  </author>
    128   <date month="June" year="1999"/>
     156  <date month="December" year="2007"/>
    132160   The Hypertext Transfer Protocol (HTTP) is an application-level
    133161   protocol for distributed, collaborative, hypermedia information
    134    systems. It is a generic, stateless, protocol which can be used for
    135    many tasks beyond its use for hypertext, such as name servers and
    136    distributed object management systems, through extension of its
    137    request methods, error codes and headers <xref target="RFC2324"/>. A feature of HTTP is
    138    the typing and negotiation of data representation, allowing systems
    139    to be built independently of the data being transferred.
    140 </t>
    141 <t>
    142    HTTP has been in use by the World-Wide Web global information
    143    initiative since 1990. This specification defines the protocol
    144    referred to as "HTTP/1.1", and is an update to RFC 2068 <xref target="RFC2068"/>.
     162   systems. HTTP has been in use by the World Wide Web global information
     163   initiative since 1990. This document is Part 1 of the eight-part specification
     164   that defines the protocol referred to as "HTTP/1.1" and, taken together,
     165   updates RFC 2616 and RFC 2617.  Part 1 provides an overview of HTTP and
     166   its associated terminology, defines the "http" and "https" Uniform
     167   Resource Identifier (URI) schemes, defines the generic message syntax
     168   and parsing requirements for HTTP message frames, and describes
     169   general security concerns for implementations.
    149174<section title="Introduction" anchor="introduction">
     176   This document will define aspects of HTTP related to overall network
     177   operation, message framing, interaction with transport protocols, and
     178   URI schemes. Right now it only includes the extracted relevant sections
     179   of <xref target="RFC2616"/> and <xref target="RFC2617"/>.
    151181<section title="Purpose" anchor="intro.purpose">
    273303      response. An entity consists of metainformation in the form of
    274304      entity-header fields and content in the form of an entity-body, as
    275       described in <xref target="entity"/>.
     305      described in &entity;.
    276306    </t>
    277307  </list>
    283313    <t>
    284314      An entity included with a response that is subject to content
    285       negotiation, as described in <xref target="content.negotiation"/>. There may exist multiple
     315      negotiation, as described in &content.negotiation;. There may exist multiple
    286316      representations associated with a particular response status.
    287317    </t>
    294324    <t>
    295325      The mechanism for selecting the appropriate representation when
    296       servicing a request, as described in <xref target="content.negotiation"/>. The
     326      servicing a request, as described in &content.negotiation;. The
    297327      representation of entities in any response can be negotiated
    298328      (including error responses).
    427457      the response message for use in answering subsequent requests. The
    428458      rules for determining the cacheability of HTTP responses are
    429       defined in <xref target="caching"/>. Even if a resource is cacheable, there may
     459      defined in &caching;. Even if a resource is cacheable, there may
    430460      be additional constraints on whether a cache can use the cached
    431461      copy for a particular request.
    432     </t>
    433   </list>
    434 </t>
    435 <t>
    436   <iref item="first-hand"/>
    437   <x:dfn>first-hand</x:dfn>
    438   <list>
    439     <t>
    440       A response is first-hand if it comes directly and without
    441       unnecessary delay from the origin server, perhaps via one or more
    442       proxies. A response is also first-hand if its validity has just
    443       been checked directly with the origin server.
    444     </t>
    445   </list>
    446 </t>
    447 <t>
    448   <iref item="explicit expiration time"/>
    449   <x:dfn>explicit expiration time</x:dfn>
    450   <list>
    451     <t>
    452       The time at which the origin server intends that an entity should
    453       no longer be returned by a cache without further validation.
    454     </t>
    455   </list>
    456 </t>
    457 <t>
    458   <iref item="heuristic expiration time"/>
    459   <x:dfn>heuristic expiration time</x:dfn>
    460   <list>
    461     <t>
    462       An expiration time assigned by a cache when no explicit expiration
    463       time is available.
    464     </t>
    465   </list>
    466 </t>
    467 <t>
    468   <iref item="age"/>
    469   <x:dfn>age</x:dfn>
    470   <list>
    471     <t>
    472       The age of a response is the time since it was sent by, or
    473       successfully validated with, the origin server.
    474     </t>
    475   </list>
    476 </t>
    477 <t>
    478   <iref item="freshness lifetime"/>
    479   <x:dfn>freshness lifetime</x:dfn>
    480   <list>
    481     <t>
    482       The length of time between the generation of a response and its
    483       expiration time.
    484     </t>
    485   </list>
    486 </t>
    487 <t>
    488   <iref item="fresh"/>
    489   <x:dfn>fresh</x:dfn>
    490   <list>
    491     <t>
    492       A response is fresh if its age has not yet exceeded its freshness
    493       lifetime.
    494     </t>
    495   </list>
    496 </t>
    497 <t>
    498   <iref item="stale"/>
    499   <x:dfn>stale</x:dfn>
    500   <list>
    501     <t>
    502       A response is stale if its age has passed its freshness lifetime.
    503     </t>
    504   </list>
    505 </t>
    506 <t>
    507   <iref item="semantically transparent"/>
    508   <x:dfn>semantically transparent</x:dfn>
    509   <list>
    510     <t>
    511       A cache behaves in a "semantically transparent" manner, with
    512       respect to a particular response, when its use affects neither the
    513       requesting client nor the origin server, except to improve
    514       performance. When a cache is semantically transparent, the client
    515       receives exactly the same response (except for hop-by-hop headers)
    516       that it would have received had its request been handled directly
    517       by the origin server.
    518     </t>
    519   </list>
    520 </t>
    521 <t>
    522   <iref item="validator"/>
    523   <x:dfn>validator</x:dfn>
    524   <list>
    525     <t>
    526       A protocol element (e.g., an entity tag or a Last-Modified time)
    527       that is used to find out whether a cache entry is an equivalent
    528       copy of an entity.
    529462    </t>
    530463  </list>
    565498   followed by a MIME-like message containing server information, entity
    566499   metainformation, and possible entity-body content. The relationship
    567    between HTTP and MIME is described in <xref target="differences.between.http.entities.and.rfc.2045.entities"/>.
     500   between HTTP and MIME is described in &diff2045entity;.
    629562   contain modifiers which place special requirements on cache behavior.
    630563   HTTP requirements for cache behavior and cacheable responses are
    631    defined in <xref target="caching"/>.
     564   defined in &caching;.
    856789   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
    857790   tolerant applications). The end-of-line marker within an entity-body
    858    is defined by its associated media type, as described in <xref target="media.types"/>.
     791   is defined by its associated media type, as described in &payload;.
    860793<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="CRLF"/>
    1037970   provide GET-based forms that could generate such URIs. A server
    1038971   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
    1039    than the server can handle (see <xref target="status.414"/>).
     972   than the server can handle (see &status-414;).
    11051038<section title="Date/Time Formats" anchor="date.time.formats">
    11071039<section title="Full Date" anchor="">
    1172 <section title="Delta Seconds" anchor="delta.seconds">
    1173 <t>
    1174    Some HTTP header fields allow a time value to be specified as an
    1175    integer number of seconds, represented in decimal, after the time
    1176    that the message was received.
    1177 </t>
    1178 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
    1179     delta-seconds  = 1*DIGIT
    1180 </artwork></figure>
    1181 </section>
    1182 </section>
    1184 <section title="Character Sets" anchor="character.sets">
    1185 <t>
    1186    HTTP uses the same definition of the term "character set" as that
    1187    described for MIME:
    1188 </t>
    1189 <t>
    1190    The term "character set" is used in this document to refer to a
    1191    method used with one or more tables to convert a sequence of octets
    1192    into a sequence of characters. Note that unconditional conversion in
    1193    the other direction is not required, in that not all characters may
    1194    be available in a given character set and a character set may provide
    1195    more than one sequence of octets to represent a particular character.
    1196    This definition is intended to allow various kinds of character
    1197    encoding, from simple single-table mappings such as US-ASCII to
    1198    complex table switching methods such as those that use ISO-2022's
    1199    techniques. However, the definition associated with a MIME character
    1200    set name &MUST; fully specify the mapping to be performed from octets
    1201    to characters. In particular, use of external profiling information
    1202    to determine the exact mapping is not permitted.
    1203 </t>
    1204 <t><list><t>
    1205       <x:h>Note:</x:h> This use of the term "character set" is more commonly
    1206       referred to as a "character encoding." However, since HTTP and
    1207       MIME share the same registry, it is important that the terminology
    1208       also be shared.
    1209 </t></list></t>
    1210 <t>
    1211    HTTP character sets are identified by case-insensitive tokens. The
    1212    complete set of tokens is defined by the IANA Character Set registry
    1213    <xref target="RFC1700"/>.
    1214 </t>
    1215 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="charset"/>
    1216     charset = token
    1217 </artwork></figure>
    1218 <t>
    1219    Although HTTP allows an arbitrary token to be used as a charset
    1220    value, any token that has a predefined value within the IANA
    1221    Character Set registry <xref target="RFC1700"/> &MUST; represent the character set defined
    1222    by that registry. Applications &SHOULD; limit their use of character
    1223    sets to those defined by the IANA registry.
    1224 </t>
    1225 <t>
    1226    Implementors should be aware of IETF character set requirements <xref target="RFC2279"/>
    1227    <xref target="RFC2277"/>.
    1228 </t>
    1230 <section title="Missing Charset" anchor="missing.charset">
    1231 <t>
    1232    Some HTTP/1.0 software has interpreted a Content-Type header without
    1233    charset parameter incorrectly to mean "recipient should guess."
    1234    Senders wishing to defeat this behavior &MAY; include a charset
    1235    parameter even when the charset is ISO-8859-1 and &SHOULD; do so when
    1236    it is known that it will not confuse the recipient.
    1237 </t>
    1238 <t>
    1239    Unfortunately, some older HTTP/1.0 clients did not deal properly with
    1240    an explicit charset parameter. HTTP/1.1 recipients &MUST; respect the
    1241    charset label provided by the sender; and those user agents that have
    1242    a provision to "guess" a charset &MUST; use the charset from the
    1243    content-type field if they support that charset, rather than the
    1244    recipient's preference, when initially displaying a document. See
    1245    <xref target="canonicalization.and.text.defaults"/>.
    1246 </t>
    1247 </section>
    1248 </section>
    1250 <section title="Content Codings" anchor="content.codings">
    1251 <t>
    1252    Content coding values indicate an encoding transformation that has
    1253    been or can be applied to an entity. Content codings are primarily
    1254    used to allow a document to be compressed or otherwise usefully
    1255    transformed without losing the identity of its underlying media type
    1256    and without loss of information. Frequently, the entity is stored in
    1257    coded form, transmitted directly, and only decoded by the recipient.
    1258 </t>
    1259 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="content-coding"/>
    1260     content-coding   = token
    1261 </artwork></figure>
    1262 <t>
    1263    All content-coding values are case-insensitive. HTTP/1.1 uses
    1264    content-coding values in the Accept-Encoding (<xref target="header.accept-encoding"/>) and
    1265    Content-Encoding (<xref target="header.content-encoding"/>) header fields. Although the value
    1266    describes the content-coding, what is more important is that it
    1267    indicates what decoding mechanism will be required to remove the
    1268    encoding.
    1269 </t>
    1270 <t>
    1271    The Internet Assigned Numbers Authority (IANA) acts as a registry for
    1272    content-coding value tokens. Initially, the registry contains the
    1273    following tokens:
    1274 </t>
    1275 <t>
    1276    gzip<iref item="gzip"/>
    1277   <list>
    1278     <t>
    1279         An encoding format produced by the file compression program
    1280         "gzip" (GNU zip) as described in RFC 1952 <xref target="RFC1952"/>. This format is a
    1281         Lempel-Ziv coding (LZ77) with a 32 bit CRC.
    1282     </t>
    1283   </list>
    1284 </t>
    1285 <t>
    1286    compress<iref item="compress"/>
    1287   <list><t>
    1288         The encoding format produced by the common UNIX file compression
    1289         program "compress". This format is an adaptive Lempel-Ziv-Welch
    1290         coding (LZW).
    1291 </t><t>
    1292         Use of program names for the identification of encoding formats
    1293         is not desirable and is discouraged for future encodings. Their
    1294         use here is representative of historical practice, not good
    1295         design. For compatibility with previous implementations of HTTP,
    1296         applications &SHOULD; consider "x-gzip" and "x-compress" to be
    1297         equivalent to "gzip" and "compress" respectively.
    1298   </t></list>
    1299 </t>
    1300 <t>
    1301    deflate<iref item="deflate"/>
    1302   <list><t>
    1303         The "zlib" format defined in RFC 1950 <xref target="RFC1950"/> in combination with
    1304         the "deflate" compression mechanism described in RFC 1951 <xref target="RFC1951"/>.
    1305   </t></list>
    1306 </t>
    1307 <t>
    1308    identity<iref item="identity"/>
    1309   <list><t>
    1310         The default (identity) encoding; the use of no transformation
    1311         whatsoever. This content-coding is used only in the Accept-Encoding
    1312         header, and &SHOULD-NOT;  be used in the Content-Encoding
    1313         header.
    1314   </t></list>
    1315 </t>
    1316 <t>
    1317    New content-coding value tokens &SHOULD; be registered; to allow
    1318    interoperability between clients and servers, specifications of the
    1319    content coding algorithms needed to implement a new value &SHOULD; be
    1320    publicly available and adequate for independent implementation, and
    1321    conform to the purpose of content coding defined in this section.
    1322 </t>
    13631143   has a different focus for an 8bit-clean transfer protocol. In HTTP,
    13641144   the only unsafe characteristic of message-bodies is the difficulty in
    1365    determining the exact body length (<xref target="entity.length"/>), or the desire to
     1145   determining the exact body length (<xref target="message.length"/>), or the desire to
    13661146   encrypt data over a shared transport.
    13701150   transfer-coding value tokens. Initially, the registry contains the
    13711151   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>), "identity" (section
    1372    3.6.2), "gzip" (<xref target="content.codings"/>), "compress" (<xref target="content.codings"/>), and "deflate"
    1373    (<xref target="content.codings"/>).
     1152   3.6.2), "gzip" (&payload;), "compress" (&payload;), and "deflate"
     1153   (&payload;).
    13761156   New transfer-coding value tokens &SHOULD; be registered in the same way
    1377    as new content-coding value tokens (<xref target="content.codings"/>).
     1157   as new content-coding value tokens (&payload;).
    1448    An example process for decoding a Chunked-Body is presented in
    1449    <xref target="introduction.of.transfer-encoding"/>.
    1450 </t>
     1228   A process for decoding the "chunked" transfer-coding
     1229   can be represented in pseudo-code as:
     1231<figure><artwork type="code">
     1232    length := 0
     1233    read chunk-size, chunk-extension (if any) and CRLF
     1234    while (chunk-size &gt; 0) {
     1235       read chunk-data and CRLF
     1236       append chunk-data to entity-body
     1237       length := length + chunk-size
     1238       read chunk-size and CRLF
     1239    }
     1240    read entity-header
     1241    while (entity-header not empty) {
     1242       append entity-header to existing header fields
     1243       read entity-header
     1244    }
     1245    Content-Length := length
     1246    Remove "chunked" from Transfer-Encoding
    14521249   All HTTP/1.1 applications &MUST; be able to receive and decode the
    1459 <section title="Media Types" anchor="media.types">
    1460 <t>
    1461    HTTP uses Internet Media Types <xref target="RFC1590"/> in the Content-Type (<xref target="header.content-type"/>)
    1462    and Accept (<xref target="header.accept"/>) header fields in order to provide
    1463    open and extensible data typing and type negotiation.
    1464 </t>
    1465 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="media-type"/><iref primary="true" item="Grammar" subitem="type"/><iref primary="true" item="Grammar" subitem="subtype"/>
    1466     media-type     = type "/" subtype *( ";" parameter )
    1467     type           = token
    1468     subtype        = token
    1469 </artwork></figure>
    1470 <t>
    1471    Parameters &MAY; follow the type/subtype in the form of attribute/value
    1472    pairs (as defined in <xref target="transfer.codings"/>).
    1473 </t>
    1474 <t>
    1475    The type, subtype, and parameter attribute names are case-insensitive.
    1476    Parameter values might or might not be case-sensitive,
    1477    depending on the semantics of the parameter name. Linear white space
    1478    (LWS) &MUST-NOT; be used between the type and subtype, nor between an
    1479    attribute and its value. The presence or absence of a parameter might
    1480    be significant to the processing of a media-type, depending on its
    1481    definition within the media type registry.
    1482 </t>
    1483 <t>
    1484    Note that some older HTTP applications do not recognize media type
    1485    parameters. When sending data to older HTTP applications,
    1486    implementations &SHOULD; only use media type parameters when they are
    1487    required by that type/subtype definition.
    1488 </t>
    1489 <t>
    1490    Media-type values are registered with the Internet Assigned Number
    1491    Authority (IANA <xref target="RFC1700"/>). The media type registration process is
    1492    outlined in RFC 1590 <xref target="RFC1590"/>. Use of non-registered media types is
    1493    discouraged.
    1494 </t>
    1496 <section title="Canonicalization and Text Defaults" anchor="canonicalization.and.text.defaults">
    1497 <t>
    1498    Internet media types are registered with a canonical form. An
    1499    entity-body transferred via HTTP messages &MUST; be represented in the
    1500    appropriate canonical form prior to its transmission except for
    1501    "text" types, as defined in the next paragraph.
    1502 </t>
    1503 <t>
    1504    When in canonical form, media subtypes of the "text" type use CRLF as
    1505    the text line break. HTTP relaxes this requirement and allows the
    1506    transport of text media with plain CR or LF alone representing a line
    1507    break when it is done consistently for an entire entity-body. HTTP
    1508    applications &MUST; accept CRLF, bare CR, and bare LF as being
    1509    representative of a line break in text media received via HTTP. In
    1510    addition, if the text is represented in a character set that does not
    1511    use octets 13 and 10 for CR and LF respectively, as is the case for
    1512    some multi-byte character sets, HTTP allows the use of whatever octet
    1513    sequences are defined by that character set to represent the
    1514    equivalent of CR and LF for line breaks. This flexibility regarding
    1515    line breaks applies only to text media in the entity-body; a bare CR
    1516    or LF &MUST-NOT; be substituted for CRLF within any of the HTTP control
    1517    structures (such as header fields and multipart boundaries).
    1518 </t>
    1519 <t>
    1520    If an entity-body is encoded with a content-coding, the underlying
    1521    data &MUST; be in a form defined above prior to being encoded.
    1522 </t>
    1523 <t>
    1524    The "charset" parameter is used with some media types to define the
    1525    character set (<xref target="character.sets"/>) of the data. When no explicit charset
    1526    parameter is provided by the sender, media subtypes of the "text"
    1527    type are defined to have a default charset value of "ISO-8859-1" when
    1528    received via HTTP. Data in character sets other than "ISO-8859-1" or
    1529    its subsets &MUST; be labeled with an appropriate charset value. See
    1530    <xref target="missing.charset"/> for compatibility problems.
    1531 </t>
    1532 </section>
    1534 <section title="Multipart Types" anchor="multipart.types">
    1535 <t>
    1536    MIME provides for a number of "multipart" types -- encapsulations of
    1537    one or more entities within a single message-body. All multipart
    1538    types share a common syntax, as defined in section <xref target="RFC2046" x:sec="5.1.1" x:fmt="number"/> of RFC 2046
    1539    <xref target="RFC2046"/>, and &MUST; include a boundary parameter as part of the media type
    1540    value. The message body is itself a protocol element and &MUST;
    1541    therefore use only CRLF to represent line breaks between body-parts.
    1542    Unlike in RFC 2046, the epilogue of any multipart message &MUST; be
    1543    empty; HTTP applications &MUST-NOT; transmit the epilogue (even if the
    1544    original multipart contains an epilogue). These restrictions exist in
    1545    order to preserve the self-delimiting nature of a multipart message-body,
    1546    wherein the "end" of the message-body is indicated by the
    1547    ending multipart boundary.
    1548 </t>
    1549 <t>
    1550    In general, HTTP treats a multipart message-body no differently than
    1551    any other media type: strictly as payload. The one exception is the
    1552    "multipart/byteranges" type (<xref target=""/>) when it appears in a 206
    1553    (Partial Content) response, which will be interpreted by some HTTP
    1554    caching mechanisms as described in sections <xref target="combining.byte.ranges" format="counter"/>
    1555    and <xref target="header.content-range" format="counter"/>. In all
    1556    other cases, an HTTP user agent &SHOULD; follow the same or similar
    1557    behavior as a MIME user agent would upon receipt of a multipart type.
    1558    The MIME header fields within each body-part of a multipart message-body
    1559    do not have any significance to HTTP beyond that defined by
    1560    their MIME semantics.
    1561 </t>
    1562 <t>
    1563    In general, an HTTP user agent &SHOULD; follow the same or similar
    1564    behavior as a MIME user agent would upon receipt of a multipart type.
    1565    If an application receives an unrecognized multipart subtype, the
    1566    application &MUST; treat it as being equivalent to "multipart/mixed".
    1567 </t>
    1568 <t><list><t>
    1569       <x:h>Note:</x:h> The "multipart/form-data" type has been specifically defined
    1570       for carrying form data suitable for processing via the POST
    1571       request method, as described in RFC 1867 <xref target="RFC1867"/>.
    1572 </t></list></t>
    1573 </section>
    1574 </section>
    1576 <section title="Product Tokens" anchor="product.tokens">
    1577 <t>
    1578    Product tokens are used to allow communicating applications to
    1579    identify themselves by software name and version. Most fields using
    1580    product tokens also allow sub-products which form a significant part
    1581    of the application to be listed, separated by white space. By
    1582    convention, the products are listed in order of their significance
    1583    for identifying the application.
    1584 </t>
    1585 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
    1586     product         = token ["/" product-version]
    1587     product-version = token
    1588 </artwork></figure>
    1589 <t>
    1590    Examples:
    1591 </t>
    1592 <figure><artwork type="example">
    1593     User-Agent: CERN-LineMode/2.15 libwww/2.17b3
    1594     Server: Apache/0.8.4
    1595 </artwork></figure>
    1596 <t>
    1597    Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
    1598    used for advertising or other non-essential information. Although any
    1599    token character &MAY; appear in a product-version, this token &SHOULD;
    1600    only be used for a version identifier (i.e., successive versions of
    1601    the same product &SHOULD; only differ in the product-version portion of
    1602    the product value).
    1603 </t>
    1604 </section>
    1606 <section title="Quality Values" anchor="quality.values">
    1607 <t>
    1608    HTTP content negotiation (<xref target="content.negotiation"/>) uses short "floating point"
    1609    numbers to indicate the relative importance ("weight") of various
    1610    negotiable parameters.  A weight is normalized to a real number in
    1611    the range 0 through 1, where 0 is the minimum and 1 the maximum
    1612    value. If a parameter has a quality value of 0, then content with
    1613    this parameter is `not acceptable' for the client. HTTP/1.1
    1614    applications &MUST-NOT; generate more than three digits after the
    1615    decimal point. User configuration of these values &SHOULD; also be
    1616    limited in this fashion.
    1617 </t>
    1618 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="qvalue"/>
    1619     qvalue         = ( "0" [ "." 0*3DIGIT ] )
    1620                    | ( "1" [ "." 0*3("0") ] )
    1621 </artwork></figure>
    1622 <t>
    1623    "Quality values" is a misnomer, since these values merely represent
    1624    relative degradation in desired quality.
    1625 </t>
    1626 </section>
    1628 <section title="Language Tags" anchor="language.tags">
    1629 <t>
    1630    A language tag identifies a natural language spoken, written, or
    1631    otherwise conveyed by human beings for communication of information
    1632    to other human beings. Computer languages are explicitly excluded.
    1633    HTTP uses language tags within the Accept-Language and Content-Language
    1634    fields.
    1635 </t>
    1636 <t>
    1637    The syntax and registry of HTTP language tags is the same as that
    1638    defined by RFC 1766 <xref target="RFC1766"/>. In summary, a language tag is composed of 1
    1639    or more parts: A primary language tag and a possibly empty series of
    1640    subtags:
    1641 </t>
    1642 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="language-tag"/><iref primary="true" item="Grammar" subitem="primary-tag"/><iref primary="true" item="Grammar" subitem="subtag"/>
    1643      language-tag  = primary-tag *( "-" subtag )
    1644      primary-tag   = 1*8ALPHA
    1645      subtag        = 1*8ALPHA
    1646 </artwork></figure>
    1647 <t>
    1648    White space is not allowed within the tag and all tags are case-insensitive.
    1649    The name space of language tags is administered by the
    1650    IANA. Example tags include:
    1651 </t>
    1652 <figure><artwork type="example">
    1653     en, en-US, en-cockney, i-cherokee, x-pig-latin
    1654 </artwork></figure>
    1655 <t>
    1656    where any two-letter primary-tag is an ISO-639 language abbreviation
    1657    and any two-letter initial subtag is an ISO-3166 country code. (The
    1658    last three tags above are not registered tags; all but the last are
    1659    examples of tags which could be registered in future.)
    1660 </t>
    1661 </section>
    1663 <section title="Entity Tags" anchor="entity.tags">
    1664 <t>
    1665    Entity tags are used for comparing two or more entities from the same
    1666    requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
    1667    If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
    1668    If-Range (<xref target="header.if-range"/>) header fields. The definition of how they
    1669    are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
    1670    entity tag consists of an opaque quoted string, possibly prefixed by
    1671    a weakness indicator.
    1672 </t>
    1673 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/>
    1674    entity-tag = [ weak ] opaque-tag
    1675    weak       = "W/"
    1676    opaque-tag = quoted-string
    1677 </artwork></figure>
    1678 <t>
    1679    A "strong entity tag" &MAY; be shared by two entities of a resource
    1680    only if they are equivalent by octet equality.
    1681 </t>
    1682 <t>
    1683    A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
    1684    two entities of a resource only if the entities are equivalent and
    1685    could be substituted for each other with no significant change in
    1686    semantics. A weak entity tag can only be used for weak comparison.
    1687 </t>
    1688 <t>
    1689    An entity tag &MUST; be unique across all versions of all entities
    1690    associated with a particular resource. A given entity tag value &MAY;
    1691    be used for entities obtained by requests on different URIs. The use
    1692    of the same entity tag value in conjunction with entities obtained by
    1693    requests on different URIs does not imply the equivalence of those
    1694    entities.
    1695 </t>
    1696 </section>
    1698 <section title="Range Units" anchor="range.units">
    1699 <t>
    1700    HTTP/1.1 allows a client to request that only part (a range of) the
    1701    response entity be included within the response. HTTP/1.1 uses range
    1702    units in the Range (<xref target="header.range"/>) and Content-Range (<xref target="header.content-range"/>)
    1703    header fields. An entity can be broken down into subranges according
    1704    to various structural units.
    1705 </t>
    1706 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="range-unit"/><iref primary="true" item="Grammar" subitem="bytes-unit"/><iref primary="true" item="Grammar" subitem="other-range-unit"/>
    1707    range-unit       = bytes-unit | other-range-unit
    1708    bytes-unit       = "bytes"
    1709    other-range-unit = token
    1710 </artwork></figure>
    1711 <t>
    1712    The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
    1713    implementations &MAY; ignore ranges specified using other units.
    1714 </t>
    1715 <t>
    1716    HTTP/1.1 has been designed to allow implementations of applications
    1717    that do not depend on knowledge of ranges.
    1718 </t>
    1719 </section>
    1720 </section>
    17251258<section title="HTTP Message" anchor="http.message">
    17661299   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
    1767    request-header (<xref target="request.header.fields"/>), response-header (<xref target="response.header.fields"/>), and
    1768    entity-header (<xref target="entity.header.fields"/>) fields, follow the same generic format as
     1300   request-header (&request-header-fields;), response-header (&response-header-fields;), and
     1301   entity-header (&entity-header-fields;) fields, follow the same generic format as
    17691302   that given in <xref target="RFC822" x:fmt="sec" x:sec="3.1"/> of RFC 822 <xref target="RFC822"/>. Each header field consists
    17701303   of a name followed by a colon (":") and the field value. Field names
    18421375   inclusion of a Content-Length or Transfer-Encoding header field in
    18431376   the request's message-headers. A message-body &MUST-NOT; be included in
    1844    a request if the specification of the request method (<xref target="method"/>)
     1377   a request if the specification of the request method (&method;)
    18451378   does not allow sending an entity-body in requests. A server &SHOULD;
    18461379   read and forward a message-body on any request; if the request method
    19511484<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
    1952     general-header = Cache-Control            ; <xref target="header.cache-control"/>
     1485    general-header = Cache-Control            ; &header-cache-control;
    19531486                   | Connection               ; <xref target="header.connection"/>
    19541487                   | Date                     ; <xref target=""/>
    1955                    | Pragma                   ; <xref target="header.pragma"/>
     1488                   | Pragma                   ; &header-pragma;
    19561489                   | Trailer                  ; <xref target="header.trailer"/>
    19571490                   | Transfer-Encoding        ; <xref target="header.transfer-encoding"/>
    19581491                   | Upgrade                  ; <xref target="header.upgrade"/>
    19591492                   | Via                      ; <xref target="header.via"/>
    1960                    | Warning                  ; <xref target="header.warning"/>
     1493                   | Warning                  ; &header-warning;
    19771510   the identifier of the resource, and the protocol version in use.
     1512<!--                 Host                      ; should be moved here eventually -->
    19791513<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
    19801514     Request       = Request-Line              ; <xref target="request-line"/>
    19811515                     *(( general-header        ; <xref target="general.header.fields"/>
    1982                       | request-header         ; <xref target="request.header.fields"/>
    1983                       | entity-header ) CRLF)  ; <xref target="entity.header.fields"/>
     1516                      | request-header         ; &request-header-fields;
     1517                      | entity-header ) CRLF)  ; &entity-header-fields;
    19841518                     CRLF
    19851519                     [ message-body ]          ; <xref target="message.body"/>
    20041538<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
    2005     Method         = "OPTIONS"                ; <xref target="OPTIONS"/>
    2006                    | "GET"                    ; <xref target="GET"/>
    2007                    | "HEAD"                   ; <xref target="HEAD"/>
    2008                    | "POST"                   ; <xref target="POST"/>
    2009                    | "PUT"                    ; <xref target="PUT"/>
    2010                    | "DELETE"                 ; <xref target="DELETE"/>
    2011                    | "TRACE"                  ; <xref target="TRACE"/>
    2012                    | "CONNECT"                ; <xref target="CONNECT"/>
    2013                    | extension-method
    2014     extension-method = token
    2015 </artwork></figure>
    2016 <t>
    2017    The list of methods allowed by a resource can be specified in an
    2018    Allow header field (<xref target="header.allow"/>). The return code of the response
    2019    always notifies the client whether a method is currently allowed on a
    2020    resource, since the set of allowed methods can change dynamically. An
    2021    origin server &SHOULD; return the status code 405 (Method Not Allowed)
    2022    if the method is known by the origin server but not allowed for the
    2023    requested resource, and 501 (Not Implemented) if the method is
    2024    unrecognized or not implemented by the origin server. The methods GET
    2025    and HEAD &MUST; be supported by all general-purpose servers. All other
    2026    methods are &OPTIONAL;; however, if the above methods are implemented,
    2027    they &MUST; be implemented with the same semantics as those specified
    2028    in <xref target="method.definitions"/>.
    2029 </t>
     1539    Method         = token
    2070    The authority form is only used by the CONNECT method (<xref target="CONNECT"/>).
     1581   The authority form is only used by the CONNECT method (&CONNECT;).
    2151 <section title="Request Header Fields" anchor="request.header.fields">
    2152 <t>
    2153    The request-header fields allow the client to pass additional
    2154    information about the request, and about the client itself, to the
    2155    server. These fields act as request modifiers, with semantics
    2156    equivalent to the parameters on a programming language method
    2157    invocation.
