source: draft-ietf-httpbis/latest/p1-messaging.xml @ 382

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Prepare release of draft 05.

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "November">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY caching                "<xref target='Part6' x:rel='#caching' xmlns:x=''/>">
18  <!ENTITY payload                "<xref target='Part3' xmlns:x=''/>">
19  <!ENTITY media-types            "<xref target='Part3' x:rel='#media.types' xmlns:x=''/>">
20  <!ENTITY content-codings        "<xref target='Part3' x:rel='#content.codings' xmlns:x=''/>">
21  <!ENTITY CONNECT                "<xref target='Part2' x:rel='#CONNECT' xmlns:x=''/>">
22  <!ENTITY content.negotiation    "<xref target='Part3' x:rel='#content.negotiation' xmlns:x=''/>">
23  <!ENTITY diff2045entity         "<xref target='Part3' x:rel='#differences.between.http.entities.and.rfc.2045.entities' xmlns:x=''/>">
24  <!ENTITY entity                 "<xref target='Part3' x:rel='#entity' xmlns:x=''/>">
25  <!ENTITY entity-body            "<xref target='Part3' x:rel='#entity.body' xmlns:x=''/>">
26  <!ENTITY entity-header-fields   "<xref target='Part3' x:rel='#entity.header.fields' xmlns:x=''/>">
27  <!ENTITY header-accept          "<xref target='Part3' x:rel='#header.accept' xmlns:x=''/>">
28  <!ENTITY header-cache-control   "<xref target='Part6' x:rel='#header.cache-control' xmlns:x=''/>">
29  <!ENTITY header-expect          "<xref target='Part2' x:rel='#header.expect' xmlns:x=''/>">
30  <!ENTITY header-pragma          "<xref target='Part6' x:rel='#header.pragma' xmlns:x=''/>">
31  <!ENTITY header-warning         "<xref target='Part6' x:rel='#header.warning' xmlns:x=''/>">
32  <!ENTITY idempotent-methods     "<xref target='Part2' x:rel='#idempotent.methods' xmlns:x=''/>">
33  <!ENTITY qvalue                 "<xref target='Part3' x:rel='#quality.values' xmlns:x=''/>">
34  <!ENTITY request-header-fields  "<xref target='Part2' x:rel='#request.header.fields' xmlns:x=''/>">
35  <!ENTITY response-header-fields "<xref target='Part2' x:rel='#response.header.fields' xmlns:x=''/>">
36  <!ENTITY method                 "<xref target='Part2' x:rel='#method' xmlns:x=''/>">
37  <!ENTITY status-codes           "<xref target='Part2' x:rel='' xmlns:x=''/>">
38  <!ENTITY status-100             "<xref target='Part2' x:rel='#status.100' xmlns:x=''/>">
39  <!ENTITY status-1xx             "<xref target='Part2' x:rel='#status.1xx' xmlns:x=''/>">
40  <!ENTITY status-414             "<xref target='Part2' x:rel='#status.414' xmlns:x=''/>">
42<?rfc toc="yes" ?>
43<?rfc symrefs="yes" ?>
44<?rfc sortrefs="yes" ?>
45<?rfc compact="yes"?>
46<?rfc subcompact="no" ?>
47<?rfc linkmailto="no" ?>
48<?rfc editing="no" ?>
49<?rfc comments="yes"?>
50<?rfc inline="yes"?>
51<?rfc-ext allow-markup-in-artwork="yes" ?>
52<?rfc-ext include-references-in-index="yes" ?>
53<rfc obsoletes="2616" category="std" x:maturity-level="draft"
54     ipr="full3978" docName="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"
55     xmlns:x=''>
58  <title abbrev="HTTP/1.1, Part 1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
60  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
61    <organization abbrev="Day Software">Day Software</organization>
62    <address>
63      <postal>
64        <street>23 Corporate Plaza DR, Suite 280</street>
65        <city>Newport Beach</city>
66        <region>CA</region>
67        <code>92660</code>
68        <country>USA</country>
69      </postal>
70      <phone>+1-949-706-5300</phone>
71      <facsimile>+1-949-706-5305</facsimile>
72      <email></email>
73      <uri></uri>
74    </address>
75  </author>
77  <author initials="J." surname="Gettys" fullname="Jim Gettys">
78    <organization>One Laptop per Child</organization>
79    <address>
80      <postal>
81        <street>21 Oak Knoll Road</street>
82        <city>Carlisle</city>
83        <region>MA</region>
84        <code>01741</code>
85        <country>USA</country>
86      </postal>
87      <email></email>
88      <uri></uri>
89    </address>
90  </author>
92  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
93    <organization abbrev="HP">Hewlett-Packard Company</organization>
94    <address>
95      <postal>
96        <street>HP Labs, Large Scale Systems Group</street>
97        <street>1501 Page Mill Road, MS 1177</street>
98        <city>Palo Alto</city>
99        <region>CA</region>
100        <code>94304</code>
101        <country>USA</country>
102      </postal>
103      <email></email>
