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.

1 edited


  • draft-ietf-httpbis/latest/p2-semantics.xml

    r19 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='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
    1111  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
    1212  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
     13  <!ENTITY ID-VERSION "latest">
     14  <!ENTITY messaging                  "[Part 1]">
     15  <!ENTITY payload                    "[Part 3]">
     16  <!ENTITY conditional                "[Part 4]">
     17  <!ENTITY range                      "[Part 5]">
     18  <!ENTITY caching                    "[Part 6]">
     19  <!ENTITY auth                       "[Part 7]">
     20  <!ENTITY content-negotiation        "[Part 3]">
     21  <!ENTITY diff2045entity             "[Part 3]">
     22  <!ENTITY uri                        "[Part 1]">
     23  <!ENTITY http-url                   "[Part 1]">
     24  <!ENTITY http-version               "[Part 1]">
     25  <!ENTITY use100                     "[Part 1]">
     26  <!ENTITY qvalue                     "[Part 3]">
     27  <!ENTITY header-accept              "[Part 3]">
     28  <!ENTITY header-accept-charset      "[Part 3]">
     29  <!ENTITY header-accept-encoding     "[Part 3]">
     30  <!ENTITY header-accept-language     "[Part 3]">
     31  <!ENTITY header-accept-ranges       "[Part 3]">
     32  <!ENTITY header-age                 "[Part 6]">
     33  <!ENTITY header-authorization       "[Part 7]">
     34  <!ENTITY header-cache-control       "[Part 6]">
     35  <!ENTITY header-content-location    "[Part 3]">
     36  <!ENTITY header-content-range       "[Part 5]">
     37  <!ENTITY header-etag                "[Part 4]">
     38  <!ENTITY header-expires             "[Part 6]">
     39  <!ENTITY header-host                "[Part 1]">
     40  <!ENTITY header-if-match            "[Part 4]">
     41  <!ENTITY header-if-modified-since   "[Part 4]">
     42  <!ENTITY header-if-none-match       "[Part 4]">
     43  <!ENTITY header-if-range            "[Part 5]">
     44  <!ENTITY header-if-unmodified-since "[Part 4]">
     45  <!ENTITY header-pragma              "[Part 6]">
     46  <!ENTITY header-proxy-authenticate  "[Part 7]">
     47  <!ENTITY header-proxy-authorization "[Part 7]">
     48  <!ENTITY header-range               "[Part 5]">
     49  <!ENTITY header-upgrade             "[Part 1]">
     50  <!ENTITY header-te                  "[Part 1]">
     51  <!ENTITY header-vary                "[Part 6]">
     52  <!ENTITY header-via                 "[Part 1]">
     53  <!ENTITY header-warning             "[Part 6]">
     54  <!ENTITY header-www-authenticate    "[Part 7]">
     55  <!ENTITY message-body               "[Part 1]">
    1457<?rfc toc="yes" ?>
    15 <?rfc symrefs="no" ?>
     58<?rfc symrefs="yes" ?>
     59<?rfc sortrefs="yes" ?>
    1660<?rfc compact="yes"?>
    1761<?rfc subcompact="no" ?>
    2064<?rfc-ext allow-markup-in-artwork="yes" ?>
    2165<?rfc-ext include-references-in-index="yes" ?>
    22 <rfc number="2616" category="std" obsoletes="2068" xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:ed="http://greenbytes.de/2002/rfcedit" >
    23 <x:assign-section-number number="18" builtin-target="authors"/>
     66<rfc obsoletes="2068, 2616, 2617" category="std"
     67     ipr="full3978" docName="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"
     68     xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:ed="http://greenbytes.de/2002/rfcedit">
    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>
     71  <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
     73  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
     74    <organization abbrev="Day Software">Day Software</organization>
    3075    <address>
    3176      <postal>
    32         <street>University of California, Irvine</street>
    33         <city>Irvine</city>
     77        <street>23 Corporate Plaza DR, Suite 280</street>
     78        <city>Newport Beach</city>
    3479        <region>CA</region>
    35         <code>92697-3425</code>
     80        <code>92660</code>
     81        <country>USA</country>
    3682      </postal>
    37       <facsimile>+1(949)824-1715</facsimile>
    38       <email>fielding@ics.uci.edu</email>
     83      <phone>+1-949-706-5300</phone>
     84      <facsimile>+1-949-706-5305</facsimile>
     85      <email>fielding@gbiv.com</email>
     86      <uri>http://roy.gbiv.com/</uri>
    3987    </address>
    4088  </author>
    42   <author initials="J." surname="Gettys" fullname="James Gettys">
    43     <organization abbrev="Compaq/W3C">World Wide Web Consortium</organization>
     90  <author initials="J." surname="Gettys" fullname="Jim Gettys">
     91    <organization>One Laptop per Child</organization>
    4492    <address>
    4593      <postal>
    46         <street>MIT Laboratory for Computer Science, NE43-356</street>
    47         <street>545 Technology Square</street>
    48         <city>Cambridge</city>
     94        <street>21 Oak Knoll Road</street>
     95        <city>Carlisle</city>
    4996        <region>MA</region>
    50         <code>02139</code>
     97        <code>01741</code>
     98        <country>USA</country>
    5199      </postal>
    52       <facsimile>+1(617)258-8682</facsimile>
    53       <email>jg@w3.org</email>
     100      <email>jg@laptop.org</email>
     101      <uri>http://www.laptop.org/</uri>
    54102    </address>
    55103  </author>
    57105  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    58     <organization abbrev="Compaq">Compaq Computer Corporation</organization>
     106    <organization abbrev="HP">Hewlett-Packard Company</organization>
    59107    <address>
    60108      <postal>
    61         <street>Western Research Laboratory</street>
    62         <street>250 University Avenue</street>
     109        <street>HP Labs, Large Scale Systems Group</street>
     110        <street>1501 Page Mill Road, MS 1177</street>
    63111        <city>Palo Alto</city>
    64112        <region>CA</region>
    65         <code>94305</code>
     113        <code>94304</code>
     114        <country>USA</country>
    66115      </postal>
    67       <email>mogul@wrl.dec.com</email>
     116      <email>JeffMogul@acm.org</email>
    68117    </address>
    69118  </author>
    71120  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    72     <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
     121    <organization abbrev="Microsoft">Microsoft Corporation</organization>
    73122    <address>
    74123      <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>
     124        <street>1 Microsoft Way</street>
     125        <city>Redmond</city>
     126        <region>WA</region>
     127        <code>98052</code>
     128        <country>USA</country>
    80129      </postal>
    81       <facsimile>+1(617)258-8682</facsimile>
    82       <email>frystyk@w3.org</email>
     130      <email>henrikn@microsoft.com</email>
    83131    </address>
    84132  </author>
    86134  <author initials="L." surname="Masinter" fullname="Larry Masinter">
    87     <organization abbrev="Xerox">Xerox Corporation</organization>
     135    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    88136    <address>
    89137      <postal>
    90         <street>MIT Laboratory for Computer Science, NE43-356</street>
    91         <street>3333 Coyote Hill Road</street>
    92         <city>Palo Alto</city>
     138        <street>345 Park Ave</street>
     139        <city>San Jose</city>
    93140        <region>CA</region>
    94         <code>94034</code>
     141        <code>95110</code>
     142        <country>USA</country>
    95143      </postal>
    96       <email>masinter@parc.xerox.com</email>
     144      <email>LMM@acm.org</email>
     145      <uri>http://larry.masinter.net/</uri>
    97146    </address>
    98147  </author>
    115164    <address>
    116165      <postal>
    117         <street>MIT Laboratory for Computer Science, NE43-356</street>
     166        <street>MIT Laboratory for Computer Science</street>
    118167        <street>545 Technology Square</street>
    119168        <city>Cambridge</city>
    120169        <region>MA</region>
    121170        <code>02139</code>
     171        <country>USA</country>
    122172      </postal>
    123       <facsimile>+1(617)258-8682</facsimile>
     173      <facsimile>+1 (617) 258 8682</facsimile>
    124174      <email>timbl@w3.org</email>
    125175    </address>
    126176  </author>
    128   <date month="June" year="1999"/>
     178  <date month="December" year="2007"/>
    132182   The Hypertext Transfer Protocol (HTTP) is an application-level
    133183   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"/>.
     184   systems. HTTP has been in use by the World Wide Web global information
     185   initiative since 1990. This document is Part 2 of the eight-part specification
     186   that defines the protocol referred to as "HTTP/1.1" and, taken together,
     187   updates RFC 2616 and RFC 2617.  Part 2 defines the semantics of HTTP messages
     188   as expressed by request methods, request-header fields, response status codes,
     189   and response-header fields.
    149194<section title="Introduction" anchor="introduction">
    151 <section title="Purpose" anchor="intro.purpose">
    152 <t>
    153    The Hypertext Transfer Protocol (HTTP) is an application-level
    154    protocol for distributed, collaborative, hypermedia information
    155    systems. HTTP has been in use by the World-Wide Web global
    156    information initiative since 1990. The first version of HTTP,
    157    referred to as HTTP/0.9, was a simple protocol for raw data transfer
    158    across the Internet. HTTP/1.0, as defined by RFC 1945 <xref target="RFC1945"/>, improved
    159    the protocol by allowing messages to be in the format of MIME-like
    160    messages, containing metainformation about the data transferred and
    161    modifiers on the request/response semantics. However, HTTP/1.0 does
    162    not sufficiently take into consideration the effects of hierarchical
    163    proxies, caching, the need for persistent connections, or virtual
    164    hosts. In addition, the proliferation of incompletely-implemented
    165    applications calling themselves "HTTP/1.0" has necessitated a
    166    protocol version change in order for two communicating applications
    167    to determine each other's true capabilities.
    168 </t>
    169 <t>
    170    This specification defines the protocol referred to as "HTTP/1.1".
    171    This protocol includes more stringent requirements than HTTP/1.0 in
    172    order to ensure reliable implementation of its features.
    173 </t>
    174 <t>
    175    Practical information systems require more functionality than simple
    176    retrieval, including search, front-end update, and annotation. HTTP
    177    allows an open-ended set of methods and headers that indicate the
    178    purpose of a request <xref target="RFC2324"/>. It builds on the discipline of reference
    179    provided by the Uniform Resource Identifier (URI) <xref target="RFC1630"/>, as a location
    180    (URL) <xref target="RFC1738"/> or name (URN) <xref target="RFC1737"/>, for indicating the resource to which a
    181    method is to be applied. Messages are passed in a format similar to
    182    that used by Internet mail <xref target="RFC822"/> as defined by the Multipurpose
    183    Internet Mail Extensions (MIME) <xref target="RFC2045"/>.
    184 </t>
    185 <t>
    186    HTTP is also used as a generic protocol for communication between
    187    user agents and proxies/gateways to other Internet systems, including
    188    those supported by the SMTP <xref target="RFC821"/>, NNTP <xref target="RFC977"/>, FTP <xref target="RFC959"/>, Gopher <xref target="RFC1436"/>,
    189    and WAIS <xref target="WAIS"/> protocols. In this way, HTTP allows basic hypermedia
    190    access to resources available from diverse applications.
    191 </t>
    192 </section>
    194 <section title="Requirements" anchor="intro.requirements">
    195 <t>
    196    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    197    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    198    document are to be interpreted as described in RFC 2119 <xref target="RFC2119"/>.
    199 </t>
    200 <t>
    201    An implementation is not compliant if it fails to satisfy one or more
    202    of the &MUST; or &REQUIRED; level requirements for the protocols it
    203    implements. An implementation that satisfies all the &MUST; or &REQUIRED;
    204    level and all the &SHOULD; level requirements for its protocols is said
    205    to be "unconditionally compliant"; one that satisfies all the &MUST;
    206    level requirements but not all the &SHOULD; level requirements for its
    207    protocols is said to be "conditionally compliant."
    208 </t>
    209 </section>
    211 <section title="Terminology" anchor="intro.terminology">
    212 <t>
    213    This specification uses a number of terms to refer to the roles
    214    played by participants in, and objects of, the HTTP communication.
    215 </t>
    216 <t>
    217   <iref item="connection"/>
    218   <x:dfn>connection</x:dfn>
    219   <list>
    220     <t>
    221       A transport layer virtual circuit established between two programs
    222       for the purpose of communication.
    223     </t>
    224   </list>
    225 </t>
    226 <t>
    227   <iref item="message"/>
    228   <x:dfn>message</x:dfn>
    229   <list>
    230     <t>
    231       The basic unit of HTTP communication, consisting of a structured
    232       sequence of octets matching the syntax defined in <xref target="http.message"/> and
    233       transmitted via the connection.