    2158 </t>
    2159 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="request-header"/>
    2160     request-header = Accept                   ; <xref target="header.accept"/>
    2161                    | Accept-Charset           ; <xref target="header.accept-charset"/>
    2162                    | Accept-Encoding          ; <xref target="header.accept-encoding"/>
    2163                    | Accept-Language          ; <xref target="header.accept-language"/>
    2164                    | Authorization            ; <xref target="header.authorization"/>
    2165                    | Expect                   ; <xref target="header.expect"/>
    2166                    | From                     ; <xref target="header.from"/>
    2167                    | Host                     ; <xref target=""/>
    2168                    | If-Match                 ; <xref target="header.if-match"/>
    2169                    | If-Modified-Since        ; <xref target="header.if-modified-since"/>
    2170                    | If-None-Match            ; <xref target="header.if-none-match"/>
    2171                    | If-Range                 ; <xref target="header.if-range"/>
    2172                    | If-Unmodified-Since      ; <xref target="header.if-unmodified-since"/>
    2173                    | Max-Forwards             ; <xref target="header.max-forwards"/>
    2174                    | Proxy-Authorization      ; <xref target="header.proxy-authorization"/>
    2175                    | Range                    ; <xref target="header.range"/>
    2176                    | Referer                  ; <xref target="header.referer"/>
    2177                    | TE                       ; <xref target="header.te"/>
    2178                    | User-Agent               ; <xref target="header.user-agent"/>
    2179 </artwork></figure>
    2180 <t>
    2181    Request-header field names can be extended reliably only in
    2182    combination with a change in the protocol version. However, new or
    2183    experimental header fields &MAY; be given the semantics of request-header
    2184    fields if all parties in the communication recognize them to
    2185    be request-header fields. Unrecognized header fields are treated as
    2186    entity-header fields.
    2187 </t>
    2188 </section>
    21981671    Response      = Status-Line               ; <xref target="status-line"/>
    21991672                    *(( general-header        ; <xref target="general.header.fields"/>
    2200                      | response-header        ; <xref target="response.header.fields"/>
    2201                      | entity-header ) CRLF)  ; <xref target="entity.header.fields"/>
     1673                     | response-header        ; &response-header-fields;
     1674                     | entity-header ) CRLF)  ; &entity-header-fields;
    22021675                    CRLF
    2203                     [ message-body ]          ; <xref target="entity.body"/>
     1676                    [ message-body ]          ; <xref target="message.body"/>
    22191692   The Status-Code element is a 3-digit integer result code of the
    22201693   attempt to understand and satisfy the request. These codes are fully
    2221    defined in <xref target=""/>. The Reason-Phrase is intended to give a short
     1694   defined in &status-codes;. The Reason-Phrase is intended to give a short
    22221695   textual description of the Status-Code. The Status-Code is intended
    22231696   for use by automata and the Reason-Phrase is intended for the human
    22501723  </list>
    2252 <t> 
    2253    The individual values of the numeric status codes defined for
    2254    HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
    2255    presented below. The reason phrases listed here are only
    2256    recommendations -- they &MAY; be replaced by local equivalents without
    2257    affecting the protocol.
    2258 </t>
    22591725<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Code"/><iref primary="true" item="Grammar" subitem="extension-code"/><iref primary="true" item="Grammar" subitem="Reason-Phrase"/>
    2260    Status-Code    =
    2261          "100"  ; <xref target="status.100"/>: Continue
    2262        | "101"  ; <xref target="status.101"/>: Switching Protocols
    2263        | "200"  ; <xref target="status.200"/>: OK
    2264        | "201"  ; <xref target="status.201"/>: Created
    2265        | "202"  ; <xref target="status.202"/>: Accepted
    2266        | "203"  ; <xref target="status.203"/>: Non-Authoritative Information
    2267        | "204"  ; <xref target="status.204"/>: No Content
    2268        | "205"  ; <xref target="status.205"/>: Reset Content
    2269        | "206"  ; <xref target="status.206"/>: Partial Content
    2270        | "300"  ; <xref target="status.300"/>: Multiple Choices
    2271        | "301"  ; <xref target="status.301"/>: Moved Permanently
    2272        | "302"  ; <xref target="status.302"/>: Found
    2273        | "303"  ; <xref target="status.303"/>: See Other
    2274        | "304"  ; <xref target="status.304"/>: Not Modified
    2275        | "305"  ; <xref target="status.305"/>: Use Proxy
    2276        | "307"  ; <xref target="status.307"/>: Temporary Redirect
    2277        | "400"  ; <xref target="status.400"/>: Bad Request
    2278        | "401"  ; <xref target="status.401"/>: Unauthorized
    2279        | "402"  ; <xref target="status.402"/>: Payment Required
    2280        | "403"  ; <xref target="status.403"/>: Forbidden
    2281        | "404"  ; <xref target="status.404"/>: Not Found
    2282        | "405"  ; <xref target="status.405"/>: Method Not Allowed
    2283        | "406"  ; <xref target="status.406"/>: Not Acceptable
    2284        | "407"  ; <xref target="status.407"/>: Proxy Authentication Required
    2285        | "408"  ; <xref target="status.408"/>: Request Time-out
    2286        | "409"  ; <xref target="status.409"/>: Conflict
    2287        | "410"  ; <xref target="status.410"/>: Gone
    2288        | "411"  ; <xref target="status.411"/>: Length Required
    2289        | "412"  ; <xref target="status.412"/>: Precondition Failed
    2290        | "413"  ; <xref target="status.413"/>: Request Entity Too Large
    2291        | "414"  ; <xref target="status.414"/>: Request-URI Too Large
    2292        | "415"  ; <xref target="status.415"/>: Unsupported Media Type
    2293        | "416"  ; <xref target="status.416"/>: Requested range not satisfiable
    2294        | "417"  ; <xref target="status.417"/>: Expectation Failed
    2295        | "500"  ; <xref target="status.500"/>: Internal Server Error
    2296        | "501"  ; <xref target="status.501"/>: Not Implemented
    2297        | "502"  ; <xref target="status.502"/>: Bad Gateway
    2298        | "503"  ; <xref target="status.503"/>: Service Unavailable
    2299        | "504"  ; <xref target="status.504"/>: Gateway Time-out
    2300        | "505"  ; <xref target="status.505"/>: HTTP Version not supported
    2301        | extension-code
    2303    extension-code = 3DIGIT
     1726   Status-Code    = 3DIGIT
    23041727   Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
    2306 <t>
    2307    HTTP status codes are extensible. HTTP applications are not required
    2308    to understand the meaning of all registered status codes, though such
    2309    understanding is obviously desirable. However, applications &MUST;
    2310    understand the class of any status code, as indicated by the first
    2311    digit, and treat any unrecognized response as being equivalent to the
    2312    x00 status code of that class, with the exception that an
    2313    unrecognized response &MUST-NOT; be cached. For example, if an
    2314    unrecognized status code of 431 is received by the client, it can
    2315    safely assume that there was something wrong with its request and
    2316    treat the response as if it had received a 400 status code. In such
    2317    cases, user agents &SHOULD; present to the user the entity returned
    2318    with the response, since that entity is likely to include human-readable
    2319    information which will explain the unusual status.
    2320 </t>
    2321 </section>
    2322 </section>
    2324 <section title="Response Header Fields" anchor="response.header.fields">
    2325 <t>
    2326    The response-header fields allow the server to pass additional
    2327    information about the response which cannot be placed in the Status-Line.
    2328    These header fields give information about the server and about
    2329    further access to the resource identified by the Request-URI.
    2330 </t>
    2331 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="response-header"/>
    2332     response-header = Accept-Ranges           ; <xref target="header.accept-ranges"/>
    2333                     | Age                     ; <xref target="header.age"/>
    2334                     | ETag                    ; <xref target="header.etag"/>
    2335                     | Location                ; <xref target="header.location"/>
    2336                     | Proxy-Authenticate      ; <xref target="header.proxy-authenticate"/>
    2337                     | Retry-After             ; <xref target="header.retry-after"/>
    2338                     | Server                  ; <xref target="header.server"/>
    2339                     | Vary                    ; <xref target="header.vary"/>
    2340                     | WWW-Authenticate        ; <xref target="header.www-authenticate"/>
    2341 </artwork></figure>
    2342 <t>
    2343    Response-header field names can be extended reliably only in
    2344    combination with a change in the protocol version. However, new or
    2345    experimental header fields &MAY; be given the semantics of response-header
    2346    fields if all parties in the communication recognize them to
    2347    be response-header fields. Unrecognized header fields are treated as
    2348    entity-header fields.
    2349 </t>
    2350 </section>
    2351 </section>
    2354 <section title="Entity" anchor="entity">
    2355 <t>
    2356    Request and Response messages &MAY; transfer an entity if not otherwise
    2357    restricted by the request method or response status code. An entity
    2358    consists of entity-header fields and an entity-body, although some
    2359    responses will only include the entity-headers.
    2360 </t>
    2361 <t>
    2362    In this section, both sender and recipient refer to either the client
    2363    or the server, depending on who sends and who receives the entity.
    2364 </t>
    2366 <section title="Entity Header Fields" anchor="entity.header.fields">
    2367 <t>
    2368    Entity-header fields define metainformation about the entity-body or,
    2369    if no body is present, about the resource identified by the request.
    2370    Some of this metainformation is &OPTIONAL;; some might be &REQUIRED; by
    2371    portions of this specification.
    2372 </t>
    2373 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-header"/><iref primary="true" item="Grammar" subitem="extension-header"/>
    2374     entity-header  = Allow                    ; <xref target="header.allow"/>
    2375                    | Content-Encoding         ; <xref target="header.content-encoding"/>
    2376                    | Content-Language         ; <xref target="header.content-language"/>
    2377                    | Content-Length           ; <xref target="header.content-length"/>
    2378                    | Content-Location         ; <xref target="header.content-location"/>
    2379                    | Content-MD5              ; <xref target="header.content-md5"/>
    2380                    | Content-Range            ; <xref target="header.content-range"/>
    2381                    | Content-Type             ; <xref target="header.content-type"/>
    2382                    | Expires                  ; <xref target="header.expires"/>
    2383                    | Last-Modified            ; <xref target="header.last-modified"/>
    2384                    | extension-header
    2386     extension-header = message-header
    2387 </artwork></figure>
    2388 <t>
    2389    The extension-header mechanism allows additional entity-header fields
    2390    to be defined without changing the protocol, but these fields cannot
    2391    be assumed to be recognizable by the recipient. Unrecognized header
    2392    fields &SHOULD; be ignored by the recipient and &MUST; be forwarded by
    2393    transparent proxies.
    2394 </t>
    2395 </section>
    2397 <section title="Entity Body" anchor="entity.body">
    2398 <t>
    2399    The entity-body (if any) sent with an HTTP request or response is in
    2400    a format and encoding defined by the entity-header fields.
    2401 </t>
    2402 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-body"/>
    2403     entity-body    = *OCTET
    2404 </artwork></figure>
    2405 <t>
    2406    An entity-body is only present in a message when a message-body is
    2407    present, as described in <xref target="message.body"/>. The entity-body is obtained
    2408    from the message-body by decoding any Transfer-Encoding that might
    2409    have been applied to ensure safe and proper transfer of the message.
    2410 </t>
    2412 <section title="Type" anchor="type">
    2413 <t>
    2414    When an entity-body is included with a message, the data type of that
    2415    body is determined via the header fields Content-Type and Content-Encoding.
    2416    These define a two-layer, ordered encoding model:
    2417 </t>
    2418 <figure><artwork type="example">
    2419     entity-body := Content-Encoding( Content-Type( data ) )
    2420 </artwork></figure>
    2421 <t>
    2422    Content-Type specifies the media type of the underlying data.
    2423    Content-Encoding may be used to indicate any additional content
    2424    codings applied to the data, usually for the purpose of data
    2425    compression, that are a property of the requested resource. There is
    2426    no default encoding.
    2427 </t>
    2428 <t>
    2429    Any HTTP/1.1 message containing an entity-body &SHOULD; include a
    2430    Content-Type header field defining the media type of that body. If
    2431    and only if the media type is not given by a Content-Type field, the
    2432    recipient &MAY; attempt to guess the media type via inspection of its
    2433    content and/or the name extension(s) of the URI used to identify the
    2434    resource. If the media type remains unknown, the recipient &SHOULD;
    2435    treat it as type "application/octet-stream".
    2436 </t>
    2437 </section>
    2439 <section title="Entity Length" anchor="entity.length">
    2440 <t>
    2441    The entity-length of a message is the length of the message-body
    2442    before any transfer-codings have been applied. <xref target="message.length"/> defines
    2443    how the transfer-length of a message-body is determined.
    2444 </t>
    2445 </section>
    2446 </section>
    2447 </section>
    25741858   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
    2575    non-idempotent sequences of methods (see <xref target="idempotent.methods"/>). Otherwise, a
     1859   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
    25761860   premature termination of the transport connection could lead to
    25771861   indeterminate results. A client wishing to send a non-idempotent
    26311915   transport connection and retransmit the aborted sequence of requests
    26321916   without user interaction so long as the request sequence is
    2633    idempotent (see <xref target="idempotent.methods"/>). Non-idempotent methods or sequences
     1917   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
    26341918   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
    26351919   human operator the choice of retrying the request(s). Confirmation by
    26821966<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
    2684    The purpose of the 100 (Continue) status (see <xref target="status.100"/>) is to
     1968   The purpose of the 100 (Continue) status (see &status-100;) is to
    26851969   allow a client that is sending a request message with a request body
    26861970   to determine if the origin server is willing to accept the request
    26961980        If a client will wait for a 100 (Continue) response before
    26971981        sending the request body, it &MUST; send an Expect request-header
    2698         field (<xref target="header.expect"/>) with the "100-continue" expectation.
    2699     </t>
    2700     <t>
    2701         A client &MUST-NOT; send an Expect request-header field (<xref target="header.expect"/>)
     1982        field (&header-expect;) with the "100-continue" expectation.
     1983    </t>
     1984    <t>
     1985        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
    27021986        with the "100-continue" expectation if it does not intend
    27031987        to send a request body.
    27852069        client and did not include an Expect request-header field with
    27862070        the "100-continue" expectation. This requirement overrides the
    2787         general rule for forwarding of 1xx responses (see <xref target="status.1xx"/>).
     2071        general rule for forwarding of 1xx responses (see &status-1xx;).
    27882072    </t>
    27892073  </list>
    2851 <section title="Method Definitions" anchor="method.definitions">
    2852 <t>
    2853    The set of common methods for HTTP/1.1 is defined below. Although
    2854    this set can be expanded, additional methods cannot be assumed to
    2855    share the same semantics for separately extended clients and servers.
    2857    The Host request-header field (<xref target=""/>) &MUST; accompany all
    2858    HTTP/1.1 requests.
    2859 </t>
    2861 <section title="Safe and Idempotent Methods" anchor="safe.and.idempotent">
    2863 <section title="Safe Methods" anchor="safe.methods">
    2864 <t>
    2865    Implementors should be aware that the software represents the user in
    2866    their interactions over the Internet, and should be careful to allow
    2867    the user to be aware of any actions they might take which may have an
    2868    unexpected significance to themselves or others.
    2869 </t>
    2870 <t>
    2871    In particular, the convention has been established that the GET and
    2872    HEAD methods &SHOULD-NOT;  have the significance of taking an action
    2873    other than retrieval. These methods ought to be considered "safe".
    2874    This allows user agents to represent other methods, such as POST, PUT
    2875    and DELETE, in a special way, so that the user is made aware of the
    2876    fact that a possibly unsafe action is being requested.
    2877 </t>
    2878 <t>
    2879    Naturally, it is not possible to ensure that the server does not
    2880    generate side-effects as a result of performing a GET request; in
    2881    fact, some dynamic resources consider that a feature. The important
    2882    distinction here is that the user did not request the side-effects,
    2883    so therefore cannot be held accountable for them.
    2884 </t>
    2885 </section>
    2887 <section title="Idempotent Methods" anchor="idempotent.methods">
    2888 <t>
    2889    Methods can also have the property of "idempotence" in that (aside
    2890    from error or expiration issues) the side-effects of N &gt; 0 identical
    2891    requests is the same as for a single request. The methods GET, HEAD,
    2892    PUT and DELETE share this property. Also, the methods OPTIONS and
    2893    TRACE &SHOULD-NOT;  have side effects, and so are inherently idempotent.
    2894 </t>
    2895 <t>
    2896    However, it is possible that a sequence of several requests is non-idempotent,
    2897    even if all of the methods executed in that sequence are
    2898    idempotent. (A sequence is idempotent if a single execution of the
    2899    entire sequence always yields a result that is not changed by a
    2900    reexecution of all, or part, of that sequence.) For example, a
    2901    sequence is non-idempotent if its result depends on a value that is
    2902    later modified in the same sequence.
    2903 </t>
    2904 <t>
    2905    A sequence that never has side effects is idempotent, by definition
    2906    (provided that no concurrent operations are being executed on the
    2907    same set of resources).
    2908 </t>
    2909 </section>
    2910 </section>
    2912 <section title="OPTIONS" anchor="OPTIONS">
    2913   <iref primary="true" item="OPTIONS method" x:for-anchor=""/>
    2914   <iref primary="true" item="Methods" subitem="OPTIONS" x:for-anchor=""/>
    2915 <t>
    2916    The OPTIONS method represents a request for information about the
    2917    communication options available on the request/response chain
    2918    identified by the Request-URI. This method allows the client to
    2919    determine the options and/or requirements associated with a resource,
    2920    or the capabilities of a server, without implying a resource action
    2921    or initiating a resource retrieval.
    2922 </t>
    2923 <t>
    2924    Responses to this method are not cacheable.
    2925 </t>
    2926 <t>
    2927    If the OPTIONS request includes an entity-body (as indicated by the
    2928    presence of Content-Length or Transfer-Encoding), then the media type
    2929    &MUST; be indicated by a Content-Type field. Although this
    2930    specification does not define any use for such a body, future
    2931    extensions to HTTP might use the OPTIONS body to make more detailed
    2932    queries on the server. A server that does not support such an
    2933    extension &MAY; discard the request body.
    2934 </t>
    2935 <t>
    2936    If the Request-URI is an asterisk ("*"), the OPTIONS request is
    2937    intended to apply to the server in general rather than to a specific
    2938    resource. Since a server's communication options typically depend on
    2939    the resource, the "*" request is only useful as a "ping" or "no-op"
    2940    type of method; it does nothing beyond allowing the client to test
    2941    the capabilities of the server. For example, this can be used to test
    2942    a proxy for HTTP/1.1 compliance (or lack thereof).
    2943 </t>
    2944 <t>
    2945    If the Request-URI is not an asterisk, the OPTIONS request applies
    2946    only to the options that are available when communicating with that
    2947    resource.
    2948 </t>
    2949 <t>
    2950    A 200 response &SHOULD; include any header fields that indicate
    2951    optional features implemented by the server and applicable to that
    2952    resource (e.g., Allow), possibly including extensions not defined by
    2953    this specification. The response body, if any, &SHOULD; also include
    2954    information about the communication options. The format for such a
    2955    body is not defined by this specification, but might be defined by
    2956    future extensions to HTTP. Content negotiation &MAY; be used to select
    2957    the appropriate response format. If no response body is included, the
    2958    response &MUST; include a Content-Length field with a field-value of
    2959    "0".
    2960 </t>
    2961 <t>
    2962    The Max-Forwards request-header field &MAY; be used to target a
    2963    specific proxy in the request chain. When a proxy receives an OPTIONS
    2964    request on an absoluteURI for which request forwarding is permitted,
    2965    the proxy &MUST; check for a Max-Forwards field. If the Max-Forwards
    2966    field-value is zero ("0"), the proxy &MUST-NOT; forward the message;
    2967    instead, the proxy &SHOULD; respond with its own communication options.
    2968    If the Max-Forwards field-value is an integer greater than zero, the
    2969    proxy &MUST; decrement the field-value when it forwards the request. If
    2970    no Max-Forwards field is present in the request, then the forwarded
    2971    request &MUST-NOT; include a Max-Forwards field.
    2972 </t>
    2973 </section>
    2975 <section title="GET" anchor="GET">
    2976   <iref primary="true" item="GET method" x:for-anchor=""/>
    2977   <iref primary="true" item="Methods" subitem="GET" x:for-anchor=""/>
    2978 <t>
    2979    The GET method means retrieve whatever information (in the form of an
    2980    entity) is identified by the Request-URI. If the Request-URI refers
    2981    to a data-producing process, it is the produced data which shall be
    2982    returned as the entity in the response and not the source text of the
    2983    process, unless that text happens to be the output of the process.
    2984 </t>
    2985 <t>
    2986    The semantics of the GET method change to a "conditional GET" if the
    2987    request message includes an If-Modified-Since, If-Unmodified-Since,
    2988    If-Match, If-None-Match, or If-Range header field. A conditional GET
    2989    method requests that the entity be transferred only under the
    2990    circumstances described by the conditional header field(s). The
    2991    conditional GET method is intended to reduce unnecessary network
    2992    usage by allowing cached entities to be refreshed without requiring
    2993    multiple requests or transferring data already held by the client.
    2994 </t>
    2995 <t>
    2996    The semantics of the GET method change to a "partial GET" if the
    2997    request message includes a Range header field. A partial GET requests
    2998    that only part of the entity be transferred, as described in <xref target="header.range"/>.
    2999    The partial GET method is intended to reduce unnecessary
    3000    network usage by allowing partially-retrieved entities to be
    3001    completed without transferring data already held by the client.
    3002 </t>
    3003 <t>
    3004    The response to a GET request is cacheable if and only if it meets
    3005    the requirements for HTTP caching described in <xref target="caching"/>.
    3006 </t>
    3007 <t>
    3008    See <xref target=""/> for security considerations when used for forms.
    3009 </t>
    3010 </section>
    3012 <section title="HEAD" anchor="HEAD">
    3013   <iref primary="true" item="HEAD method" x:for-anchor=""/>
    3014   <iref primary="true" item="Methods" subitem="HEAD" x:for-anchor=""/>
    3015 <t>
    3016    The HEAD method is identical to GET except that the server &MUST-NOT;
    3017    return a message-body in the response. The metainformation contained
    3018    in the HTTP headers in response to a HEAD request &SHOULD; be identical
    3019    to the information sent in response to a GET request. This method can
    3020    be used for obtaining metainformation about the entity implied by the
    3021    request without transferring the entity-body itself. This method is
    3022    often used for testing hypertext links for validity, accessibility,
    3023    and recent modification.
    3024 </t>
    3025 <t>
    3026    The response to a HEAD request &MAY; be cacheable in the sense that the
    3027    information contained in the response &MAY; be used to update a
    3028    previously cached entity from that resource. If the new field values
    3029    indicate that the cached entity differs from the current entity (as
    3030    would be indicated by a change in Content-Length, Content-MD5, ETag
    3031    or Last-Modified), then the cache &MUST; treat the cache entry as
    3032    stale.
    3033 </t>
    3034 </section>
    3036 <section title="POST" anchor="POST">
    3037   <iref primary="true" item="POST method" x:for-anchor=""/>
    3038   <iref primary="true" item="Methods" subitem="POST" x:for-anchor=""/>
    3039 <t>
    3040    The POST method is used to request that the origin server accept the
    3041    entity enclosed in the request as a new subordinate of the resource
    3042    identified by the Request-URI in the Request-Line. POST is designed
    3043    to allow a uniform method to cover the following functions:
    3044   <list style="symbols">
    3045     <t>
    3046       Annotation of existing resources;
    3047     </t>
    3048     <t>
    3049         Posting a message to a bulletin board, newsgroup, mailing list,
    3050         or similar group of articles;
    3051     </t>
    3052     <t>
    3053         Providing a block of data, such as the result of submitting a
    3054         form, to a data-handling process;
    3055     </t>
    3056     <t>
    3057         Extending a database through an append operation.
    3058     </t>
    3059   </list>
    3060 </t>
    3061 <t>
    3062    The actual function performed by the POST method is determined by the
    3063    server and is usually dependent on the Request-URI. The posted entity
    3064    is subordinate to that URI in the same way that a file is subordinate
    3065    to a directory containing it, a news article is subordinate to a
    3066    newsgroup to which it is posted, or a record is subordinate to a
    3067    database.
    3068 </t>
    3069 <t>
    3070    The action performed by the POST method might not result in a
    3071    resource that can be identified by a URI. In this case, either 200
    3072    (OK) or 204 (No Content) is the appropriate response status,
    3073    depending on whether or not the response includes an entity that
    3074    describes the result.
    3075 </t>
    3076 <t>
    3077    If a resource has been created on the origin server, the response
    3078    &SHOULD; be 201 (Created) and contain an entity which describes the
    3079    status of the request and refers to the new resource, and a Location
    3080    header (see <xref target="header.location"/>).
    3081 </t>
    3082 <t>
    3083    Responses to this method are not cacheable, unless the response
    3084    includes appropriate Cache-Control or Expires header fields. However,
    3085    the 303 (See Other) response can be used to direct the user agent to
    3086    retrieve a cacheable resource.
    3087 </t>
    3088 <t>
    3089    POST requests &MUST; obey the message transmission requirements set out
    3090    in <xref target="message.transmission.requirements"/>.
    3091 </t>
    3092 <t>
    3093    See <xref target=""/> for security considerations.
    3094 </t>
    3095 </section>
    3097 <section title="PUT" anchor="PUT">
    3098   <iref primary="true" item="PUT method" x:for-anchor=""/>
    3099   <iref primary="true" item="Methods" subitem="PUT" x:for-anchor=""/>
    3100 <t>
    3101    The PUT method requests that the enclosed entity be stored under the
    3102    supplied Request-URI. If the Request-URI refers to an already
    3103    existing resource, the enclosed entity &SHOULD; be considered as a
    3104    modified version of the one residing on the origin server. If the
    3105    Request-URI does not point to an existing resource, and that URI is
    3106    capable of being defined as a new resource by the requesting user
    3107    agent, the origin server can create the resource with that URI. If a
    3108    new resource is created, the origin server &MUST; inform the user agent
    3109    via the 201 (Created) response. If an existing resource is modified,
    3110    either the 200 (OK) or 204 (No Content) response codes &SHOULD; be sent
    3111    to indicate successful completion of the request. If the resource
    3112    could not be created or modified with the Request-URI, an appropriate
    3113    error response &SHOULD; be given that reflects the nature of the
    3114    problem. The recipient of the entity &MUST-NOT; ignore any Content-*
    3115    (e.g. Content-Range) headers that it does not understand or implement
    3116    and &MUST; return a 501 (Not Implemented) response in such cases.
    3117 </t>
    3118 <t>
    3119    If the request passes through a cache and the Request-URI identifies
    3120    one or more currently cached entities, those entries &SHOULD; be
    3121    treated as stale. Responses to this method are not cacheable.
    3122 </t>
    3123 <t>
    3124    The fundamental difference between the POST and PUT requests is
    3125    reflected in the different meaning of the Request-URI. The URI in a
    3126    POST request identifies the resource that will handle the enclosed
    3127    entity. That resource might be a data-accepting process, a gateway to
    3128    some other protocol, or a separate entity that accepts annotations.
    3129    In contrast, the URI in a PUT request identifies the entity enclosed
    3130    with the request -- the user agent knows what URI is intended and the
    3131    server &MUST-NOT; attempt to apply the request to some other resource.
    3132    If the server desires that the request be applied to a different URI,
    3133    it &MUST; send a 301 (Moved Permanently) response; the user agent &MAY;
    3134    then make its own decision regarding whether or not to redirect the
    3135    request.
    3136 </t>
    3137 <t>
    3138    A single resource &MAY; be identified by many different URIs. For
    3139    example, an article might have a URI for identifying "the current
    3140    version" which is separate from the URI identifying each particular
    3141    version. In this case, a PUT request on a general URI might result in
    3142    several other URIs being defined by the origin server.
    3143 </t>
    3144 <t>
    3145    HTTP/1.1 does not define how a PUT method affects the state of an
    3146    origin server.
    3147 </t>
    3148 <t>
    3149    PUT requests &MUST; obey the message transmission requirements set out
    3150    in <xref target="message.transmission.requirements"/>.
    3151 </t>
    3152 <t>
    3153    Unless otherwise specified for a particular entity-header, the
    3154    entity-headers in the PUT request &SHOULD; be applied to the resource
    3155    created or modified by the PUT.
    3156 </t>
    3157 </section>
    3159 <section title="DELETE" anchor="DELETE">
    3160   <iref primary="true" item="DELETE method" x:for-anchor=""/>
    3161   <iref primary="true" item="Methods" subitem="DELETE" x:for-anchor=""/>
    3162 <t>
    3163    The DELETE method requests that the origin server delete the resource
    3164    identified by the Request-URI. This method &MAY; be overridden by human
    3165    intervention (or other means) on the origin server. The client cannot
    3166    be guaranteed that the operation has been carried out, even if the
    3167    status code returned from the origin server indicates that the action
    3168    has been completed successfully. However, the server &SHOULD-NOT;
    3169    indicate success unless, at the time the response is given, it
    3170    intends to delete the resource or move it to an inaccessible
    3171    location.
    3172 </t>
    3173 <t>
    3174    A successful response &SHOULD; be 200 (OK) if the response includes an
    3175    entity describing the status, 202 (Accepted) if the action has not
    3176    yet been enacted, or 204 (No Content) if the action has been enacted
    3177    but the response does not include an entity.
    3178 </t>
    3179 <t>
    3180    If the request passes through a cache and the Request-URI identifies
    3181    one or more currently cached entities, those entries &SHOULD; be
    3182    treated as stale. Responses to this method are not cacheable.
    3183 </t>
    3184 </section>
    3186 <section title="TRACE" anchor="TRACE">
    3187   <iref primary="true" item="TRACE method" x:for-anchor=""/>
    3188   <iref primary="true" item="Methods" subitem="TRACE" x:for-anchor=""/>
    3189 <t>
    3190    The TRACE method is used to invoke a remote, application-layer loop-back
    3191    of the request message. The final recipient of the request
    3192    &SHOULD; reflect the message received back to the client as the
    3193    entity-body of a 200 (OK) response. The final recipient is either the
    3194    origin server or the first proxy or gateway to receive a Max-Forwards
    3195    value of zero (0) in the request (see <xref target="header.max-forwards"/>). A TRACE request
    3196    &MUST-NOT; include an entity.
    3197 </t>
    3198 <t>
    3199    TRACE allows the client to see what is being received at the other
    3200    end of the request chain and use that data for testing or diagnostic
    3201    information. The value of the Via header field (<xref target="header.via"/>) is of
    3202    particular interest, since it acts as a trace of the request chain.
    3203    Use of the Max-Forwards header field allows the client to limit the
    3204    length of the request chain, which is useful for testing a chain of
    3205    proxies forwarding messages in an infinite loop.
    3206 </t>
    3207 <t>
    3208    If the request is valid, the response &SHOULD; contain the entire
    3209    request message in the entity-body, with a Content-Type of
    3210    "message/http". Responses to this method &MUST-NOT; be cached.
    3211 </t>
    3212 </section>
    3214 <section title="CONNECT" anchor="CONNECT">
    3215   <iref primary="true" item="CONNECT method" x:for-anchor=""/>
    3216   <iref primary="true" item="Methods" subitem="CONNECT" x:for-anchor=""/>
    3217 <t>
    3218    This specification reserves the method name CONNECT for use with a
    3219    proxy that can dynamically switch to being a tunnel (e.g. SSL
    3220    tunneling <xref target="Luo1998"/>).
    3221 </t>
    3222 </section>
    3223 </section>
    3226 <section title="Status Code Definitions" anchor="">
    3227 <t>
    3228    Each Status-Code is described below, including a description of which
    3229    method(s) it can follow and any metainformation required in the
    3230    response.