104    </address>
105  </author>
107  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
108    <organization abbrev="Microsoft">Microsoft Corporation</organization>
109    <address>
110      <postal>
111        <street>1 Microsoft Way</street>
112        <city>Redmond</city>
113        <region>WA</region>
114        <code>98052</code>
115        <country>USA</country>
116      </postal>
117      <email></email>
118    </address>
119  </author>
121  <author initials="L." surname="Masinter" fullname="Larry Masinter">
122    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
123    <address>
124      <postal>
125        <street>345 Park Ave</street>
126        <city>San Jose</city>
127        <region>CA</region>
128        <code>95110</code>
129        <country>USA</country>
130      </postal>
131      <email></email>
132      <uri></uri>
133    </address>
134  </author>
136  <author initials="P." surname="Leach" fullname="Paul J. Leach">
137    <organization abbrev="Microsoft">Microsoft Corporation</organization>
138    <address>
139      <postal>
140        <street>1 Microsoft Way</street>
141        <city>Redmond</city>
142        <region>WA</region>
143        <code>98052</code>
144      </postal>
145      <email></email>
146    </address>
147  </author>
149  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
150    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
151    <address>
152      <postal>
153        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
154        <street>The Stata Center, Building 32</street>
155        <street>32 Vassar Street</street>
156        <city>Cambridge</city>
157        <region>MA</region>
158        <code>02139</code>
159        <country>USA</country>
160      </postal>
161      <email></email>
162      <uri></uri>
163    </address>
164  </author>
166  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
167    <organization abbrev="W3C">World Wide Web Consortium</organization>
168    <address>
169      <postal>
170        <street>W3C / ERCIM</street>
171        <street>2004, rte des Lucioles</street>
172        <city>Sophia-Antipolis</city>
173        <region>AM</region>
174        <code>06902</code>
175        <country>France</country>
176      </postal>
177      <email></email>
178      <uri></uri>
179    </address>
180  </author>
182  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
183    <organization abbrev="greenbytes">greenbytes GmbH</organization>
184    <address>
185      <postal>
186        <street>Hafenweg 16</street>
187        <city>Muenster</city><region>NW</region><code>48155</code>
188        <country>Germany</country>
189      </postal>
190      <phone>+49 251 2807760</phone>   
191      <facsimile>+49 251 2807761</facsimile>   
192      <email></email>       
193      <uri></uri>     
194    </address>
195  </author>
197  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
201   The Hypertext Transfer Protocol (HTTP) is an application-level
202   protocol for distributed, collaborative, hypermedia information
203   systems. HTTP has been in use by the World Wide Web global information
204   initiative since 1990. This document is Part 1 of the seven-part specification
205   that defines the protocol referred to as "HTTP/1.1" and, taken together,
206   obsoletes RFC 2616.  Part 1 provides an overview of HTTP and
207   its associated terminology, defines the "http" and "https" Uniform
208   Resource Identifier (URI) schemes, defines the generic message syntax
209   and parsing requirements for HTTP message frames, and describes
210   general security concerns for implementations.
214<note title="Editorial Note (To be removed by RFC Editor)">
215  <t>
216    Discussion of this draft should take place on the HTTPBIS working group
217    mailing list ( The current issues list is
218    at <eref target=""/>
219    and related documents (including fancy diffs) can be found at
220    <eref target=""/>.
221  </t>
222  <t>
223    The changes in this draft are summarized in <xref target="changes.since.05"/>.
224  </t>
228<section title="Introduction" anchor="introduction">
230   The Hypertext Transfer Protocol (HTTP) is an application-level
231   request/response protocol that uses extensible semantics and MIME-like
232   message payloads for flexible interaction with network-based hypermedia
233   information systems. HTTP relies upon the Uniform Resource Identifier (URI)
234   standard <xref target="RFC3986"/> to indicate resource targets for
235   interaction and to identify other resources.