    234     </t>
    235   </list>
    236 </t>
    237 <t>
    238   <iref item="request"/>
    239   <x:dfn>request</x:dfn>
    240   <list>
    241     <t>
    242       An HTTP request message, as defined in <xref target="request"/>.
    243     </t>
    244   </list>
    245 </t>
    246 <t>
    247   <iref item="response"/>
    248   <x:dfn>response</x:dfn>
    249   <list>
    250     <t>
    251       An HTTP response message, as defined in <xref target="response"/>.
    252     </t>
    253   </list>
    254 </t>
    255 <t>
    256   <iref item="resource"/>
    257   <x:dfn>resource</x:dfn>
    258   <list>
    259     <t>
    260       A network data object or service that can be identified by a URI,
    261       as defined in <xref target="uri"/>. Resources may be available in multiple
    262       representations (e.g. multiple languages, data formats, size, and
    263       resolutions) or vary in other ways.
    264     </t>
    265   </list>
    266 </t>
    267 <t>
    268   <iref item="entity"/>
    269   <x:dfn>entity</x:dfn>
    270   <list>
    271     <t>
    272       The information transferred as the payload of a request or
    273       response. An entity consists of metainformation in the form of
    274       entity-header fields and content in the form of an entity-body, as
    275       described in <xref target="entity"/>.
    276     </t>
    277   </list>
    278 </t>
    279 <t>
    280   <iref item="representation"/>
    281   <x:dfn>representation</x:dfn>
    282   <list>
    283     <t>
    284       An entity included with a response that is subject to content
    285       negotiation, as described in <xref target="content.negotiation"/>. There may exist multiple
    286       representations associated with a particular response status.
    287     </t>
    288   </list>
    289 </t>
    290 <t>
    291   <iref item="content negotiation"/>
    292   <x:dfn>content negotiation</x:dfn>
    293   <list>
    294     <t>
    295       The mechanism for selecting the appropriate representation when
    296       servicing a request, as described in <xref target="content.negotiation"/>. The
    297       representation of entities in any response can be negotiated
    298       (including error responses).
    299     </t>
    300   </list>
    301 </t>
    302 <t>
    303   <iref item="variant"/>
    304   <x:dfn>variant</x:dfn>
    305   <list>
    306     <t>
    307       A resource may have one, or more than one, representation(s)
    308       associated with it at any given instant. Each of these
    309       representations is termed a `varriant'.  Use of the term `variant'
    310       does not necessarily imply that the resource is subject to content
    311       negotiation.
    312     </t>
    313   </list>
    314 </t>
    315 <t>
    316   <iref item="client"/>
    317   <x:dfn>client</x:dfn>
    318   <list>
    319     <t>
    320       A program that establishes connections for the purpose of sending
    321       requests.
    322     </t>
    323   </list>
    324 </t>
    325 <t>
    326   <iref item="user agent"/>
    327   <x:dfn>user agent</x:dfn>
    328   <list>
    329     <t>
    330       The client which initiates a request. These are often browsers,
    331       editors, spiders (web-traversing robots), or other end user tools.
    332     </t>
    333   </list>
    334 </t>
    335 <t>
    336   <iref item="server"/>
    337   <x:dfn>server</x:dfn>
    338   <list>
    339     <t>
    340       An application program that accepts connections in order to
    341       service requests by sending back responses. Any given program may
    342       be capable of being both a client and a server; our use of these
    343       terms refers only to the role being performed by the program for a
    344       particular connection, rather than to the program's capabilities
    345       in general. Likewise, any server may act as an origin server,
    346       proxy, gateway, or tunnel, switching behavior based on the nature
    347       of each request.
    348     </t>
    349   </list>
    350 </t>
    351 <t>
    352   <iref item="origin server"/>
    353   <x:dfn>origin server</x:dfn>
    354   <list>
    355     <t>
    356       The server on which a given resource resides or is to be created.
    357     </t>
    358   </list>
    359 </t>
    360 <t>
    361   <iref item="proxy"/>
    362   <x:dfn>proxy</x:dfn>
    363   <list>
    364     <t>
    365       An intermediary program which acts as both a server and a client
    366       for the purpose of making requests on behalf of other clients.
    367       Requests are serviced internally or by passing them on, with
    368       possible translation, to other servers. A proxy &MUST; implement
    369       both the client and server requirements of this specification. A
    370       "transparent proxy" is a proxy that does not modify the request or
    371       response beyond what is required for proxy authentication and
    372       identification. A "non-transparent proxy" is a proxy that modifies
    373       the request or response in order to provide some added service to
    374       the user agent, such as group annotation services, media type
    375       transformation, protocol reduction, or anonymity filtering. Except
    376       where either transparent or non-transparent behavior is explicitly
    377       stated, the HTTP proxy requirements apply to both types of
    378       proxies.
    379     </t>
    380   </list>
    381 </t>
    382 <t>
    383   <iref item="gateway"/>
    384   <x:dfn>gateway</x:dfn>
    385   <list>
    386     <t>
    387       A server which acts as an intermediary for some other server.
    388       Unlike a proxy, a gateway receives requests as if it were the
    389       origin server for the requested resource; the requesting client
    390       may not be aware that it is communicating with a gateway.
    391     </t>
    392   </list>
    393 </t>
    394 <t>
    395   <iref item="tunnel"/>
    396   <x:dfn>tunnel</x:dfn>
    397   <list>
    398     <t>
    399       An intermediary program which is acting as a blind relay between
    400       two connections. Once active, a tunnel is not considered a party
    401       to the HTTP communication, though the tunnel may have been
    402       initiated by an HTTP request. The tunnel ceases to exist when both
    403       ends of the relayed connections are closed.
    404     </t>
    405   </list>
    406 </t>
    407 <t>
    408   <iref item="cache"/>
    409   <x:dfn>cache</x:dfn>
    410   <list>
    411     <t>
    412       A program's local store of response messages and the subsystem
    413       that controls its message storage, retrieval, and deletion. A
    414       cache stores cacheable responses in order to reduce the response
    415       time and network bandwidth consumption on future, equivalent
    416       requests. Any client or server may include a cache, though a cache
    417       cannot be used by a server that is acting as a tunnel.
    418     </t>
    419   </list>
    420 </t>
    421 <t>
    422   <iref item="cacheable"/>
    423   <x:dfn>cacheable</x:dfn>
    424   <list>
    425     <t>
    426       A response is cacheable if a cache is allowed to store a copy of
    427       the response message for use in answering subsequent requests. The
    428       rules for determining the cacheability of HTTP responses are
    429       defined in <xref target="caching"/>. Even if a resource is cacheable, there may
    430       be additional constraints on whether a cache can use the cached
    431       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.
    529     </t>
    530   </list>
    531 </t>
    532 <t>
    533   <iref item="upstream"/>
    534   <iref item="downstream"/>
    535   <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
    536   <list>
    537     <t>
    538       Upstream and downstream describe the flow of a message: all
    539       messages flow from upstream to downstream.
    540     </t>
    541   </list>
    542 </t>
    543 <t>
    544   <iref item="inbound"/>
    545   <iref item="outbound"/>
    546   <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
    547   <list>
    548     <t>
    549       Inbound and outbound refer to the request and response paths for
    550       messages: "inbound" means "traveling toward the origin server",
    551       and "outbound" means "traveling toward the user agent"
    552     </t>
    553   </list>
    554 </t>
    555 </section>
    557 <section title="Overall Operation" anchor="intro.overall.operation">
     196   This document will define aspects of HTTP related to request and response
     197   semantics. Right now it only includes the extracted relevant sections of
     198   RFC 2616 with only minor edits.
    559201   The HTTP protocol is a request/response protocol. A client sends a
    565207   followed by a MIME-like message containing server information, entity
    566208   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"/>.
    568 </t>
    569 <t>
    570    Most HTTP communication is initiated by a user agent and consists of
    571    a request to be applied to a resource on some origin server. In the
    572    simplest case, this may be accomplished via a single connection (v)
    573    between the user agent (UA) and the origin server (O).
    574 </t>
    575 <figure><artwork type="drawing">
    576        request chain ------------------------&gt;
    577     UA -------------------v------------------- O
    578        &lt;----------------------- response chain
    579 </artwork></figure>
    580 <t>
    581    A more complicated situation occurs when one or more intermediaries
    582    are present in the request/response chain. There are three common
    583    forms of intermediary: proxy, gateway, and tunnel. A proxy is a
    584    forwarding agent, receiving requests for a URI in its absolute form,
    585    rewriting all or part of the message, and forwarding the reformatted
    586    request toward the server identified by the URI. A gateway is a
    587    receiving agent, acting as a layer above some other server(s) and, if
    588    necessary, translating the requests to the underlying server's
    589    protocol. A tunnel acts as a relay point between two connections
    590    without changing the messages; tunnels are used when the
    591    communication needs to pass through an intermediary (such as a
    592    firewall) even when the intermediary cannot understand the contents
    593    of the messages.
    594 </t>
    595 <figure><artwork type="drawing">
    596        request chain --------------------------------------&gt;
    597     UA -----v----- A -----v----- B -----v----- C -----v----- O
    598        &lt;------------------------------------- response chain
    599 </artwork></figure>
    600 <t>
    601    The figure above shows three intermediaries (A, B, and C) between the
    602    user agent and origin server. A request or response message that
    603    travels the whole chain will pass through four separate connections.
    604    This distinction is important because some HTTP communication options
    605    may apply only to the connection with the nearest, non-tunnel
    606    neighbor, only to the end-points of the chain, or to all connections
    607    along the chain. Although the diagram is linear, each participant may
    608    be engaged in multiple, simultaneous communications. For example, B
    609    may be receiving requests from many clients other than A, and/or
    610    forwarding requests to servers other than C, at the same time that it
    611    is handling A's request.
    612 </t>
    613 <t>
    614    Any party to the communication which is not acting as a tunnel may
    615    employ an internal cache for handling requests. The effect of a cache
    616    is that the request/response chain is shortened if one of the
    617    participants along the chain has a cached response applicable to that
    618    request. The following illustrates the resulting chain if B has a
    619    cached copy of an earlier response from O (via C) for a request which
    620    has not been cached by UA or A.
    621 </t>
    622 <figure><artwork type="drawing">
    623           request chain ----------&gt;
    624        UA -----v----- A -----v----- B - - - - - - C - - - - - - O
    625           &lt;--------- response chain
    626 </artwork></figure>
    627 <t>
    628    Not all responses are usefully cacheable, and some requests may
    629    contain modifiers which place special requirements on cache behavior.
    630    HTTP requirements for cache behavior and cacheable responses are
    631    defined in <xref target="caching"/>.
    632 </t>
    633 <t>
    634    In fact, there are a wide variety of architectures and configurations
    635    of caches and proxies currently being experimented with or deployed
    636    across the World Wide Web. These systems include national hierarchies
    637    of proxy caches to save transoceanic bandwidth, systems that
    638    broadcast or multicast cache entries, organizations that distribute
    639    subsets of cached data via CD-ROM, and so on. HTTP systems are used
    640    in corporate intranets over high-bandwidth links, and for access via
    641    PDAs with low-power radio links and intermittent connectivity. The
    642    goal of HTTP/1.1 is to support the wide diversity of configurations
    643    already deployed while introducing protocol constructs that meet the
    644    needs of those who build web applications that require high
    645    reliability and, failing that, at least reliable indications of
    646    failure.
    647 </t>
    648 <t>
    649    HTTP communication usually takes place over TCP/IP connections. The
    650    default port is TCP 80 <xref target="RFC1700"/>, but other ports can be used. This does
    651    not preclude HTTP from being implemented on top of any other protocol
    652    on the Internet, or on other networks. HTTP only presumes a reliable
    653    transport; any protocol that provides such guarantees can be used;
    654    the mapping of the HTTP/1.1 request and response structures onto the
    655    transport data units of the protocol in question is outside the scope
    656    of this specification.
    657 </t>
    658 <t>
    659    In HTTP/1.0, most implementations used a new connection for each
    660    request/response exchange. In HTTP/1.1, a connection may be used for
    661    one or more request/response exchanges, although connections may be
    662    closed for a variety of reasons (see <xref target="persistent.connections"/>).