    3231 </t>
    3233 <section title="Informational 1xx" anchor="status.1xx">
    3234 <t>
    3235    This class of status code indicates a provisional response,
    3236    consisting only of the Status-Line and optional headers, and is
    3237    terminated by an empty line. There are no required headers for this
    3238    class of status code. Since HTTP/1.0 did not define any 1xx status
    3239    codes, servers &MUST-NOT; send a 1xx response to an HTTP/1.0 client
    3240    except under experimental conditions.
    3241 </t>
    3242 <t>
    3243    A client &MUST; be prepared to accept one or more 1xx status responses
    3244    prior to a regular response, even if the client does not expect a 100
    3245    (Continue) status message. Unexpected 1xx status responses &MAY; be
    3246    ignored by a user agent.
    3247 </t>
    3248 <t>
    3249    Proxies &MUST; forward 1xx responses, unless the connection between the
    3250    proxy and its client has been closed, or unless the proxy itself
    3251    requested the generation of the 1xx response. (For example, if a
    3252    proxy adds a "Expect: 100-continue" field when it forwards a request,
    3253    then it need not forward the corresponding 100 (Continue)
    3254    response(s).)
    3255 </t>
    3257 <section title="100 Continue" anchor="status.100">
    3258   <iref primary="true" item="100 Continue (status code)" x:for-anchor=""/>
    3259   <iref primary="true" item="Status Codes" subitem="100 Continue" x:for-anchor=""/>
    3260 <t>
    3261    The client &SHOULD; continue with its request. This interim response is
    3262    used to inform the client that the initial part of the request has
    3263    been received and has not yet been rejected by the server. The client
    3264    &SHOULD; continue by sending the remainder of the request or, if the
    3265    request has already been completed, ignore this response. The server
    3266    &MUST; send a final response after the request has been completed. See
    3267    <xref target="use.of.the.100.status"/> for detailed discussion of the use and handling of this
    3268    status code.
    3269 </t>
    3270 </section>
    3272 <section title="101 Switching Protocols" anchor="status.101">
    3273   <iref primary="true" item="101 Switching Protocols (status code)" x:for-anchor=""/>
    3274   <iref primary="true" item="Status Codes" subitem="101 Switching Protocols" x:for-anchor=""/>
    3275 <t>
    3276    The server understands and is willing to comply with the client's
    3277    request, via the Upgrade message header field (<xref target="header.upgrade"/>), for a
    3278    change in the application protocol being used on this connection. The
    3279    server will switch protocols to those defined by the response's
    3280    Upgrade header field immediately after the empty line which
    3281    terminates the 101 response.
    3282 </t>
    3283 <t>
    3284    The protocol &SHOULD; be switched only when it is advantageous to do
    3285    so. For example, switching to a newer version of HTTP is advantageous
    3286    over older versions, and switching to a real-time, synchronous
    3287    protocol might be advantageous when delivering resources that use
    3288    such features.
    3289 </t>
    3290 </section>
    3291 </section>
    3293 <section title="Successful 2xx" anchor="status.2xx">
    3294 <t>
    3295    This class of status code indicates that the client's request was
    3296    successfully received, understood, and accepted.
    3297 </t>
    3299 <section title="200 OK" anchor="status.200">
    3300   <iref primary="true" item="200 OK (status code)" x:for-anchor=""/>
    3301   <iref primary="true" item="Status Codes" subitem="200 OK" x:for-anchor=""/>
    3302 <t>
    3303    The request has succeeded. The information returned with the response
    3304    is dependent on the method used in the request, for example:
    3305   <list style="hanging">
    3306     <t hangText="GET">
    3307           an entity corresponding to the requested resource is sent in
    3308           the response;
    3309     </t>
    3310     <t hangText="HEAD">
    3311           the entity-header fields corresponding to the requested
    3312           resource are sent in the response without any message-body;
    3313     </t>
    3314     <t hangText="POST">
    3315       an entity describing or containing the result of the action;
    3316     </t>
    3317     <t hangText="TRACE">
    3318       an entity containing the request message as received by the
    3319       end server.
    3320     </t>
    3321   </list>
    3322 </t>
    3323 </section>
    3325 <section title="201 Created" anchor="status.201">
    3326   <iref primary="true" item="201 Created (status code)" x:for-anchor=""/>
    3327   <iref primary="true" item="Status Codes" subitem="201 Created" x:for-anchor=""/>
    3328 <t>
    3329    The request has been fulfilled and resulted in a new resource being
    3330    created. The newly created resource can be referenced by the URI(s)
    3331    returned in the entity of the response, with the most specific URI
    3332    for the resource given by a Location header field. The response
    3333    &SHOULD; include an entity containing a list of resource
    3334    characteristics and location(s) from which the user or user agent can
    3335    choose the one most appropriate. The entity format is specified by
    3336    the media type given in the Content-Type header field. The origin
    3337    server &MUST; create the resource before returning the 201 status code.
    3338    If the action cannot be carried out immediately, the server &SHOULD;
    3339    respond with 202 (Accepted) response instead.
    3340 </t>
    3341 <t>
    3342    A 201 response &MAY; contain an ETag response header field indicating
    3343    the current value of the entity tag for the requested variant just
    3344    created, see <xref target="header.etag"/>.
    3345 </t>
    3346 </section>
    3348 <section title="202 Accepted" anchor="status.202">
    3349   <iref primary="true" item="202 Accepted (status code)" x:for-anchor=""/>
    3350   <iref primary="true" item="Status Codes" subitem="202 Accepted" x:for-anchor=""/>
    3351 <t>
    3352    The request has been accepted for processing, but the processing has
    3353    not been completed.  The request might or might not eventually be
    3354    acted upon, as it might be disallowed when processing actually takes
    3355    place. There is no facility for re-sending a status code from an
    3356    asynchronous operation such as this.
    3357 </t>
    3358 <t>
    3359    The 202 response is intentionally non-committal. Its purpose is to
    3360    allow a server to accept a request for some other process (perhaps a
    3361    batch-oriented process that is only run once per day) without
    3362    requiring that the user agent's connection to the server persist
    3363    until the process is completed. The entity returned with this
    3364    response &SHOULD; include an indication of the request's current status
    3365    and either a pointer to a status monitor or some estimate of when the
    3366    user can expect the request to be fulfilled.
    3367 </t>
    3368 </section>
    3370 <section title="203 Non-Authoritative Information" anchor="status.203">
    3371   <iref primary="true" item="203 Non-Authoritative Information (status code)" x:for-anchor=""/>
    3372   <iref primary="true" item="Status Codes" subitem="203 Non-Authoritative Information" x:for-anchor=""/>
    3373 <t>
    3374    The returned metainformation in the entity-header is not the
    3375    definitive set as available from the origin server, but is gathered
    3376    from a local or a third-party copy. The set presented &MAY; be a subset
    3377    or superset of the original version. For example, including local
    3378    annotation information about the resource might result in a superset
    3379    of the metainformation known by the origin server. Use of this
    3380    response code is not required and is only appropriate when the
    3381    response would otherwise be 200 (OK).
    3382 </t>
    3383 </section>
    3385 <section title="204 No Content" anchor="status.204">
    3386   <iref primary="true" item="204 No Content (status code)" x:for-anchor=""/>
    3387   <iref primary="true" item="Status Codes" subitem="204 No Content" x:for-anchor=""/>
    3388 <t>
    3389    The server has fulfilled the request but does not need to return an
    3390    entity-body, and might want to return updated metainformation. The
    3391    response &MAY; include new or updated metainformation in the form of
    3392    entity-headers, which if present &SHOULD; be associated with the
    3393    requested variant.
    3394 </t>
    3395 <t>
    3396    If the client is a user agent, it &SHOULD-NOT;  change its document view
    3397    from that which caused the request to be sent. This response is
    3398    primarily intended to allow input for actions to take place without
    3399    causing a change to the user agent's active document view, although
    3400    any new or updated metainformation &SHOULD; be applied to the document
    3401    currently in the user agent's active view.
    3402 </t>
    3403 <t>
    3404    The 204 response &MUST-NOT; include a message-body, and thus is always
    3405    terminated by the first empty line after the header fields.
    3406 </t>
    3407 </section>
    3409 <section title="205 Reset Content" anchor="status.205">
    3410   <iref primary="true" item="205 Reset Content (status code)" x:for-anchor=""/>
    3411   <iref primary="true" item="Status Codes" subitem="205 Reset Content" x:for-anchor=""/>
    3412 <t>
    3413    The server has fulfilled the request and the user agent &SHOULD; reset
    3414    the document view which caused the request to be sent. This response
    3415    is primarily intended to allow input for actions to take place via
    3416    user input, followed by a clearing of the form in which the input is
    3417    given so that the user can easily initiate another input action. The
    3418    response &MUST-NOT; include an entity.
    3419 </t>
    3420 </section>
    3422 <section title="206 Partial Content" anchor="status.206">
    3423   <iref primary="true" item="206 Partial Content (status code)" x:for-anchor=""/>
    3424   <iref primary="true" item="Status Codes" subitem="206 Partial Content" x:for-anchor=""/>
    3425 <t>
    3426    The server has fulfilled the partial GET request for the resource.
    3427    The request &MUST; have included a Range header field (<xref target="header.range"/>)
    3428    indicating the desired range, and &MAY; have included an If-Range
    3429    header field (<xref target="header.if-range"/>) to make the request conditional.
    3430 </t>
    3431 <t>
    3432    The response &MUST; include the following header fields:
    3433   <list style="symbols">
    3434     <t>
    3435         Either a Content-Range header field (<xref target="header.content-range"/>) indicating
    3436         the range included with this response, or a multipart/byteranges
    3437         Content-Type including Content-Range fields for each part. If a
    3438         Content-Length header field is present in the response, its
    3439         value &MUST; match the actual number of OCTETs transmitted in the
    3440         message-body.
    3441     </t>
    3442     <t>
    3443         Date
    3444     </t>
    3445     <t>
    3446         ETag and/or Content-Location, if the header would have been sent
    3447         in a 200 response to the same request
    3448     </t>
    3449     <t>
    3450         Expires, Cache-Control, and/or Vary, if the field-value might
    3451         differ from that sent in any previous response for the same
    3452         variant
    3453     </t>
    3454   </list>
    3455 </t>
    3456 <t>
    3457    If the 206 response is the result of an If-Range request that used a
    3458    strong cache validator (see <xref target="weak.and.strong.validators"/>), the response &SHOULD-NOT;
    3459    include other entity-headers. If the response is the result of an
    3460    If-Range request that used a weak validator, the response &MUST-NOT;
    3461    include other entity-headers; this prevents inconsistencies between
    3462    cached entity-bodies and updated headers. Otherwise, the response
    3463    &MUST; include all of the entity-headers that would have been returned
    3464    with a 200 (OK) response to the same request.
    3465 </t>
    3466 <t>
    3467    A cache &MUST-NOT; combine a 206 response with other previously cached
    3468    content if the ETag or Last-Modified headers do not match exactly,
    3469    see <xref target="combining.byte.ranges" format="counter"/>.
    3470 </t>
    3471 <t>
    3472    A cache that does not support the Range and Content-Range headers
    3473    &MUST-NOT; cache 206 (Partial) responses.
    3474 </t>
    3475 </section>
    3476 </section>
    3478 <section title="Redirection 3xx" anchor="status.3xx">
    3479 <t>
    3480    This class of status code indicates that further action needs to be
    3481    taken by the user agent in order to fulfill the request.  The action
    3482    required &MAY; be carried out by the user agent without interaction
    3483    with the user if and only if the method used in the second request is
    3484    GET or HEAD. A client &SHOULD; detect infinite redirection loops, since
    3485    such loops generate network traffic for each redirection.
    3486   <list><t>
    3487       <x:h>Note:</x:h> previous versions of this specification recommended a
    3488       maximum of five redirections. Content developers should be aware
    3489       that there might be clients that implement such a fixed
    3490       limitation.
    3491   </t></list>
    3492 </t>
    3494 <section title="300 Multiple Choices" anchor="status.300">
    3495   <iref primary="true" item="300 Multiple Choices (status code)" x:for-anchor=""/>
    3496   <iref primary="true" item="Status Codes" subitem="300 Multiple Choices" x:for-anchor=""/>
    3497 <t>
    3498    The requested resource corresponds to any one of a set of
    3499    representations, each with its own specific location, and agent-driven
    3500    negotiation information (<xref target="content.negotiation"/>) is being provided so that
    3501    the user (or user agent) can select a preferred representation and
    3502    redirect its request to that location.
    3503 </t>
    3504 <t>
    3505    Unless it was a HEAD request, the response &SHOULD; include an entity
    3506    containing a list of resource characteristics and location(s) from
    3507    which the user or user agent can choose the one most appropriate. The
    3508    entity format is specified by the media type given in the Content-Type
    3509    header field. Depending upon the format and the capabilities of
    3510    the user agent, selection of the most appropriate choice &MAY; be
    3511    performed automatically. However, this specification does not define
    3512    any standard for such automatic selection.
    3513 </t>
    3514 <t>
    3515    If the server has a preferred choice of representation, it &SHOULD;
    3516    include the specific URI for that representation in the Location
    3517    field; user agents &MAY; use the Location field value for automatic
    3518    redirection. This response is cacheable unless indicated otherwise.
    3519 </t>
    3520 </section>
    3522 <section title="301 Moved Permanently" anchor="status.301">
    3523   <iref primary="true" item="301 Moved Permanently (status code)" x:for-anchor=""/>
    3524   <iref primary="true" item="Status Codes" subitem="301 Moved Permanently" x:for-anchor=""/>
    3525 <t>
    3526    The requested resource has been assigned a new permanent URI and any
    3527    future references to this resource &SHOULD; use one of the returned
    3528    URIs.  Clients with link editing capabilities ought to automatically
    3529    re-link references to the Request-URI to one or more of the new
    3530    references returned by the server, where possible. This response is
    3531    cacheable unless indicated otherwise.
    3532 </t>
    3533 <t>
    3534    The new permanent URI &SHOULD; be given by the Location field in the
    3535    response. Unless the request method was HEAD, the entity of the
    3536    response &SHOULD; contain a short hypertext note with a hyperlink to
    3537    the new URI(s).
    3538 </t>
    3539 <t>
    3540    If the 301 status code is received in response to a request other
    3541    than GET or HEAD, the user agent &MUST-NOT; automatically redirect the
    3542    request unless it can be confirmed by the user, since this might
    3543    change the conditions under which the request was issued.
    3544   <list><t>
    3545       <x:h>Note:</x:h> When automatically redirecting a POST request after
    3546       receiving a 301 status code, some existing HTTP/1.0 user agents
    3547       will erroneously change it into a GET request.
    3548   </t></list>
    3549 </t>
    3550 </section>
    3552 <section title="302 Found" anchor="status.302">
    3553   <iref primary="true" item="302 Found (status code)" x:for-anchor=""/>
    3554   <iref primary="true" item="Status Codes" subitem="302 Found" x:for-anchor=""/>
    3555 <t>
    3556    The requested resource resides temporarily under a different URI.
    3557    Since the redirection might be altered on occasion, the client &SHOULD;
    3558    continue to use the Request-URI for future requests.  This response
    3559    is only cacheable if indicated by a Cache-Control or Expires header
    3560    field.
    3561 </t>
    3562 <t>
    3563    The temporary URI &SHOULD; be given by the Location field in the
    3564    response. Unless the request method was HEAD, the entity of the
    3565    response &SHOULD; contain a short hypertext note with a hyperlink to
    3566    the new URI(s).
    3567 </t>
    3568 <t>
    3569    If the 302 status code is received in response to a request other
    3570    than GET or HEAD, the user agent &MUST-NOT; automatically redirect the
    3571    request unless it can be confirmed by the user, since this might
    3572    change the conditions under which the request was issued.
    3573   <list><t>
    3574       <x:h>Note:</x:h> RFC 1945 and RFC 2068 specify that the client is not allowed
    3575       to change the method on the redirected request.  However, most
    3576       existing user agent implementations treat 302 as if it were a 303
    3577       response, performing a GET on the Location field-value regardless
    3578       of the original request method. The status codes 303 and 307 have
    3579       been added for servers that wish to make unambiguously clear which
    3580       kind of reaction is expected of the client.
    3581   </t></list>
    3582 </t>
    3583 </section>
    3585 <section title="303 See Other" anchor="status.303">
    3586   <iref primary="true" item="303 See Other (status code)" x:for-anchor=""/>
    3587   <iref primary="true" item="Status Codes" subitem="303 See Other" x:for-anchor=""/>
    3588 <t>
    3589    The response to the request can be found under a different URI and
    3590    &SHOULD; be retrieved using a GET method on that resource. This method
    3591    exists primarily to allow the output of a POST-activated script to
    3592    redirect the user agent to a selected resource. The new URI is not a
    3593    substitute reference for the originally requested resource. The 303
    3594    response &MUST-NOT; be cached, but the response to the second
    3595    (redirected) request might be cacheable.
    3596 </t>
    3597 <t>
    3598    The different URI &SHOULD; be given by the Location field in the
    3599    response. Unless the request method was HEAD, the entity of the
    3600    response &SHOULD; contain a short hypertext note with a hyperlink to
    3601    the new URI(s).
    3602   <list><t>
    3603       <x:h>Note:</x:h> Many pre-HTTP/1.1 user agents do not understand the 303
    3604       status. When interoperability with such clients is a concern, the
    3605       302 status code may be used instead, since most user agents react
    3606       to a 302 response as described here for 303.
    3607   </t></list>
    3608 </t>
    3609 </section>
    3611 <section title="304 Not Modified" anchor="status.304">
    3612   <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
    3613   <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
    3614 <t>
    3615    If the client has performed a conditional GET request and access is
    3616    allowed, but the document has not been modified, the server &SHOULD;
    3617    respond with this status code. The 304 response &MUST-NOT; contain a
    3618    message-body, and thus is always terminated by the first empty line
    3619    after the header fields.
    3620 </t>
    3621 <t>
    3622    The response &MUST; include the following header fields:
    3623   <list style="symbols">
    3624     <t>Date, unless its omission is required by <xref target="clockless.origin.server.operation"/></t>
    3625   </list>
    3626 </t>
    3627 <t>
    3628    If a clockless origin server obeys these rules, and proxies and
    3629    clients add their own Date to any response received without one (as
    3630    already specified by [RFC 2068], section <xref target="RFC2068" x:sec="14.19" x:fmt="number"/>), caches will operate
    3631    correctly.
    3632   <list style="symbols">
    3633     <t>ETag and/or Content-Location, if the header would have been sent
    3634         in a 200 response to the same request</t>
    3635     <t>Expires, Cache-Control, and/or Vary, if the field-value might
    3636         differ from that sent in any previous response for the same
    3637         variant</t>
    3638   </list>
    3639 </t>
    3640 <t>
    3641    If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
    3642    the response &SHOULD-NOT;  include other entity-headers.
    3643    Otherwise (i.e., the conditional GET used a weak validator), the
    3644    response &MUST-NOT; include other entity-headers; this prevents
    3645    inconsistencies between cached entity-bodies and updated headers.
    3646 </t>
    3647 <t>
    3648    If a 304 response indicates an entity not currently cached, then the
    3649    cache &MUST; disregard the response and repeat the request without the
    3650    conditional.
    3651 </t>
    3652 <t>
    3653    If a cache uses a received 304 response to update a cache entry, the
    3654    cache &MUST; update the entry to reflect any new field values given in
    3655    the response.
    3656 </t>
    3657 </section>
    3659 <section title="305 Use Proxy" anchor="status.305">
    3660   <iref primary="true" item="305 Use Proxy (status code)" x:for-anchor=""/>
    3661   <iref primary="true" item="Status Codes" subitem="305 Use Proxy" x:for-anchor=""/>
    3662 <t>
    3663    The requested resource &MUST; be accessed through the proxy given by
    3664    the Location field. The Location field gives the URI of the proxy.
    3665    The recipient is expected to repeat this single request via the
    3666    proxy. 305 responses &MUST; only be generated by origin servers.
    3667   <list><t>
    3668       <x:h>Note:</x:h> RFC 2068 was not clear that 305 was intended to redirect a
    3669       single request, and to be generated by origin servers only.  Not
    3670       observing these limitations has significant security consequences.
    3671   </t></list>
    3672 </t>
    3673 </section>
    3675 <section title="306 (Unused)" anchor="status.306">
    3676   <iref primary="true" item="306 (Unused) (status code)" x:for-anchor=""/>
    3677   <iref primary="true" item="Status Codes" subitem="306 (Unused)" x:for-anchor=""/>
    3678 <t>
    3679    The 306 status code was used in a previous version of the
    3680    specification, is no longer used, and the code is reserved.
    3681 </t>
    3682 </section>
    3684 <section title="307 Temporary Redirect" anchor="status.307">
    3685   <iref primary="true" item="307 Temporary Redirect (status code)" x:for-anchor=""/>
    3686   <iref primary="true" item="Status Codes" subitem="307 Temporary Redirect" x:for-anchor=""/>
    3687 <t>
    3688    The requested resource resides temporarily under a different URI.
    3689    Since the redirection &MAY; be altered on occasion, the client &SHOULD;
    3690    continue to use the Request-URI for future requests.  This response
    3691    is only cacheable if indicated by a Cache-Control or Expires header
    3692    field.
    3693 </t>
    3694 <t>
    3695    The temporary URI &SHOULD; be given by the Location field in the
    3696    response. Unless the request method was HEAD, the entity of the
    3697    response &SHOULD; contain a short hypertext note with a hyperlink to
    3698    the new URI(s) , since many pre-HTTP/1.1 user agents do not
    3699    understand the 307 status. Therefore, the note &SHOULD; contain the
    3700    information necessary for a user to repeat the original request on
    3701    the new URI.
    3702 </t>
    3703 <t>
    3704    If the 307 status code is received in response to a request other
    3705    than GET or HEAD, the user agent &MUST-NOT; automatically redirect the
    3706    request unless it can be confirmed by the user, since this might
    3707    change the conditions under which the request was issued.
    3708 </t>
    3709 </section>
    3710 </section>
    3712 <section title="Client Error 4xx" anchor="status.4xx">
    3713 <t>
    3714    The 4xx class of status code is intended for cases in which the
    3715    client seems to have erred. Except when responding to a HEAD request,
    3716    the server &SHOULD; include an entity containing an explanation of the
    3717    error situation, and whether it is a temporary or permanent
    3718    condition. These status codes are applicable to any request method.
    3719    User agents &SHOULD; display any included entity to the user.
    3720 </t>
    3721 <t>
    3722    If the client is sending data, a server implementation using TCP
    3723    &SHOULD; be careful to ensure that the client acknowledges receipt of
    3724    the packet(s) containing the response, before the server closes the
    3725    input connection. If the client continues sending data to the server
    3726    after the close, the server's TCP stack will send a reset packet to
    3727    the client, which may erase the client's unacknowledged input buffers
    3728    before they can be read and interpreted by the HTTP application.
    3729 </t>
    3731 <section title="400 Bad Request" anchor="status.400">
    3732   <iref primary="true" item="400 Bad Request (status code)" x:for-anchor=""/>
    3733   <iref primary="true" item="Status Codes" subitem="400 Bad Request" x:for-anchor=""/>
    3734 <t>
    3735    The request could not be understood by the server due to malformed
    3736    syntax. The client &SHOULD-NOT;  repeat the request without
    3737    modifications.
    3738 </t>
    3739 </section>
    3741 <section title="401 Unauthorized" anchor="status.401">
    3742   <iref primary="true" item="401 Unauthorized (status code)" x:for-anchor=""/>
    3743   <iref primary="true" item="Status Codes" subitem="401 Unauthorized" x:for-anchor=""/>
    3744 <t>
    3745    The request requires user authentication. The response &MUST; include a
    3746    WWW-Authenticate header field (<xref target="header.www-authenticate"/>) containing a challenge
    3747    applicable to the requested resource. The client &MAY; repeat the
    3748    request with a suitable Authorization header field (<xref target="header.authorization"/>). If
    3749    the request already included Authorization credentials, then the 401
    3750    response indicates that authorization has been refused for those
    3751    credentials. If the 401 response contains the same challenge as the
    3752    prior response, and the user agent has already attempted
    3753    authentication at least once, then the user &SHOULD; be presented the
    3754    entity that was given in the response, since that entity might
    3755    include relevant diagnostic information. HTTP access authentication
    3756    is explained in "HTTP Authentication: Basic and Digest Access
    3757    Authentication" <xref target="RFC2617"/>.
    3758 </t>
    3759 </section>
    3761 <section title="402 Payment Required" anchor="status.402">
    3762   <iref primary="true" item="402 Payment Required (status code)" x:for-anchor=""/>
    3763   <iref primary="true" item="Status Codes" subitem="402 Payment Required" x:for-anchor=""/>
    3764 <t>
    3765    This code is reserved for future use.
    3766 </t>
    3767 </section>
    3769 <section title="403 Forbidden" anchor="status.403">
    3770   <iref primary="true" item="403 Forbidden (status code)" x:for-anchor=""/>
    3771   <iref primary="true" item="Status Codes" subitem="403 Forbidden" x:for-anchor=""/>
    3772 <t>
    3773    The server understood the request, but is refusing to fulfill it.
    3774    Authorization will not help and the request &SHOULD-NOT;  be repeated.
    3775    If the request method was not HEAD and the server wishes to make
    3776    public why the request has not been fulfilled, it &SHOULD; describe the
    3777    reason for the refusal in the entity.  If the server does not wish to
    3778    make this information available to the client, the status code 404
    3779    (Not Found) can be used instead.
    3780 </t>
    3781 </section>
    3783 <section title="404 Not Found" anchor="status.404">
    3784   <iref primary="true" item="404 Not Found (status code)" x:for-anchor=""/>
    3785   <iref primary="true" item="Status Codes" subitem="404 Not Found" x:for-anchor=""/>
    3786 <t>
    3787    The server has not found anything matching the Request-URI. No
    3788    indication is given of whether the condition is temporary or
    3789    permanent. The 410 (Gone) status code &SHOULD; be used if the server
    3790    knows, through some internally configurable mechanism, that an old
    3791    resource is permanently unavailable and has no forwarding address.
    3792    This status code is commonly used when the server does not wish to
    3793    reveal exactly why the request has been refused, or when no other
    3794    response is applicable.
    3795 </t>
    3796 </section>
    3798 <section title="405 Method Not Allowed" anchor="status.405">
    3799   <iref primary="true" item="405 Method Not Allowed (status code)" x:for-anchor=""/>
    3800   <iref primary="true" item="Status Codes" subitem="405 Method Not Allowed" x:for-anchor=""/>
    3801 <t>
    3802    The method specified in the Request-Line is not allowed for the
    3803    resource identified by the Request-URI. The response &MUST; include an
    3804    Allow header containing a list of valid methods for the requested
    3805    resource.
    3806 </t>
    3807 </section>
    3809 <section title="406 Not Acceptable" anchor="status.406">
    3810   <iref primary="true" item="406 Not Acceptable (status code)" x:for-anchor=""/>
    3811   <iref primary="true" item="Status Codes" subitem="406 Not Acceptable" x:for-anchor=""/>
    3812 <t>
    3813    The resource identified by the request is only capable of generating
    3814    response entities which have content characteristics not acceptable
    3815    according to the accept headers sent in the request.
    3816 </t>
    3817 <t>
    3818    Unless it was a HEAD request, the response &SHOULD; include an entity
    3819    containing a list of available entity characteristics and location(s)
    3820    from which the user or user agent can choose the one most
    3821    appropriate. The entity format is specified by the media type given
    3822    in the Content-Type header field. Depending upon the format and the
    3823    capabilities of the user agent, selection of the most appropriate
    3824    choice &MAY; be performed automatically. However, this specification
    3825    does not define any standard for such automatic selection.
    3826   <list><t>
    3827       <x:h>Note:</x:h> HTTP/1.1 servers are allowed to return responses which are
    3828       not acceptable according to the accept headers sent in the
    3829       request. In some cases, this may even be preferable to sending a
    3830       406 response. User agents are encouraged to inspect the headers of
    3831       an incoming response to determine if it is acceptable.
    3832   </t></list>
    3833 </t>
    3834 <t>
    3835    If the response could be unacceptable, a user agent &SHOULD;
    3836    temporarily stop receipt of more data and query the user for a
    3837    decision on further actions.
    3838 </t>
    3839 </section>
    3841 <section title="407 Proxy Authentication Required" anchor="status.407">
    3842   <iref primary="true" item="407 Proxy Authentication Required (status code)" x:for-anchor=""/>
    3843   <iref primary="true" item="Status Codes" subitem="407 Proxy Authentication Required" x:for-anchor=""/>
    3844 <t>
    3845    This code is similar to 401 (Unauthorized), but indicates that the
    3846    client must first authenticate itself with the proxy. The proxy &MUST;
    3847    return a Proxy-Authenticate header field (<xref target="header.proxy-authenticate"/>) containing a
    3848    challenge applicable to the proxy for the requested resource. The
    3849    client &MAY; repeat the request with a suitable Proxy-Authorization
    3850    header field (<xref target="header.proxy-authorization"/>). HTTP access authentication is explained
    3851    in "HTTP Authentication: Basic and Digest Access Authentication"
    3852    <xref target="RFC2617"/>.
    3853 </t>
    3854 </section>
    3856 <section title="408 Request Timeout" anchor="status.408">
    3857   <iref primary="true" item="408 Request Timeout (status code)" x:for-anchor=""/>
    3858   <iref primary="true" item="Status Codes" subitem="408 Request Timeout" x:for-anchor=""/>
    3859 <t>
    3860    The client did not produce a request within the time that the server
    3861    was prepared to wait. The client &MAY; repeat the request without
    3862    modifications at any later time.
    3863 </t>
    3864 </section>
    3866 <section title="409 Conflict" anchor="status.409">
    3867   <iref primary="true" item="409 Conflict (status code)" x:for-anchor=""/>
    3868   <iref primary="true" item="Status Codes" subitem="409 Conflict" x:for-anchor=""/>
    3869 <t>
    3870    The request could not be completed due to a conflict with the current
    3871    state of the resource. This code is only allowed in situations where
    3872    it is expected that the user might be able to resolve the conflict
    3873    and resubmit the request. The response body &SHOULD; include enough
    3874    information for the user to recognize the source of the conflict.
    3875    Ideally, the response entity would include enough information for the
    3876    user or user agent to fix the problem; however, that might not be
    3877    possible and is not required.
    3878 </t>
    3879 <t>
    3880    Conflicts are most likely to occur in response to a PUT request. For
    3881    example, if versioning were being used and the entity being PUT
    3882    included changes to a resource which conflict with those made by an
    3883    earlier (third-party) request, the server might use the 409 response
    3884    to indicate that it can't complete the request. In this case, the
    3885    response entity would likely contain a list of the differences
    3886    between the two versions in a format defined by the response
    3887    Content-Type.
    3888 </t>
    3889 </section>
    3891 <section title="410 Gone" anchor="status.410">
    3892   <iref primary="true" item="410 Gone (status code)" x:for-anchor=""/>
    3893   <iref primary="true" item="Status Codes" subitem="410 Gone" x:for-anchor=""/>
    3894 <t>
    3895    The requested resource is no longer available at the server and no
    3896    forwarding address is known. This condition is expected to be
    3897    considered permanent. Clients with link editing capabilities &SHOULD;
    3898    delete references to the Request-URI after user approval. If the
    3899    server does not know, or has no facility to determine, whether or not
    3900    the condition is permanent, the status code 404 (Not Found) &SHOULD; be
    3901    used instead. This response is cacheable unless indicated otherwise.
    3902 </t>
    3903 <t>
    3904    The 410 response is primarily intended to assist the task of web
    3905    maintenance by notifying the recipient that the resource is
    3906    intentionally unavailable and that the server owners desire that
    3907    remote links to that resource be removed. Such an event is common for
    3908    limited-time, promotional services and for resources belonging to
    3909    individuals no longer working at the server's site. It is not
    3910    necessary to mark all permanently unavailable resources as "gone" or
    3911    to keep the mark for any length of time -- that is left to the
    3912    discretion of the server owner.