236   Messages are passed in a format similar to that used by Internet mail
237   <xref target="RFC5322"/> and the Multipurpose Internet Mail Extensions
238   (MIME) <xref target="RFC2045"/> (see &diff2045entity; for the differences
239   between HTTP and MIME messages).
242   HTTP is also designed for use as a generic protocol for translating
243   communication to and from other Internet information systems, such as
244   USENET news services via NNTP <xref target="RFC3977"/>,
245   file services via FTP <xref target="RFC959"/>,
246   Gopher <xref target="RFC1436"/>, and WAIS <xref target="WAIS"/>.
247   HTTP proxies and gateways provide access to alternative information
248   services by translating their diverse protocols into a hypermedia
249   format that can be viewed and manipulated by clients in the same way
250   as HTTP services.
253   This document is Part 1 of the seven-part specification of HTTP,
254   defining the protocol referred to as "HTTP/1.1" and obsoleting
255   <xref target="RFC2616"/>.
256   Part 1 defines how clients determine when to use HTTP, the URI schemes
257   specific to HTTP-based resources, overall network operation with
258   transport protocol connection management, and HTTP message framing.
259   Our goal is to define all of the mechanisms necessary for HTTP message
260   handling that are independent of message semantics, thereby defining the
261   complete set of requirements for an HTTP message relay or generic
262   message parser.
265<section title="Requirements" anchor="intro.requirements">
267   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
268   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
269   document are to be interpreted as described in <xref target="RFC2119"/>.
272   An implementation is not compliant if it fails to satisfy one or more
273   of the &MUST; or &REQUIRED; level requirements for the protocols it
274   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
275   level and all the &SHOULD; level requirements for its protocols is said
276   to be "unconditionally compliant"; one that satisfies all the &MUST;
277   level requirements but not all the &SHOULD; level requirements for its
278   protocols is said to be "conditionally compliant."
282<section title="Overall Operation" anchor="intro.overall.operation">
284   HTTP is a request/response protocol. A client sends a
285   request to the server in the form of a request method, URI, and
286   protocol version, followed by a MIME-like message containing request
287   modifiers, client information, and possible body content over a
288   connection with a server. The server responds with a status line,
289   including the message's protocol version and a success or error code,
290   followed by a MIME-like message containing server information, entity
291   metainformation, and possible entity-body content. The relationship
292   between HTTP and MIME is described in &diff2045entity;.
295   Most HTTP communication is initiated by a user agent and consists of
296   a request to be applied to a resource on some origin server. In the
297   simplest case, this may be accomplished via a single connection (v)
298   between the user agent (UA) and the origin server (O).
300<figure><artwork type="drawing">
301       request chain ------------------------&gt;
302    UA -------------------v------------------- O
303       &lt;----------------------- response chain
306   A more complicated situation occurs when one or more intermediaries
307   are present in the request/response chain. There are three common
308   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
309   forwarding agent, receiving requests for a URI in its absolute form,
310   rewriting all or part of the message, and forwarding the reformatted
311   request toward the server identified by the URI. A gateway is a
312   receiving agent, acting as a layer above some other server(s) and, if
313   necessary, translating the requests to the underlying server's
314   protocol. A tunnel acts as a relay point between two connections
315   without changing the messages; tunnels are used when the
316   communication needs to pass through an intermediary (such as a
317   firewall) even when the intermediary cannot understand the contents
318   of the messages.
320<figure><artwork type="drawing">
321       request chain --------------------------------------&gt;
322    UA -----v----- A -----v----- B -----v----- C -----v----- O
323       &lt;------------------------------------- response chain
326   The figure above shows three intermediaries (A, B, and C) between the
327   user agent and origin server. A request or response message that
328   travels the whole chain will pass through four separate connections.
329   This distinction is important because some HTTP communication options
330   may apply only to the connection with the nearest, non-tunnel
331   neighbor, only to the end-points of the chain, or to all connections
332   along the chain. Although the diagram is linear, each participant may
333   be engaged in multiple, simultaneous communications. For example, B
334   may be receiving requests from many clients other than A, and/or
335   forwarding requests to servers other than C, at the same time that it
336   is handling A's request.