    663 </t>
    664 </section>
    665 </section>
    667 <section title="Notational Conventions and Generic Grammar" anchor="notation">
    669 <section title="Augmented BNF" anchor="notation.abnf">
    670 <t>
    671    All of the mechanisms specified in this document are described in
    672    both prose and an augmented Backus-Naur Form (BNF) similar to that
    673    used by RFC 822 <xref target="RFC822"/>. Implementors will need to be familiar with the
    674    notation in order to understand this specification. The augmented BNF
    675    includes the following constructs:
    676 </t>
    677 <t>
    678    name = definition
    679   <list>
    680     <t>
    681       The name of a rule is simply the name itself (without any
    682       enclosing "&lt;" and "&gt;") and is separated from its definition by the
    683       equal "=" character. White space is only significant in that
    684       indentation of continuation lines is used to indicate a rule
    685       definition that spans more than one line. Certain basic rules are
    686       in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle
    687       brackets are used within definitions whenever their presence will
    688       facilitate discerning the use of rule names.
    689     </t>
    690   </list>
    691 </t>
    692 <t>
    693    "literal"
    694   <list>
    695     <t>
    696       Quotation marks surround literal text. Unless stated otherwise,
    697       the text is case-insensitive.
    698     </t>
    699   </list>
    700 </t>
    701 <t>
    702    rule1 | rule2
    703   <list>
    704     <t>
    705       Elements separated by a bar ("|") are alternatives, e.g., "yes |
    706       no" will accept yes or no.
    707     </t>
    708   </list>
    709 </t>
    710 <t>
    711    (rule1 rule2)
    712   <list>
    713     <t>
    714       Elements enclosed in parentheses are treated as a single element.
    715       Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
    716       foo elem" and "elem bar elem".
    717     </t>
    718   </list>
    719 </t>
    720 <t>
    721    *rule
    722   <list>
    723     <t>
    724       The character "*" preceding an element indicates repetition. The
    725       full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and at most
    726       &lt;m&gt; occurrences of element. Default values are 0 and infinity so
    727       that "*(element)" allows any number, including zero; "1*element"
    728       requires at least one; and "1*2element" allows one or two.
    729     </t>
    730   </list>
    731 </t>
    732 <t>
    733    [rule]
    734   <list>
    735     <t>
    736       Square brackets enclose optional elements; "[foo bar]" is
    737       equivalent to "*1(foo bar)".
    738     </t>
    739   </list>
    740 </t>
    741 <t>
    742    N rule
    743   <list>
    744     <t>
    745       Specific repetition: "&lt;n&gt;(element)" is equivalent to
    746       "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element).
    747       Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
    748       alphabetic characters.
    749     </t>
    750   </list>
    751 </t>
    752 <t>
    753    #rule
    754   <list>
    755     <t>
    756       A construct "#" is defined, similar to "*", for defining lists of
    757       elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
    758       &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
    759       (",") and &OPTIONAL; linear white space (LWS). This makes the usual
    760       form of lists very easy; a rule such as
    761     </t>
    762     <t>
    763          ( *LWS element *( *LWS "," *LWS element ))
    764     </t>
    765     <t>
    766       can be shown as
    767     </t>
    768     <t>
    769          1#element
    770     </t>
    771     <t>
    772       Wherever this construct is used, null elements are allowed, but do
    773       not contribute to the count of elements present. That is,
    774       "(element), , (element) " is permitted, but counts as only two
    775       elements. Therefore, where at least one element is required, at
    776       least one non-null element &MUST; be present. Default values are 0
    777       and infinity so that "#element" allows any number, including zero;
    778       "1#element" requires at least one; and "1#2element" allows one or
    779       two.
    780     </t>
    781   </list>
    782 </t>
    783 <t>
    784    ; comment
    785   <list>
    786     <t>
    787       A semi-colon, set off some distance to the right of rule text,
    788       starts a comment that continues to the end of line. This is a
    789       simple way of including useful notes in parallel with the
    790       specifications.
    791     </t>
    792   </list>
    793 </t>
    794 <t>
    795    implied *LWS
    796   <list>
    797     <t>
    798       The grammar described by this specification is word-based. Except
    799       where noted otherwise, linear white space (LWS) can be included
    800       between any two adjacent words (token or quoted-string), and
    801       between adjacent words and separators, without changing the
    802       interpretation of a field. At least one delimiter (LWS and/or
    803       separators) &MUST; exist between any two tokens (for the definition
    804       of "token" below), since they would otherwise be interpreted as a
    805       single token.
    806     </t>
    807   </list>
    808 </t>
    809 </section>
    811 <section title="Basic Rules" anchor="basic.rules">
    812 <x:anchor-alias value="OCTET"/>
    813 <x:anchor-alias value="CHAR"/>
    814 <x:anchor-alias value="UPALPHA"/>
    815 <x:anchor-alias value="LOALPHA"/>
    816 <x:anchor-alias value="ALPHA"/>
    817 <x:anchor-alias value="DIGIT"/>
    818 <x:anchor-alias value="CTL"/>
    819 <x:anchor-alias value="CR"/>
    820 <x:anchor-alias value="LF"/>
    821 <x:anchor-alias value="SP"/>
    822 <x:anchor-alias value="HT"/>
    823 <x:anchor-alias value="CRLF"/>
    824 <x:anchor-alias value="LWS"/>
    825 <x:anchor-alias value="TEXT"/>
    826 <x:anchor-alias value="HEX"/>
    827 <x:anchor-alias value="token"/>
    828 <x:anchor-alias value="separators"/>
    829 <x:anchor-alias value="comment"/>
    830 <x:anchor-alias value="ctext"/>
    831 <x:anchor-alias value="quoted-string"/>
    832 <x:anchor-alias value="qdtext"/>
    833 <x:anchor-alias value="quoted-pair"/>
    834 <t>
    835    The following rules are used throughout this specification to
    836    describe basic parsing constructs. The US-ASCII coded character set
    837    is defined by ANSI X3.4-1986 <xref target="USASCII"/>.
    838 </t>
    839 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="OCTET"/><iref primary="true" item="Grammar" subitem="CHAR"/><iref primary="true" item="Grammar" subitem="UPALPHA"/><iref primary="true" item="Grammar" subitem="LOALPHA"/><iref primary="true" item="Grammar" subitem="ALPHA"/><iref primary="true" item="Grammar" subitem="DIGIT"/><iref primary="true" item="Grammar" subitem="CTL"/><iref primary="true" item="Grammar" subitem="CR"/><iref primary="true" item="Grammar" subitem="LF"/><iref primary="true" item="Grammar" subitem="SP"/><iref primary="true" item="Grammar" subitem="HT"/>
    840     OCTET          = &lt;any 8-bit sequence of data&gt;
    841     CHAR           = &lt;any US-ASCII character (octets 0 - 127)&gt;
    842     UPALPHA        = &lt;any US-ASCII uppercase letter "A".."Z"&gt;
    843     LOALPHA        = &lt;any US-ASCII lowercase letter "a".."z"&gt;
    844     ALPHA          = UPALPHA | LOALPHA
    845     DIGIT          = &lt;any US-ASCII digit "0".."9"&gt;
    846     CTL            = &lt;any US-ASCII control character
    847                      (octets 0 - 31) and DEL (127)&gt;
    848     CR             = &lt;US-ASCII CR, carriage return (13)&gt;
    849     LF             = &lt;US-ASCII LF, linefeed (10)&gt;
    850     SP             = &lt;US-ASCII SP, space (32)&gt;
    851     HT             = &lt;US-ASCII HT, horizontal-tab (9)&gt;
    852     &lt;"&gt;            = &lt;US-ASCII double-quote mark (34)&gt;
    853 </artwork></figure>
    854 <t>
    855    HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
    856    protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
    857    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"/>.
    859 </t>
    860 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="CRLF"/>
    861     CRLF           = CR LF
    862 </artwork></figure>
    863 <t>
    864    HTTP/1.1 header field values can be folded onto multiple lines if the
    865    continuation line begins with a space or horizontal tab. All linear
    866    white space, including folding, has the same semantics as SP. A
    867    recipient &MAY; replace any linear white space with a single SP before
    868    interpreting the field value or forwarding the message downstream.
    869 </t>
    870 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="LWS"/>
    871     LWS            = [CRLF] 1*( SP | HT )
    872 </artwork></figure>
    873 <t>
    874    The TEXT rule is only used for descriptive field contents and values
    875    that are not intended to be interpreted by the message parser. Words
    876    of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
    877    <xref target="ISO-8859"/> only when encoded according to the rules of RFC 2047
    878    <xref target="RFC2047"/>.
    879 </t>
    880 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
    881     TEXT           = &lt;any OCTET except CTLs,
    882                      but including LWS&gt;
    883 </artwork></figure>
    884 <t>
    885    A CRLF is allowed in the definition of TEXT only as part of a header
    886    field continuation. It is expected that the folding LWS will be
    887    replaced with a single SP before interpretation of the TEXT value.
    888 </t>
    889 <t>
    890    Hexadecimal numeric characters are used in several protocol elements.
    891 </t>
    892 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HEX"/>
    893     HEX            = "A" | "B" | "C" | "D" | "E" | "F"
    894                    | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
    895 </artwork></figure>
    896 <t>
    897    Many HTTP/1.1 header field values consist of words separated by LWS
    898    or special characters. These special characters &MUST; be in a quoted
    899    string to be used within a parameter value (as defined in
    900    <xref target="transfer.codings"/>).
    901 </t>
    902 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="separators"/>
    903     token          = 1*&lt;any CHAR except CTLs or separators&gt;
    904     separators     = "(" | ")" | "&lt;" | "&gt;" | "@"
    905                    | "," | ";" | ":" | "\" | &lt;"&gt;
    906                    | "/" | "[" | "]" | "?" | "="
    907                    | "{" | "}" | SP | HT
    908 </artwork></figure>
    909 <t>
    910    Comments can be included in some HTTP header fields by surrounding
    911    the comment text with parentheses. Comments are only allowed in
    912    fields containing "comment" as part of their field value definition.
    913    In all other fields, parentheses are considered part of the field
    914    value.
    915 </t>
    916 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
    917     comment        = "(" *( ctext | quoted-pair | comment ) ")"
    918     ctext          = &lt;any TEXT excluding "(" and ")"&gt;
    919 </artwork></figure>
    920 <t>
    921    A string of text is parsed as a single word if it is quoted using
    922    double-quote marks.
    923 </t>
    924 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
    925     quoted-string  = ( &lt;"&gt; *(qdtext | quoted-pair ) &lt;"&gt; )
    926     qdtext         = &lt;any TEXT except &lt;"&gt;&gt;
    927 </artwork></figure>
    928 <t>
    929    The backslash character ("\") &MAY; be used as a single-character
    930    quoting mechanism only within quoted-string and comment constructs.
    931 </t>
    932 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-pair"/>
    933     quoted-pair    = "\" CHAR
    934 </artwork></figure>
    935 </section>
    936 </section>
    938 <section title="Protocol Parameters" anchor="protocol.parameters">
    940 <section title="HTTP Version" anchor="http.version">
    941 <t>
    942    HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
    943    of the protocol. The protocol versioning policy is intended to allow
    944    the sender to indicate the format of a message and its capacity for
    945    understanding further HTTP communication, rather than the features
    946    obtained via that communication. No change is made to the version
    947    number for the addition of message components which do not affect
    948    communication behavior or which only add to extensible field values.
    949    The &lt;minor&gt; number is incremented when the changes made to the
    950    protocol add features which do not change the general message parsing
    951    algorithm, but which may add to the message semantics and imply
    952    additional capabilities of the sender. The &lt;major&gt; number is
    953    incremented when the format of a message within the protocol is
    954    changed. See RFC 2145 <xref target="RFC2145"/> for a fuller explanation.
    955 </t>
    956 <t>
    957    The version of an HTTP message is indicated by an HTTP-Version field
    958    in the first line of the message.
    959 </t>
    960 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/>
    961        HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
    962 </artwork></figure>
    963 <t>
    964    Note that the major and minor numbers &MUST; be treated as separate
    965    integers and that each &MAY; be incremented higher than a single digit.
    966    Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
    967    lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
    968    &MUST-NOT; be sent.
    969 </t>
    970 <t>
    971    An application that sends a request or response message that includes
    972    HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
    973    with this specification. Applications that are at least conditionally
    974    compliant with this specification &SHOULD; use an HTTP-Version of
    975    "HTTP/1.1" in their messages, and &MUST; do so for any message that is
    976    not compatible with HTTP/1.0. For more details on when to send
    977    specific HTTP-Version values, see RFC 2145 <xref target="RFC2145"/>.
    978 </t>
    979 <t>
    980    The HTTP version of an application is the highest HTTP version for
    981    which the application is at least conditionally compliant.
    982 </t>
    983 <t>
    984    Proxy and gateway applications need to be careful when forwarding
    985    messages in protocol versions different from that of the application.