    3913 </t>
    3914 </section>
    3916 <section title="411 Length Required" anchor="status.411">
    3917   <iref primary="true" item="411 Length Required (status code)" x:for-anchor=""/>
    3918   <iref primary="true" item="Status Codes" subitem="411 Length Required" x:for-anchor=""/>
    3919 <t>
    3920    The server refuses to accept the request without a defined Content-Length.
    3921    The client &MAY; repeat the request if it adds a valid
    3922    Content-Length header field containing the length of the message-body
    3923    in the request message.
    3924 </t>
    3925 </section>
    3927 <section title="412 Precondition Failed" anchor="status.412">
    3928   <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
    3929   <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
    3930 <t>
    3931    The precondition given in one or more of the request-header fields
    3932    evaluated to false when it was tested on the server. This response
    3933    code allows the client to place preconditions on the current resource
    3934    metainformation (header field data) and thus prevent the requested
    3935    method from being applied to a resource other than the one intended.
    3936 </t>
    3937 </section>
    3939 <section title="413 Request Entity Too Large" anchor="status.413">
    3940   <iref primary="true" item="413 Request Entity Too Large (status code)" x:for-anchor=""/>
    3941   <iref primary="true" item="Status Codes" subitem="413 Request Entity Too Large" x:for-anchor=""/>
    3942 <t>
    3943    The server is refusing to process a request because the request
    3944    entity is larger than the server is willing or able to process. The
    3945    server &MAY; close the connection to prevent the client from continuing
    3946    the request.
    3947 </t>
    3948 <t>
    3949    If the condition is temporary, the server &SHOULD; include a Retry-After
    3950    header field to indicate that it is temporary and after what
    3951    time the client &MAY; try again.
    3952 </t>
    3953 </section>
    3955 <section title="414 Request-URI Too Long" anchor="status.414">
    3956   <iref primary="true" item="414 Request-URI Too Long (status code)" x:for-anchor=""/>
    3957   <iref primary="true" item="Status Codes" subitem="414 Request-URI Too Long" x:for-anchor=""/>
    3958 <t>
    3959    The server is refusing to service the request because the Request-URI
    3960    is longer than the server is willing to interpret. This rare
    3961    condition is only likely to occur when a client has improperly
    3962    converted a POST request to a GET request with long query
    3963    information, when the client has descended into a URI "black hole" of
    3964    redirection (e.g., a redirected URI prefix that points to a suffix of
    3965    itself), or when the server is under attack by a client attempting to
    3966    exploit security holes present in some servers using fixed-length
    3967    buffers for reading or manipulating the Request-URI.
    3968 </t>
    3969 </section>
    3971 <section title="415 Unsupported Media Type" anchor="status.415">
    3972   <iref primary="true" item="415 Unsupported Media Type (status code)" x:for-anchor=""/>
    3973   <iref primary="true" item="Status Codes" subitem="415 Unsupported Media Type" x:for-anchor=""/>
    3974 <t>
    3975    The server is refusing to service the request because the entity of
    3976    the request is in a format not supported by the requested resource
    3977    for the requested method.
    3978 </t>
    3979 </section>
    3981 <section title="416 Requested Range Not Satisfiable" anchor="status.416">
    3982   <iref primary="true" item="416 Requested Range Not Satisfiable (status code)" x:for-anchor=""/>
    3983   <iref primary="true" item="Status Codes" subitem="416 Requested Range Not Satisfiable" x:for-anchor=""/>
    3984 <t>
    3985    A server &SHOULD; return a response with this status code if a request
    3986    included a Range request-header field (<xref target="header.range"/>), and none of
    3987    the range-specifier values in this field overlap the current extent
    3988    of the selected resource, and the request did not include an If-Range
    3989    request-header field. (For byte-ranges, this means that the first-byte-pos
    3990    of all of the byte-range-spec values were greater than the
    3991    current length of the selected resource.)
    3992 </t>
    3993 <t>
    3994    When this status code is returned for a byte-range request, the
    3995    response &SHOULD; include a Content-Range entity-header field
    3996    specifying the current length of the selected resource (see <xref target="header.content-range"/>).
    3997    This response &MUST-NOT; use the multipart/byteranges content-type.
    3998 </t>
    3999 </section>
    4001 <section title="417 Expectation Failed" anchor="status.417">
    4002   <iref primary="true" item="417 Expectation Failed (status code)" x:for-anchor=""/>
    4003   <iref primary="true" item="Status Codes" subitem="417 Expectation Failed" x:for-anchor=""/>
    4004 <t>
    4005    The expectation given in an Expect request-header field (see <xref target="header.expect"/>)
    4006    could not be met by this server, or, if the server is a proxy,
    4007    the server has unambiguous evidence that the request could not be met
    4008    by the next-hop server.
    4009 </t>
    4010 </section>
    4011 </section>
    4013 <section title="Server Error 5xx" anchor="status.5xx">
    4014 <t>
    4015    Response status codes beginning with the digit "5" indicate cases in
    4016    which the server is aware that it has erred or is incapable of
    4017    performing the request. Except when responding to a HEAD request, the
    4018    server &SHOULD; include an entity containing an explanation of the
    4019    error situation, and whether it is a temporary or permanent
    4020    condition. User agents &SHOULD; display any included entity to the
    4021    user. These response codes are applicable to any request method.
    4022 </t>
    4024 <section title="500 Internal Server Error" anchor="status.500">
    4025   <iref primary="true" item="500 Internal Server Error (status code)" x:for-anchor=""/>
    4026   <iref primary="true" item="Status Codes" subitem="500 Internal Server Error" x:for-anchor=""/>
    4027 <t>
    4028    The server encountered an unexpected condition which prevented it
    4029    from fulfilling the request.
    4030 </t>
    4031 </section>
    4033 <section title="501 Not Implemented" anchor="status.501">
    4034   <iref primary="true" item="501 Not Implemented (status code)" x:for-anchor=""/>
    4035   <iref primary="true" item="Status Codes" subitem="501 Not Implemented" x:for-anchor=""/>
    4036 <t>
    4037    The server does not support the functionality required to fulfill the
    4038    request. This is the appropriate response when the server does not
    4039    recognize the request method and is not capable of supporting it for
    4040    any resource.
    4041 </t>
    4042 </section>
    4044 <section title="502 Bad Gateway" anchor="status.502">
    4045   <iref primary="true" item="502 Bad Gateway (status code)" x:for-anchor=""/>
    4046   <iref primary="true" item="Status Codes" subitem="502 Bad Gateway" x:for-anchor=""/>
    4047 <t>
    4048    The server, while acting as a gateway or proxy, received an invalid
    4049    response from the upstream server it accessed in attempting to
    4050    fulfill the request.
    4051 </t>
    4052 </section>
    4054 <section title="503 Service Unavailable" anchor="status.503">
    4055   <iref primary="true" item="503 Service Unavailable (status code)" x:for-anchor=""/>
    4056   <iref primary="true" item="Status Codes" subitem="503 Service Unavailable" x:for-anchor=""/>
    4057 <t>
    4058    The server is currently unable to handle the request due to a
    4059    temporary overloading or maintenance of the server. The implication
    4060    is that this is a temporary condition which will be alleviated after
    4061    some delay. If known, the length of the delay &MAY; be indicated in a
    4062    Retry-After header. If no Retry-After is given, the client &SHOULD;
    4063    handle the response as it would for a 500 response.
    4064   <list><t>
    4065       <x:h>Note:</x:h> The existence of the 503 status code does not imply that a
    4066       server must use it when becoming overloaded. Some servers may wish
    4067       to simply refuse the connection.
    4068   </t></list>
    4069 </t>
    4070 </section>
    4072 <section title="504 Gateway Timeout" anchor="status.504">
    4073   <iref primary="true" item="504 Gateway Timeout (status code)" x:for-anchor=""/>
    4074   <iref primary="true" item="Status Codes" subitem="504 Gateway Timeout" x:for-anchor=""/>
    4075 <t>
    4076    The server, while acting as a gateway or proxy, did not receive a
    4077    timely response from the upstream server specified by the URI (e.g.
    4078    HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
    4079    to access in attempting to complete the request.
    4080   <list><t>
    4081       <x:h>Note:</x:h> Note to implementors: some deployed proxies are known to
    4082       return 400 or 500 when DNS lookups time out.
    4083   </t></list>
    4084 </t>
    4085 </section>
    4087 <section title="505 HTTP Version Not Supported" anchor="status.505">
    4088   <iref primary="true" item="505 HTTP Version Not Supported (status code)" x:for-anchor=""/>
    4089   <iref primary="true" item="Status Codes" subitem="505 HTTP Version Not Supported" x:for-anchor=""/>
    4090 <t>
    4091    The server does not support, or refuses to support, the HTTP protocol
    4092    version that was used in the request message. The server is
    4093    indicating that it is unable or unwilling to complete the request
    4094    using the same major version as the client, as described in <xref target="http.version"/>,
    4095    other than with this error message. The response &SHOULD; contain
    4096    an entity describing why that version is not supported and what other
    4097    protocols are supported by that server.
    4098 </t>
    4100 </section>
    4101 </section>
    4102 </section>
    4105 <section title="Access Authentication" anchor="access.authentication">
    4106 <t>
    4107    HTTP provides several &OPTIONAL; challenge-response authentication
    4109    mechanisms which can be used by a server to challenge a client
    4110    request and by a client to provide authentication information. The
    4111    general framework for access authentication, and the specification of
    4112    "basic" and "digest" authentication, are specified in "HTTP
    4113    Authentication: Basic and Digest Access Authentication" <xref target="RFC2617"/>. This
    4114    specification adopts the definitions of "challenge" and "credentials"
    4115    from that specification.
    4116 </t>
    4117 </section>
    4119 <section title="Content Negotiation" anchor="content.negotiation">
    4120 <t>
    4121    Most HTTP responses include an entity which contains information for
    4122    interpretation by a human user. Naturally, it is desirable to supply
    4123    the user with the "best available" entity corresponding to the
    4124    request. Unfortunately for servers and caches, not all users have the
    4125    same preferences for what is "best," and not all user agents are
    4126    equally capable of rendering all entity types. For that reason, HTTP
    4127    has provisions for several mechanisms for "content negotiation" --
    4128    the process of selecting the best representation for a given response
    4129    when there are multiple representations available.
    4130   <list><t>
    4131       <x:h>Note:</x:h> This is not called "format negotiation" because the
    4132       alternate representations may be of the same media type, but use
    4133       different capabilities of that type, be in different languages,
    4134       etc.
    4135   </t></list>
    4136 </t>
    4137 <t>
    4138    Any response containing an entity-body &MAY; be subject to negotiation,
    4139    including error responses.
    4140 </t>
    4141 <t>
    4142    There are two kinds of content negotiation which are possible in
    4143    HTTP: server-driven and agent-driven negotiation. These two kinds of
    4144    negotiation are orthogonal and thus may be used separately or in
    4145    combination. One method of combination, referred to as transparent
    4146    negotiation, occurs when a cache uses the agent-driven negotiation
    4147    information provided by the origin server in order to provide
    4148    server-driven negotiation for subsequent requests.
    4149 </t>
    4151 <section title="Server-driven Negotiation" anchor="server-driven.negotiation">
    4152 <t>
    4153    If the selection of the best representation for a response is made by
    4154    an algorithm located at the server, it is called server-driven
    4155    negotiation. Selection is based on the available representations of
    4156    the response (the dimensions over which it can vary; e.g. language,
    4157    content-coding, etc.) and the contents of particular header fields in
    4158    the request message or on other information pertaining to the request
    4159    (such as the network address of the client).
    4160 </t>
    4161 <t>
    4162    Server-driven negotiation is advantageous when the algorithm for
    4163    selecting from among the available representations is difficult to
    4164    describe to the user agent, or when the server desires to send its
    4165    "best guess" to the client along with the first response (hoping to
    4166    avoid the round-trip delay of a subsequent request if the "best
    4167    guess" is good enough for the user). In order to improve the server's
    4168    guess, the user agent &MAY; include request header fields (Accept,
    4169    Accept-Language, Accept-Encoding, etc.) which describe its
    4170    preferences for such a response.
    4171 </t>
    4172 <t>
    4173    Server-driven negotiation has disadvantages:
    4174   <list style="numbers">
    4175     <t>
    4176          It is impossible for the server to accurately determine what
    4177          might be "best" for any given user, since that would require
    4178          complete knowledge of both the capabilities of the user agent
    4179          and the intended use for the response (e.g., does the user want
    4180          to view it on screen or print it on paper?).
    4181     </t>
    4182     <t>
    4183          Having the user agent describe its capabilities in every
    4184          request can be both very inefficient (given that only a small
    4185          percentage of responses have multiple representations) and a
    4186          potential violation of the user's privacy.
    4187     </t>
    4188     <t>
    4189          It complicates the implementation of an origin server and the
    4190          algorithms for generating responses to a request.
    4191     </t>
    4192     <t>
    4193          It may limit a public cache's ability to use the same response
    4194          for multiple user's requests.
    4195     </t>
    4196   </list>
    4197 </t>
    4198 <t>
    4199    HTTP/1.1 includes the following request-header fields for enabling
    4200    server-driven negotiation through description of user agent
    4201    capabilities and user preferences: Accept (<xref target="header.accept"/>), Accept-Charset
    4202    (<xref target="header.accept-charset"/>), Accept-Encoding (<xref target="header.accept-encoding"/>), Accept-Language
    4203    (<xref target="header.accept-language"/>), and User-Agent (<xref target="header.user-agent"/>). However, an
    4204    origin server is not limited to these dimensions and &MAY; vary the
    4205    response based on any aspect of the request, including information
    4206    outside the request-header fields or within extension header fields
    4207    not defined by this specification.
    4208 </t>
    4209 <t>
    4210    The Vary  header field can be used to express the parameters the
    4211    server uses to select a representation that is subject to server-driven
    4212    negotiation. See <xref target="caching.negotiated.responses"/> for use of the Vary header field
    4213    by caches and <xref target="header.vary"/> for use of the Vary header field by
    4214    servers.
    4215 </t>
    4216 </section>
    4218 <section title="Agent-driven Negotiation" anchor="agent-driven.negotiation">
    4219 <t>
    4220    With agent-driven negotiation, selection of the best representation
    4221    for a response is performed by the user agent after receiving an
    4222    initial response from the origin server. Selection is based on a list
    4223    of the available representations of the response included within the
    4224    header fields or entity-body of the initial response, with each
    4225    representation identified by its own URI. Selection from among the
    4226    representations may be performed automatically (if the user agent is
    4227    capable of doing so) or manually by the user selecting from a
    4228    generated (possibly hypertext) menu.
    4229 </t>
    4230 <t>
    4231    Agent-driven negotiation is advantageous when the response would vary
    4232    over commonly-used dimensions (such as type, language, or encoding),
    4233    when the origin server is unable to determine a user agent's
    4234    capabilities from examining the request, and generally when public
    4235    caches are used to distribute server load and reduce network usage.
    4236 </t>
    4237 <t>
    4238    Agent-driven negotiation suffers from the disadvantage of needing a
    4239    second request to obtain the best alternate representation. This
    4240    second request is only efficient when caching is used. In addition,
    4241    this specification does not define any mechanism for supporting
    4242    automatic selection, though it also does not prevent any such
    4243    mechanism from being developed as an extension and used within
    4244    HTTP/1.1.
    4245 </t>
    4246 <t>
    4247    HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
    4248    status codes for enabling agent-driven negotiation when the server is
    4249    unwilling or unable to provide a varying response using server-driven
    4250    negotiation.
    4251 </t>
    4252 </section>
    4254 <section title="Transparent Negotiation" anchor="transparent.negotiation">
    4255 <t>
    4256    Transparent negotiation is a combination of both server-driven and
    4257    agent-driven negotiation. When a cache is supplied with a form of the
    4258    list of available representations of the response (as in agent-driven
    4259    negotiation) and the dimensions of variance are completely understood
    4260    by the cache, then the cache becomes capable of performing server-driven
    4261    negotiation on behalf of the origin server for subsequent
    4262    requests on that resource.
    4263 </t>
    4264 <t>
    4265    Transparent negotiation has the advantage of distributing the
    4266    negotiation work that would otherwise be required of the origin
    4267    server and also removing the second request delay of agent-driven
    4268    negotiation when the cache is able to correctly guess the right
    4269    response.
    4270 </t>
    4271 <t>
    4272    This specification does not define any mechanism for transparent
    4273    negotiation, though it also does not prevent any such mechanism from
    4274    being developed as an extension that could be used within HTTP/1.1.
    4275 </t>
    4276 </section>
    4277 </section>
    4280 <section title="Caching in HTTP" anchor="caching">
    4282 <t>
    4283    HTTP is typically used for distributed information systems, where
    4284    performance can be improved by the use of response caches. The
    4285    HTTP/1.1 protocol includes a number of elements intended to make
    4286    caching work as well as possible. Because these elements are
    4287    inextricable from other aspects of the protocol, and because they
    4288    interact with each other, it is useful to describe the basic caching
    4289    design of HTTP separately from the detailed descriptions of methods,
    4290    headers, response codes, etc.
    4291 </t>
    4292 <t>
    4293    Caching would be useless if it did not significantly improve
    4294    performance. The goal of caching in HTTP/1.1 is to eliminate the need
    4295    to send requests in many cases, and to eliminate the need to send
    4296    full responses in many other cases. The former reduces the number of
    4297    network round-trips required for many operations; we use an
    4298    "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
    4299    latter reduces network bandwidth requirements; we use a "validation"
    4300    mechanism for this purpose (see <xref target="validation.model"/>).
    4301 </t>
    4302 <t>
    4303    Requirements for performance, availability, and disconnected
    4304    operation require us to be able to relax the goal of semantic
    4305    transparency. The HTTP/1.1 protocol allows origin servers, caches,
    4306    and clients to explicitly reduce transparency when necessary.
    4307    However, because non-transparent operation may confuse non-expert
    4308    users, and might be incompatible with certain server applications
    4309    (such as those for ordering merchandise), the protocol requires that
    4310    transparency be relaxed
    4311   <list style="symbols">
    4312      <t>only by an explicit protocol-level request when relaxed by
    4313         client or origin server</t>
    4315      <t>only with an explicit warning to the end user when relaxed by
    4316         cache or client</t>
    4317   </list>
    4318 </t>
    4319 <t>
    4320    Therefore, the HTTP/1.1 protocol provides these important elements:
    4321   <list style="numbers">
    4322       <t>Protocol features that provide full semantic transparency when
    4323          this is required by all parties.</t>
    4325       <t>Protocol features that allow an origin server or user agent to
    4326          explicitly request and control non-transparent operation.</t>
    4328       <t>Protocol features that allow a cache to attach warnings to
    4329          responses that do not preserve the requested approximation of
    4330          semantic transparency.</t>
    4331   </list>
    4332 </t>
    4333 <t>
    4334    A basic principle is that it must be possible for the clients to
    4335    detect any potential relaxation of semantic transparency.
    4336   <list><t>
    4337       <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
    4338       design decisions not explicitly discussed in this specification.
    4339       If a decision might affect semantic transparency, the implementor
    4340       ought to err on the side of maintaining transparency unless a
    4341       careful and complete analysis shows significant benefits in
    4342       breaking transparency.
    4343     </t></list>
    4344 </t>
    4346 <section title="">
    4348 <section title="Cache Correctness" anchor="cache.correctness">
    4349 <t>
    4350    A correct cache &MUST; respond to a request with the most up-to-date
    4351    response held by the cache that is appropriate to the request (see
    4352    sections <xref target="disambiguating.expiration.values" format="counter"/>,
    4353    <xref target="disambiguating.multiple.responses" format="counter"/>,
    4354    and <xref target="cache.replacement" format="counter"/>) which meets one of the following
    4355    conditions:
    4356   <list style="numbers">
    4357       <t>It has been checked for equivalence with what the origin server
    4358          would have returned by revalidating the response with the
    4359          origin server (<xref target="validation.model"/>);</t>
    4361       <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
    4362          this means it meets the least restrictive freshness requirement
    4363          of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
    4364          the origin server so specifies, it is the freshness requirement
    4365          of the origin server alone.
    4367          If a stored response is not "fresh enough" by the most
    4368          restrictive freshness requirement of both the client and the
    4369          origin server, in carefully considered circumstances the cache
    4370          &MAY; still return the response with the appropriate Warning
    4371          header (see section <xref target="" format="counter"/>
    4372          and <xref target="header.warning" format="counter"/>), unless such a response
    4373          is prohibited (e.g., by a "no-store" cache-directive, or by a
    4374          "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
    4376       <t>It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),
    4377          or error (4xx or 5xx) response message.</t>
    4378   </list>
    4379 </t>
    4380 <t>
    4381    If the cache can not communicate with the origin server, then a
    4382    correct cache &SHOULD; respond as above if the response can be
    4383    correctly served from the cache; if not it &MUST; return an error or
    4384    warning indicating that there was a communication failure.
    4385 </t>
    4386 <t>
    4387    If a cache receives a response (either an entire response, or a 304
    4388    (Not Modified) response) that it would normally forward to the
    4389    requesting client, and the received response is no longer fresh, the
    4390    cache &SHOULD; forward it to the requesting client without adding a new
    4391    Warning (but without removing any existing Warning headers). A cache
    4392    &SHOULD-NOT;  attempt to revalidate a response simply because that
    4393    response became stale in transit; this might lead to an infinite
    4394    loop. A user agent that receives a stale response without a Warning
    4395    &MAY; display a warning indication to the user.
    4396 </t>
    4397 </section>
    4399 <section title="Warnings" anchor="warnings">
    4400 <t>
    4401    Whenever a cache returns a response that is neither first-hand nor
    4402    "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
    4403    &MUST; attach a warning to that effect, using a Warning general-header.
    4404    The Warning header and the currently defined warnings are described
    4405    in <xref target="header.warning"/>. The warning allows clients to take appropriate
    4406    action.
    4407 </t>
    4408 <t>
    4409    Warnings &MAY; be used for other purposes, both cache-related and
    4410    otherwise. The use of a warning, rather than an error status code,
    4411    distinguish these responses from true failures.
    4412 </t>
    4413 <t>
    4414    Warnings are assigned three digit warn-codes. The first digit
    4415    indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
    4416    stored cache entry after a successful revalidation:
    4417 </t>
    4418 <t>
    4419   <list style="hanging">
    4420     <t hangText="1xx">Warnings that describe the freshness or revalidation status of
    4421      the response, and so &MUST; be deleted after a successful
    4422      revalidation. 1XX warn-codes &MAY; be generated by a cache only when
    4423      validating a cached entry. It &MUST-NOT; be generated by clients.</t>
    4425     <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
    4426      headers that is not rectified by a revalidation (for example, a
    4427      lossy compression of the entity bodies) and which &MUST-NOT; be
    4428      deleted after a successful revalidation.</t>
    4429     </list>
    4430 </t>
    4431 <t>
    4432    See <xref target="header.warning"/> for the definitions of the codes themselves.
    4433 </t>
    4434 <t>
    4435    HTTP/1.0 caches will cache all Warnings in responses, without
    4436    deleting the ones in the first category. Warnings in responses that
    4437    are passed to HTTP/1.0 caches carry an extra warning-date field,
    4438    which prevents a future HTTP/1.1 recipient from believing an
    4439    erroneously cached Warning.
    4440 </t>
    4441 <t>
    4442    Warnings also carry a warning text. The text &MAY; be in any
    4443    appropriate natural language (perhaps based on the client's Accept
    4444    headers), and include an &OPTIONAL; indication of what character set is
    4445    used.
    4446 </t>
    4447 <t>
    4448    Multiple warnings &MAY; be attached to a response (either by the origin
    4449    server or by a cache), including multiple warnings with the same code
    4450    number. For example, a server might provide the same warning with
    4451    texts in both English and Basque.
    4452 </t>
    4453 <t>
    4454    When multiple warnings are attached to a response, it might not be
    4455    practical or reasonable to display all of them to the user. This
    4456    version of HTTP does not specify strict priority rules for deciding
    4457    which warnings to display and in what order, but does suggest some
    4458    heuristics.
    4459 </t>
    4460 </section>
    4462 <section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
    4463 <t>
    4464    The basic cache mechanisms in HTTP/1.1 (server-specified expiration
    4465    times and validators) are implicit directives to caches. In some
    4466    cases, a server or client might need to provide explicit directives
    4467    to the HTTP caches. We use the Cache-Control header for this purpose.
    4468 </t>
    4469 <t>
    4470    The Cache-Control header allows a client or server to transmit a
    4471    variety of directives in either requests or responses. These
    4472    directives typically override the default caching algorithms. As a
    4473    general rule, if there is any apparent conflict between header
    4474    values, the most restrictive interpretation is applied (that is, the
    4475    one that is most likely to preserve semantic transparency). However,
    4476    in some cases, cache-control directives are explicitly specified as
    4477    weakening the approximation of semantic transparency (for example,
    4478    "max-stale" or "public").
    4479 </t>
    4480 <t>
    4481    The cache-control directives are described in detail in <xref target="header.cache-control"/>.
    4482 </t>
    4483 </section>
    4485 <section title="Explicit User Agent Warnings" anchor="">
    4486 <t>
    4487    Many user agents make it possible for users to override the basic
    4488    caching mechanisms. For example, the user agent might allow the user
    4489    to specify that cached entities (even explicitly stale ones) are
    4490    never validated. Or the user agent might habitually add "Cache-Control:
    4491    max-stale=3600" to every request. The user agent &SHOULD-NOT;
    4492    default to either non-transparent behavior, or behavior that results
    4493    in abnormally ineffective caching, but &MAY; be explicitly configured
    4494    to do so by an explicit action of the user.
    4495 </t>
    4496 <t>
    4497    If the user has overridden the basic caching mechanisms, the user
    4498    agent &SHOULD; explicitly indicate to the user whenever this results in
    4499    the display of information that might not meet the server's
    4500    transparency requirements (in particular, if the displayed entity is
    4501    known to be stale). Since the protocol normally allows the user agent
    4502    to determine if responses are stale or not, this indication need only
    4503    be displayed when this actually happens. The indication need not be a
    4504    dialog box; it could be an icon (for example, a picture of a rotting
    4505    fish) or some other indicator.
    4506 </t>
    4507 <t>
    4508    If the user has overridden the caching mechanisms in a way that would
    4509    abnormally reduce the effectiveness of caches, the user agent &SHOULD;
    4510    continually indicate this state to the user (for example, by a
    4511    display of a picture of currency in flames) so that the user does not
    4512    inadvertently consume excess resources or suffer from excessive
    4513    latency.
    4514 </t>
    4515 </section>
    4517 <section title="Exceptions to the Rules and Warnings" anchor="">
    4518 <t>
    4519    In some cases, the operator of a cache &MAY; choose to configure it to
    4520    return stale responses even when not requested by clients. This
    4521    decision ought not be made lightly, but may be necessary for reasons
    4522    of availability or performance, especially when the cache is poorly
    4523    connected to the origin server. Whenever a cache returns a stale
    4524    response, it &MUST; mark it as such (using a Warning header) enabling
    4525    the client software to alert the user that there might be a potential
    4526    problem.
    4527 </t>
    4528 <t>
    4529    It also allows the user agent to take steps to obtain a first-hand or
    4530    fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
    4531    response if the client explicitly requests a first-hand or fresh one,
    4532    unless it is impossible to comply for technical or policy reasons.
    4533 </t>
    4534 </section>
    4536 <section title="Client-controlled Behavior" anchor="client-controlled.behavior">
    4537 <t>
    4538    While the origin server (and to a lesser extent, intermediate caches,
    4539    by their contribution to the age of a response) are the primary
    4540    source of expiration information, in some cases the client might need
    4541    to control a cache's decision about whether to return a cached
    4542    response without validating it. Clients do this using several
    4543    directives of the Cache-Control header.
    4544 </t>
    4545 <t>
    4546    A client's request &MAY; specify the maximum age it is willing to
    4547    accept of an unvalidated response; specifying a value of zero forces
    4548    the cache(s) to revalidate all responses. A client &MAY; also specify
    4549    the minimum time remaining before a response expires. Both of these
    4550    options increase constraints on the behavior of caches, and so cannot
    4551    further relax the cache's approximation of semantic transparency.
    4552 </t>
    4553 <t>
    4554    A client &MAY; also specify that it will accept stale responses, up to
    4555    some maximum amount of staleness. This loosens the constraints on the
    4556    caches, and so might violate the origin server's specified
    4557    constraints on semantic transparency, but might be necessary to
    4558    support disconnected operation, or high availability in the face of
    4559    poor connectivity.
    4560 </t>
    4561 </section>
    4562 </section>
    4564 <section title="Expiration Model" anchor="expiration.model">
    4566 <section title="Server-Specified Expiration" anchor="server-specified.expiration">
    4567 <t>
    4568    HTTP caching works best when caches can entirely avoid making
    4569    requests to the origin server. The primary mechanism for avoiding
    4570    requests is for an origin server to provide an explicit expiration
    4571    time in the future, indicating that a response &MAY; be used to satisfy
    4572    subsequent requests. In other words, a cache can return a fresh
    4573    response without first contacting the server.
    4574 </t>
    4575 <t>
    4576    Our expectation is that servers will assign future explicit
    4577    expiration times to responses in the belief that the entity is not
    4578    likely to change, in a semantically significant way, before the
    4579    expiration time is reached. This normally preserves semantic
    4580    transparency, as long as the server's expiration times are carefully
    4581    chosen.
    4582 </t>
    4583 <t>
    4584    The expiration mechanism applies only to responses taken from a cache
    4585    and not to first-hand responses forwarded immediately to the
    4586    requesting client.
    4587 </t>
    4588 <t>
    4589    If an origin server wishes to force a semantically transparent cache
    4590    to validate every request, it &MAY; assign an explicit expiration time
    4591    in the past. This means that the response is always stale, and so the
    4592    cache &SHOULD; validate it before using it for subsequent requests. See
    4593    <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
    4594 </t>
    4595 <t>
    4596    If an origin server wishes to force any HTTP/1.1 cache, no matter how
    4597    it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
    4598    cache-control directive (see <xref target="header.cache-control"/>).
    4599 </t>
    4600 <t>
    4601    Servers specify explicit expiration times using either the Expires
    4602    header, or the max-age directive of the Cache-Control header.
    4603 </t>
    4604 <t>
    4605    An expiration time cannot be used to force a user agent to refresh
    4606    its display or reload a resource; its semantics apply only to caching
    4607    mechanisms, and such mechanisms need only check a resource's
    4608    expiration status when a new request for that resource is initiated.
    4609    See <xref target="history.lists"/> for an explanation of the difference between caches
    4610    and history mechanisms.
    4611 </t>
    4612 </section>
    4614 <section title="Heuristic Expiration" anchor="heuristic.expiration">
    4615 <t>
    4616    Since origin servers do not always provide explicit expiration times,
    4617    HTTP caches typically assign heuristic expiration times, employing
    4618    algorithms that use other header values (such as the Last-Modified
    4619    time) to estimate a plausible expiration time. The HTTP/1.1
    4620    specification does not provide specific algorithms, but does impose
    4621    worst-case constraints on their results. Since heuristic expiration
    4622    times might compromise semantic transparency, they ought to used
    4623    cautiously, and we encourage origin servers to provide explicit
    4624    expiration times as much as possible.
    4625 </t>
    4626 </section>
    4628 <section title="Age Calculations" anchor="age.calculations">
    4629 <t>
    4630    In order to know if a cached entry is fresh, a cache needs to know if
    4631    its age exceeds its freshness lifetime. We discuss how to calculate
    4632    the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
    4633    the age of a response or cache entry.
    4634 </t>
    4635 <t>
    4636    In this discussion, we use the term "now" to mean "the current value
    4637    of the clock at the host performing the calculation." Hosts that use
    4638    HTTP, but especially hosts running origin servers and caches, &SHOULD;
    4639    use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
    4640    a globally accurate time standard.
    4641 </t>
    4642 <t>
    4643    HTTP/1.1 requires origin servers to send a Date header, if possible,
    4644    with every response, giving the time at which the response was
    4645    generated (see <xref target=""/>). We use the term "date_value" to denote
    4646    the value of the Date header, in a form appropriate for arithmetic
    4647    operations.
    4648 </t>
    4649 <t>
    4650    HTTP/1.1 uses the Age response-header to convey the estimated age of
    4651    the response message when obtained from a cache. The Age field value
    4652    is the cache's estimate of the amount of time since the response was
    4653    generated or revalidated by the origin server.
    4654 </t>
    4655 <t>
    4656    In essence, the Age value is the sum of the time that the response
    4657    has been resident in each of the caches along the path from the
    4658    origin server, plus the amount of time it has been in transit along
    4659    network paths.
    4660 </t>
    4661 <t>
    4662    We use the term "age_value" to denote the value of the Age header, in
    4663    a form appropriate for arithmetic operations.
    4664 </t>
    4665 <t>
    4666    A response's age can be calculated in two entirely independent ways:
    4667   <list style="numbers">
    4668       <t>now minus date_value, if the local clock is reasonably well
    4669          synchronized to the origin server's clock. If the result is
    4670          negative, the result is replaced by zero.</t>
    4672       <t>age_value, if all of the caches along the response path
    4673          implement HTTP/1.1.</t>
    4674   </list>
    4675 </t>
    4676 <t>
    4677    Given that we have two independent ways to compute the age of a
    4678    response when it is received, we can combine these as
    4679 </t>
    4680 <figure><artwork type="code">
    4681     corrected_received_age = max(now - date_value, age_value)
    4682 </artwork></figure>
    4683 <t>
    4684    and as long as we have either nearly synchronized clocks or all-HTTP/1.1
    4685    paths, one gets a reliable (conservative) result.
    4686 </t>
    4687 <t>
    4688    Because of network-imposed delays, some significant interval might
    4689    pass between the time that a server generates a response and the time
    4690    it is received at the next outbound cache or client. If uncorrected,
    4691    this delay could result in improperly low ages.
    4692 </t>
    4693 <t>
    4694    Because the request that resulted in the returned Age value must have
    4695    been initiated prior to that Age value's generation, we can correct
    4696    for delays imposed by the network by recording the time at which the
    4697    request was initiated. Then, when an Age value is received, it &MUST;
    4698    be interpreted relative to the time the request was initiated, not
    4699    the time that the response was received. This algorithm results in
    4700    conservative behavior no matter how much delay is experienced. So, we
    4701    compute:
    4702 </t>
    4703 <figure><artwork type="code">
    4704    corrected_initial_age = corrected_received_age
    4705                          + (now - request_time)
    4706 </artwork></figure>
    4707 <t>
    4708    where "request_time" is the time (according to the local clock) when
    4709    the request that elicited this response was sent.
    4710 </t>
    4711 <t>
    4712    Summary of age calculation algorithm, when a cache receives a
    4713    response:
    4714 </t>
    4715 <figure><artwork type="code">
    4716    /*
    4717     * age_value
    4718     *      is the value of Age: header received by the cache with
    4719     *              this response.
    4720     * date_value
    4721     *      is the value of the origin server's Date: header
    4722     * request_time
    4723     *      is the (local) time when the cache made the request
    4724     *              that resulted in this cached response
    4725     * response_time
    4726     *      is the (local) time when the cache received the
    4727     *              response
    4728     * now
    4729     *      is the current (local) time
    4730     */
    4732    apparent_age = max(0, response_time - date_value);
    4733    corrected_received_age = max(apparent_age, age_value);
    4734    response_delay = response_time - request_time;
    4735    corrected_initial_age = corrected_received_age + response_delay;
    4736    resident_time = now - response_time;
    4737    current_age   = corrected_initial_age + resident_time;
    4738 </artwork></figure>
    4739 <t>
    4740    The current_age of a cache entry is calculated by adding the amount
    4741    of time (in seconds) since the cache entry was last validated by the
    4742    origin server to the corrected_initial_age. When a response is
    4743    generated from a cache entry, the cache &MUST; include a single Age
    4744    header field in the response with a value equal to the cache entry's
    4745    current_age.
    4746 </t>
    4747 <t>
    4748    The presence of an Age header field in a response implies that a
    4749    response is not first-hand. However, the converse is not true, since
    4750    the lack of an Age header field in a response does not imply that the
    4751    response is first-hand unless all caches along the request path are
    4752    compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
    4753    the Age header field).
    4754 </t>
    4755 </section>
    4757 <section title="Expiration Calculations" anchor="expiration.calculations">
    4758 <t>
    4759    In order to decide whether a response is fresh or stale, we need to
    4760    compare its freshness lifetime to its age. The age is calculated as
    4761    described in <xref target="age.calculations"/>; this section describes how to calculate
    4762    the freshness lifetime, and to determine if a response has expired.
    4763    In the discussion below, the values can be represented in any form
    4764    appropriate for arithmetic operations.
    4765 </t>
    4766 <t>
    4767    We use the term "expires_value" to denote the value of the Expires
    4768    header. We use the term "max_age_value" to denote an appropriate
    4769    value of the number of seconds carried by the "max-age" directive of
    4770    the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
    4771 </t>
    4772 <t>
    4773    The max-age directive takes priority over Expires, so if max-age is
    4774    present in a response, the calculation is simply:
    4775 </t>
    4776 <figure><artwork type="code">
    4777    freshness_lifetime = max_age_value
    4778 </artwork></figure>
    4779 <t>
    4780    Otherwise, if Expires is present in the response, the calculation is:
    4781 </t>
    4782 <figure><artwork type="code">
    4783    freshness_lifetime = expires_value - date_value
    4784 </artwork></figure>
    4785 <t>
    4786    Note that neither of these calculations is vulnerable to clock skew,
    4787    since all of the information comes from the origin server.
    4788 </t>
    4789 <t>
    4790    If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
    4791    (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
    4792    does not include other restrictions on caching, the cache &MAY; compute
    4793    a freshness lifetime using a heuristic. The cache &MUST; attach Warning
    4794    113 to any response whose age is more than 24 hours if such warning
    4795    has not already been added.
    4796 </t>
    4797 <t>
    4798    Also, if the response does have a Last-Modified time, the heuristic
    4799    expiration value &SHOULD; be no more than some fraction of the interval
    4800    since that time. A typical setting of this fraction might be 10%.
    4801 </t>
    4802 <t>
    4803    The calculation to determine if a response has expired is quite
    4804    simple:
    4805 </t>
    4806 <figure><artwork type="code">
    4807    response_is_fresh = (freshness_lifetime &gt; current_age)
    4808 </artwork></figure>
    4809 </section>
    4811 <section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
    4812 <t>
    4813    Because expiration values are assigned optimistically, it is possible
    4814    for two caches to contain fresh values for the same resource that are
    4815    different.
    4816 </t>
    4817 <t>
    4818    If a client performing a retrieval receives a non-first-hand response
    4819    for a request that was already fresh in its own cache, and the Date
    4820    header in its existing cache entry is newer than the Date on the new
    4821    response, then the client &MAY; ignore the response. If so, it &MAY;
    4822    retry the request with a "Cache-Control: max-age=0" directive (see
    4823    <xref target="header.cache-control"/>), to force a check with the origin server.
    4824 </t>
    4825 <t>
    4826    If a cache has two fresh responses for the same representation with
    4827    different validators, it &MUST; use the one with the more recent Date
    4828    header. This situation might arise because the cache is pooling
    4829    responses from other caches, or because a client has asked for a
    4830    reload or a revalidation of an apparently fresh cache entry.
    4831 </t>
    4832 </section>
    4834 <section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
    4835 <t>
    4836    Because a client might be receiving responses via multiple paths, so
    4837    that some responses flow through one set of caches and other
    4838    responses flow through a different set of caches, a client might
    4839    receive responses in an order different from that in which the origin
    4840    server sent them. We would like the client to use the most recently
    4841    generated response, even if older responses are still apparently
    4842    fresh.
    4843 </t>
    4844 <t>
    4845    Neither the entity tag nor the expiration value can impose an
    4846    ordering on responses, since it is possible that a later response
    4847    intentionally carries an earlier expiration time. The Date values are
    4848    ordered to a granularity of one second.
    4849 </t>
    4850 <t>
    4851    When a client tries to revalidate a cache entry, and the response it
    4852    receives contains a Date header that appears to be older than the one
    4853    for the existing entry, then the client &SHOULD; repeat the request
    4854    unconditionally, and include
    4855 </t>
    4856 <figure><artwork type="example">
    4857     Cache-Control: max-age=0
    4858 </artwork></figure>
    4859 <t>
    4860    to force any intermediate caches to validate their copies directly
    4861    with the origin server, or
    4862 </t>
    4863 <figure><artwork type="example">
    4864     Cache-Control: no-cache
    4865 </artwork></figure>
    4866 <t>
    4867    to force any intermediate caches to obtain a new copy from the origin
    4868    server.
    4869 </t>
    4870 <t>
    4871    If the Date values are equal, then the client &MAY; use either response
    4872    (or &MAY;, if it is being extremely prudent, request a new response).
    4873    Servers &MUST-NOT; depend on clients being able to choose
    4874    deterministically between responses generated during the same second,
    4875    if their expiration times overlap.
    4876 </t>
    4877 </section>
    4878 </section>
    4880 <section title="Validation Model" anchor="validation.model">
    4881 <t>
    4882    When a cache has a stale entry that it would like to use as a
    4883    response to a client's request, it first has to check with the origin
    4884    server (or possibly an intermediate cache with a fresh response) to
    4885    see if its cached entry is still usable. We call this "validating"
    4886    the cache entry. Since we do not want to have to pay the overhead of
    4887    retransmitting the full response if the cached entry is good, and we
    4888    do not want to pay the overhead of an extra round trip if the cached
    4889    entry is invalid, the HTTP/1.1 protocol supports the use of
    4890    conditional methods.
    4891 </t>
    4892 <t>
    4893    The key protocol features for supporting conditional methods are
    4894    those concerned with "cache validators." When an origin server
    4895    generates a full response, it attaches some sort of validator to it,
    4896    which is kept with the cache entry. When a client (user agent or
    4897    proxy cache) makes a conditional request for a resource for which it
    4898    has a cache entry, it includes the associated validator in the
    4899    request.
    4900 </t>
    4901 <t>
    4902    The server then checks that validator against the current validator
    4903    for the entity, and, if they match (see <xref target="weak.and.strong.validators"/>), it responds
    4904    with a special status code (usually, 304 (Not Modified)) and no
    4905    entity-body. Otherwise, it returns a full response (including
    4906    entity-body). Thus, we avoid transmitting the full response if the
    4907    validator matches, and we avoid an extra round trip if it does not
    4908    match.
    4909 </t>
    4910 <t>
    4911    In HTTP/1.1, a conditional request looks exactly the same as a normal
    4912    request for the same resource, except that it carries a special
    4913    header (which includes the validator) that implicitly turns the
    4914    method (usually, GET) into a conditional.
    4915 </t>
    4916 <t>
    4917    The protocol includes both positive and negative senses of cache-validating
    4918    conditions. That is, it is possible to request either that
    4919    a method be performed if and only if a validator matches or if and
    4920    only if no validators match.
    4921   <list><t>
    4922       <x:h>Note:</x:h> a response that lacks a validator may still be cached, and
    4923       served from cache until it expires, unless this is explicitly
    4924       prohibited by a cache-control directive. However, a cache cannot
    4925       do a conditional retrieval if it does not have a validator for the
    4926       entity, which means it will not be refreshable after it expires.
    4927   </t></list>
    4928 </t>
    4930 <section title="Last-Modified Dates" anchor="last-modified.dates">
    4931 <t>
    4932    The Last-Modified entity-header field value is often used as a cache
    4933    validator. In simple terms, a cache entry is considered to be valid
    4934    if the entity has not been modified since the Last-Modified value.
    4935 </t>
    4936 </section>
    4938 <section title="Entity Tag Cache Validators" anchor="entity.tag.cache.validators">
    4939 <t>
    4940    The ETag response-header field value, an entity tag, provides for an
    4941    "opaque" cache validator. This might allow more reliable validation
    4942    in situations where it is inconvenient to store modification dates,
    4943    where the one-second resolution of HTTP date values is not
    4944    sufficient, or where the origin server wishes to avoid certain
    4945    paradoxes that might arise from the use of modification dates.
    4946 </t>
    4947 <t>
    4948    Entity Tags are described in <xref target="entity.tags"/>. The headers used with
    4949    entity tags are described in sections <xref target="header.etag" format="counter"/>,
    4950    <xref target="header.if-match" format="counter"/>, <xref target="header.if-none-match" format="counter"/>
    4951    and <xref target="header.vary" format="counter"/>.
    4952 </t>
    4953 </section>
    4955 <section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
    4956 <t>
    4957    Since both origin servers and caches will compare two validators to
    4958    decide if they represent the same or different entities, one normally
    4959    would expect that if the entity (the entity-body or any entity-headers)
    4960    changes in any way, then the associated validator would
    4961    change as well. If this is true, then we call this validator a
    4962    "strong validator."
    4963 </t>
    4964 <t>
    4965    However, there might be cases when a server prefers to change the
    4966    validator only on semantically significant changes, and not when
    4967    insignificant aspects of the entity change. A validator that does not
    4968    always change when the resource changes is a "weak validator."
    4969 </t>
    4970 <t>
    4971    Entity tags are normally "strong validators," but the protocol
    4972    provides a mechanism to tag an entity tag as "weak." One can think of
    4973    a strong validator as one that changes whenever the bits of an entity
    4974    changes, while a weak value changes whenever the meaning of an entity
    4975    changes. Alternatively, one can think of a strong validator as part
    4976    of an identifier for a specific entity, while a weak validator is
    4977    part of an identifier for a set of semantically equivalent entities.
    4978   <list><t>
    4979       <x:h>Note:</x:h> One example of a strong validator is an integer that is
    4980       incremented in stable storage every time an entity is changed.
    4981     </t><t>
    4982       An entity's modification time, if represented with one-second
    4983       resolution, could be a weak validator, since it is possible that
    4984       the resource might be modified twice during a single second.
    4985     </t><t>
    4986       Support for weak validators is optional. However, weak validators
    4987       allow for more efficient caching of equivalent objects; for
    4988       example, a hit counter on a site is probably good enough if it is
    4989       updated every few days or weeks, and any value during that period
    4990       is likely "good enough" to be equivalent.
    4991     </t></list>
    4992 </t>
    4993 <t>
    4994    A "use" of a validator is either when a client generates a request
    4995    and includes the validator in a validating header field, or when a
    4996    server compares two validators.
    4997 </t>
    4998 <t>
    4999    Strong validators are usable in any context. Weak validators are only
    5000    usable in contexts that do not depend on exact equality of an entity.
    5001    For example, either kind is usable for a conditional GET of a full
    5002    entity. However, only a strong validator is usable for a sub-range
    5003    retrieval, since otherwise the client might end up with an internally
    5004    inconsistent entity.
    5005 </t>
    5006 <t>
    5007    Clients &MAY; issue simple (non-subrange) GET requests with either weak
    5008    validators or strong validators. Clients &MUST-NOT; use weak validators
    5009    in other forms of request.
    5010 </t>
    5011 <t>
    5012    The only function that the HTTP/1.1 protocol defines on validators is
    5013    comparison. There are two validator comparison functions, depending
    5014    on whether the comparison context allows the use of weak validators
    5015    or not:
    5016   <list style="symbols">
    5017      <t>The strong comparison function: in order to be considered equal,
    5018         both validators &MUST; be identical in every way, and both &MUST-NOT;
    5019         be weak.</t>
    5020      <t>The weak comparison function: in order to be considered equal,
    5021         both validators &MUST; be identical in every way, but either or
    5022         both of them &MAY; be tagged as "weak" without affecting the
    5023         result.</t>
    5024   </list>
    5025 </t>
    5026 <t>
    5027    An entity tag is strong unless it is explicitly tagged as weak.
    5028    <xref target="entity.tags"/> gives the syntax for entity tags.
    5029 </t>
    5030 <t>
    5031    A Last-Modified time, when used as a validator in a request, is
    5032    implicitly weak unless it is possible to deduce that it is strong,
    5033    using the following rules:
    5034   <list style="symbols">
    5035      <t>The validator is being compared by an origin server to the
    5036         actual current validator for the entity and,</t>
    5037      <t>That origin server reliably knows that the associated entity did
    5038         not change twice during the second covered by the presented
    5039         validator.</t>
    5040   </list>
    5041 </t>
    5042 <t>
    5043    or
    5044   <list style="symbols">
    5045      <t>The validator is about to be used by a client in an If-Modified-Since
    5046         or If-Unmodified-Since header, because the client
    5047         has a cache entry for the associated entity, and</t>
    5048      <t>That cache entry includes a Date value, which gives the time
    5049         when the origin server sent the original response, and</t>
    5050      <t>The presented Last-Modified time is at least 60 seconds before
    5051         the Date value.</t>
    5052   </list>
    5053 </t>
    5054 <t>
    5055    or
    5056   <list style="symbols">
    5057      <t>The validator is being compared by an intermediate cache to the
    5058         validator stored in its cache entry for the entity, and</t>
    5059      <t>That cache entry includes a Date value, which gives the time
    5060         when the origin server sent the original response, and</t>
    5061      <t>The presented Last-Modified time is at least 60 seconds before
    5062         the Date value.</t>
    5063   </list>
    5064 </t>
    5065 <t>
    5066    This method relies on the fact that if two different responses were
    5067    sent by the origin server during the same second, but both had the
    5068    same Last-Modified time, then at least one of those responses would
    5069    have a Date value equal to its Last-Modified time. The arbitrary 60-second
    5070    limit guards against the possibility that the Date and Last-Modified
    5071    values are generated from different clocks, or at somewhat
    5072    different times during the preparation of the response. An
    5073    implementation &MAY; use a value larger than 60 seconds, if it is
    5074    believed that 60 seconds is too short.
    5075 </t>
    5076 <t>
    5077    If a client wishes to perform a sub-range retrieval on a value for
    5078    which it has only a Last-Modified time and no opaque validator, it
    5079    &MAY; do this only if the Last-Modified time is strong in the sense
    5080    described here.
    5081 </t>
    5082 <t>
    5083    A cache or origin server receiving a conditional request, other than
    5084    a full-body GET request, &MUST; use the strong comparison function to
    5085    evaluate the condition.
    5086 </t>
    5087 <t>
    5088    These rules allow HTTP/1.1 caches and clients to safely perform sub-range
    5089    retrievals on values that have been obtained from HTTP/1.0
    5090    servers.
    5091 </t>
    5092 </section>
    5094 <section title="Rules for When to Use Entity Tags and Last-Modified Dates" anchor="">
    5095 <t>
    5096    We adopt a set of rules and recommendations for origin servers,
    5097    clients, and caches regarding when various validator types ought to
    5098    be used, and for what purposes.
    5099 </t>
    5100 <t>
    5101    HTTP/1.1 origin servers:
    5102   <list style="symbols">
    5103      <t>&SHOULD; send an entity tag validator unless it is not feasible to
    5104         generate one.</t>
    5106      <t>&MAY; send a weak entity tag instead of a strong entity tag, if
    5107         performance considerations support the use of weak entity tags,
    5108         or if it is unfeasible to send a strong entity tag.</t>
    5110      <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
    5111         unless the risk of a breakdown in semantic transparency that
    5112         could result from using this date in an If-Modified-Since header
    5113         would lead to serious problems.</t>
    5114   </list>
    5115 </t>
    5116 <t>
    5117    In other words, the preferred behavior for an HTTP/1.1 origin server
    5118    is to send both a strong entity tag and a Last-Modified value.
    5119 </t>
    5120 <t>
    5121    In order to be legal, a strong entity tag &MUST; change whenever the
    5122    associated entity value changes in any way. A weak entity tag &SHOULD;
    5123    change whenever the associated entity changes in a semantically
    5124    significant way.
    5125   <list><t>
    5126       <x:h>Note:</x:h> in order to provide semantically transparent caching, an
    5127       origin server must avoid reusing a specific strong entity tag
    5128       value for two different entities, or reusing a specific weak
    5129       entity tag value for two semantically different entities. Cache
    5130       entries might persist for arbitrarily long periods, regardless of
    5131       expiration times, so it might be inappropriate to expect that a
    5132       cache will never again attempt to validate an entry using a
    5133       validator that it obtained at some point in the past.
    5134   </t></list>
    5135 </t>
    5136 <t>
    5137    HTTP/1.1 clients:
    5138   <list style="symbols">
    5139      <t>If an entity tag has been provided by the origin server, &MUST;
    5140         use that entity tag in any cache-conditional request (using If-Match
    5141         or If-None-Match).</t>
    5143      <t>If only a Last-Modified value has been provided by the origin
    5144         server, &SHOULD; use that value in non-subrange cache-conditional
    5145         requests (using If-Modified-Since).</t>
    5147      <t>If only a Last-Modified value has been provided by an HTTP/1.0
    5148         origin server, &MAY; use that value in subrange cache-conditional
    5149         requests (using If-Unmodified-Since:). The user agent &SHOULD;
    5150         provide a way to disable this, in case of difficulty.</t>
    5152      <t>If both an entity tag and a Last-Modified value have been
    5153         provided by the origin server, &SHOULD; use both validators in
    5154         cache-conditional requests. This allows both HTTP/1.0 and
    5155         HTTP/1.1 caches to respond appropriately.</t>
    5156   </list>
    5157 </t>
    5158 <t>
    5159    An HTTP/1.1 origin server, upon receiving a conditional request that
    5160    includes both a Last-Modified date (e.g., in an If-Modified-Since or
    5161    If-Unmodified-Since header field) and one or more entity tags (e.g.,
    5162    in an If-Match, If-None-Match, or If-Range header field) as cache
    5163    validators, &MUST-NOT; return a response status of 304 (Not Modified)
    5164    unless doing so is consistent with all of the conditional header
    5165    fields in the request.
    5166 </t>
    5167 <t>
    5168    An HTTP/1.1 caching proxy, upon receiving a conditional request that
    5169    includes both a Last-Modified date and one or more entity tags as
    5170    cache validators, &MUST-NOT; return a locally cached response to the
    5171    client unless that cached response is consistent with all of the
    5172    conditional header fields in the request.
    5173   <list><t>
    5174       <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
    5175       servers and clients should transmit as much non-redundant
    5176       information as is available in their responses and requests.
    5177       HTTP/1.1 systems receiving this information will make the most
    5178       conservative assumptions about the validators they receive.
    5179   </t><t>
    5180       HTTP/1.0 clients and caches will ignore entity tags. Generally,
    5181       last-modified values received or used by these systems will
    5182       support transparent and efficient caching, and so HTTP/1.1 origin
    5183       servers should provide Last-Modified values. In those rare cases
    5184       where the use of a Last-Modified value as a validator by an
    5185       HTTP/1.0 system could result in a serious problem, then HTTP/1.1
    5186       origin servers should not provide one.
    5187   </t></list>
    5188 </t>
    5189 </section>
    5191 <section title="Non-validating Conditionals" anchor="non-validating.conditionals">
    5192 <t>
    5193    The principle behind entity tags is that only the service author
    5194    knows the semantics of a resource well enough to select an
    5195    appropriate cache validation mechanism, and the specification of any
    5196    validator comparison function more complex than byte-equality would
    5197    open up a can of worms. Thus, comparisons of any other headers
    5198    (except Last-Modified, for compatibility with HTTP/1.0) are never
    5199    used for purposes of validating a cache entry.
    5200 </t>
    5201 </section>
    5202 </section>
    5204 <section title="Response Cacheability" anchor="response.cacheability">
    5205 <t>
    5206    Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
    5207    directive, a caching system &MAY; always store a successful response
    5208    (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
    5209    if it is fresh, and &MAY; return it after successful validation. If
    5210    there is neither a cache validator nor an explicit expiration time
    5211    associated with a response, we do not expect it to be cached, but
    5212    certain caches &MAY; violate this expectation (for example, when little
    5213    or no network connectivity is available). A client can usually detect
    5214    that such a response was taken from a cache by comparing the Date
    5215    header to the current time.
    5216   <list><t>
    5217       <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
    5218       without providing any Warning.
    5219   </t></list>
    5220 </t>
    5221 <t>
    5222    However, in some cases it might be inappropriate for a cache to
    5223    retain an entity, or to return it in response to a subsequent
    5224    request. This might be because absolute semantic transparency is
    5225    deemed necessary by the service author, or because of security or
    5226    privacy considerations. Certain cache-control directives are
    5227    therefore provided so that the server can indicate that certain
    5228    resource entities, or portions thereof, are not to be cached
    5229    regardless of other considerations.
    5230 </t>
    5231 <t>
    5232    Note that <xref target="header.authorization"/> normally prevents a shared cache from saving
    5233    and returning a response to a previous request if that request
    5234    included an Authorization header.
    5235 </t>
    5236 <t>
    5237    A response received with a status code of 200, 203, 206, 300, 301 or
    5238    410 &MAY; be stored by a cache and used in reply to a subsequent
    5239    request, subject to the expiration mechanism, unless a cache-control
    5240    directive prohibits caching. However, a cache that does not support
    5241    the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
    5242    Content) responses.
    5243 </t>
    5244 <t>
    5245    A response received with any other status code (e.g. status codes 302
    5246    and 307) &MUST-NOT; be returned in a reply to a subsequent request
    5247    unless there are cache-control directives or another header(s) that
    5248    explicitly allow it. For example, these include the following: an
    5249    Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
    5250    "proxy-revalidate", "public" or "private" cache-control
    5251    directive (<xref target="header.cache-control"/>).
    5252 </t>
    5253 </section>
    5255 <section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
    5256 <t>
    5257    The purpose of an HTTP cache is to store information received in
    5258    response to requests for use in responding to future requests. In
    5259    many cases, a cache simply returns the appropriate parts of a
    5260    response to the requester. However, if the cache holds a cache entry
    5261    based on a previous response, it might have to combine parts of a new
    5262    response with what is held in the cache entry.
    5263 </t>
    5265 <section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
    5266 <t>
    5267    For the purpose of defining the behavior of caches and non-caching
    5268    proxies, we divide HTTP headers into two categories:
    5269   <list style="symbols">
    5270       <t>End-to-end headers, which are  transmitted to the ultimate
    5271         recipient of a request or response. End-to-end headers in
    5272         responses &MUST; be stored as part of a cache entry and &MUST; be
    5273         transmitted in any response formed from a cache entry.</t>
    5275       <t>Hop-by-hop headers, which are meaningful only for a single
    5276         transport-level connection, and are not stored by caches or
    5277         forwarded by proxies.</t>
    5278   </list>
    5279 </t>
    5280 <t>
    5281    The following HTTP/1.1 headers are hop-by-hop headers:
    5282   <list style="symbols">
    5283       <t>Connection</t>
    5284       <t>Keep-Alive</t>
    5285       <t>Proxy-Authenticate</t>
    5286       <t>Proxy-Authorization</t>
    5287       <t>TE</t>
    5288       <t>Trailers</t>
    5289       <t>Transfer-Encoding</t>
    5290       <t>Upgrade</t>
    5291   </list>
    5292 </t>
    5293 <t>
    5294    All other headers defined by HTTP/1.1 are end-to-end headers.
    5295 </t>
    5296 <t>
    5297    Other hop-by-hop headers &MUST; be listed in a Connection header,
    5298    (<xref target="header.connection"/>) to be introduced into HTTP/1.1 (or later).
    5299 </t>
    5300 </section>
    5302 <section title="Non-modifiable Headers" anchor="non-modifiable.headers">
    5303 <t>
    5304    Some features of the HTTP/1.1 protocol, such as Digest
    5305    Authentication, depend on the value of certain end-to-end headers. A
    5306    transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
    5307    definition of that header requires or specifically allows that.
    5308 </t>
    5309 <t>
    5310    A transparent proxy &MUST-NOT; modify any of the following fields in a
    5311    request or response, and it &MUST-NOT; add any of these fields if not
    5312    already present:
    5313   <list style="symbols">
    5314       <t>Content-Location</t>
    5315       <t>Content-MD5</t>
    5316       <t>ETag</t>
    5317       <t>Last-Modified</t>
    5318   </list>
    5319 </t>
    5320 <t>
    5321    A transparent proxy &MUST-NOT; modify any of the following fields in a
    5322    response:
    5323   <list style="symbols">
    5324     <t>Expires</t>
    5325   </list>
    5326 </t>
    5327 <t>
    5328    but it &MAY; add any of these fields if not already present. If an
    5329    Expires header is added, it &MUST; be given a field-value identical to
    5330    that of the Date header in that response.
    5331 </t>
    5332 <t>
    5333    A  proxy &MUST-NOT; modify or add any of the following fields in a
    5334    message that contains the no-transform cache-control directive, or in
    5335    any request:
    5336   <list style="symbols">
    5337     <t>Content-Encoding</t>
    5338     <t>Content-Range</t>
    5339     <t>Content-Type</t>
    5340   </list>
    5341 </t>
    5342 <t>
    5343    A non-transparent proxy &MAY; modify or add these fields to a message
    5344    that does not include no-transform, but if it does so, it &MUST; add a
    5345    Warning 214 (Transformation applied) if one does not already appear
    5346    in the message (see <xref target="header.warning"/>).
    5347   <list><t>
    5348       Warning: unnecessary modification of end-to-end headers might
    5349       cause authentication failures if stronger authentication
    5350       mechanisms are introduced in later versions of HTTP. Such
    5351       authentication mechanisms &MAY; rely on the values of header fields
    5352       not listed here.
    5353     </t></list>
    5354 </t>
    5355 <t>
    5356    The Content-Length field of a request or response is added or deleted
    5357    according to the rules in <xref target="message.length"/>. A transparent proxy &MUST;
    5358    preserve the entity-length (<xref target="entity.length"/>) of the entity-body,
    5359    although it &MAY; change the transfer-length (<xref target="message.length"/>).
    5360 </t>
    5361 </section>
    5363 <section title="Combining Headers" anchor="combining.headers">
    5364 <t>
    5365    When a cache makes a validating request to a server, and the server
    5366    provides a 304 (Not Modified) response or a 206 (Partial Content)
    5367    response, the cache then constructs a response to send to the
    5368    requesting client.
    5369 </t>
    5370 <t>
    5371    If the status code is 304 (Not Modified), the cache uses the entity-body
    5372    stored in the cache entry as the entity-body of this outgoing
    5373    response. If the status code is 206 (Partial Content) and the ETag or
    5374    Last-Modified headers match exactly, the cache &MAY; combine the
    5375    contents stored in the cache entry with the new contents received in
    5376    the response and use the result as the entity-body of this outgoing
    5377    response, (see <xref target="combining.byte.ranges" format="counter"/>).
    5378 </t>
    5379 <t>
    5380    The end-to-end headers stored in the cache entry are used for the
    5381    constructed response, except that
    5382   <list style="symbols">
    5383     <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
    5384       &MUST; be deleted from the cache entry and the forwarded response.</t>
    5385     <t>any stored Warning headers with warn-code 2xx &MUST; be retained
    5386         in the cache entry and the forwarded response.</t>
    5387     <t>any end-to-end headers provided in the 304 or 206 response &MUST;
    5388         replace the corresponding headers from the cache entry.</t>
    5389   </list>
    5390 </t>
    5391 <t>
    5392    Unless the cache decides to remove the cache entry, it &MUST; also
    5393    replace the end-to-end headers stored with the cache entry with
    5394    corresponding headers received in the incoming response, except for
    5395    Warning headers as described immediately above. If a header field-name
    5396    in the incoming response matches more than one header in the
    5397    cache entry, all such old headers &MUST; be replaced.
    5398 </t>
    5399 <t>
    5400    In other words, the set of end-to-end headers received in the
    5401    incoming response overrides all corresponding end-to-end headers
    5402    stored with the cache entry (except for stored Warning headers with
    5403    warn-code 1xx, which are deleted even if not overridden).