    986    Since the protocol version indicates the protocol capability of the
    987    sender, a proxy/gateway &MUST-NOT; send a message with a version
    988    indicator which is greater than its actual version. If a higher
    989    version request is received, the proxy/gateway &MUST; either downgrade
    990    the request version, or respond with an error, or switch to tunnel
    991    behavior.
    992 </t>
    993 <t>
    994    Due to interoperability problems with HTTP/1.0 proxies discovered
    995    since the publication of RFC 2068 <xref target="RFC2068"/>, caching proxies MUST, gateways
    996    &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
    997    they support. The proxy/gateway's response to that request &MUST; be in
    998    the same major version as the request.
    999 </t>
    1000 <t>
    1001   <list>
    1002     <t>
    1003       <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
    1004       of header fields required or forbidden by the versions involved.
    1005     </t>
    1006   </list>
    1007 </t>
    1008 </section>
    1010 <section title="Uniform Resource Identifiers" anchor="uri">
    1011 <t>
    1012    URIs have been known by many names: WWW addresses, Universal Document
    1013    Identifiers, Universal Resource Identifiers <xref target="RFC1630"/>, and finally the
    1014    combination of Uniform Resource Locators (URL) <xref target="RFC1738"/> and Names (URN)
    1015    <xref target="RFC1737"/>. As far as HTTP is concerned, Uniform Resource Identifiers are
    1016    simply formatted strings which identify--via name, location, or any
    1017    other characteristic--a resource.
    1018 </t>
    1020 <section title="General Syntax" anchor="general.syntax">
    1021 <t>
    1022    URIs in HTTP can be represented in absolute form or relative to some
    1023    known base URI <xref target="RFC1808"/>, depending upon the context of their use. The two
    1024    forms are differentiated by the fact that absolute URIs always begin
    1025    with a scheme name followed by a colon. For definitive information on
    1026    URL syntax and semantics, see "Uniform Resource Identifiers (URI):
    1027    Generic Syntax and Semantics," RFC 2396 <xref target="RFC2396"/> (which replaces RFCs
    1028    1738 <xref target="RFC1738"/> and RFC 1808 <xref target="RFC1808"/>). This specification adopts the
    1029    definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
    1030    "host","abs_path", "rel_path", and "authority" from that
    1031    specification.
    1032 </t>
    1033 <t>
    1034    The HTTP protocol does not place any a priori limit on the length of
    1035    a URI. Servers &MUST; be able to handle the URI of any resource they
    1036    serve, and &SHOULD; be able to handle URIs of unbounded length if they
    1037    provide GET-based forms that could generate such URIs. A server
    1038    &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
    1039    than the server can handle (see <xref target="status.414"/>).
    1040 </t>
    1041 <t>
    1042   <list>
    1043     <t>
    1044       <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
    1045       above 255 bytes, because some older client or proxy
    1046       implementations might not properly support these lengths.
    1047     </t>
    1048   </list>
    1049 </t>
    1050 </section>
    1052 <section title="http URL" anchor="http.url">
    1053 <t>
    1054    The "http" scheme is used to locate network resources via the HTTP
    1055    protocol. This section defines the scheme-specific syntax and
    1056    semantics for http URLs.
    1057 </t>
    1058 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http_URL"/>
    1059 http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
    1060 </artwork></figure>
    1061 <t>
    1062    If the port is empty or not given, port 80 is assumed. The semantics
    1063    are that the identified resource is located at the server listening
    1064    for TCP connections on that port of that host, and the Request-URI
    1065    for the resource is abs_path (<xref target="request-uri"/>). The use of IP addresses
    1066    in URLs &SHOULD; be avoided whenever possible (see RFC 1900 <xref target="RFC1900"/>). If
    1067    the abs_path is not present in the URL, it &MUST; be given as "/" when
    1068    used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
    1070    receives a host name which is not a fully qualified domain name, it
    1071    &MAY; add its domain to the host name it received. If a proxy receives
    1072    a fully qualified domain name, the proxy &MUST-NOT; change the host
    1073    name.
    1074 </t>
    1075 </section>
    1077 <section title="URI Comparison" anchor="uri.comparison">
    1078 <t>
    1079    When comparing two URIs to decide if they match or not, a client
    1080    &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
    1081    URIs, with these exceptions:
    1082   <list style="symbols">
    1083     <t>A port that is empty or not given is equivalent to the default
    1084         port for that URI-reference;</t>
    1085     <t>Comparisons of host names &MUST; be case-insensitive;</t>
    1086     <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
    1087     <t>An empty abs_path is equivalent to an abs_path of "/".</t>
    1088   </list>
    1089 </t>
    1090 <t>
    1091    Characters other than those in the "reserved" and "unsafe" sets (see
    1092    RFC 2396 <xref target="RFC2396"/>) are equivalent to their ""%" HEX HEX" encoding.
    1093 </t>
    1094 <t>
    1095    For example, the following three URIs are equivalent:
    1096 </t>
    1097 <figure><artwork type="example">
    1098    http://abc.com:80/~smith/home.html
    1099    http://ABC.com/%7Esmith/home.html
    1100    http://ABC.com:/%7esmith/home.html
    1101 </artwork></figure>
    1102 </section>
    1103 </section>
    1105 <section title="Date/Time Formats" anchor="date.time.formats">
    1107 <section title="Full Date" anchor="full.date">
    1108 <t>
    1109    HTTP applications have historically allowed three different formats
    1110    for the representation of date/time stamps:
    1111 </t>
    1112 <figure><artwork type="example">
    1113    Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
    1114    Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
    1115    Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
    1116 </artwork></figure>
    1117 <t>
    1118    The first format is preferred as an Internet standard and represents
    1119    a fixed-length subset of that defined by RFC 1123 <xref target="RFC1123"/> (an update to
    1120    RFC 822 <xref target="RFC822"/>). The second format is in common use, but is based on the
    1121    obsolete RFC 850 <xref target="RFC1036"/> date format and lacks a four-digit year.
    1122    HTTP/1.1 clients and servers that parse the date value &MUST; accept
    1123    all three formats (for compatibility with HTTP/1.0), though they &MUST;
    1124    only generate the RFC 1123 format for representing HTTP-date values
    1125    in header fields. See <xref target="tolerant.applications"/> for further information.
    1126 </t>
    1127 <t><list><t>
    1128       <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
    1129       accepting date values that may have been sent by non-HTTP
    1130       applications, as is sometimes the case when retrieving or posting
    1131       messages via proxies/gateways to SMTP or NNTP.
    1132 </t></list></t>
    1133 <t>
    1134    All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
    1135    (GMT), without exception. For the purposes of HTTP, GMT is exactly
    1136    equal to UTC (Coordinated Universal Time). This is indicated in the
    1137    first two formats by the inclusion of "GMT" as the three-letter
    1138    abbreviation for time zone, and &MUST; be assumed when reading the
    1139    asctime format. HTTP-date is case sensitive and &MUST-NOT; include
    1140    additional LWS beyond that specifically included as SP in the
    1141    grammar.
    1142 </t>
    1143 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-date"/><iref primary="true" item="Grammar" subitem="rfc1123-date"/><iref primary="true" item="Grammar" subitem="rfc850-date"/><iref primary="true" item="Grammar" subitem="asctime-date"/><iref primary="true" item="Grammar" subitem="date1"/><iref primary="true" item="Grammar" subitem="date2"/><iref primary="true" item="Grammar" subitem="date3"/><iref primary="true" item="Grammar" subitem="time"/><iref primary="true" item="Grammar" subitem="wkday"/><iref primary="true" item="Grammar" subitem="weekday"/><iref primary="true" item="Grammar" subitem="month"/>
    1144     HTTP-date    = rfc1123-date | rfc850-date | asctime-date
    1145     rfc1123-date = wkday "," SP date1 SP time SP "GMT"
    1146     rfc850-date  = weekday "," SP date2 SP time SP "GMT"
    1147     asctime-date = wkday SP date3 SP time SP 4DIGIT
    1148     date1        = 2DIGIT SP month SP 4DIGIT
    1149                    ; day month year (e.g., 02 Jun 1982)
    1150     date2        = 2DIGIT "-" month "-" 2DIGIT
    1151                    ; day-month-year (e.g., 02-Jun-82)
    1152     date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
    1153                    ; month day (e.g., Jun  2)
    1154     time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
    1155                    ; 00:00:00 - 23:59:59
    1156     wkday        = "Mon" | "Tue" | "Wed"
    1157                  | "Thu" | "Fri" | "Sat" | "Sun"
    1158     weekday      = "Monday" | "Tuesday" | "Wednesday"
    1159                  | "Thursday" | "Friday" | "Saturday" | "Sunday"
    1160     month        = "Jan" | "Feb" | "Mar" | "Apr"
    1161                  | "May" | "Jun" | "Jul" | "Aug"
    1162                  | "Sep" | "Oct" | "Nov" | "Dec"
    1163 </artwork></figure>
    1164 <t>
    1165       <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
    1166       to their usage within the protocol stream. Clients and servers are
    1167       not required to use these formats for user presentation, request
    1168       logging, etc.
    1169 </t>
    1170 </section>
    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>
    1323 </section>
    1325 <section title="Transfer Codings" anchor="transfer.codings">
    1326 <t>
    1327    Transfer-coding values are used to indicate an encoding
    1328    transformation that has been, can be, or may need to be applied to an
    1329    entity-body in order to ensure "safe transport" through the network.
    1330    This differs from a content coding in that the transfer-coding is a
    1331    property of the message, not of the original entity.
    1332 </t>
    1333 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
    1334     transfer-coding         = "chunked" | transfer-extension
    1335     transfer-extension      = token *( ";" parameter )
    1336 </artwork></figure>
    1337 <t>
    1338    Parameters are in  the form of attribute/value pairs.
    1339 </t>
    1340 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="parameter"/><iref primary="true" item="Grammar" subitem="attribute"/><iref primary="true" item="Grammar" subitem="value"/>
    1341     parameter               = attribute "=" value
    1342     attribute               = token
    1343     value                   = token | quoted-string
    1344 </artwork></figure>
    1345 <t>
    1346    All transfer-coding values are case-insensitive. HTTP/1.1 uses
    1347    transfer-coding values in the TE header field (<xref target="header.te"/>) and in
    1348    the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
    1349 </t>
    1350 <t>
    1351    Whenever a transfer-coding is applied to a message-body, the set of
    1352    transfer-codings &MUST; include "chunked", unless the message is
    1353    terminated by closing the connection. When the "chunked" transfer-coding
    1354    is used, it &MUST; be the last transfer-coding applied to the
    1355    message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
    1356    than once to a message-body. These rules allow the recipient to
    1357    determine the transfer-length of the message (<xref target="message.length"/>).
    1358 </t>
    1359 <t>
    1360    Transfer-codings are analogous to the Content-Transfer-Encoding
    1361    values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
    1362    binary data over a 7-bit transport service. However, safe transport
    1363    has a different focus for an 8bit-clean transfer protocol. In HTTP,
    1364    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
    1366    encrypt data over a shared transport.
    1367 </t>
    1368 <t>
    1369    The Internet Assigned Numbers Authority (IANA) acts as a registry for
    1370    transfer-coding value tokens. Initially, the registry contains the
    1371    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"/>).
    1374 </t>
    1375 <t>
    1376    New transfer-coding value tokens &SHOULD; be registered in the same way
    1377    as new content-coding value tokens (<xref target="content.codings"/>).
    1378 </t>
    1379 <t>
    1380    A server which receives an entity-body with a transfer-coding it does
    1381    not understand &SHOULD; return 501 (Unimplemented), and close the
    1382    connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
    1383    client.
    1384 </t>
    1386 <section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
    1387 <t>
    1388    The chunked encoding modifies the body of a message in order to
    1389    transfer it as a series of chunks, each with its own size indicator,
    1390    followed by an &OPTIONAL; trailer containing entity-header fields. This
    1391    allows dynamically produced content to be transferred along with the
    1392    information necessary for the recipient to verify that it has
    1393    received the full message.
    1394 </t>
    1395 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Chunked-Body"/><iref primary="true" item="Grammar" subitem="chunk"/><iref primary="true" item="Grammar" subitem="chunk-size"/><iref primary="true" item="Grammar" subitem="last-chunk"/><iref primary="true" item="Grammar" subitem="chunk-extension"/><iref primary="true" item="Grammar" subitem="chunk-ext-name"/><iref primary="true" item="Grammar" subitem="chunk-ext-val"/><iref primary="true" item="Grammar" subitem="chunk-data"/><iref primary="true" item="Grammar" subitem="trailer"/>
    1396     Chunked-Body   = *chunk
    1397                      last-chunk
    1398                      trailer
    1399                      CRLF
    1401     chunk          = chunk-size [ chunk-extension ] CRLF
    1402                      chunk-data CRLF
    1403     chunk-size     = 1*HEX
    1404     last-chunk     = 1*("0") [ chunk-extension ] CRLF
    1406     chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
    1407     chunk-ext-name = token
    1408     chunk-ext-val  = token | quoted-string
    1409     chunk-data     = chunk-size(OCTET)
    1410     trailer        = *(entity-header CRLF)
    1411 </artwork></figure>
    1412 <t>
    1413    The chunk-size field is a string of hex digits indicating the size of
    1414    the chunk. The chunked encoding is ended by any chunk whose size is
    1415    zero, followed by the trailer, which is terminated by an empty line.
    1416 </t>
    1417 <t>
    1418    The trailer allows the sender to include additional HTTP header
    1419    fields at the end of the message. The Trailer header field can be
    1420    used to indicate which header fields are included in a trailer (see
    1421    <xref target="header.trailer"/>).
    1422 </t>
    1423 <t>
    1424    A server using chunked transfer-coding in a response &MUST-NOT; use the
    1425    trailer for any header fields unless at least one of the following is
    1426    true:
    1427   <list style="numbers">
    1428     <t>the request included a TE header field that indicates "trailers" is
    1429      acceptable in the transfer-coding of the  response, as described in
    1430      <xref target="header.te"/>; or,</t>
    1432     <t>the server is the origin server for the response, the trailer
    1433      fields consist entirely of optional metadata, and the recipient
    1434      could use the message (in a manner acceptable to the origin server)
    1435      without receiving this metadata.  In other words, the origin server
    1436      is willing to accept the possibility that the trailer fields might
    1437      be silently discarded along the path to the client.</t>
    1438   </list>
    1439 </t>
    1440 <t>
    1441    This requirement prevents an interoperability failure when the
    1442    message is being received by an HTTP/1.1 (or later) proxy and
    1443    forwarded to an HTTP/1.0 recipient. It avoids a situation where
    1444    compliance with the protocol would have necessitated a possibly
    1445    infinite buffer on the proxy.
    1446 </t>
    1447 <t>
    1448    An example process for decoding a Chunked-Body is presented in
    1449    <xref target="introduction.of.transfer-encoding"/>.
    1450 </t>
    1451 <t>
    1452    All HTTP/1.1 applications &MUST; be able to receive and decode the
    1453    "chunked" transfer-coding, and &MUST; ignore chunk-extension extensions
    1454    they do not understand.
    1455 </t>
    1456 </section>
    1457 </section>
    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="internet.media.type.multipart.byteranges"/>) 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>
     209   between HTTP and MIME is described in &diff2045entity;.
    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>
    1725 <section title="HTTP Message" anchor="http.message">
    1727 <section title="Message Types" anchor="message.types">
    1728 <t>
    1729    HTTP messages consist of requests from client to server and responses
    1730    from server to client.
    1731 </t>
    1732 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
    1733     HTTP-message   = Request | Response     ; HTTP/1.1 messages
    1734 </artwork></figure>
    1735 <t>
    1736    Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
    1737    message format of RFC 822 <xref target="RFC822"/> for transferring entities (the payload
    1738    of the message). Both types of message consist of a start-line, zero
    1739    or more header fields (also known as "headers"), an empty line (i.e.,
    1740    a line with nothing preceding the CRLF) indicating the end of the
    1741    header fields, and possibly a message-body.
    1742 </t>
    1743 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
    1744      generic-message = start-line
    1745                        *(message-header CRLF)
    1746                        CRLF
    1747                        [ message-body ]
    1748      start-line      = Request-Line | Status-Line
    1749 </artwork></figure>
    1750 <t>
    1751    In the interest of robustness, servers &SHOULD; ignore any empty
    1752    line(s) received where a Request-Line is expected. In other words, if
    1753    the server is reading the protocol stream at the beginning of a
    1754    message and receives a CRLF first, it should ignore the CRLF.
    1755 </t>
    1756 <t>
    1757    Certain buggy HTTP/1.0 client implementations generate extra CRLF's
    1758    after a POST request. To restate what is explicitly forbidden by the
    1759    BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
    1760    extra CRLF.
    1761 </t>
    1762 </section>
    1764 <section title="Message Headers" anchor="message.headers">
    1765 <t>
    1766    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
    1769    that given in <xref target="RFC822" x:fmt="sec" x:sec="3.1"/> of RFC 822 <xref target="RFC822"/>. Each header field consists
    1770    of a name followed by a colon (":") and the field value. Field names
    1771    are case-insensitive. The field value &MAY; be preceded by any amount
    1772    of LWS, though a single SP is preferred. Header fields can be
    1773    extended over multiple lines by preceding each extra line with at
    1774    least one SP or HT. Applications ought to follow "common form", where
    1775    one is known or indicated, when generating HTTP constructs, since
    1776    there might exist some implementations that fail to accept anything
    1777    beyond the common forms.
    1778 </t>
    1779 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-header"/><iref primary="true" item="Grammar" subitem="field-name"/><iref primary="true" item="Grammar" subitem="field-value"/><iref primary="true" item="Grammar" subitem="field-content"/>
    1780     message-header = field-name ":" [ field-value ]
    1781     field-name     = token
    1782     field-value    = *( field-content | LWS )
    1783     field-content  = &lt;the OCTETs making up the field-value
    1784                      and consisting of either *TEXT or combinations
    1785                      of token, separators, and quoted-string&gt;
    1786 </artwork></figure>
    1787 <t>
    1788    The field-content does not include any leading or trailing LWS:
    1789    linear white space occurring before the first non-whitespace
    1790    character of the field-value or after the last non-whitespace
    1791    character of the field-value. Such leading or trailing LWS &MAY; be
    1792    removed without changing the semantics of the field value. Any LWS
    1793    that occurs between field-content &MAY; be replaced with a single SP
    1794    before interpreting the field value or forwarding the message
    1795    downstream.
    1796 </t>
    1797 <t>
    1798    The order in which header fields with differing field names are
    1799    received is not significant. However, it is "good practice" to send
    1800    general-header fields first, followed by request-header or response-header
    1801    fields, and ending with the entity-header fields.
    1802 </t>
    1803 <t>
    1804    Multiple message-header fields with the same field-name &MAY; be
    1805    present in a message if and only if the entire field-value for that
    1806    header field is defined as a comma-separated list [i.e., #(values)].
    1807    It &MUST; be possible to combine the multiple header fields into one
    1808    "field-name: field-value" pair, without changing the semantics of the
    1809    message, by appending each subsequent field-value to the first, each
    1810    separated by a comma. The order in which header fields with the same
    1811    field-name are received is therefore significant to the
    1812    interpretation of the combined field value, and thus a proxy &MUST-NOT;
    1813    change the order of these field values when a message is forwarded.
    1814 </t>
    1815 </section>
    1817 <section title="Message Body" anchor="message.body">
    1818 <t>
    1819    The message-body (if any) of an HTTP message is used to carry the
    1820    entity-body associated with the request or response. The message-body
    1821    differs from the entity-body only when a transfer-coding has been
    1822    applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
    1823 </t>
    1824 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
    1825     message-body = entity-body
    1826                  | &lt;entity-body encoded as per Transfer-Encoding&gt;
    1827 </artwork></figure>
    1828 <t>
    1829    Transfer-Encoding &MUST; be used to indicate any transfer-codings
    1830    applied by an application to ensure safe and proper transfer of the
    1831    message. Transfer-Encoding is a property of the message, not of the
    1832    entity, and thus &MAY; be added or removed by any application along the
    1833    request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
    1834    when certain transfer-codings may be used.)
    1835 </t>
    1836 <t>
    1837    The rules for when a message-body is allowed in a message differ for
    1838    requests and responses.
    1839 </t>
    1840 <t>
    1841    The presence of a message-body in a request is signaled by the
    1842    inclusion of a Content-Length or Transfer-Encoding header field in
    1843    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"/>)
    1845    does not allow sending an entity-body in requests. A server &SHOULD;
    1846    read and forward a message-body on any request; if the request method
    1847    does not include defined semantics for an entity-body, then the
    1848    message-body &SHOULD; be ignored when handling the request.
    1849 </t>
    1850 <t>
    1851    For response messages, whether or not a message-body is included with
    1852    a message is dependent on both the request method and the response
    1853    status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
    1854    &MUST-NOT; include a message-body, even though the presence of entity-header
    1855    fields might lead one to believe they do. All 1xx
    1856    (informational), 204 (no content), and 304 (not modified) responses
    1857    &MUST-NOT; include a message-body. All other responses do include a
    1858    message-body, although it &MAY; be of zero length.
    1859 </t>
    1860 </section>
    1862 <section title="Message Length" anchor="message.length">
    1863 <t>
    1864    The transfer-length of a message is the length of the message-body as
    1865    it appears in the message; that is, after any transfer-codings have
    1866    been applied. When a message-body is included with a message, the
    1867    transfer-length of that body is determined by one of the following
    1868    (in order of precedence):
    1869 </t>
    1870 <t>
    1871   <list style="numbers">
    1872     <x:lt><t>
    1873      Any response message which "&MUST-NOT;" include a message-body (such
    1874      as the 1xx, 204, and 304 responses and any response to a HEAD
    1875      request) is always terminated by the first empty line after the
    1876      header fields, regardless of the entity-header fields present in
    1877      the message.
    1878     </t></x:lt>
    1879     <x:lt><t>
    1880      If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>) is present and
    1881      has any value other than "identity", then the transfer-length is
    1882      defined by use of the "chunked" transfer-coding (<xref target="transfer.codings"/>),
    1883      unless the message is terminated by closing the connection.
    1884     </t></x:lt>
    1885     <x:lt><t>
    1886      If a Content-Length header field (<xref target="header.content-length"/>) is present, its
    1887      decimal value in OCTETs represents both the entity-length and the
    1888      transfer-length. The Content-Length header field &MUST-NOT; be sent
    1889      if these two lengths are different (i.e., if a Transfer-Encoding
    1890      header field is present). If a message is received with both a
    1891      Transfer-Encoding header field and a Content-Length header field,
    1892      the latter &MUST; be ignored.
    1893     </t></x:lt>
    1894     <x:lt><t>
    1895      If the message uses the media type "multipart/byteranges", and the
    1896      ransfer-length is not otherwise specified, then this self-elimiting
    1897      media type defines the transfer-length. This media type
    1898      UST NOT be used unless the sender knows that the recipient can arse
    1899      it; the presence in a request of a Range header with ultiple byte-range
    1900      specifiers from a 1.1 client implies that the lient can parse
    1901      multipart/byteranges responses.
    1902     <list style="empty"><t>
    1903        A range header might be forwarded by a 1.0 proxy that does not
    1904        understand multipart/byteranges; in this case the server &MUST;
    1905        delimit the message using methods defined in items 1, 3 or 5 of
    1906        this section.
    1907     </t></list>
    1908     </t></x:lt>
    1909     <x:lt><t>
    1910      By the server closing the connection. (Closing the connection
    1911      cannot be used to indicate the end of a request body, since that
    1912      would leave no possibility for the server to send back a response.)
    1913     </t></x:lt>
    1914   </list>
    1915 </t>
    1916 <t>
    1917    For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
    1918    containing a message-body &MUST; include a valid Content-Length header
    1919    field unless the server is known to be HTTP/1.1 compliant. If a
    1920    request contains a message-body and a Content-Length is not given,
    1921    the server &SHOULD; respond with 400 (bad request) if it cannot
    1922    determine the length of the message, or with 411 (length required) if
    1923    it wishes to insist on receiving a valid Content-Length.
    1924 </t>
    1925 <t>
    1926    All HTTP/1.1 applications that receive entities &MUST; accept the
    1927    "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
    1928    to be used for messages when the message length cannot be determined
    1929    in advance.
    1930 </t>
    1931 <t>
    1932    Messages &MUST-NOT; include both a Content-Length header field and a
    1933    non-identity transfer-coding. If the message does include a non-identity
    1934    transfer-coding, the Content-Length &MUST; be ignored.
    1935 </t>
    1936 <t>
    1937    When a Content-Length is given in a message where a message-body is
    1938    allowed, its field value &MUST; exactly match the number of OCTETs in
    1939    the message-body. HTTP/1.1 user agents &MUST; notify the user when an
    1940    invalid length is received and detected.
    1941 </t>
    1942 </section>
    1944 <section title="General Header Fields" anchor="general.header.fields">
    1945 <t>
    1946    There are a few header fields which have general applicability for
    1947    both request and response messages, but which do not apply to the
    1948    entity being transferred. These header fields apply only to the
    1949    message being transmitted.