    5404   <list><t>
    5405       <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
    5406       Modified) or a 206 (Partial Content) response to update any header
    5407       associated with a previous response for the same entity or sub-ranges
    5408       thereof, although it might not always be meaningful or
    5409       correct to do so. This rule does not allow an origin server to use
    5410       a 304 (Not Modified) or a 206 (Partial Content) response to
    5411       entirely delete a header that it had provided with a previous
    5412       response.
    5413   </t></list>
    5414 </t>
    5415 </section>
    5417 <section title="Combining Byte Ranges" anchor="combining.byte.ranges">
    5418 <t>
    5419    A response might transfer only a subrange of the bytes of an entity-body,
    5420    either because the request included one or more Range
    5421    specifications, or because a connection was broken prematurely. After
    5422    several such transfers, a cache might have received several ranges of
    5423    the same entity-body.
    5424 </t>
    5425 <t>
    5426    If a cache has a stored non-empty set of subranges for an entity, and
    5427    an incoming response transfers another subrange, the cache &MAY;
    5428    combine the new subrange with the existing set if both the following
    5429    conditions are met:
    5430   <list style="symbols">
    5431     <t>Both the incoming response and the cache entry have a cache
    5432         validator.</t>
    5433     <t>The two cache validators match using the strong comparison
    5434         function (see <xref target="weak.and.strong.validators"/>).</t>
    5435   </list>
    5436 </t>
    5437 <t>
    5438    If either requirement is not met, the cache &MUST; use only the most
    5439    recent partial response (based on the Date values transmitted with
    5440    every response, and using the incoming response if these values are
    5441    equal or missing), and &MUST; discard the other partial information.
    5442 </t>
    5443 </section>
    5444 </section>
    5446 <section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
    5447 <t>
    5448    Use of server-driven content negotiation (<xref target="server-driven.negotiation"/>), as indicated
    5449    by the presence of a Vary header field in a response, alters the
    5450    conditions and procedure by which a cache can use the response for
    5451    subsequent requests. See <xref target="header.vary"/> for use of the Vary header
    5452    field by servers.
    5453 </t>
    5454 <t>
    5455    A server &SHOULD; use the Vary header field to inform a cache of what
    5456    request-header fields were used to select among multiple
    5457    representations of a cacheable response subject to server-driven
    5458    negotiation. The set of header fields named by the Vary field value
    5459    is known as the "selecting" request-headers.
    5460 </t>
    5461 <t>
    5462    When the cache receives a subsequent request whose Request-URI
    5463    specifies one or more cache entries including a Vary header field,
    5464    the cache &MUST-NOT; use such a cache entry to construct a response to
    5465    the new request unless all of the selecting request-headers present
    5466    in the new request match the corresponding stored request-headers in
    5467    the original request.
    5468 </t>
    5469 <t>
    5470    The selecting request-headers from two requests are defined to match
    5471    if and only if the selecting request-headers in the first request can
    5472    be transformed to the selecting request-headers in the second request
    5473    by adding or removing linear white space (LWS) at places where this
    5474    is allowed by the corresponding BNF, and/or combining multiple
    5475    message-header fields with the same field name following the rules
    5476    about message headers in <xref target="message.headers"/>.
    5477 </t>
    5478 <t>
    5479    A Vary header field-value of "*" always fails to match and subsequent
    5480    requests on that resource can only be properly interpreted by the
    5481    origin server.
    5482 </t>
    5483 <t>
    5484    If the selecting request header fields for the cached entry do not
    5485    match the selecting request header fields of the new request, then
    5486    the cache &MUST-NOT; use a cached entry to satisfy the request unless
    5487    it first relays the new request to the origin server in a conditional
    5488    request and the server responds with 304 (Not Modified), including an
    5489    entity tag or Content-Location that indicates the entity to be used.
    5490 </t>
    5491 <t>
    5492    If an entity tag was assigned to a cached representation, the
    5493    forwarded request &SHOULD; be conditional and include the entity tags
    5494    in an If-None-Match header field from all its cache entries for the
    5495    resource. This conveys to the server the set of entities currently
    5496    held by the cache, so that if any one of these entities matches the
    5497    requested entity, the server can use the ETag header field in its 304
    5498    (Not Modified) response to tell the cache which entry is appropriate.
    5499    If the entity-tag of the new response matches that of an existing
    5500    entry, the new response &SHOULD; be used to update the header fields of
    5501    the existing entry, and the result &MUST; be returned to the client.
    5502 </t>
    5503 <t>
    5504    If any of the existing cache entries contains only partial content
    5505    for the associated entity, its entity-tag &SHOULD-NOT;  be included in
    5506    the If-None-Match header field unless the request is for a range that
    5507    would be fully satisfied by that entry.
    5508 </t>
    5509 <t>
    5510    If a cache receives a successful response whose Content-Location
    5511    field matches that of an existing cache entry for the same Request-URI,
    5512    whose entity-tag differs from that of the existing entry, and
    5513    whose Date is more recent than that of the existing entry, the
    5514    existing entry &SHOULD-NOT;  be returned in response to future requests
    5515    and &SHOULD; be deleted from the cache.
    5516 </t>
    5517 </section>
    5519 <section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
    5520 <t>
    5521    For reasons of security and privacy, it is necessary to make a
    5522    distinction between "shared" and "non-shared" caches. A non-shared
    5523    cache is one that is accessible only to a single user. Accessibility
    5524    in this case &SHOULD; be enforced by appropriate security mechanisms.
    5525    All other caches are considered to be "shared." Other sections of
    5526    this specification place certain constraints on the operation of
    5527    shared caches in order to prevent loss of privacy or failure of
    5528    access controls.
    5529 </t>
    5530 </section>
    5532 <section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
    5533 <t>
    5534    A cache that receives an incomplete response (for example, with fewer
    5535    bytes of data than specified in a Content-Length header) &MAY; store
    5536    the response. However, the cache &MUST; treat this as a partial
    5537    response. Partial responses &MAY; be combined as described in <xref target="combining.byte.ranges"/>;
    5538    the result might be a full response or might still be
    5539    partial. A cache &MUST-NOT; return a partial response to a client
    5540    without explicitly marking it as such, using the 206 (Partial
    5541    Content) status code. A cache &MUST-NOT; return a partial response
    5542    using a status code of 200 (OK).
    5543 </t>
    5544 <t>
    5545    If a cache receives a 5xx response while attempting to revalidate an
    5546    entry, it &MAY; either forward this response to the requesting client,
    5547    or act as if the server failed to respond. In the latter case, it &MAY;
    5548    return a previously received response unless the cached entry
    5549    includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
    5550 </t>
    5551 </section>
    5553 <section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
    5554 <t>
    5555    Unless the origin server explicitly prohibits the caching of their
    5556    responses, the application of GET and HEAD methods to any resources
    5557    &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
    5558    these responses are taken from a cache. They &MAY; still have side
    5559    effects, but a cache is not required to consider such side effects in
    5560    its caching decisions. Caches are always expected to observe an
    5561    origin server's explicit restrictions on caching.
    5562 </t>
    5563 <t>
    5564    We note one exception to this rule: since some applications have
    5565    traditionally used GETs and HEADs with query URLs (those containing a
    5566    "?" in the rel_path part) to perform operations with significant side
    5567    effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
    5568    the server provides an explicit expiration time. This specifically
    5569    means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT;
    5570    be taken from a cache. See <xref target="safe.methods"/> for related information.
    5571 </t>
    5572 </section>
    5574 <section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
    5575 <t>
    5576    The effect of certain methods performed on a resource at the origin
    5577    server might cause one or more existing cache entries to become non-transparently
    5578    invalid. That is, although they might continue to be
    5579    "fresh," they do not accurately reflect what the origin server would
    5580    return for a new request on that resource.
    5581 </t>
    5582 <t>
    5583    There is no way for the HTTP protocol to guarantee that all such
    5584    cache entries are marked invalid. For example, the request that
    5585    caused the change at the origin server might not have gone through
    5586    the proxy where a cache entry is stored. However, several rules help
    5587    reduce the likelihood of erroneous behavior.
    5588 </t>
    5589 <t>
    5590    In this section, the phrase "invalidate an entity" means that the
    5591    cache will either remove all instances of that entity from its
    5592    storage, or will mark these as "invalid" and in need of a mandatory
    5593    revalidation before they can be returned in response to a subsequent
    5594    request.
    5595 </t>
    5596 <t>
    5597    Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
    5598    either the entity referred to by the Request-URI, or by the Location
    5599    or Content-Location headers (if present). These methods are:
    5600   <list style="symbols">
    5601       <t>PUT</t>
    5602       <t>DELETE</t>
    5603       <t>POST</t>
    5604   </list>
    5605 </t> 
    5606 <t>
    5607    In order to prevent denial of service attacks, an invalidation based
    5608    on the URI in a Location or Content-Location header &MUST; only be
    5609    performed if the host part is the same as in the Request-URI.
    5610 </t>
    5611 <t>
    5612    A cache that passes through requests for methods it does not
    5613    understand &SHOULD; invalidate any entities referred to by the
    5614    Request-URI.
    5615 </t>
    5616 </section>
    5618 <section title="Write-Through Mandatory" anchor="write-through.mandatory">
    5619 <t>
    5620    All methods that might be expected to cause modifications to the
    5621    origin server's resources &MUST; be written through to the origin
    5622    server. This currently includes all methods except for GET and HEAD.
    5623    A cache &MUST-NOT; reply to such a request from a client before having
    5624    transmitted the request to the inbound server, and having received a
    5625    corresponding response from the inbound server. This does not prevent
    5626    a proxy cache from sending a 100 (Continue) response before the
    5627    inbound server has sent its final reply.
    5628 </t>
    5629 <t>
    5630    The alternative (known as "write-back" or "copy-back" caching) is not
    5631    allowed in HTTP/1.1, due to the difficulty of providing consistent
    5632    updates and the problems arising from server, cache, or network
    5633    failure prior to write-back.
    5634 </t>
    5635 </section>
    5637 <section title="Cache Replacement" anchor="cache.replacement">
    5638 <t>
    5639    If a new cacheable (see sections <xref target="" format="counter"/>,
    5640    <xref target="disambiguating.expiration.values" format="counter"/>,
    5641    <xref target="disambiguating.multiple.responses" format="counter"/>
    5642    and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
    5643    response is received from a resource while any existing responses for
    5644    the same resource are cached, the cache &SHOULD; use the new response
    5645    to reply to the current request. It &MAY; insert it into cache storage
    5646    and &MAY;, if it meets all other requirements, use it to respond to any
    5647    future requests that would previously have caused the old response to
    5648    be returned. If it inserts the new response into cache storage  the
    5649    rules in <xref target="combining.headers"/> apply.
    5650   <list><t>
    5651       <x:h>Note:</x:h> a new response that has an older Date header value than
    5652       existing cached responses is not cacheable.
    5653   </t></list>
    5654 </t>
    5655 </section>
    5657 <section title="History Lists" anchor="history.lists">
    5658 <t>
    5659    User agents often have history mechanisms, such as "Back" buttons and
    5660    history lists, which can be used to redisplay an entity retrieved
    5661    earlier in a session.
    5662 </t>
    5663 <t>
    5664    History mechanisms and caches are different. In particular history
    5665    mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
    5666    the current state of a resource. Rather, a history mechanism is meant
    5667    to show exactly what the user saw at the time when the resource was
    5668    retrieved.
    5669 </t>
    5670 <t>
    5671    By default, an expiration time does not apply to history mechanisms.
    5672    If the entity is still in storage, a history mechanism &SHOULD; display
    5673    it even if the entity has expired, unless the user has specifically
    5674    configured the agent to refresh expired history documents.
    5675 </t>
    5676 <t>
    5677    This is not to be construed to prohibit the history mechanism from
    5678    telling the user that a view might be stale.
    5679   <list><t>
    5680       <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
    5681       viewing stale resources, this will tend to force service authors
    5682       to avoid using HTTP expiration controls and cache controls when
    5683       they would otherwise like to. Service authors may consider it
    5684       important that users not be presented with error messages or
    5685       warning messages when they use navigation controls (such as BACK)
    5686       to view previously fetched resources. Even though sometimes such
    5687       resources ought not to cached, or ought to expire quickly, user
    5688       interface considerations may force service authors to resort to
    5689       other means of preventing caching (e.g. "once-only" URLs) in order
    5690       not to suffer the effects of improperly functioning history
    5691       mechanisms.
    5692   </t></list>
    5693 </t>
    5694 </section>
    5695 </section>
    57002134<section title="Header Field Definitions" anchor="header.fields">
    57052139   sends and who receives the entity.
    5708 <section title="Accept" anchor="header.accept">
    5709   <iref primary="true" item="Accept header" x:for-anchor=""/>
    5710   <iref primary="true" item="Headers" subitem="Accept" x:for-anchor=""/>
    5711 <t>
    5712    The Accept request-header field can be used to specify certain media
    5713    types which are acceptable for the response. Accept headers can be
    5714    used to indicate that the request is specifically limited to a small
    5715    set of desired types, as in the case of a request for an in-line
    5716    image.
    5717 </t>
    5718 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept"/><iref primary="true" item="Grammar" subitem="media-range"/><iref primary="true" item="Grammar" subitem="accept-params"/><iref primary="true" item="Grammar" subitem="accept-extension"/>
    5719     Accept         = "Accept" ":"
    5720                      #( media-range [ accept-params ] )
    5722     media-range    = ( "*/*"
    5723                      | ( type "/" "*" )
    5724                      | ( type "/" subtype )
    5725                      ) *( ";" parameter )
    5726     accept-params  = ";" "q" "=" qvalue *( accept-extension )
    5727     accept-extension = ";" token [ "=" ( token | quoted-string ) ]
    5728 </artwork></figure>
    5729 <t>
    5730    The asterisk "*" character is used to group media types into ranges,
    5731    with "*/*" indicating all media types and "type/*" indicating all
    5732    subtypes of that type. The media-range &MAY; include media type
    5733    parameters that are applicable to that range.
    5734 </t>
    5735 <t>
    5736    Each media-range &MAY; be followed by one or more accept-params,
    5737    beginning with the "q" parameter for indicating a relative quality
    5738    factor. The first "q" parameter (if any) separates the media-range
    5739    parameter(s) from the accept-params. Quality factors allow the user
    5740    or user agent to indicate the relative degree of preference for that
    5741    media-range, using the qvalue scale from 0 to 1 (<xref target="quality.values"/>). The
    5742    default value is q=1.
    5743   <list><t>
    5744       <x:h>Note:</x:h> Use of the "q" parameter name to separate media type
    5745       parameters from Accept extension parameters is due to historical
    5746       practice. Although this prevents any media type parameter named
    5747       "q" from being used with a media range, such an event is believed
    5748       to be unlikely given the lack of any "q" parameters in the IANA
    5749       media type registry and the rare usage of any media type
    5750       parameters in Accept. Future media types are discouraged from
    5751       registering any parameter named "q".
    5752   </t></list>
    5753 </t>
    5754 <t>
    5755    The example
    5756 </t>
    5757 <figure><artwork type="example">
    5758     Accept: audio/*; q=0.2, audio/basic
    5759 </artwork></figure>
    5760 <t>
    5761    &SHOULD; be interpreted as "I prefer audio/basic, but send me any audio
    5762    type if it is the best available after an 80% mark-down in quality."
    5763 </t>
    5764 <t>
    5765    If no Accept header field is present, then it is assumed that the
    5766    client accepts all media types. If an Accept header field is present,
    5767    and if the server cannot send a response which is acceptable
    5768    according to the combined Accept field value, then the server &SHOULD;
    5769    send a 406 (not acceptable) response.
    5770 </t>
    5771 <t>
    5772    A more elaborate example is
    5773 </t>
    5774 <figure><artwork type="example">
    5775     Accept: text/plain; q=0.5, text/html,
    5776             text/x-dvi; q=0.8, text/x-c
    5777 </artwork></figure>
    5778 <t>
    5779    Verbally, this would be interpreted as "text/html and text/x-c are
    5780    the preferred media types, but if they do not exist, then send the
    5781    text/x-dvi entity, and if that does not exist, send the text/plain
    5782    entity."
    5783 </t>
    5784 <t>
    5785    Media ranges can be overridden by more specific media ranges or
    5786    specific media types. If more than one media range applies to a given
    5787    type, the most specific reference has precedence. For example,
    5788 </t>
    5789 <figure><artwork type="example">
    5790     Accept: text/*, text/html, text/html;level=1, */*
    5791 </artwork></figure>
    5792 <t>
    5793    have the following precedence:
    5794 </t>
    5795 <figure><artwork type="example">
    5796     1) text/html;level=1
    5797     2) text/html
    5798     3) text/*
    5799     4) */*
    5800 </artwork></figure>
    5801 <t>
    5802    The media type quality factor associated with a given type is
    5803    determined by finding the media range with the highest precedence
    5804    which matches that type. For example,
    5805 </t>
    5806 <figure><artwork type="example">
    5807     Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
    5808             text/html;level=2;q=0.4, */*;q=0.5
    5809 </artwork></figure>
    5810 <t>
    5811    would cause the following values to be associated:
    5812 </t>
    5813 <figure><artwork type="example">
    5814     text/html;level=1         = 1
    5815     text/html                 = 0.7
    5816     text/plain                = 0.3
    5817     image/jpeg                = 0.5
    5818     text/html;level=2         = 0.4
    5819     text/html;level=3         = 0.7
    5820 </artwork></figure>
    5821 <t>
    5822       <x:h>Note:</x:h> A user agent might be provided with a default set of quality
    5823       values for certain media ranges. However, unless the user agent is
    5824       a closed system which cannot interact with other rendering agents,
    5825       this default set ought to be configurable by the user.
    5826 </t>
    5827 </section>
    5829 <section title="Accept-Charset" anchor="header.accept-charset">
    5830   <iref primary="true" item="Accept-Charset header" x:for-anchor=""/>
    5831   <iref primary="true" item="Headers" subitem="Accept-Charset" x:for-anchor=""/>
    5832 <t>
    5833    The Accept-Charset request-header field can be used to indicate what
    5834    character sets are acceptable for the response. This field allows
    5835    clients capable of understanding more comprehensive or special-purpose
    5836    character sets to signal that capability to a server which is
    5837    capable of representing documents in those character sets.
    5838 </t>
    5839 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Charset"/>
    5840    Accept-Charset = "Accept-Charset" ":"
    5841            1#( ( charset | "*" )[ ";" "q" "=" qvalue ] )
    5842 </artwork></figure>
    5843 <t>
    5844    Character set values are described in <xref target="character.sets"/>. Each charset &MAY;
    5845    be given an associated quality value which represents the user's
    5846    preference for that charset. The default value is q=1. An example is
    5847 </t>
    5848 <figure><artwork type="example">
    5849    Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
    5850 </artwork></figure>
    5851 <t>
    5852    The special value "*", if present in the Accept-Charset field,
    5853    matches every character set (including ISO-8859-1) which is not
    5854    mentioned elsewhere in the Accept-Charset field. If no "*" is present
    5855    in an Accept-Charset field, then all character sets not explicitly
    5856    mentioned get a quality value of 0, except for ISO-8859-1, which gets
    5857    a quality value of 1 if not explicitly mentioned.
    5858 </t>
    5859 <t>
    5860    If no Accept-Charset header is present, the default is that any
    5861    character set is acceptable. If an Accept-Charset header is present,
    5862    and if the server cannot send a response which is acceptable
    5863    according to the Accept-Charset header, then the server &SHOULD; send
    5864    an error response with the 406 (not acceptable) status code, though
    5865    the sending of an unacceptable response is also allowed.
    5866 </t>
    5867 </section>
    5869 <section title="Accept-Encoding" anchor="header.accept-encoding">
    5870   <iref primary="true" item="Accept-Encoding header" x:for-anchor=""/>
    5871   <iref primary="true" item="Headers" subitem="Accept-Encoding" x:for-anchor=""/>
    5872 <t>
    5873    The Accept-Encoding request-header field is similar to Accept, but
    5874    restricts the content-codings (<xref target="content.codings"/>) that are acceptable in
    5875    the response.
    5876 </t>
    5877 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Encoding"/><iref primary="true" item="Grammar" subitem="codings"/>
    5878     Accept-Encoding  = "Accept-Encoding" ":"
    5879                        1#( codings [ ";" "q" "=" qvalue ] )
    5880     codings          = ( content-coding | "*" )
    5881 </artwork></figure>
    5882 <t>
    5883    Examples of its use are:
    5884 </t>
    5885 <figure><artwork type="example">
    5886     Accept-Encoding: compress, gzip
    5887     Accept-Encoding:
    5888     Accept-Encoding: *
    5889     Accept-Encoding: compress;q=0.5, gzip;q=1.0
    5890     Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
    5891 </artwork></figure>
    5892 <t>
    5893    A server tests whether a content-coding is acceptable, according to
    5894    an Accept-Encoding field, using these rules:
    5895   <list style="numbers">
    5896       <t>If the content-coding is one of the content-codings listed in
    5897          the Accept-Encoding field, then it is acceptable, unless it is
    5898          accompanied by a qvalue of 0. (As defined in <xref target="quality.values"/>, a
    5899          qvalue of 0 means "not acceptable.")</t>
    5901       <t>The special "*" symbol in an Accept-Encoding field matches any
    5902          available content-coding not explicitly listed in the header
    5903          field.</t>
    5905       <t>If multiple content-codings are acceptable, then the acceptable
    5906          content-coding with the highest non-zero qvalue is preferred.</t>
    5908       <t>The "identity" content-coding is always acceptable, unless
    5909          specifically refused because the Accept-Encoding field includes
    5910          "identity;q=0", or because the field includes "*;q=0" and does
    5911          not explicitly include the "identity" content-coding. If the
    5912          Accept-Encoding field-value is empty, then only the "identity"
    5913          encoding is acceptable.</t>
    5914   </list>
    5915 </t>
    5916 <t>
    5917    If an Accept-Encoding field is present in a request, and if the
    5918    server cannot send a response which is acceptable according to the
    5919    Accept-Encoding header, then the server &SHOULD; send an error response
    5920    with the 406 (Not Acceptable) status code.
    5921 </t>
    5922 <t>
    5923    If no Accept-Encoding field is present in a request, the server &MAY;
    5924    assume that the client will accept any content coding. In this case,
    5925    if "identity" is one of the available content-codings, then the
    5926    server &SHOULD; use the "identity" content-coding, unless it has
    5927    additional information that a different content-coding is meaningful
    5928    to the client.
    5929   <list><t>
    5930       <x:h>Note:</x:h> If the request does not include an Accept-Encoding field,
    5931       and if the "identity" content-coding is unavailable, then
    5932       content-codings commonly understood by HTTP/1.0 clients (i.e.,
    5933       "gzip" and "compress") are preferred; some older clients
    5934       improperly display messages sent with other content-codings.  The
    5935       server might also make this decision based on information about
    5936       the particular user-agent or client.
    5937     </t><t>
    5938       <x:h>Note:</x:h> Most HTTP/1.0 applications do not recognize or obey qvalues
    5939       associated with content-codings. This means that qvalues will not
    5940       work and are not permitted with x-gzip or x-compress.
    5941     </t></list>
    5942 </t>
    5943 </section>
    5945 <section title="Accept-Language" anchor="header.accept-language">
    5946   <iref primary="true" item="Accept-Language header" x:for-anchor=""/>
    5947   <iref primary="true" item="Headers" subitem="Accept-Language" x:for-anchor=""/>
    5948 <t>
    5949    The Accept-Language request-header field is similar to Accept, but
    5950    restricts the set of natural languages that are preferred as a
    5951    response to the request. Language tags are defined in <xref target="language.tags"/>.
    5952 </t>
    5953 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Language"/><iref primary="true" item="Grammar" subitem="language-range"/>
    5954     Accept-Language = "Accept-Language" ":"
    5955                       1#( language-range [ ";" "q" "=" qvalue ] )
    5956     language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
    5957 </artwork></figure>
    5958 <t>
    5959    Each language-range &MAY; be given an associated quality value which
    5960    represents an estimate of the user's preference for the languages
    5961    specified by that range. The quality value defaults to "q=1". For
    5962    example,
    5963 </t>
    5964 <figure><artwork type="example">
    5965     Accept-Language: da, en-gb;q=0.8, en;q=0.7
    5966 </artwork></figure>
    5967 <t>
    5968    would mean: "I prefer Danish, but will accept British English and
    5969    other types of English." A language-range matches a language-tag if
    5970    it exactly equals the tag, or if it exactly equals a prefix of the
    5971    tag such that the first tag character following the prefix is "-".
    5972    The special range "*", if present in the Accept-Language field,
    5973    matches every tag not matched by any other range present in the
    5974    Accept-Language field.
    5975   <list><t>
    5976       <x:h>Note:</x:h> This use of a prefix matching rule does not imply that
    5977       language tags are assigned to languages in such a way that it is
    5978       always true that if a user understands a language with a certain
    5979       tag, then this user will also understand all languages with tags
    5980       for which this tag is a prefix. The prefix rule simply allows the
    5981       use of prefix tags if this is the case.
    5982   </t></list>
    5983 </t>
    5984 <t>
    5985    The language quality factor assigned to a language-tag by the
    5986    Accept-Language field is the quality value of the longest language-range
    5987    in the field that matches the language-tag. If no language-range
    5988    in the field matches the tag, the language quality factor
    5989    assigned is 0. If no Accept-Language header is present in the
    5990    request, the server
    5991    &SHOULD; assume that all languages are equally acceptable. If an
    5992    Accept-Language header is present, then all languages which are
    5993    assigned a quality factor greater than 0 are acceptable.
    5994 </t>
    5995 <t>
    5996    It might be contrary to the privacy expectations of the user to send
    5997    an Accept-Language header with the complete linguistic preferences of
    5998    the user in every request. For a discussion of this issue, see
    5999    <xref target=""/>.
    6000 </t>
    6001 <t>
    6002    As intelligibility is highly dependent on the individual user, it is
    6003    recommended that client applications make the choice of linguistic
    6004    preference available to the user. If the choice is not made
    6005    available, then the Accept-Language header field &MUST-NOT; be given in
    6006    the request.
    6007   <list><t>
    6008       <x:h>Note:</x:h> When making the choice of linguistic preference available to
    6009       the user, we remind implementors of  the fact that users are not
    6010       familiar with the details of language matching as described above,
    6011       and should provide appropriate guidance. As an example, users
    6012       might assume that on selecting "en-gb", they will be served any
    6013       kind of English document if British English is not available. A
    6014       user agent might suggest in such a case to add "en" to get the
    6015       best matching behavior.
    6016   </t></list>
    6017 </t>
    6018 </section>
    6020 <section title="Accept-Ranges" anchor="header.accept-ranges">
    6021   <iref primary="true" item="Accept-Ranges header" x:for-anchor=""/>
    6022   <iref primary="true" item="Headers" subitem="Accept-Ranges" x:for-anchor=""/>
    6023 <t>
    6024       The Accept-Ranges response-header field allows the server to
    6025       indicate its acceptance of range requests for a resource:
    6026 </t>
    6027 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Ranges"/><iref primary="true" item="Grammar" subitem="acceptable-ranges"/>
    6028        Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges
    6029        acceptable-ranges = 1#range-unit | "none"
    6030 </artwork></figure>
    6031 <t>
    6032       Origin servers that accept byte-range requests &MAY; send
    6033 </t>
    6034 <figure><artwork type="example">
    6035        Accept-Ranges: bytes
    6036 </artwork></figure>
    6037 <t>
    6038       but are not required to do so. Clients &MAY; generate byte-range
    6039       requests without having received this header for the resource
    6040       involved. Range units are defined in <xref target="range.units"/>.
    6041 </t>
    6042 <t>
    6043       Servers that do not accept any kind of range request for a
    6044       resource &MAY; send
    6045 </t>
    6046 <figure><artwork type="example">
    6047        Accept-Ranges: none
    6048 </artwork></figure>
    6049 <t>
    6050       to advise the client not to attempt a range request.
    6051 </t>
    6052 </section>
    6054 <section title="Age" anchor="header.age">
    6055   <iref primary="true" item="Age header" x:for-anchor=""/>
    6056   <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
    6057 <t>
    6058       The Age response-header field conveys the sender's estimate of the
    6059       amount of time since the response (or its revalidation) was
    6060       generated at the origin server. A cached response is "fresh" if
    6061       its age does not exceed its freshness lifetime. Age values are
    6062       calculated as specified in <xref target="age.calculations"/>.
    6063 </t>
    6064 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
    6065         Age = "Age" ":" age-value
    6066         age-value = delta-seconds
    6067 </artwork></figure>
    6068 <t>
    6069       Age values are non-negative decimal integers, representing time in
    6070       seconds.
    6071 </t>
    6072 <t>
    6073       If a cache receives a value larger than the largest positive
    6074       integer it can represent, or if any of its age calculations
    6075       overflows, it &MUST; transmit an Age header with a value of
    6076       2147483648 (2^31). An HTTP/1.1 server that includes a cache &MUST;
    6077       include an Age header field in every response generated from its
    6078       own cache. Caches &SHOULD; use an arithmetic type of at least 31
    6079       bits of range.
    6080 </t>
    6081 </section>
    6083 <section title="Allow" anchor="header.allow">
    6084   <iref primary="true" item="Allow header" x:for-anchor=""/>
    6085   <iref primary="true" item="Headers" subitem="Allow" x:for-anchor=""/>
    6086 <t>
    6087       The Allow entity-header field lists the set of methods supported
    6088       by the resource identified by the Request-URI. The purpose of this
    6089       field is strictly to inform the recipient of valid methods
    6090       associated with the resource. An Allow header field &MUST; be
    6091       present in a 405 (Method Not Allowed) response.
    6092 </t>
    6093 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Allow"/>
    6094        Allow   = "Allow" ":" #Method
    6095 </artwork></figure>
    6096 <t>
    6097       Example of use:
    6098 </t>
    6099 <figure><artwork type="example">
    6100        Allow: GET, HEAD, PUT
    6101 </artwork></figure>
    6102 <t>
    6103       This field cannot prevent a client from trying other methods.
    6104       However, the indications given by the Allow header field value
    6105       &SHOULD; be followed. The actual set of allowed methods is defined
    6106       by the origin server at the time of each request.
    6107 </t>
    6108 <t>
    6109       The Allow header field &MAY; be provided with a PUT request to
    6110       recommend the methods to be supported by the new or modified
    6111       resource. The server is not required to support these methods and
    6112       &SHOULD; include an Allow header in the response giving the actual
    6113       supported methods.
    6114 </t>
    6115 <t>
    6116       A proxy &MUST-NOT; modify the Allow header field even if it does not
    6117       understand all the methods specified, since the user agent might
    6118       have other means of communicating with the origin server.
    6119 </t>
    6120 </section>
    6122 <section title="Authorization" anchor="header.authorization">
    6123   <iref primary="true" item="Authorization header" x:for-anchor=""/>
    6124   <iref primary="true" item="Headers" subitem="Authorization" x:for-anchor=""/>
    6125 <t>
    6126       A user agent that wishes to authenticate itself with a server--
    6127       usually, but not necessarily, after receiving a 401 response--does
    6128       so by including an Authorization request-header field with the
    6129       request.  The Authorization field value consists of credentials
    6130       containing the authentication information of the user agent for
    6131       the realm of the resource being requested.
    6132 </t>
    6133 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Authorization"/>
    6134        Authorization  = "Authorization" ":" credentials
    6135 </artwork></figure>
    6136 <t>
    6137       HTTP access authentication is described in "HTTP Authentication:
    6138       Basic and Digest Access Authentication" <xref target="RFC2617"/>. If a request is
    6139       authenticated and a realm specified, the same credentials &SHOULD;
    6140       be valid for all other requests within this realm (assuming that
    6141       the authentication scheme itself does not require otherwise, such
    6142       as credentials that vary according to a challenge value or using
    6143       synchronized clocks).