    1950 </t>
    1951 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
    1952     general-header = Cache-Control            ; <xref target="header.cache-control"/>
    1953                    | Connection               ; <xref target="header.connection"/>
    1954                    | Date                     ; <xref target="header.date"/>
    1955                    | Pragma                   ; <xref target="header.pragma"/>
    1956                    | Trailer                  ; <xref target="header.trailer"/>
    1957                    | Transfer-Encoding        ; <xref target="header.transfer-encoding"/>
    1958                    | Upgrade                  ; <xref target="header.upgrade"/>
    1959                    | Via                      ; <xref target="header.via"/>
    1960                    | Warning                  ; <xref target="header.warning"/>
    1961 </artwork></figure>
    1962 <t>
    1963    General-header field names can be extended reliably only in
    1964    combination with a change in the protocol version. However, new or
    1965    experimental header fields may be given the semantics of general
    1966    header fields if all parties in the communication recognize them to
    1967    be general-header fields. Unrecognized header fields are treated as
    1968    entity-header fields.
    1969 </t>
    1970 </section>
    1971 </section>
    1973 <section title="Request" anchor="request">
    1974 <t>
    1975    A request message from a client to a server includes, within the
    1976    first line of that message, the method to be applied to the resource,
    1977    the identifier of the resource, and the protocol version in use.
    1978 </t>
    1979 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
    1980      Request       = Request-Line              ; <xref target="request-line"/>
    1981                      *(( general-header        ; <xref target="general.header.fields"/>
    1982                       | request-header         ; <xref target="request.header.fields"/>
    1983                       | entity-header ) CRLF)  ; <xref target="entity.header.fields"/>
    1984                      CRLF
    1985                      [ message-body ]          ; <xref target="message.body"/>
    1986 </artwork></figure>
    1988 <section title="Request-Line" anchor="request-line">
    1989 <t>
    1990    The Request-Line begins with a method token, followed by the
    1991    Request-URI and the protocol version, and ending with CRLF. The
    1992    elements are separated by SP characters. No CR or LF is allowed
    1993    except in the final CRLF sequence.
    1994 </t>
    1995 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
    1996      Request-Line   = Method SP Request-URI SP HTTP-Version CRLF
    1997 </artwork></figure>
    1999243<section title="Method" anchor="method">
    2032 <section title="Request-URI" anchor="request-uri">
    2033 <t>
    2034    The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
    2035    identifies the resource upon which to apply the request.
    2036 </t>
    2037 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
    2038     Request-URI    = "*" | absoluteURI | abs_path | authority
    2039 </artwork></figure>
    2040 <t>
    2041    The four options for Request-URI are dependent on the nature of the
    2042    request. The asterisk "*" means that the request does not apply to a
    2043    particular resource, but to the server itself, and is only allowed
    2044    when the method used does not necessarily apply to a resource. One
    2045    example would be
    2046 </t>
    2047 <figure><artwork type="example">
    2048     OPTIONS * HTTP/1.1
    2049 </artwork></figure>
    2050 <t>
    2051    The absoluteURI form is &REQUIRED; when the request is being made to a
    2052    proxy. The proxy is requested to forward the request or service it
    2053    from a valid cache, and return the response. Note that the proxy &MAY;
    2054    forward the request on to another proxy or directly to the server
    2055    specified by the absoluteURI. In order to avoid request loops, a
    2056    proxy &MUST; be able to recognize all of its server names, including
    2057    any aliases, local variations, and the numeric IP address. An example
    2058    Request-Line would be:
    2059 </t>
    2060 <figure><artwork type="example">
    2061     GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
    2062 </artwork></figure>
    2063 <t>
    2064    To allow for transition to absoluteURIs in all requests in future
    2065    versions of HTTP, all HTTP/1.1 servers &MUST; accept the absoluteURI
    2066    form in requests, even though HTTP/1.1 clients will only generate
    2067    them in requests to proxies.
    2068 </t>
    2069 <t>
    2070    The authority form is only used by the CONNECT method (<xref target="CONNECT"/>).
    2071 </t>
    2072 <t>
    2073    The most common form of Request-URI is that used to identify a
    2074    resource on an origin server or gateway. In this case the absolute
    2075    path of the URI &MUST; be transmitted (see <xref target="general.syntax"/>, abs_path) as
    2076    the Request-URI, and the network location of the URI (authority) &MUST;
    2077    be transmitted in a Host header field. For example, a client wishing
    2078    to retrieve the resource above directly from the origin server would
    2079    create a TCP connection to port 80 of the host "www.w3.org" and send
    2080    the lines:
    2081 </t>
    2082 <figure><artwork type="example">
    2083     GET /pub/WWW/TheProject.html HTTP/1.1
    2084     Host: www.w3.org
    2085 </artwork></figure>
    2086 <t>
    2087    followed by the remainder of the Request. Note that the absolute path
    2088    cannot be empty; if none is present in the original URI, it &MUST; be
    2089    given as "/" (the server root).
    2090 </t>
    2091 <t>
    2092    The Request-URI is transmitted in the format specified in
    2093    <xref target="general.syntax"/>. If the Request-URI is encoded using the "% HEX HEX" encoding
    2094    <xref target="RFC2396"/>, the origin server &MUST; decode the Request-URI in order to
    2095    properly interpret the request. Servers &SHOULD; respond to invalid
    2096    Request-URIs with an appropriate status code.
    2097 </t>
    2098 <t>
    2099    A transparent proxy &MUST-NOT; rewrite the "abs_path" part of the
    2100    received Request-URI when forwarding it to the next inbound server,
    2101    except as noted above to replace a null abs_path with "/".
    2102 </t>
    2103 <t>
    2104   <list><t>
    2105       <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
    2106       meaning of the request when the origin server is improperly using
    2107       a non-reserved URI character for a reserved purpose.  Implementors
    2108       should be aware that some pre-HTTP/1.1 proxies have been known to
    2109       rewrite the Request-URI.
    2110   </t></list>
    2111 </t>
    2112 </section>
    2113 </section>
    2115 <section title="The Resource Identified by a Request" anchor="the.resource.identified.by.a.request">
    2116 <t>
    2117    The exact resource identified by an Internet request is determined by
    2118    examining both the Request-URI and the Host header field.
    2119 </t>
    2120 <t>
    2121    An origin server that does not allow resources to differ by the
    2122    requested host &MAY; ignore the Host header field value when
    2123    determining the resource identified by an HTTP/1.1 request. (But see
    2124    <xref target="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses"/>
    2125    for other requirements on Host support in HTTP/1.1.)
    2126 </t>
    2127 <t>
    2128    An origin server that does differentiate resources based on the host
    2129    requested (sometimes referred to as virtual hosts or vanity host
    2130    names) &MUST; use the following rules for determining the requested
    2131    resource on an HTTP/1.1 request:
    2132   <list style="numbers">
    2133     <t>If Request-URI is an absoluteURI, the host is part of the
    2134      Request-URI. Any Host header field value in the request &MUST; be
    2135      ignored.</t>
    2136     <t>If the Request-URI is not an absoluteURI, and the request includes
    2137      a Host header field, the host is determined by the Host header
    2138      field value.</t>
    2139     <t>If the host as determined by rule 1 or 2 is not a valid host on
    2140      the server, the response &MUST; be a 400 (Bad Request) error message.</t>
    2141   </list>
    2142 </t>
    2143 <t>
    2144    Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
    2145    attempt to use heuristics (e.g., examination of the URI path for
    2146    something unique to a particular host) in order to determine what
    2147    exact resource is being requested.
    2148 </t>
    2149 </section>
    2151276<section title="Request Header Fields" anchor="request.header.fields">
    2159284<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"/>
     285    request-header = Accept                   ; &header-accept;
     286                   | Accept-Charset           ; &header-accept-charset;
     287                   | Accept-Encoding          ; &header-accept-encoding;
     288                   | Accept-Language          ; &header-accept-language;
     289                   | Authorization            ; &header-authorization;
    2165290                   | Expect                   ; <xref target="header.expect"/>
    2166291                   | From                     ; <xref target="header.from"/>
    2167                    | Host                     ; <xref target="header.host"/>
    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"/>
     292                   | Host                     ; &header-host;
     293                   | If-Match                 ; &header-if-match;
     294                   | If-Modified-Since        ; &header-if-modified-since;
     295                   | If-None-Match            ; &header-if-none-match;
     296                   | If-Range                 ; &header-if-range;
     297                   | If-Unmodified-Since      ; &header-if-unmodified-since;
    2173298                   | Max-Forwards             ; <xref target="header.max-forwards"/>
    2174                    | Proxy-Authorization      ; <xref target="header.proxy-authorization"/>
    2175                    | Range                    ; <xref target="header.range"/>
     299                   | Proxy-Authorization      ; &header-proxy-authorization;
     300                   | Range                    ; &header-range;
    2176301                   | Referer                  ; <xref target="header.referer"/>
    2177                    | TE                       ; <xref target="header.te"/>
     302                   | TE                       ; &header-te;
    2178303                   | User-Agent               ; <xref target="header.user-agent"/>
    2189 </section>
    2192 <section title="Response" anchor="response">
    2193 <t>
    2194    After receiving and interpreting a request message, a server responds
    2195    with an HTTP response message.
    2196 </t>
    2197 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
    2198     Response      = Status-Line               ; <xref target="status-line"/>
    2199                     *(( general-header        ; <xref target="general.header.fields"/>
    2200                      | response-header        ; <xref target="response.header.fields"/>
    2201                      | entity-header ) CRLF)  ; <xref target="entity.header.fields"/>
    2202                     CRLF
    2203                     [ message-body ]          ; <xref target="entity.body"/>
    2204 </artwork></figure>
    2206 <section title="Status-Line" anchor="status-line">
    2207 <t>
    2208    The first line of a Response message is the Status-Line, consisting
    2209    of the protocol version followed by a numeric status code and its
    2210    associated textual phrase, with each element separated by SP
    2211    characters. No CR or LF is allowed except in the final CRLF sequence.
    2212 </t>
    2213 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
    2214     Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
    2215 </artwork></figure>
    2217315<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
    2223321   for use by automata and the Reason-Phrase is intended for the human
    2224322   user. The client is not required to examine or display the Reason-Phrase.
    2225 </t>
    2226 <t>
    2227    The first digit of the Status-Code defines the class of response. The
    2228    last two digits do not have any categorization role. There are 5
    2229    values for the first digit:
    2230   <list style="symbols">
    2231     <t>
    2232       1xx: Informational - Request received, continuing process
    2233     </t>
    2234     <t>
    2235       2xx: Success - The action was successfully received,
    2236         understood, and accepted
    2237     </t>
    2238     <t>
    2239       3xx: Redirection - Further action must be taken in order to
    2240         complete the request
    2241     </t>
    2242     <t>
    2243       4xx: Client Error - The request contains bad syntax or cannot
    2244         be fulfilled
    2245     </t>
    2246     <t>
    2247       5xx: Server Error - The server failed to fulfill an apparently
    2248         valid request
    2249     </t>
    2250   </list>
    2322 </section>
    2324395<section title="Response Header Fields" anchor="response.header.fields">
    2331402<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"/>
     403    response-header = Accept-Ranges           ; &header-accept-ranges;
     404                    | Age                     ; &header-age;
     405                    | ETag                    ; &header-etag;
    2335406                    | Location                ; <xref target="header.location"/>
    2336                     | Proxy-Authenticate      ; <xref target="header.proxy-authenticate"/>
     407                    | Proxy-Authenticate      ; &header-proxy-authenticate;
    2337408                    | Retry-After             ; <xref target="header.retry-after"/>
    2338409                    | Server                  ; <xref target="header.server"/>
    2339                     | Vary                    ; <xref target="header.vary"/>
    2340                     | WWW-Authenticate        ; <xref target="header.www-authenticate"/>
     410                    | Vary                    ; &header-vary;
     411                    | WWW-Authenticate        ; &header-www-authenticate;
    2351 </section>
    2354423<section title="Entity" anchor="entity">
    2357426   restricted by the request method or response status code. An entity
    2358427   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>
     428   responses will only include the entity-headers. HTTP entity-body and
     429   entity-header fields are defined in &payload;.
    2406432   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
     433   present, as described in &message-body;. The entity-body is obtained
    2408434   from the message-body by decoding any Transfer-Encoding that might
    2409435   have been applied to ensure safe and proper transfer of the message.
    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>
    2451 <section title="Connections" anchor="connections">
    2453 <section title="Persistent Connections" anchor="persistent.connections">
    2455 <section title="Purpose" anchor="persistent.purpose">
    2456 <t>
    2457    Prior to persistent connections, a separate TCP connection was
    2458    established to fetch each URL, increasing the load on HTTP servers
    2459    and causing congestion on the Internet. The use of inline images and
    2460    other associated data often require a client to make multiple
    2461    requests of the same server in a short amount of time. Analysis of
    2462    these performance problems and results from a prototype
    2463    implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
    2464    measurements of actual HTTP/1.1 (RFC 2068) implementations show good
    2465    results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
    2466    T/TCP <xref target="Tou1998"/>.
    2467 </t>
    2468 <t>
    2469    Persistent HTTP connections have a number of advantages:
    2470   <list style="symbols">
    2471       <t>
    2472         By opening and closing fewer TCP connections, CPU time is saved
    2473         in routers and hosts (clients, servers, proxies, gateways,
    2474         tunnels, or caches), and memory used for TCP protocol control
    2475         blocks can be saved in hosts.
    2476       </t>
    2477       <t>
    2478         HTTP requests and responses can be pipelined on a connection.
    2479         Pipelining allows a client to make multiple requests without
    2480         waiting for each response, allowing a single TCP connection to
    2481         be used much more efficiently, with much lower elapsed time.
    2482       </t>
    2483       <t>
    2484         Network congestion is reduced by reducing the number of packets
    2485         caused by TCP opens, and by allowing TCP sufficient time to
    2486         determine the congestion state of the network.
    2487       </t>
    2488       <t>
    2489         Latency on subsequent requests is reduced since there is no time
    2490         spent in TCP's connection opening handshake.
    2491       </t>
    2492       <t>
    2493         HTTP can evolve more gracefully, since errors can be reported
    2494         without the penalty of closing the TCP connection. Clients using
    2495         future versions of HTTP might optimistically try a new feature,
    2496         but if communicating with an older server, retry with old
    2497         semantics after an error is reported.
    2498       </t>
    2499     </list>
    2500 </t>
    2501 <t>
    2502    HTTP implementations &SHOULD; implement persistent connections.
    2503 </t>
    2504 </section>
    2506 <section title="Overall Operation" anchor="persistent.overall">
    2507 <t>
    2508    A significant difference between HTTP/1.1 and earlier versions of
    2509    HTTP is that persistent connections are the default behavior of any
    2510    HTTP connection. That is, unless otherwise indicated, the client
    2511    &SHOULD; assume that the server will maintain a persistent connection,
    2512    even after error responses from the server.
    2513 </t>
    2514 <t>
    2515    Persistent connections provide a mechanism by which a client and a
    2516    server can signal the close of a TCP connection. This signaling takes
    2517    place using the Connection header field (<xref target="header.connection"/>). Once a close
    2518    has been signaled, the client &MUST-NOT; send any more requests on that
    2519    connection.
    2520 </t>
    2522 <section title="Negotiation" anchor="persistent.negotiation">
    2523 <t>
    2524    An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
    2525    maintain a persistent connection unless a Connection header including
    2526    the connection-token "close" was sent in the request. If the server
    2527    chooses to close the connection immediately after sending the
    2528    response, it &SHOULD; send a Connection header including the
    2529    connection-token close.
    2530 </t>
    2531 <t>
    2532    An HTTP/1.1 client &MAY; expect a connection to remain open, but would
    2533    decide to keep it open based on whether the response from a server
    2534    contains a Connection header with the connection-token close. In case
    2535    the client does not want to maintain a connection for more than that
    2536    request, it &SHOULD; send a Connection header including the
    2537    connection-token close.
    2538 </t>
    2539 <t>
    2540    If either the client or the server sends the close token in the
    2541    Connection header, that request becomes the last one for the
    2542    connection.
    2543 </t>
    2544 <t>
    2545    Clients and servers &SHOULD-NOT;  assume that a persistent connection is
    2546    maintained for HTTP versions less than 1.1 unless it is explicitly
    2547    signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
    2548    compatibility with HTTP/1.0 clients.
    2549 </t>
    2550 <t>
    2551    In order to remain persistent, all messages on the connection &MUST;
    2552    have a self-defined message length (i.e., one not defined by closure
    2553    of the connection), as described in <xref target="message.length"/>.
    2554 </t>
    2555 </section>
    2557 <section title="Pipelining" anchor="pipelining">
    2558 <t>
    2559    A client that supports persistent connections &MAY; "pipeline" its
    2560    requests (i.e., send multiple requests without waiting for each
    2561    response). A server &MUST; send its responses to those requests in the
    2562    same order that the requests were received.
    2563 </t>
    2564 <t>
    2565    Clients which assume persistent connections and pipeline immediately
    2566    after connection establishment &SHOULD; be prepared to retry their
    2567    connection if the first pipelined attempt fails. If a client does
    2568    such a retry, it &MUST-NOT; pipeline before it knows the connection is
    2569    persistent. Clients &MUST; also be prepared to resend their requests if
    2570    the server closes the connection before sending all of the
    2571    corresponding responses.
    2572 </t>
    2573 <t>
    2574    Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
    2575    non-idempotent sequences of methods (see <xref target="idempotent.methods"/>). Otherwise, a
    2576    premature termination of the transport connection could lead to
    2577    indeterminate results. A client wishing to send a non-idempotent
    2578    request &SHOULD; wait to send that request until it has received the
    2579    response status for the previous request.
    2580 </t>
    2581 </section>
    2582 </section>
    2584 <section title="Proxy Servers" anchor="persistent.proxy">
    2585 <t>
    2586    It is especially important that proxies correctly implement the
    2587    properties of the Connection header field as specified in <xref target="header.connection"/>.
    2588 </t>
    2589 <t>
    2590    The proxy server &MUST; signal persistent connections separately with
    2591    its clients and the origin servers (or other proxy servers) that it
    2592    connects to. Each persistent connection applies to only one transport
    2593    link.
    2594 </t>
    2595 <t>
    2596    A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
    2597    with an HTTP/1.0 client (but see RFC 2068 <xref target="RFC2068"/> for information and
    2598    discussion of the problems with the Keep-Alive header implemented by
    2599    many HTTP/1.0 clients).
    2600 </t>
    2601 </section>
    2603 <section title="Practical Considerations" anchor="persistent.practical">
    2604 <t>
    2605    Servers will usually have some time-out value beyond which they will
    2606    no longer maintain an inactive connection. Proxy servers might make
    2607    this a higher value since it is likely that the client will be making
    2608    more connections through the same server. The use of persistent
    2609    connections places no requirements on the length (or existence) of
    2610    this time-out for either the client or the server.
    2611 </t>
    2612 <t>
    2613    When a client or server wishes to time-out it &SHOULD; issue a graceful
    2614    close on the transport connection. Clients and servers &SHOULD; both
    2615    constantly watch for the other side of the transport close, and
    2616    respond to it as appropriate. If a client or server does not detect
    2617    the other side's close promptly it could cause unnecessary resource
    2618    drain on the network.
    2619 </t>
    2620 <t>
    2621    A client, server, or proxy &MAY; close the transport connection at any
    2622    time. For example, a client might have started to send a new request
    2623    at the same time that the server has decided to close the "idle"
    2624    connection. From the server's point of view, the connection is being
    2625    closed while it was idle, but from the client's point of view, a
    2626    request is in progress.
    2627 </t>
    2628 <t>
    2629    This means that clients, servers, and proxies &MUST; be able to recover
    2630    from asynchronous close events. Client software &SHOULD; reopen the
    2631    transport connection and retransmit the aborted sequence of requests
    2632    without user interaction so long as the request sequence is
    2633    idempotent (see <xref target="idempotent.methods"/>). Non-idempotent methods or sequences
    2634    &MUST-NOT; be automatically retried, although user agents &MAY; offer a
    2635    human operator the choice of retrying the request(s). Confirmation by
    2636    user-agent software with semantic understanding of the application
    2637    &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT;
    2638    be repeated if the second sequence of requests fails.
    2639 </t>
    2640 <t>
    2641    Servers &SHOULD; always respond to at least one request per connection,
    2642    if at all possible. Servers &SHOULD-NOT;  close a connection in the
    2643    middle of transmitting a response, unless a network or client failure
    2644    is suspected.
    2645 </t>
    2646 <t>
    2647    Clients that use persistent connections &SHOULD; limit the number of
    2648    simultaneous connections that they maintain to a given server. A
    2649    single-user client &SHOULD-NOT; maintain more than 2 connections with
    2650    any server or proxy. A proxy &SHOULD; use up to 2*N connections to
    2651    another server or proxy, where N is the number of simultaneously
    2652    active users. These guidelines are intended to improve HTTP response
    2653    times and avoid congestion.
    2654 </t>
    2655 </section>
    2656 </section>
    2658 <section title="Message Transmission Requirements" anchor="message.transmission.requirements">
    2660 <section title="Persistent Connections and Flow Control" anchor="persistent.flow">
    2661 <t>
    2662    HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
    2663    flow control mechanisms to resolve temporary overloads, rather than
    2664    terminating connections with the expectation that clients will retry.
    2665    The latter technique can exacerbate network congestion.
    2666 </t>
    2667 </section>
    2669 <section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
    2670 <t>
    2671    An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
    2672    the network connection for an error status while it is transmitting
    2673    the request. If the client sees an error status, it &SHOULD;
    2674    immediately cease transmitting the body. If the body is being sent
    2675    using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
    2676    empty trailer &MAY; be used to prematurely mark the end of the message.
    2677    If the body was preceded by a Content-Length header, the client &MUST;
    2678    close the connection.
    2679 </t>
    2680 </section>
    2682 <section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
    2683 <t>
    2684    The purpose of the 100 (Continue) status (see <xref target="status.100"/>) is to
    2685    allow a client that is sending a request message with a request body
    2686    to determine if the origin server is willing to accept the request
    2687    (based on the request headers) before the client sends the request
    2688    body. In some cases, it might either be inappropriate or highly
    2689    inefficient for the client to send the body if the server will reject
    2690    the message without looking at the body.
    2691 </t>
    2692 <t>
    2693    Requirements for HTTP/1.1 clients:
    2694   <list style="symbols">
    2695     <t>
    2696         If a client will wait for a 100 (Continue) response before
    2697         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"/>)
    2702         with the "100-continue" expectation if it does not intend
    2703         to send a request body.
    2704     </t>
    2705   </list>
    2706 </t>
    2707 <t>
    2708    Because of the presence of older implementations, the protocol allows
    2709    ambiguous situations in which a client may send "Expect: 100-continue"
    2710    without receiving either a 417 (Expectation Failed) status
    2711    or a 100 (Continue) status. Therefore, when a client sends this
    2712    header field to an origin server (possibly via a proxy) from which it
    2713    has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
    2714    for an indefinite period before sending the request body.
    2715 </t>
    2716 <t>
    2717    Requirements for HTTP/1.1 origin servers:
    2718   <list style="symbols">
    2719     <t> Upon receiving a request which includes an Expect request-header
    2720         field with the "100-continue" expectation, an origin server &MUST;
    2721         either respond with 100 (Continue) status and continue to read
    2722         from the input stream, or respond with a final status code. The
    2723         origin server &MUST-NOT; wait for the request body before sending
    2724         the 100 (Continue) response. If it responds with a final status
    2725         code, it &MAY; close the transport connection or it &MAY; continue
    2726         to read and discard the rest of the request.  It &MUST-NOT;
    2727         perform the requested method if it returns a final status code.
    2728     </t>
    2729     <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
    2730         the request message does not include an Expect request-header
    2731         field with the "100-continue" expectation, and &MUST-NOT; send a
    2732         100 (Continue) response if such a request comes from an HTTP/1.0
    2733         (or earlier) client. There is an exception to this rule: for
    2734         compatibility with RFC 2068, a server &MAY; send a 100 (Continue)
    2735         status in response to an HTTP/1.1 PUT or POST request that does
    2736         not include an Expect request-header field with the "100-continue"
    2737         expectation. This exception, the purpose of which is
    2738         to minimize any client processing delays associated with an
    2739         undeclared wait for 100 (Continue) status, applies only to
    2740         HTTP/1.1 requests, and not to requests with any other HTTP-version
    2741         value.
    2742     </t>
    2743     <t> An origin server &MAY; omit a 100 (Continue) response if it has
    2744         already received some or all of the request body for the
    2745         corresponding request.
    2746     </t>
    2747     <t> An origin server that sends a 100 (Continue) response &MUST;
    2748     ultimately send a final status code, once the request body is
    2749         received and processed, unless it terminates the transport
    2750         connection prematurely.