    6144 </t>
    6145 <t>
    6146       When a shared cache (see <xref target="shared.and.non-shared.caches"/>) receives a request
    6147       containing an Authorization field, it &MUST-NOT; return the
    6148       corresponding response as a reply to any other request, unless one
    6149       of the following specific exceptions holds:
    6150 </t>
    6151 <t>
    6152   <list style="numbers">
    6153       <t>If the response includes the "s-maxage" cache-control
    6154          directive, the cache &MAY; use that response in replying to a
    6155          subsequent request. But (if the specified maximum age has
    6156          passed) a proxy cache &MUST; first revalidate it with the origin
    6157          server, using the request-headers from the new request to allow
    6158          the origin server to authenticate the new request. (This is the
    6159          defined behavior for s-maxage.) If the response includes "s-maxage=0",
    6160          the proxy &MUST; always revalidate it before re-using
    6161          it.</t>
    6163       <t>If the response includes the "must-revalidate" cache-control
    6164          directive, the cache &MAY; use that response in replying to a
    6165          subsequent request. But if the response is stale, all caches
    6166          &MUST; first revalidate it with the origin server, using the
    6167          request-headers from the new request to allow the origin server
    6168          to authenticate the new request.</t>
    6170       <t>If the response includes the "public" cache-control directive,
    6171          it &MAY; be returned in reply to any subsequent request.</t>
    6172   </list>
    6173 </t>
    6174 </section>
    6176 <section title="Cache-Control" anchor="header.cache-control">
    6177   <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
    6178   <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
    6179 <t>
    6180    The Cache-Control general-header field is used to specify directives
    6181    that &MUST; be obeyed by all caching mechanisms along the
    6182    request/response chain. The directives specify behavior intended to
    6183    prevent caches from adversely interfering with the request or
    6184    response. These directives typically override the default caching
    6185    algorithms. Cache directives are unidirectional in that the presence
    6186    of a directive in a request does not imply that the same directive is
    6187    to be given in the response.
    6188   <list><t>
    6189       Note that HTTP/1.0 caches might not implement Cache-Control and
    6190       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
    6191   </t></list>
    6192 </t>
    6193 <t>
    6194    Cache directives &MUST; be passed through by a proxy or gateway
    6195    application, regardless of their significance to that application,
    6196    since the directives might be applicable to all recipients along the
    6197    request/response chain. It is not possible to specify a cache-directive
    6198    for a specific cache.
    6199 </t>
    6200 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/><iref primary="true" item="Grammar" subitem="cache-request-directive"/><iref primary="true" item="Grammar" subitem="cache-response-directive"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
    6201    Cache-Control   = "Cache-Control" ":" 1#cache-directive
    6203    cache-directive = cache-request-directive
    6204         | cache-response-directive
    6206    cache-request-directive =
    6207           "no-cache"                          ; <xref target=""/>
    6208         | "no-store"                          ; <xref target=""/>
    6209         | "max-age" "=" delta-seconds         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
    6210         | "max-stale" [ "=" delta-seconds ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    6211         | "min-fresh" "=" delta-seconds       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    6212         | "no-transform"                      ; <xref target="no-transform.directive"/>
    6213         | "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
    6214         | cache-extension                     ; <xref target="cache.control.extensions"/>
    6216     cache-response-directive =
    6217           "public"                               ; <xref target=""/>
    6218         | "private" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ] ; <xref target=""/>
    6219         | "no-cache" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ]; <xref target=""/>
    6220         | "no-store"                             ; <xref target=""/>
    6221         | "no-transform"                         ; <xref target="no-transform.directive"/>
    6222         | "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
    6223         | "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
    6224         | "max-age" "=" delta-seconds            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    6225         | "s-maxage" "=" delta-seconds           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
    6226         | cache-extension                        ; <xref target="cache.control.extensions"/>
    6228    cache-extension = token [ "=" ( token | quoted-string ) ]
    6229 </artwork></figure>
    6230 <t>
    6231    When a directive appears without any 1#field-name parameter, the
    6232    directive applies to the entire request or response. When such a
    6233    directive appears with a 1#field-name parameter, it applies only to
    6234    the named field or fields, and not to the rest of the request or
    6235    response. This mechanism supports extensibility; implementations of
    6236    future versions of the HTTP protocol might apply these directives to
    6237    header fields not defined in HTTP/1.1.
    6238 </t>
    6239 <t>
    6240    The cache-control directives can be broken down into these general
    6241    categories:
    6242   <list style="symbols">
    6243      <t>Restrictions on what are cacheable; these may only be imposed by
    6244         the origin server.</t>
    6246      <t>Restrictions on what may be stored by a cache; these may be
    6247         imposed by either the origin server or the user agent.</t>
    6249      <t>Modifications of the basic expiration mechanism; these may be
    6250         imposed by either the origin server or the user agent.</t>
    6252      <t>Controls over cache revalidation and reload; these may only be
    6253         imposed by a user agent.</t>
    6255      <t>Control over transformation of entities.</t>
    6257      <t>Extensions to the caching system.</t>
    6258   </list>
    6259 </t>
    6261 <section title="What is Cacheable" anchor="">
    6262 <t>
    6263    By default, a response is cacheable if the requirements of the
    6264    request method, request header fields, and the response status
    6265    indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
    6266    for cacheability. The following Cache-Control response directives
    6267    allow an origin server to override the default cacheability of a
    6268    response:
    6269 </t>
    6270 <t>
    6271   <iref item="Cache Directives" subitem="public" primary="true"/>
    6272   <iref item="public" subitem="Cache Directive" primary="true"/>
    6273    public
    6274   <list><t>
    6275       Indicates that the response &MAY; be cached by any cache, even if it
    6276       would normally be non-cacheable or cacheable only within a non-shared
    6277       cache. (See also Authorization, <xref target="header.authorization"/>, for
    6278       additional details.)
    6279   </t></list>
    6280 </t>
    6281 <t>
    6282   <iref item="Cache Directives" subitem="private" primary="true"/>
    6283   <iref item="private" subitem="Cache Directive" primary="true"/>
    6284    private
    6285   <list><t>
    6286       Indicates that all or part of the response message is intended for
    6287       a single user and &MUST-NOT; be cached by a shared cache. This
    6288       allows an origin server to state that the specified parts of the
    6289       response are intended for only one user and are not a valid
    6290       response for requests by other users. A private (non-shared) cache
    6291       &MAY; cache the response.
    6292     </t><t>
    6293        <x:h>Note:</x:h> This usage of the word private only controls where the
    6294        response may be cached, and cannot ensure the privacy of the
    6295        message content.
    6296   </t></list>
    6297 </t>
    6298 <t>
    6299   <iref item="Cache Directives" subitem="no-cache" primary="true"/>
    6300   <iref item="no-cache" subitem="Cache Directive" primary="true"/>
    6301    no-cache
    6302   <list><t>
    6303        If the no-cache directive does not specify a field-name, then a
    6304       cache &MUST-NOT; use the response to satisfy a subsequent request
    6305       without successful revalidation with the origin server. This
    6306       allows an origin server to prevent caching even by caches that
    6307       have been configured to return stale responses to client requests.
    6308     </t><t>
    6309       If the no-cache directive does specify one or more field-names,
    6310       then a cache &MAY; use the response to satisfy a subsequent request,
    6311       subject to any other restrictions on caching. However, the
    6312       specified field-name(s) &MUST-NOT; be sent in the response to a
    6313       subsequent request without successful revalidation with the origin
    6314       server. This allows an origin server to prevent the re-use of
    6315       certain header fields in a response, while still allowing caching
    6316       of the rest of the response.
    6317     <list><t>
    6318        <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
    6319        directive.
    6320     </t></list>
    6321   </t></list>
    6322 </t>
    6323 </section>
    6325 <section title="What May be Stored by Caches" anchor="">
    6326 <t>
    6327   <iref item="Cache Directives" subitem="no-store" primary="true"/>
    6328   <iref item="no-store" subitem="Cache Directive" primary="true"/>
    6329    no-store
    6330   <list><t>   
    6331       The purpose of the no-store directive is to prevent the
    6332       inadvertent release or retention of sensitive information (for
    6333       example, on backup tapes). The no-store directive applies to the
    6334       entire message, and &MAY; be sent either in a response or in a
    6335       request. If sent in a request, a cache &MUST-NOT; store any part of
    6336       either this request or any response to it. If sent in a response,
    6337       a cache &MUST-NOT; store any part of either this response or the
    6338       request that elicited it. This directive applies to both non-shared
    6339       and shared caches. "&MUST-NOT; store" in this context means
    6340       that the cache &MUST-NOT; intentionally store the information in
    6341       non-volatile storage, and &MUST; make a best-effort attempt to
    6342       remove the information from volatile storage as promptly as
    6343       possible after forwarding it.
    6344   </t><t>
    6345       Even when this directive is associated with a response, users
    6346       might explicitly store such a response outside of the caching
    6347       system (e.g., with a "Save As" dialog). History buffers &MAY; store
    6348       such responses as part of their normal operation.
    6349   </t><t>
    6350       The purpose of this directive is to meet the stated requirements
    6351       of certain users and service authors who are concerned about
    6352       accidental releases of information via unanticipated accesses to
    6353       cache data structures. While the use of this directive might
    6354       improve privacy in some cases, we caution that it is NOT in any
    6355       way a reliable or sufficient mechanism for ensuring privacy. In
    6356       particular, malicious or compromised caches might not recognize or
    6357       obey this directive, and communications networks might be
    6358       vulnerable to eavesdropping.
    6359   </t></list>
    6360 </t>
    6361 </section>
    6363 <section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
    6364 <t>
    6365    The expiration time of an entity &MAY; be specified by the origin
    6366    server using the Expires header (see <xref target="header.expires"/>). Alternatively,
    6367    it &MAY; be specified using the max-age directive in a response. When
    6368    the max-age cache-control directive is present in a cached response,
    6369    the response is stale if its current age is greater than the age
    6370    value given (in seconds) at the time of a new request for that
    6371    resource. The max-age directive on a response implies that the
    6372    response is cacheable (i.e., "public") unless some other, more
    6373    restrictive cache directive is also present.
    6374 </t>
    6375 <t>
    6376    If a response includes both an Expires header and a max-age
    6377    directive, the max-age directive overrides the Expires header, even
    6378    if the Expires header is more restrictive. This rule allows an origin
    6379    server to provide, for a given response, a longer expiration time to
    6380    an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
    6381    useful if certain HTTP/1.0 caches improperly calculate ages or
    6382    expiration times, perhaps due to desynchronized clocks.
    6383 </t>
    6384 <t>
    6385    Many HTTP/1.0 cache implementations will treat an Expires value that
    6386    is less than or equal to the response Date value as being equivalent
    6387    to the Cache-Control response directive "no-cache". If an HTTP/1.1
    6388    cache receives such a response, and the response does not include a
    6389    Cache-Control header field, it &SHOULD; consider the response to be
    6390    non-cacheable in order to retain compatibility with HTTP/1.0 servers.
    6391   <list><t>
    6392        <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
    6393        cache control feature, such as the "private" directive, on a
    6394        network including older caches that do not understand that
    6395        feature. The origin server will need to combine the new feature
    6396        with an Expires field whose value is less than or equal to the
    6397        Date value. This will prevent older caches from improperly
    6398        caching the response.
    6399   </t></list>
    6400 </t>
    6401 <t>
    6402   <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
    6403   <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
    6404    s-maxage
    6405   <list><t>
    6406        If a response includes an s-maxage directive, then for a shared
    6407        cache (but not for a private cache), the maximum age specified by
    6408        this directive overrides the maximum age specified by either the
    6409        max-age directive or the Expires header. The s-maxage directive
    6410        also implies the semantics of the proxy-revalidate directive (see
    6411        <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
    6412        entry after it becomes stale to respond to a subsequent request
    6413        without first revalidating it with the origin server. The s-maxage
    6414        directive is always ignored by a private cache.
    6415   </t></list>
    6416 </t>
    6417 <t>
    6418    Note that most older caches, not compliant with this specification,
    6419    do not implement any cache-control directives. An origin server
    6420    wishing to use a cache-control directive that restricts, but does not
    6421    prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
    6422    requirement that the max-age directive overrides the Expires header,
    6423    and the fact that pre-HTTP/1.1-compliant caches do not observe the
    6424    max-age directive.
    6425 </t>
    6426 <t>
    6427    Other directives allow a user agent to modify the basic expiration
    6428    mechanism. These directives &MAY; be specified on a request:
    6429 </t>
    6430 <t>
    6431   <iref item="Cache Directives" subitem="max-age" primary="true"/>
    6432   <iref item="max-age" subitem="Cache Directive" primary="true"/>
    6433    max-age
    6434   <list><t>
    6435       Indicates that the client is willing to accept a response whose
    6436       age is no greater than the specified time in seconds. Unless max-stale
    6437       directive is also included, the client is not willing to
    6438       accept a stale response.
    6439   </t></list>
    6440 </t>
    6441 <t>
    6442   <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
    6443   <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
    6444    min-fresh
    6445   <list><t>
    6446       Indicates that the client is willing to accept a response whose
    6447       freshness lifetime is no less than its current age plus the
    6448       specified time in seconds. That is, the client wants a response
    6449       that will still be fresh for at least the specified number of
    6450       seconds.
    6451   </t></list>
    6452 </t>
    6453 <t>
    6454   <iref item="Cache Directives" subitem="max-stale" primary="true"/>
    6455   <iref item="max-stale" subitem="Cache Directive" primary="true"/>
    6456    max-stale
    6457   <list><t>
    6458       Indicates that the client is willing to accept a response that has
    6459       exceeded its expiration time. If max-stale is assigned a value,
    6460       then the client is willing to accept a response that has exceeded
    6461       its expiration time by no more than the specified number of
    6462       seconds. If no value is assigned to max-stale, then the client is
    6463       willing to accept a stale response of any age.
    6464   </t></list>
    6465 </t>
    6466 <t>
    6467    If a cache returns a stale response, either because of a max-stale
    6468    directive on a request, or because the cache is configured to
    6469    override the expiration time of a response, the cache &MUST; attach a
    6470    Warning header to the stale response, using Warning 110 (Response is
    6471    stale).
    6472 </t>
    6473 <t>
    6474    A cache &MAY; be configured to return stale responses without
    6475    validation, but only if this does not conflict with any "MUST"-level
    6476    requirements concerning cache validation (e.g., a "must-revalidate"
    6477    cache-control directive).
    6478 </t>
    6479 <t>
    6480    If both the new request and the cached entry include "max-age"
    6481    directives, then the lesser of the two values is used for determining
    6482    the freshness of the cached entry for that request.
    6483 </t>
    6484 </section>
    6486 <section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
    6487 <t>
    6488    Sometimes a user agent might want or need to insist that a cache
    6489    revalidate its cache entry with the origin server (and not just with
    6490    the next cache along the path to the origin server), or to reload its
    6491    cache entry from the origin server. End-to-end revalidation might be
    6492    necessary if either the cache or the origin server has overestimated
    6493    the expiration time of the cached response. End-to-end reload may be
    6494    necessary if the cache entry has become corrupted for some reason.
    6495 </t>
    6496 <t>
    6497    End-to-end revalidation may be requested either when the client does
    6498    not have its own local cached copy, in which case we call it
    6499    "unspecified end-to-end revalidation", or when the client does have a
    6500    local cached copy, in which case we call it "specific end-to-end
    6501    revalidation."
    6502 </t>
    6503 <t>
    6504    The client can specify these three kinds of action using Cache-Control
    6505    request directives:
    6506 </t>
    6507 <t>
    6508    End-to-end reload
    6509   <list><t>
    6510       The request includes a "no-cache" cache-control directive or, for
    6511       compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
    6512       names &MUST-NOT; be included with the no-cache directive in a
    6513       request. The server &MUST-NOT; use a cached copy when responding to
    6514       such a request.
    6515   </t></list>
    6516 </t>
    6517 <t>
    6518    Specific end-to-end revalidation
    6519   <list><t>
    6520       The request includes a "max-age=0" cache-control directive, which
    6521       forces each cache along the path to the origin server to
    6522       revalidate its own entry, if any, with the next cache or server.
    6523       The initial request includes a cache-validating conditional with
    6524       the client's current validator.
    6525   </t></list>
    6526 </t>
    6527 <t>
    6528    Unspecified end-to-end revalidation
    6529   <list><t>
    6530       The request includes "max-age=0" cache-control directive, which
    6531       forces each cache along the path to the origin server to
    6532       revalidate its own entry, if any, with the next cache or server.
    6533       The initial request does not include a cache-validating
    6534       conditional; the first cache along the path (if any) that holds a
    6535       cache entry for this resource includes a cache-validating
    6536       conditional with its current validator.
    6537   </t></list>
    6538 </t>
    6539 <t>
    6540   <iref item="Cache Directives" subitem="max-age" primary="true"/>
    6541   <iref item="max-age" subitem="Cache Directive" primary="true"/>
    6542    max-age
    6543   <list><t>
    6544       When an intermediate cache is forced, by means of a max-age=0
    6545       directive, to revalidate its own cache entry, and the client has
    6546       supplied its own validator in the request, the supplied validator
    6547       might differ from the validator currently stored with the cache
    6548       entry. In this case, the cache &MAY; use either validator in making
    6549       its own request without affecting semantic transparency.
    6550   </t><t>
    6551       However, the choice of validator might affect performance. The
    6552       best approach is for the intermediate cache to use its own
    6553       validator when making its request. If the server replies with 304
    6554       (Not Modified), then the cache can return its now validated copy
    6555       to the client with a 200 (OK) response. If the server replies with
    6556       a new entity and cache validator, however, the intermediate cache
    6557       can compare the returned validator with the one provided in the
    6558       client's request, using the strong comparison function. If the
    6559       client's validator is equal to the origin server's, then the
    6560       intermediate cache simply returns 304 (Not Modified). Otherwise,
    6561       it returns the new entity with a 200 (OK) response.
    6562   </t><t>
    6563       If a request includes the no-cache directive, it &SHOULD-NOT;
    6564       include min-fresh, max-stale, or max-age.
    6565   </t></list>
    6566 </t>
    6567 <t>
    6568   <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
    6569   <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
    6570    only-if-cached
    6571   <list><t>
    6572       In some cases, such as times of extremely poor network
    6573       connectivity, a client may want a cache to return only those
    6574       responses that it currently has stored, and not to reload or
    6575       revalidate with the origin server. To do this, the client may
    6576       include the only-if-cached directive in a request. If it receives
    6577       this directive, a cache &SHOULD; either respond using a cached entry
    6578       that is consistent with the other constraints of the request, or
    6579       respond with a 504 (Gateway Timeout) status. However, if a group
    6580       of caches is being operated as a unified system with good internal
    6581       connectivity, such a request &MAY; be forwarded within that group of
    6582       caches.
    6583   </t></list>
    6584 </t>
    6585 <t>
    6586   <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
    6587   <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
    6588    must-revalidate
    6589   <list><t>
    6590       Because a cache &MAY; be configured to ignore a server's specified
    6591       expiration time, and because a client request &MAY; include a max-stale
    6592       directive (which has a similar effect), the protocol also
    6593       includes a mechanism for the origin server to require revalidation
    6594       of a cache entry on any subsequent use. When the must-revalidate
    6595       directive is present in a response received by a cache, that cache
    6596       &MUST-NOT; use the entry after it becomes stale to respond to a
    6597       subsequent request without first revalidating it with the origin
    6598       server. (I.e., the cache &MUST; do an end-to-end revalidation every
    6599       time, if, based solely on the origin server's Expires or max-age
    6600       value, the cached response is stale.)
    6601   </t><t>
    6602       The must-revalidate directive is necessary to support reliable
    6603       operation for certain protocol features. In all circumstances an
    6604       HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
    6605       particular, if the cache cannot reach the origin server for any
    6606       reason, it &MUST; generate a 504 (Gateway Timeout) response.
    6607   </t><t>
    6608       Servers &SHOULD; send the must-revalidate directive if and only if
    6609       failure to revalidate a request on the entity could result in
    6610       incorrect operation, such as a silently unexecuted financial
    6611       transaction. Recipients &MUST-NOT; take any automated action that
    6612       violates this directive, and &MUST-NOT; automatically provide an
    6613       unvalidated copy of the entity if revalidation fails.
    6614   </t><t>
    6615       Although this is not recommended, user agents operating under
    6616       severe connectivity constraints &MAY; violate this directive but, if
    6617       so, &MUST; explicitly warn the user that an unvalidated response has
    6618       been provided. The warning &MUST; be provided on each unvalidated
    6619       access, and &SHOULD; require explicit user confirmation.
    6620   </t></list>
    6621 </t>
    6622 <t>
    6623   <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
    6624   <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
    6625    proxy-revalidate
    6626   <list><t>
    6627       The proxy-revalidate directive has the same meaning as the must-revalidate
    6628       directive, except that it does not apply to non-shared
    6629       user agent caches. It can be used on a response to an
    6630       authenticated request to permit the user's cache to store and
    6631       later return the response without needing to revalidate it (since
    6632       it has already been authenticated once by that user), while still
    6633       requiring proxies that service many users to revalidate each time
    6634       (in order to make sure that each user has been authenticated).
    6635       Note that such authenticated responses also need the public cache
    6636       control directive in order to allow them to be cached at all.
    6637   </t></list>
    6638 </t>
    6639 </section>
    6641 <section title="No-Transform Directive" anchor="no-transform.directive">
    6642 <t>
    6643   <iref item="Cache Directives" subitem="no-transform" primary="true"/>
    6644   <iref item="no-transform" subitem="Cache Directive" primary="true"/>
    6645    no-transform
    6646   <list><t>
    6647       Implementors of intermediate caches (proxies) have found it useful
    6648       to convert the media type of certain entity bodies. A non-transparent
    6649       proxy might, for example, convert between image
    6650       formats in order to save cache space or to reduce the amount of
    6651       traffic on a slow link.
    6652   </t><t>
    6653       Serious operational problems occur, however, when these
    6654       transformations are applied to entity bodies intended for certain
    6655       kinds of applications. For example, applications for medical
    6656       imaging, scientific data analysis and those using end-to-end
    6657       authentication, all depend on receiving an entity body that is bit
    6658       for bit identical to the original entity-body.
    6659   </t><t>
    6660       Therefore, if a message includes the no-transform directive, an
    6661       intermediate cache or proxy &MUST-NOT; change those headers that are
    6662       listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
    6663       directive. This implies that the cache or proxy &MUST-NOT; change
    6664       any aspect of the entity-body that is specified by these headers,
    6665       including the value of the entity-body itself.
    6666   </t></list>
    6667 </t>
    6668 </section>
    6670 <section title="Cache Control Extensions" anchor="cache.control.extensions">
    6671 <t>
    6672    The Cache-Control header field can be extended through the use of one
    6673    or more cache-extension tokens, each with an optional assigned value.
    6674    Informational extensions (those which do not require a change in
    6675    cache behavior) &MAY; be added without changing the semantics of other
    6676    directives. Behavioral extensions are designed to work by acting as
    6677    modifiers to the existing base of cache directives. Both the new
    6678    directive and the standard directive are supplied, such that
    6679    applications which do not understand the new directive will default
    6680    to the behavior specified by the standard directive, and those that
    6681    understand the new directive will recognize it as modifying the
    6682    requirements associated with the standard directive. In this way,
    6683    extensions to the cache-control directives can be made without
    6684    requiring changes to the base protocol.
    6685 </t>
    6686 <t>
    6687    This extension mechanism depends on an HTTP cache obeying all of the
    6688    cache-control directives defined for its native HTTP-version, obeying
    6689    certain extensions, and ignoring all directives that it does not
    6690    understand.
    6691 </t>
    6692 <t>
    6693    For example, consider a hypothetical new response directive called
    6694    community which acts as a modifier to the private directive. We
    6695    define this new directive to mean that, in addition to any non-shared
    6696    cache, any cache which is shared only by members of the community
    6697    named within its value may cache the response. An origin server
    6698    wishing to allow the UCI community to use an otherwise private
    6699    response in their shared cache(s) could do so by including
    6700 </t>
    6701 <figure><artwork type="example">
    6702     Cache-Control: private, community="UCI"
    6703 </artwork></figure>
    6704 <t>
    6705    A cache seeing this header field will act correctly even if the cache
    6706    does not understand the community cache-extension, since it will also
    6707    see and understand the private directive and thus default to the safe
    6708    behavior.
    6709 </t>
    6710 <t>
    6711    Unrecognized cache-directives &MUST; be ignored; it is assumed that any
    6712    cache-directive likely to be unrecognized by an HTTP/1.1 cache will
    6713    be combined with standard directives (or the response's default
    6714    cacheability) such that the cache behavior will remain minimally
    6715    correct even if the cache does not understand the extension(s).
    6716 </t>
    6717 </section>
    6718 </section>
    67202142<section title="Connection" anchor="header.connection">
    6775 <section title="Content-Encoding" anchor="header.content-encoding">
    6776   <iref primary="true" item="Content-Encoding header" x:for-anchor=""/>
    6777   <iref primary="true" item="Headers" subitem="Content-Encoding" x:for-anchor=""/>
    6778 <t>
    6779    The Content-Encoding entity-header field is used as a modifier to the
    6780    media-type. When present, its value indicates what additional content
    6781    codings have been applied to the entity-body, and thus what decoding
    6782    mechanisms must be applied in order to obtain the media-type
    6783    referenced by the Content-Type header field. Content-Encoding is
    6784    primarily used to allow a document to be compressed without losing
    6785    the identity of its underlying media type.
    6786 </t>
    6787 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Encoding"/>
    6788     Content-Encoding  = "Content-Encoding" ":" 1#content-coding
    6789 </artwork></figure>
    6790 <t>
    6791    Content codings are defined in <xref target="content.codings"/>. An example of its use is
    6792 </t>
    6793 <figure><artwork type="example">
    6794     Content-Encoding: gzip
    6795 </artwork></figure>
    6796 <t>
    6797    The content-coding is a characteristic of the entity identified by
    6798    the Request-URI. Typically, the entity-body is stored with this
    6799    encoding and is only decoded before rendering or analogous usage.
    6800    However, a non-transparent proxy &MAY; modify the content-coding if the
    6801    new coding is known to be acceptable to the recipient, unless the
    6802    "no-transform" cache-control directive is present in the message.
    6803 </t>
    6804 <t>
    6805    If the content-coding of an entity is not "identity", then the
    6806    response &MUST; include a Content-Encoding entity-header (<xref target="header.content-encoding"/>)
    6807    that lists the non-identity content-coding(s) used.
    6808 </t>
    6809 <t>
    6810    If the content-coding of an entity in a request message is not
    6811    acceptable to the origin server, the server &SHOULD; respond with a
    6812    status code of 415 (Unsupported Media Type).
    6813 </t>
    6814 <t>
    6815    If multiple encodings have been applied to an entity, the content
    6816    codings &MUST; be listed in the order in which they were applied.
    6817    Additional information about the encoding parameters &MAY; be provided
    6818    by other entity-header fields not defined by this specification.
    6819 </t>
    6820 </section>
    6822 <section title="Content-Language" anchor="header.content-language">
    6823   <iref primary="true" item="Content-Language header" x:for-anchor=""/>
    6824   <iref primary="true" item="Headers" subitem="Content-Language" x:for-anchor=""/>
    6825 <t>
    6826    The Content-Language entity-header field describes the natural
    6827    language(s) of the intended audience for the enclosed entity. Note
    6828    that this might not be equivalent to all the languages used within
    6829    the entity-body.
    6830 </t>
    6831 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Language"/>
    6832     Content-Language  = "Content-Language" ":" 1#language-tag
    6833 </artwork></figure>
    6834 <t>
    6835    Language tags are defined in <xref target="language.tags"/>. The primary purpose of
    6836    Content-Language is to allow a user to identify and differentiate
    6837    entities according to the user's own preferred language. Thus, if the
    6838    body content is intended only for a Danish-literate audience, the
    6839    appropriate field is
    6840 </t>
    6841 <figure><artwork type="example">
    6842     Content-Language: da
    6843 </artwork></figure>
    6844 <t>
    6845    If no Content-Language is specified, the default is that the content
    6846    is intended for all language audiences. This might mean that the
    6847    sender does not consider it to be specific to any natural language,
    6848    or that the sender does not know for which language it is intended.
    6849 </t>
    6850 <t>
    6851    Multiple languages &MAY; be listed for content that is intended for
    6852    multiple audiences. For example, a rendition of the "Treaty of
    6853    Waitangi," presented simultaneously in the original Maori and English
    6854    versions, would call for
    6855 </t>
    6856 <figure><artwork type="example">
    6857     Content-Language: mi, en
    6858 </artwork></figure>
    6859 <t>
    6860    However, just because multiple languages are present within an entity
    6861    does not mean that it is intended for multiple linguistic audiences.
    6862    An example would be a beginner's language primer, such as "A First
    6863    Lesson in Latin," which is clearly intended to be used by an
    6864    English-literate audience. In this case, the Content-Language would
    6865    properly only include "en".
    6866 </t>
    6867 <t>
    6868    Content-Language &MAY; be applied to any media type -- it is not
    6869    limited to textual documents.
    6870 </t>
    6871 </section>
    68732197<section title="Content-Length" anchor="header.content-length">
    68742198  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
    69052229   to being transferred, unless this is prohibited by the rules in
    69062230   <xref target="message.length"/>.
    6907 </t>
    6908 </section>
    6910 <section title="Content-Location" anchor="header.content-location">
    6911   <iref primary="true" item="Content-Location header" x:for-anchor=""/>
    6912   <iref primary="true" item="Headers" subitem="Content-Location" x:for-anchor=""/>
    6913 <t>
    6914    The Content-Location entity-header field &MAY; be used to supply the
    6915    resource location for the entity enclosed in the message when that
    6916    entity is accessible from a location separate from the requested
    6917    resource's URI. A server &SHOULD; provide a Content-Location for the
    6918    variant corresponding to the response entity; especially in the case
    6919    where a resource has multiple entities associated with it, and those
    6920    entities actually have separate locations by which they might be
    6921    individually accessed, the server &SHOULD; provide a Content-Location
    6922    for the particular variant which is returned.
    6923 </t>
    6924 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Location"/>
    6925     Content-Location = "Content-Location" ":"
    6926                       ( absoluteURI | relativeURI )
    6927 </artwork></figure>
    6928 <t>
    6929    The value of Content-Location also defines the base URI for the
    6930    entity.
    6931 </t>
    6932 <t>
    6933    The Content-Location value is not a replacement for the original
    6934    requested URI; it is only a statement of the location of the resource
    6935    corresponding to this particular entity at the time of the request.
    6936    Future requests &MAY; specify the Content-Location URI as the request-URI
    6937    if the desire is to identify the source of that particular
    6938    entity.
    6939 </t>
    6940 <t>
    6941    A cache cannot assume that an entity with a Content-Location
    6942    different from the URI used to retrieve it can be used to respond to
    6943    later requests on that Content-Location URI. However, the Content-Location
    6944    can be used to differentiate between multiple entities
    6945    retrieved from a single requested resource, as described in <xref target="caching.negotiated.responses"/>.
    6946 </t>
    6947 <t>
    6948    If the Content-Location is a relative URI, the relative URI is
    6949    interpreted relative to the Request-URI.
    6950 </t>
    6951 <t>
    6952    The meaning of the Content-Location header in PUT or POST requests is
    6953    undefined; servers are free to ignore it in those cases.
    6954 </t>
    6955 </section>
    6957 <section title="Content-MD5" anchor="header.content-md5">
    6958   <iref primary="true" item="Content-MD5 header" x:for-anchor=""/>
    6959   <iref primary="true" item="Headers" subitem="Content-MD5" x:for-anchor=""/>
    6960 <t>
    6961    The Content-MD5 entity-header field, as defined in RFC 1864 <xref target="RFC1864"/>, is
    6962    an MD5 digest of the entity-body for the purpose of providing an
    6963    end-to-end message integrity check (MIC) of the entity-body. (Note: a
    6964    MIC is good for detecting accidental modification of the entity-body
    6965    in transit, but is not proof against malicious attacks.)
    6966 </t>
    6967 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-MD5"/><iref primary="true" item="Grammar" subitem="md5-digest"/>
    6968      Content-MD5   = "Content-MD5" ":" md5-digest
    6969      md5-digest   = &lt;base64 of 128 bit MD5 digest as per RFC 1864&gt;
    6970 </artwork></figure>
    6971 <t>
    6972    The Content-MD5 header field &MAY; be generated by an origin server or
    6973    client to function as an integrity check of the entity-body. Only
    6974    origin servers or clients &MAY; generate the Content-MD5 header field;
    6975    proxies and gateways &MUST-NOT; generate it, as this would defeat its
    6976    value as an end-to-end integrity check. Any recipient of the entity-body,
    6977    including gateways and proxies, &MAY; check that the digest value
    6978    in this header field matches that of the entity-body as received.
    6979 </t>
    6980 <t>
    6981    The MD5 digest is computed based on the content of the entity-body,
    6982    including any content-coding that has been applied, but not including
    6983    any transfer-encoding applied to the message-body. If the message is
    6984    received with a transfer-encoding, that encoding &MUST; be removed
    6985    prior to checking the Content-MD5 value against the received entity.
    6986 </t>
    6987 <t>
    6988    This has the result that the digest is computed on the octets of the
    6989    entity-body exactly as, and in the order that, they would be sent if
    6990    no transfer-encoding were being applied.
    6991 </t>
    6992 <t>
    6993    HTTP extends RFC 1864 to permit the digest to be computed for MIME
    6994    composite media-types (e.g., multipart/* and message/rfc822), but
    6995    this does not change how the digest is computed as defined in the
    6996    preceding paragraph.
    6997 </t>
    6998 <t>
    6999    There are several consequences of this. The entity-body for composite
    7000    types &MAY; contain many body-parts, each with its own MIME and HTTP
    7001    headers (including Content-MD5, Content-Transfer-Encoding, and
    7002    Content-Encoding headers). If a body-part has a Content-Transfer-Encoding
    7003    or Content-Encoding header, it is assumed that the content
    7004    of the body-part has had the encoding applied, and the body-part is
    7005    included in the Content-MD5 digest as is -- i.e., after the
    7006    application. The Transfer-Encoding header field is not allowed within
    7007    body-parts.
    7008 </t>
    7009 <t>
    7010    Conversion of all line breaks to CRLF &MUST-NOT; be done before
    7011    computing or checking the digest: the line break convention used in
    7012    the text actually transmitted &MUST; be left unaltered when computing
    7013    the digest.
    7014   <list><t>
    7015       <x:h>Note:</x:h> while the definition of Content-MD5 is exactly the same for
    7016       HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
    7017       in which the application of Content-MD5 to HTTP entity-bodies
    7018       differs from its application to MIME entity-bodies. One is that
    7019       HTTP, unlike MIME, does not use Content-Transfer-Encoding, and
    7020       does use Transfer-Encoding and Content-Encoding. Another is that
    7021       HTTP more frequently uses binary content types than MIME, so it is
    7022       worth noting that, in such cases, the byte order used to compute
    7023       the digest is the transmission byte order defined for the type.
    7024       Lastly, HTTP allows transmission of text types with any of several
    7025       line break conventions and not just the canonical form using CRLF.
    7026   </t></list>
    7027 </t>
    7028 </section>
    7030 <section title="Content-Range" anchor="header.content-range">
    7031   <iref primary="true" item="Content-Range header" x:for-anchor=""/>
    7032   <iref primary="true" item="Headers" subitem="Content-Range" x:for-anchor=""/>
    7033 <t>
    7034    The Content-Range entity-header is sent with a partial entity-body to
    7035    specify where in the full entity-body the partial body should be
    7036    applied. Range units are defined in <xref target="range.units"/>.
    7037 </t>
    7038 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Range"/><iref primary="true" item="Grammar" subitem="content-range-spec"/><iref primary="true" item="Grammar" subitem="byte-content-range-spec"/><iref primary="true" item="Grammar" subitem="byte-range-resp-spec"/><iref primary="true" item="Grammar" subitem="instance-length"/>
    7039     Content-Range = "Content-Range" ":" content-range-spec
    7041     content-range-spec      = byte-content-range-spec
    7042     byte-content-range-spec = bytes-unit SP
    7043                               byte-range-resp-spec "/"
    7044                               ( instance-length | "*" )
    7046     byte-range-resp-spec = (first-byte-pos "-" last-byte-pos)
    7047                                    | "*"
    7048     instance-length           = 1*DIGIT
    7049 </artwork></figure>
    7050 <t>
    7051    The header &SHOULD; indicate the total length of the full entity-body,
    7052    unless this length is unknown or difficult to determine. The asterisk
    7053    "*" character means that the instance-length is unknown at the time
    7054    when the response was generated.
    7055 </t>
    7056 <t>
    7057    Unlike byte-ranges-specifier values (see <xref target="byte.ranges"/>), a byte-range-resp-spec
    7058    &MUST; only specify one range, and &MUST; contain
    7059    absolute byte positions for both the first and last byte of the
    7060    range.
    7061 </t>
    7062 <t>
    7063    A byte-content-range-spec with a byte-range-resp-spec whose last-byte-pos
    7064    value is less than its first-byte-pos value, or whose
    7065    instance-length value is less than or equal to its last-byte-pos
    7066    value, is invalid. The recipient of an invalid byte-content-range-spec
    7067    &MUST; ignore it and any content transferred along with it.
    7068 </t>
    7069 <t>
    7070    A server sending a response with status code 416 (Requested range not
    7071    satisfiable) &SHOULD; include a Content-Range field with a byte-range-resp-spec
    7072    of "*". The instance-length specifies the current length of
    7073    the selected resource. A response with status code 206 (Partial
    7074    Content) &MUST-NOT; include a Content-Range field with a byte-range-resp-spec of "*".
    7075 </t>
    7076 <t>
    7077    Examples of byte-content-range-spec values, assuming that the entity
    7078    contains a total of 1234 bytes:
    7079    <list style="symbols">
    7080       <t>
    7081         The first 500 bytes:
    7082 <figure><artwork type="text/plain">
    7083    bytes 0-499/1234
    7084 </artwork></figure>
    7085       </t>   
    7086       <t>
    7087         The second 500 bytes:
    7088 <figure><artwork type="text/plain">
    7089    bytes 500-999/1234
    7090 </artwork></figure>
    7091       </t>   
    7092       <t>
    7093         All except for the first 500 bytes:
    7094 <figure><artwork type="text/plain">
    7095    bytes 500-1233/1234
    7096 </artwork></figure>
    7097       </t>   
    7098       <t>
    7099         The last 500 bytes:
    7100 <figure><artwork type="text/plain">
    7101    bytes 734-1233/1234
    7102 </artwork></figure>
    7103       </t>   
    7104    </list>
    7105 </t>
    7106 <t>
    7107    When an HTTP message includes the content of a single range (for
    7108    example, a response to a request for a single range, or to a request
    7109    for a set of ranges that overlap without any holes), this content is
    7110    transmitted with a Content-Range header, and a Content-Length header
    7111    showing the number of bytes actually transferred. For example,
    7112 </t>
    7113 <figure><artwork type="example">
    7114     HTTP/1.1 206 Partial content
    7115     Date: Wed, 15 Nov 1995 06:25:24 GMT
    7116     Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
    7117     Content-Range: bytes 21010-47021/47022
    7118     Content-Length: 26012
    7119     Content-Type: image/gif
    7120 </artwork></figure>
    7121 <t>
    7122    When an HTTP message includes the content of multiple ranges (for
    7123    example, a response to a request for multiple non-overlapping
    7124    ranges), these are transmitted as a multipart message. The multipart
    7125    media type used for this purpose is "multipart/byteranges" as defined
    7126    in <xref target=""/>. See <xref target="changes.from.rfc.2068"/> for a compatibility issue.
    7127 </t>
    7128 <t>
    7129    A response to a request for a single range &MUST-NOT; be sent using the
    7130    multipart/byteranges media type.  A response to a request for
    7131    multiple ranges, whose result is a single range, &MAY; be sent as a
    7132    multipart/byteranges media type with one part. A client that cannot
    7133    decode a multipart/byteranges message &MUST-NOT; ask for multiple
    7134    byte-ranges in a single request.
    7135 </t>
    7136 <t>
    7137    When a client requests multiple byte-ranges in one request, the
    7138    server &SHOULD; return them in the order that they appeared in the
    7139    request.
    7140 </t>
    7141 <t>
    7142    If the server ignores a byte-range-spec because it is syntactically
    7143    invalid, the server &SHOULD; treat the request as if the invalid Range
    7144    header field did not exist. (Normally, this means return a 200
    7145    response containing the full entity).
    7146 </t>
    7147 <t>
    7148    If the server receives a request (other than one including an If-Range
    7149    request-header field) with an unsatisfiable Range request-header
    7150    field (that is, all of whose byte-range-spec values have a
    7151    first-byte-pos value greater than the current length of the selected
    7152    resource), it &SHOULD; return a response code of 416 (Requested range
    7153    not satisfiable) (<xref target="status.416"/>).
    7154   <list><t>
    7155       <x:h>Note:</x:h> clients cannot depend on servers to send a 416 (Requested
    7156       range not satisfiable) response instead of a 200 (OK) response for
    7157       an unsatisfiable Range request-header, since not all servers
    7158       implement this request-header.
    7159   </t></list>
    7160 </t>
    7161 </section>
    7163 <section title="Content-Type" anchor="header.content-type">
    7164   <iref primary="true" item="Content-Type header" x:for-anchor=""/>
    7165   <iref primary="true" item="Headers" subitem="Content-Type" x:for-anchor=""/>
    7166 <t>
    7167    The Content-Type entity-header field indicates the media type of the
    7168    entity-body sent to the recipient or, in the case of the HEAD method,
    7169    the media type that would have been sent had the request been a GET.
    7170 </t>
    7171 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Type"/>
    7172     Content-Type   = "Content-Type" ":" media-type
    7173 </artwork></figure>
    7174 <t>
    7175    Media types are defined in <xref target="media.types"/>. An example of the field is
    7176 </t>
    7177 <figure><artwork type="example">
    7178     Content-Type: text/html; charset=ISO-8859-4
    7179 </artwork></figure>
    7180 <t>
    7181    Further discussion of methods for identifying the media type of an
    7182    entity is provided in <xref target="type"/>.
    7260 </section>
    7262 <section title="ETag" anchor="header.etag">
    7263   <iref primary="true" item="ETag header" x:for-anchor=""/>
    7264   <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
    7265 <t>
    7266    The ETag response-header field provides the current value of the
    7267    entity tag for the requested variant. The headers used with entity
    7268    tags are described in sections <xref target="header.if-match" format="counter"/>, <xref target="header.if-none-match" format="counter"/> and <xref target="header.vary" format="counter"/>. The entity tag
    7269    &MAY; be used for comparison with other entities from the same resource
    7270    (see <xref target="weak.and.strong.validators"/>).
    7271 </t>
    7272 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
    7273     ETag = "ETag" ":" entity-tag
    7274 </artwork></figure>
    7275 <figure><preamble>
    7276    Examples:
    7277 </preamble>
    7278 <artwork type="example">
    7279    ETag: "xyzzy"
    7280    ETag: W/"xyzzy"
    7281    ETag: ""
    7282 </artwork></figure>
    7283 </section>
    7285 <section title="Expect" anchor="header.expect">
    7286   <iref primary="true" item="Expect header" x:for-anchor=""/>
    7287   <iref primary="true" item="Headers" subitem="Expect" x:for-anchor=""/>
    7288 <t>
    7289    The Expect request-header field is used to indicate that particular
    7290    server behaviors are required by the client.
    7291 </t>
    7292 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expect"/><iref primary="true" item="Grammar" subitem="expectation"/><iref primary="true" item="Grammar" subitem="expectation-extension"/><iref primary="true" item="Grammar" subitem="expect-params"/>
    7293    Expect       =  "Expect" ":" 1#expectation
    7295    expectation  =  "100-continue" | expectation-extension
    7296    expectation-extension =  token [ "=" ( token | quoted-string )
    7297                             *expect-params ]
    7298    expect-params =  ";" token [ "=" ( token | quoted-string ) ]
    7299 </artwork></figure>
    7300 <t>
    7301    A server that does not understand or is unable to comply with any of
    7302    the expectation values in the Expect field of a request &MUST; respond
    7303    with appropriate error status. The server &MUST; respond with a 417
    7304    (Expectation Failed) status if any of the expectations cannot be met
    7305    or, if there are other problems with the request, some other 4xx
    7306    status.
    7307 </t>
    7308 <t>
    7309    This header field is defined with extensible syntax to allow for
    7310    future extensions. If a server receives a request containing an
    7311    Expect field that includes an expectation-extension that it does not
    7312    support, it &MUST; respond with a 417 (Expectation Failed) status.
    7313 </t>
    7314 <t>
    7315    Comparison of expectation values is case-insensitive for unquoted
    7316    tokens (including the 100-continue token), and is case-sensitive for
    7317    quoted-string expectation-extensions.
    7318 </t>
    7319 <t>
    7320    The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy &MUST;
    7321    return a 417 (Expectation Failed) status if it receives a request
    7322    with an expectation that it cannot meet. However, the Expect
    7323    request-header itself is end-to-end; it &MUST; be forwarded if the
    7324    request is forwarded.
    7325 </t>
    7326 <t>
    7327    Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
    7328    Expect header.
    7329 </t>
    7330 <t>
    7331    See <xref target="use.of.the.100.status"/> for the use of the 100 (continue) status.
    7332 </t>
    7333 </section>
    7335 <section title="Expires" anchor="header.expires">
    7336   <iref primary="true" item="Expires header" x:for-anchor=""/>
    7337   <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
    7338 <t>
    7339    The Expires entity-header field gives the date/time after which the
    7340    response is considered stale. A stale cache entry may not normally be
    7341    returned by a cache (either a proxy cache or a user agent cache)
    7342    unless it is first validated with the origin server (or with an
    7343    intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
    7344    for further discussion of the expiration model.
    7345 </t>
    7346 <t>
    7347    The presence of an Expires field does not imply that the original
    7348    resource will change or cease to exist at, before, or after that
    7349    time.
    7350 </t>
    7351 <t>
    7352    The format is an absolute date and time as defined by HTTP-date in
    7353    <xref target=""/>; it &MUST; be in RFC 1123 date format:
    7354 </t>
    7355 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
    7356    Expires = "Expires" ":" HTTP-date
    7357 </artwork></figure>
    7358 <t>
    7359    An example of its use is
    7360 </t>
    7361 <figure><artwork type="example">
    7362    Expires: Thu, 01 Dec 1994 16:00:00 GMT
    7363 </artwork></figure>
    7364 <t>
    7365   <list><t>
    7366       <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
    7367       directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
    7368       Expires field.
    7369   </t></list>
    7370 </t>
    7371 <t>
    7372    HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
    7373    especially including the value "0", as in the past (i.e., "already
    7374    expired").
    7375 </t>
    7376 <t>
    7377    To mark a response as "already expired," an origin server sends an
    7378    Expires date that is equal to the Date header value. (See the rules
    7379    for expiration calculations in <xref target="expiration.calculations"/>.)
    7380 </t>
    7381 <t>
    7382    To mark a response as "never expires," an origin server sends an
    7383    Expires date approximately one year from the time the response is
    7384    sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
    7385    year in the future.
    7386 </t>
    7387 <t>
    7388    The presence of an Expires header field with a date value of some
    7389    time in the future on a response that otherwise would by default be
    7390    non-cacheable indicates that the response is cacheable, unless
    7391    indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
    7392 </t>
    7393 </section>
    7395 <section title="From" anchor="header.from">
    7396   <iref primary="true" item="From header" x:for-anchor=""/>
    7397   <iref primary="true" item="Headers" subitem="From" x:for-anchor=""/>
    7398 <t>
    7399    The From request-header field, if given, &SHOULD; contain an Internet
    7400    e-mail address for the human user who controls the requesting user
    7401    agent. The address &SHOULD; be machine-usable, as defined by "mailbox"
    7402    in RFC 822 <xref target="RFC822"/> as updated by RFC 1123 <xref target="RFC1123"/>:
    7403 </t>
    7404 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="From"/>
    7405     From   = "From" ":" mailbox
    7406 </artwork></figure>
    7407 <t>
    7408    An example is:
    7409 </t>
    7410 <figure><artwork type="example">
    7411     From:
    7412 </artwork></figure>
    7413 <t>
    7414    This header field &MAY; be used for logging purposes and as a means for
    7415    identifying the source of invalid or unwanted requests. It &SHOULD-NOT;
    7416    be used as an insecure form of access protection. The interpretation
    7417    of this field is that the request is being performed on behalf of the
    7418    person given, who accepts responsibility for the method performed. In
    7419    particular, robot agents &SHOULD; include this header so that the
    7420    person responsible for running the robot can be contacted if problems
    7421    occur on the receiving end.
    7422 </t>
    7423 <t>
    7424    The Internet e-mail address in this field &MAY; be separate from the
    7425    Internet host which issued the request. For example, when a request
    7426    is passed through a proxy the original issuer's address &SHOULD; be
    7427    used.
    7428 </t>
    7429 <t>
    7430    The client &SHOULD-NOT;  send the From header field without the user's
    7431    approval, as it might conflict with the user's privacy interests or
    7432    their site's security policy. It is strongly recommended that the
    7433    user be able to disable, enable, and modify the value of this field
    7434    at any time prior to a request.
    7435 </t>
    74772349   and <xref target="" format="counter"/>
    74782350   for other requirements relating to Host.
    7479 </t>
    7480 </section>
    7482 <section title="If-Match" anchor="header.if-match">
    7483   <iref primary="true" item="If-Match header" x:for-anchor=""/>
    7484   <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
    7485 <t>
    7486    The If-Match request-header field is used with a method to make it
    7487    conditional. A client that has one or more entities previously
    7488    obtained from the resource can verify that one of those entities is
    7489    current by including a list of their associated entity tags in the
    7490    If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
    7491    purpose of this feature is to allow efficient updates of cached
    7492    information with a minimum amount of transaction overhead. It is also
    7493    used, on updating requests, to prevent inadvertent modification of
    7494    the wrong version of a resource. As a special case, the value "*"
    7495    matches any current entity of the resource.
    7496 </t>
    7497 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
    7498     If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
    7499 </artwork></figure>
    7500 <t>
    7501    If any of the entity tags match the entity tag of the entity that
    7502    would have been returned in the response to a similar GET request
    7503    (without the If-Match header) on that resource, or if "*" is given
    7504    and any current entity exists for that resource, then the server &MAY;
    7505    perform the requested method as if the If-Match header field did not
    7506    exist.
    7507 </t>
    7508 <t>
    7509    A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
    7510    to compare the entity tags in If-Match.
    7511 </t>
    7512 <t>
    7513    If none of the entity tags match, or if "*" is given and no current
    7514    entity exists, the server &MUST-NOT; perform the requested method, and
    7515    &MUST; return a 412 (Precondition Failed) response. This behavior is
    7516    most useful when the client wants to prevent an updating method, such
    7517    as PUT, from modifying a resource that has changed since the client
    7518    last retrieved it.
    7519 </t>
    7520 <t>
    7521    If the request would, without the If-Match header field, result in
    7522    anything other than a 2xx or 412 status, then the If-Match header
    7523    &MUST; be ignored.
    7524 </t>
    7525 <t>
    7526    The meaning of "If-Match: *" is that the method &SHOULD; be performed
    7527    if the representation selected by the origin server (or by a cache,
    7528    possibly using the Vary mechanism, see <xref target="header.vary"/>) exists, and
    7529    &MUST-NOT; be performed if the representation does not exist.
    7530 </t>
    7531 <t>
    7532    A request intended to update a resource (e.g., a PUT) &MAY; include an
    7533    If-Match header field to signal that the request method &MUST-NOT; be
    7534    applied if the entity corresponding to the If-Match value (a single
    7535    entity tag) is no longer a representation of that resource. This
    7536    allows the user to indicate that they do not wish the request to be
    7537    successful if the resource has been changed without their knowledge.
    7538    Examples:
    7539 </t>
    7540 <figure><artwork type="example">
    7541     If-Match: "xyzzy"
    7542     If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
    7543     If-Match: *
    7544 </artwork></figure>
    7545 <t>
    7546    The result of a request having both an If-Match header field and
    7547    either an If-None-Match or an If-Modified-Since header fields is
    7548    undefined by this specification.
    7549 </t>
    7550 </section>
    7552 <section title="If-Modified-Since" anchor="header.if-modified-since">
    7553   <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
    7554   <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
    7555 <t>
    7556    The If-Modified-Since request-header field is used with a method to
    7557    make it conditional: if the requested variant has not been modified
    7558    since the time specified in this field, an entity will not be
    7559    returned from the server; instead, a 304 (not modified) response will
    7560    be returned without any message-body.
    7561 </t>
    7562 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
    7563     If-Modified-Since = "If-Modified-Since" ":" HTTP-date
    7564 </artwork></figure>
    7565 <t>
    7566    An example of the field is:
    7567 </t>
    7568 <figure><artwork type="example">
    7569     If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
    7570 </artwork></figure>
    7571 <t>
    7572    A GET method with an If-Modified-Since header and no Range header
    7573    requests that the identified entity be transferred only if it has
    7574    been modified since the date given by the If-Modified-Since header.
    7575    The algorithm for determining this includes the following cases:
    7576   <list style="numbers">
    7577       <t>If the request would normally result in anything other than a
    7578          200 (OK) status, or if the passed If-Modified-Since date is
    7579          invalid, the response is exactly the same as for a normal GET.
    7580          A date which is later than the server's current time is
    7581          invalid.</t>
    7583       <t>If the variant has been modified since the If-Modified-Since
    7584          date, the response is exactly the same as for a normal GET.</t>
    7586       <t>If the variant has not been modified since a valid If-Modified-Since
    7587          date, the server &SHOULD; return a 304 (Not
    7588          Modified) response.</t>
    7589   </list>
    7590 </t>
    7591 <t>
    7592    The purpose of this feature is to allow efficient updates of cached
    7593    information with a minimum amount of transaction overhead.
    7594   <list><t>
    7595       <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
    7596       see <xref target="header.range"/> for full details.
    7597     </t><t>
    7598       <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
    7599       clock might not be synchronized with the client.
    7600     </t><t>
    7601       <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
    7602       servers will use an exact date comparison function, rather than a
    7603       less-than function, for deciding whether to send a 304 (Not
    7604       Modified) response. To get best results when sending an If-Modified-Since
    7605       header field for cache validation, clients are
    7606       advised to use the exact date string received in a previous Last-Modified
    7607       header field whenever possible.
    7608     </t><t>
    7609       <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
    7610       header instead of a date taken from the Last-Modified header for
    7611       the same request, the client should be aware of the fact that this
    7612       date is interpreted in the server's understanding of time. The
    7613       client should consider unsynchronized clocks and rounding problems
    7614       due to the different encodings of time between the client and
    7615       server. This includes the possibility of race conditions if the
    7616       document has changed between the time it was first requested and
    7617       the If-Modified-Since date of a subsequent request, and the
    7618       possibility of clock-skew-related problems if the If-Modified-Since
    7619       date is derived from the client's clock without correction
    7620       to the server's clock. Corrections for different time bases
    7621       between client and server are at best approximate due to network
    7622       latency.
    7623     </t>
    7624   </list>
    7625 </t>
    7626 <t>
    7627    The result of a request having both an If-Modified-Since header field
    7628    and either an If-Match or an If-Unmodified-Since header fields is
    7629    undefined by this specification.
    7630 </t>
    7631 </section>
    7633 <section title="If-None-Match" anchor="header.if-none-match">
    7634   <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
    7635   <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
    7636 <t>
    7637    The If-None-Match request-header field is used with a method to make
    7638    it conditional. A client that has one or more entities previously
    7639    obtained from the resource can verify that none of those entities is
    7640    current by including a list of their associated entity tags in the
    7641    If-None-Match header field. The purpose of this feature is to allow
    7642    efficient updates of cached information with a minimum amount of
    7643    transaction overhead. It is also used to prevent a method (e.g. PUT)
    7644    from inadvertently modifying an existing resource when the client
    7645    believes that the resource does not exist.
    7646 </t>
    7647 <t>
    7648    As a special case, the value "*" matches any current entity of the
    7649    resource.
    7650 </t>
    7651 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
    7652     If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
    7653 </artwork></figure>
    7654 <t>
    7655    If any of the entity tags match the entity tag of the entity that
    7656    would have been returned in the response to a similar GET request
    7657    (without the If-None-Match header) on that resource, or if "*" is
    7658    given and any current entity exists for that resource, then the
    7659    server &MUST-NOT; perform the requested method, unless required to do
    7660    so because the resource's modification date fails to match that
    7661    supplied in an If-Modified-Since header field in the request.
    7662    Instead, if the request method was GET or HEAD, the server &SHOULD;
    7663    respond with a 304 (Not Modified) response, including the cache-related
    7664    header fields (particularly ETag) of one of the entities that
    7665    matched. For all other request methods, the server &MUST; respond with
    7666    a status of 412 (Precondition Failed).
    7667 </t>
    7668 <t>
    7669    See <xref target="weak.and.strong.validators"/> for rules on how to determine if two entities tags
    7670    match. The weak comparison function can only be used with GET or HEAD
    7671    requests.
    7672 </t>
    7673 <t>
    7674    If none of the entity tags match, then the server &MAY; perform the
    7675    requested method as if the If-None-Match header field did not exist,
    7676    but &MUST; also ignore any If-Modified-Since header field(s) in the
    7677    request. That is, if no entity tags match, then the server &MUST-NOT;
    7678    return a 304 (Not Modified) response.
    7679 </t>
    7680 <t>
    7681    If the request would, without the If-None-Match header field, result
    7682    in anything other than a 2xx or 304 status, then the If-None-Match
    7683    header &MUST; be ignored. (See <xref target=""/> for a discussion of
    7684    server behavior when both If-Modified-Since and If-None-Match appear
    7685    in the same request.)
    7686 </t>
    7687 <t>
    7688    The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
    7689    performed if the representation selected by the origin server (or by
    7690    a cache, possibly using the Vary mechanism, see <xref target="header.vary"/>)
    7691    exists, and &SHOULD; be performed if the representation does not exist.
    7692    This feature is intended to be useful in preventing races between PUT
    7693    operations.
    7694 </t>
    7695 <t>
    7696    Examples:
    7697 </t>
    7698 <figure><artwork type="example">
    7699     If-None-Match: "xyzzy"
    7700     If-None-Match: W/"xyzzy"
    7701     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
    7702     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
    7703     If-None-Match: *
    7704 </artwork></figure>
    7705 <t>
    7706    The result of a request having both an If-None-Match header field and
    7707    either an If-Match or an If-Unmodified-Since header fields is
    7708    undefined by this specification.
    7709 </t>
    7710 </section>
    7712 <section title="If-Range" anchor="header.if-range">
    7713   <iref primary="true" item="If-Range header" x:for-anchor=""/>
    7714   <iref primary="true" item="Headers" subitem="If-Range" x:for-anchor=""/>
    7715 <t>
    7716    If a client has a partial copy of an entity in its cache, and wishes
    7717    to have an up-to-date copy of the entire entity in its cache, it
    7718    could use the Range request-header with a conditional GET (using
    7719    either or both of If-Unmodified-Since and If-Match.) However, if the
    7720    condition fails because the entity has been modified, the client
    7721    would then have to make a second request to obtain the entire current
    7722    entity-body.
    7723 </t>
    7724 <t>
    7725    The If-Range header allows a client to "short-circuit" the second
    7726    request. Informally, its meaning is `if the entity is unchanged, send
    7727    me the part(s) that I am missing; otherwise, send me the entire new
    7728    entity'.
    7729 </t>
    7730 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Range"/>
    7731      If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
    7732 </artwork></figure>
    7733 <t>
    7734    If the client has no entity tag for an entity, but does have a Last-Modified
    7735    date, it &MAY; use that date in an If-Range header. (The
    7736    server can distinguish between a valid HTTP-date and any form of
    7737    entity-tag by examining no more than two characters.) The If-Range
    7738    header &SHOULD; only be used together with a Range header, and &MUST; be
    7739    ignored if the request does not include a Range header, or if the
    7740    server does not support the sub-range operation.
    7741 </t>
    7742 <t>
    7743    If the entity tag given in the If-Range header matches the current
    7744    entity tag for the entity, then the server &SHOULD; provide the
    7745    specified sub-range of the entity using a 206 (Partial content)
    7746    response. If the entity tag does not match, then the server &SHOULD;
    7747    return the entire entity using a 200 (OK) response.
    7748 </t>
    7749 </section>
    7751 <section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
    7752   <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
    7753   <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
    7754 <t>
    7755    The If-Unmodified-Since request-header field is used with a method to
    7756    make it conditional. If the requested resource has not been modified
    7757    since the time specified in this field, the server &SHOULD; perform the
    7758    requested operation as if the If-Unmodified-Since header were not
    7759    present.
    7760 </t>
    7761 <t>
    7762    If the requested variant has been modified since the specified time,
    7763    the server &MUST-NOT; perform the requested operation, and &MUST; return
    7764    a 412 (Precondition Failed).
    7765 </t>
    7766 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
    7767    If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
    7768 </artwork></figure>
    7769 <t>
    7770    An example of the field is:
    7771 </t>
    7772 <figure><artwork type="example">
    7773     If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
    7774 </artwork></figure>
    7775 <t>
    7776    If the request normally (i.e., without the If-Unmodified-Since
    7777    header) would result in anything other than a 2xx or 412 status, the
    7778    If-Unmodified-Since header &SHOULD; be ignored.
    7779 </t>
    7780 <t>
    7781    If the specified date is invalid, the header is ignored.
    7782 </t>
    7783 <t>
    7784    The result of a request having both an If-Unmodified-Since header
    7785    field and either an If-None-Match or an If-Modified-Since header
    7786    fields is undefined by this specification.
    7787 </t>
    7788 </section>
    7790 <section title="Last-Modified" anchor="header.last-modified">
    7791   <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
    7792   <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
    7793 <t>
    7794    The Last-Modified entity-header field indicates the date and time at
    7795    which the origin server believes the variant was last modified.
    7796 </t>
    7797 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
    7798     Last-Modified  = "Last-Modified" ":" HTTP-date
    7799 </artwork></figure>
    7800 <t>
    7801    An example of its use is
    7802 </t>
    7803 <figure><artwork type="example">
    7804     Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
    7805 </artwork></figure>
    7806 <t>
    7807    The exact meaning of this header field depends on the implementation
    7808    of the origin server and the nature of the original resource. For
    7809    files, it may be just the file system last-modified time. For
    7810    entities with dynamically included parts, it may be the most recent
    7811    of the set of last-modify times for its component parts. For database
    7812    gateways, it may be the last-update time stamp of the record. For
    7813    virtual objects, it may be the last time the internal state changed.
    7814 </t>
    7815 <t>
    7816    An origin server &MUST-NOT; send a Last-Modified date which is later
    7817    than the server's time of message origination. In such cases, where
    7818    the resource's last modification would indicate some time in the
    7819    future, the server &MUST; replace that date with the message
    7820    origination date.
    7821 </t>
    7822 <t>
    7823    An origin server &SHOULD; obtain the Last-Modified value of the entity
    7824    as close as possible to the time that it generates the Date value of
    7825    its response. This allows a recipient to make an accurate assessment
    7826    of the entity's modification time, especially if the entity changes
    7827    near the time that the response is generated.
    7828 </t>
    7829 <t>
    7830    HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
    7831 </t>
    7832 </section>