    2751     </t>
    2752     <t> If an origin server receives a request that does not include an
    2753         Expect request-header field with the "100-continue" expectation,
    2754         the request includes a request body, and the server responds
    2755         with a final status code before reading the entire request body
    2756         from the transport connection, then the server &SHOULD-NOT;  close
    2757         the transport connection until it has read the entire request,
    2758         or until the client closes the connection. Otherwise, the client
    2759         might not reliably receive the response message. However, this
    2760         requirement is not be construed as preventing a server from
    2761         defending itself against denial-of-service attacks, or from
    2762         badly broken client implementations.
    2763       </t>
    2764     </list>
    2765 </t>
    2766 <t>
    2767    Requirements for HTTP/1.1 proxies:
    2768   <list style="symbols">
    2769     <t> If a proxy receives a request that includes an Expect request-header
    2770         field with the "100-continue" expectation, and the proxy
    2771         either knows that the next-hop server complies with HTTP/1.1 or
    2772         higher, or does not know the HTTP version of the next-hop
    2773         server, it &MUST; forward the request, including the Expect header
    2774         field.
    2775     </t>
    2776     <t> If the proxy knows that the version of the next-hop server is
    2777         HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
    2778         respond with a 417 (Expectation Failed) status.
    2779     </t>
    2780     <t> Proxies &SHOULD; maintain a cache recording the HTTP version
    2781         numbers received from recently-referenced next-hop servers.
    2782     </t>
    2783     <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
    2784         request message was received from an HTTP/1.0 (or earlier)
    2785         client and did not include an Expect request-header field with
    2786         the "100-continue" expectation. This requirement overrides the
    2787         general rule for forwarding of 1xx responses (see <xref target="status.1xx"/>).
    2788     </t>
    2789   </list>
    2790 </t>
    2791 </section>
    2793 <section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
    2794 <t>
    2795    If an HTTP/1.1 client sends a request which includes a request body,
    2796    but which does not include an Expect request-header field with the
    2797    "100-continue" expectation, and if the client is not directly
    2798    connected to an HTTP/1.1 origin server, and if the client sees the
    2799    connection close before receiving any status from the server, the
    2800    client &SHOULD; retry the request.  If the client does retry this
    2801    request, it &MAY; use the following "binary exponential backoff"
    2802    algorithm to be assured of obtaining a reliable response:
    2803   <list style="numbers">
    2804     <t>
    2805       Initiate a new connection to the server
    2806     </t>
    2807     <t>
    2808       Transmit the request-headers
    2809     </t>
    2810     <t>
    2811       Initialize a variable R to the estimated round-trip time to the
    2812          server (e.g., based on the time it took to establish the
    2813          connection), or to a constant value of 5 seconds if the round-trip
    2814          time is not available.
    2815     </t>
    2816     <t>
    2817        Compute T = R * (2**N), where N is the number of previous
    2818          retries of this request.
    2819     </t>
    2820     <t>
    2821        Wait either for an error response from the server, or for T
    2822          seconds (whichever comes first)
    2823     </t>
    2824     <t>
    2825        If no error response is received, after T seconds transmit the
    2826          body of the request.
    2827     </t>
    2828     <t>
    2829        If client sees that the connection is closed prematurely,
    2830          repeat from step 1 until the request is accepted, an error
    2831          response is received, or the user becomes impatient and
    2832          terminates the retry process.
    2833     </t>
    2834   </list>
    2835 </t>
    2836 <t>
    2837    If at any point an error status is received, the client
    2838   <list style="symbols">
    2839       <t>&SHOULD-NOT;  continue and</t>
    2841       <t>&SHOULD; close the connection if it has not completed sending the
    2842         request message.</t>
    2843     </list>
    2844 </t>
    2845 </section>
    2846 </section>
    2847 </section>
    2855444   share the same semantics for separately extended clients and servers.
    2857    The Host request-header field (<xref target="header.host"/>) &MUST; accompany all
     446   The Host request-header field (&header-host;) &MUST; accompany all
    2858447   HTTP/1.1 requests.
    2996585   The semantics of the GET method change to a "partial GET" if the
    2997586   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"/>.
     587   that only part of the entity be transferred, as described in &header-range;.
    2999588   The partial GET method is intended to reduce unnecessary
    3000589   network usage by allowing partially-retrieved entities to be
    3004593   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"/>.
     594   the requirements for HTTP caching described in &caching;.
    3089678   POST requests &MUST; obey the message transmission requirements set out
    3090    in <xref target="message.transmission.requirements"/>.
     679   in [message.transmission.requirements].
    3149738   PUT requests &MUST; obey the message transmission requirements set out
    3150    in <xref target="message.transmission.requirements"/>.
     739   in [message.transmission.requirements].
    3199788   TRACE allows the client to see what is being received at the other
    3200789   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
     790   information. The value of the Via header field (&header-via;) is of
    3202791   particular interest, since it acts as a trace of the request chain.
    3203792   Use of the Max-Forwards header field allows the client to limit the
    3265854   request has already been completed, ignore this response. The server
    3266855   &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
     856   &use100; for detailed discussion of the use and handling of this
    3268857   status code.
    3276865   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
     866   request, via the Upgrade message header field (&header-upgrade;), for a
    3278867   change in the application protocol being used on this connection. The
    3279868   server will switch protocols to those defined by the response's
    3342931   A 201 response &MAY; contain an ETag response header field indicating
    3343932   the current value of the entity tag for the requested variant just
    3344    created, see <xref target="header.etag"/>.
     933   created, see &header-etag;.
    34241013  <iref primary="true" item="Status Codes" subitem="206 Partial Content" x:for-anchor=""/>
    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.
     1015   The server has fulfilled the partial GET request for the resource
     1016   and the enclosed entity is a partial representation as defined in &range;.
    34981041   The requested resource corresponds to any one of a set of
    34991042   representations, each with its own specific location, and agent-driven
    3500    negotiation information (<xref target="content.negotiation"/>) is being provided so that
     1043   negotiation information (&content-negotiation;) is being provided so that
    35011044   the user (or user agent) can select a preferred representation and
    35021045   redirect its request to that location.
    36221165   The response &MUST; include the following header fields:
    36231166  <list style="symbols">
    3624     <t>Date, unless its omission is required by <xref target="clockless.origin.server.operation"/></t>
     1167    <t>Date, unless its omission is required by [clockless.origin.server.operation]</t>
    36251168  </list>
    3641    If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
     1184   If the conditional GET used a strong cache validator (see &caching;),
    36421185   the response &SHOULD-NOT;  include other entity-headers.
    36431186   Otherwise (i.e., the conditional GET used a weak validator), the
    37451288   The request requires user authentication. The response &MUST; include a
    3746    WWW-Authenticate header field (<xref target="header.www-authenticate"/>) containing a challenge
     1289   WWW-Authenticate header field (&header-www-authenticate;) containing a challenge
    37471290   applicable to the requested resource. The client &MAY; repeat the
    3748    request with a suitable Authorization header field (<xref target="header.authorization"/>). If
     1291   request with a suitable Authorization header field (&header-authorization;). If
    37491292   the request already included Authorization credentials, then the 401
    37501293   response indicates that authorization has been refused for those
    38451388   This code is similar to 401 (Unauthorized), but indicates that the
    38461389   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
     1390   return a Proxy-Authenticate header field (&header-proxy-authenticate;) containing a
    38481391   challenge applicable to the proxy for the requested resource. The
    38491392   client &MAY; repeat the request with a suitable Proxy-Authorization
    3850    header field (<xref target="header.proxy-authorization"/>). HTTP access authentication is explained
     1393   header field (&header-proxy-authorization;). HTTP access authentication is explained
    38511394   in "HTTP Authentication: Basic and Digest Access Authentication"
    38521395   <xref target="RFC2617"/>.
    39831526  <iref primary="true" item="Status Codes" subitem="416 Requested Range Not Satisfiable" x:for-anchor=""/>
    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
     1528   The request included a Range request-header field (&header-range;) and none of
    39871529   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.
     1530   of the selected resource.
    40921625   version that was used in the request message. The server is
    40931626   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"/>,
     1627   using the same major version as the client, as described in &http-version;,
    40951628   other than with this error message. The response &SHOULD; contain
    40961629   an entity describing why that version is not supported and what other
    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="exceptions.to.the.rules.and.warnings" 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="explicit.ua.warnings">
    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="exceptions.to.the.rules.and.warnings">
    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="header.date"/>). 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="rules.for.when.to.use.entity.tags.and.last-modified.dates">
    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="what.may.be.stored.by.caches" 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>
    57051643   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="privacy.issues.connected.to.accept.headers"/>.
    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>
    60831646<section title="Allow" anchor="header.allow">
    61181681      have other means of communicating with the origin server.
    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="what.is.cacheable"/>
    6208         | "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
    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="what.is.cacheable"/>
    6218         | "private" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ] ; <xref target="what.is.cacheable"/>
    6219         | "no-cache" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ]; <xref target="what.is.cacheable"/>
    6220         | "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
    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="what.is.cacheable">
    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="what.may.be.stored.by.caches">
    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>
    6720 <section title="Connection" anchor="header.connection">
    6721   <iref primary="true" item="Connection header" x:for-anchor=""/>
    6722   <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
    6723 <t>
    6724    The Connection general-header field allows the sender to specify
    6725    options that are desired for that particular connection and &MUST-NOT;
    6726    be communicated by proxies over further connections.
    6727 </t>
    6728 <t>
    6729    The Connection header has the following grammar:
    6730 </t>
    6731 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="connection-token"/>
    6732     Connection = "Connection" ":" 1#(connection-token)
    6733     connection-token  = token
    6734 </artwork></figure>
    6735 <t>
    6736    HTTP/1.1 proxies &MUST; parse the Connection header field before a
    6737    message is forwarded and, for each connection-token in this field,
    6738    remove any header field(s) from the message with the same name as the
    6739    connection-token. Connection options are signaled by the presence of
    6740    a connection-token in the Connection header field, not by any
    6741    corresponding additional header field(s), since the additional header
    6742    field may not be sent if there are no parameters associated with that
    6743    connection option.
    6744 </t>
    6745 <t>
    6746    Message headers listed in the Connection header &MUST-NOT; include
    6747    end-to-end headers, such as Cache-Control.
    6748 </t>
    6749 <t>
    6750    HTTP/1.1 defines the "close" connection option for the sender to
    6751    signal that the connection will be closed after completion of the
    6752    response. For example,
    6753 </t>
    6754 <figure><artwork type="example">
    6755     Connection: close
    6756 </artwork></figure>
    6757 <t>
    6758    in either the request or the response header fields indicates that
    6759    the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
    6760    after the current request/response is complete.
    6761 </t>
    6762 <t>
    6763    HTTP/1.1 applications that do not support persistent connections &MUST;
    6764    include the "close" connection option in every message.
    6765 </t>
    6766 <t>
    6767    A system receiving an HTTP/1.0 (or lower-version) message that
    6768    includes a Connection header MUST, for each connection-token in this
    6769    field, remove and ignore any header field(s) from the message with
    6770    the same name as the connection-token. This protects against mistaken
    6771    forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
    6772 </t>
    6773 </section>
    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>
    6873 <section title="Content-Length" anchor="header.content-length">
    6874   <iref primary="true" item="Content-Length header" x:for-anchor=""/>
    6875   <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
    6876 <t>
    6877    The Content-Length entity-header field indicates the size of the
    6878    entity-body, in decimal number of OCTETs, sent to the recipient or,
    6879    in the case of the HEAD method, the size of the entity-body that
    6880    would have been sent had the request been a GET.
    6881 </t>
    6882 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/>
    6883     Content-Length    = "Content-Length" ":" 1*DIGIT
    6884 </artwork></figure>
    6885 <t>
    6886    An example is
    6887 </t>
    6888 <figure><artwork type="example">
    6889     Content-Length: 3495
    6890 </artwork></figure>
    6891 <t>
    6892    Applications &SHOULD; use this field to indicate the transfer-length of
    6893    the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
    6894 </t>
    6895 <t>
    6896    Any Content-Length greater than or equal to zero is a valid value.
    6897    <xref target="message.length"/> describes how to determine the length of a message-body
    6898    if a Content-Length is not given.
    6899 </t>
    6900 <t>
    6901    Note that the meaning of this field is significantly different from
    6902    the corresponding definition in MIME, where it is an optional field
    6903    used within the "message/external-body" content-type. In HTTP, it
    6904    &SHOULD; be sent whenever the message's length can be determined prior
    6905    to being transferred, unless this is prohibited by the rules in
    6906    <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="internet.media.type.multipart.byteranges"/>. 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