339   Any party to the communication which is not acting as a tunnel may
340   employ an internal cache for handling requests. The effect of a cache
341   is that the request/response chain is shortened if one of the
342   participants along the chain has a cached response applicable to that
343   request. The following illustrates the resulting chain if B has a
344   cached copy of an earlier response from O (via C) for a request which
345   has not been cached by UA or A.
347<figure><artwork type="drawing">
348          request chain ----------&gt;
349       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
350          &lt;--------- response chain
353   Not all responses are usefully cacheable, and some requests may
354   contain modifiers which place special requirements on cache behavior.
355   HTTP requirements for cache behavior and cacheable responses are
356   defined in &caching;.
359   In fact, there are a wide variety of architectures and configurations
360   of caches and proxies currently being experimented with or deployed
361   across the World Wide Web. These systems include national hierarchies
362   of proxy caches to save transoceanic bandwidth, systems that
363   broadcast or multicast cache entries, organizations that distribute
364   subsets of cached data via CD-ROM, and so on. HTTP systems are used
365   in corporate intranets over high-bandwidth links, and for access via
366   PDAs with low-power radio links and intermittent connectivity. The
367   goal of HTTP/1.1 is to support the wide diversity of configurations
368   already deployed while introducing protocol constructs that meet the
369   needs of those who build web applications that require high
370   reliability and, failing that, at least reliable indications of
371   failure.
374   HTTP communication usually takes place over TCP/IP connections. The
375   default port is TCP 80 (<eref target=""/>), but other ports can be used. This does
376   not preclude HTTP from being implemented on top of any other protocol
377   on the Internet, or on other networks. HTTP only presumes a reliable
378   transport; any protocol that provides such guarantees can be used;
379   the mapping of the HTTP/1.1 request and response structures onto the
380   transport data units of the protocol in question is outside the scope
381   of this specification.
384   In HTTP/1.0, most implementations used a new connection for each
385   request/response exchange. In HTTP/1.1, a connection may be used for
386   one or more request/response exchanges, although connections may be
387   closed for a variety of reasons (see <xref target="persistent.connections"/>).
392<section title="Notational Conventions and Generic Grammar" anchor="notation">
394<section title="ABNF Extension: #rule" anchor="notation.abnf">
395  <t>
396    One extension to the ABNF rules of <xref target="RFC5234"/> is used to
397    improve readability.
398  </t>
399  <t>
400    A construct "#" is defined, similar to "*", for defining lists of
401    elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least
402    &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas
403    (",") and &OPTIONAL; linear white space (OWS). This makes the usual
404    form of lists very easy; a rule such as
405    <figure><artwork type="example">
406 ( *<x:ref>OWS</x:ref> element *( *<x:ref>OWS</x:ref> "," *<x:ref>OWS</x:ref> element ))</artwork></figure>
407  </t>
408  <t>
409    can be shown as
410    <figure><artwork type="example">
411 1#element</artwork></figure>
412  </t>
413  <t>
414    Wherever this construct is used, null elements are allowed, but do
415    not contribute to the count of elements present. That is,
416    "(element), , (element) " is permitted, but counts as only two
417    elements. Therefore, where at least one element is required, at
418    least one non-null element &MUST; be present. Default values are 0
419    and infinity so that "#element" allows any number, including zero;
420    "1#element" requires at least one; and "1#2element" allows one or
421    two.
422  </t>
423  <t>
424    <cref anchor="abnf.list">
425      At a later point of time, we may want to add an appendix containing
426      the whole ABNF, with the list rules expanded to strict RFC 5234
427      notation.
428    </cref>
429  </t>
432<section title="Basic Rules" anchor="basic.rules">
433<t anchor="core.rules">
434  <x:anchor-alias value="ALPHA"/>
435  <x:anchor-alias value="CHAR"/>
436  <x:anchor-alias value="CTL"/>
437  <x:anchor-alias value="CR"/>
438  <x:anchor-alias value="CRLF"/>
439  <x:anchor-alias value="DIGIT"/>
440  <x:anchor-alias value="DQUOTE"/>
441  <x:anchor-alias value="HEXDIG"/>
442  <x:anchor-alias value="HTAB"/>
443  <x:anchor-alias value="LF"/>
444  <x:anchor-alias value="OCTET"/>
445  <x:anchor-alias value="SP"/>
446  <x:anchor-alias value="WSP"/>
447   This specification uses the Augmented Backus-Naur Form (ABNF) notation
448   of <xref target="RFC5234"/>.  The following core rules are included by
449   reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
450   ALPHA (letters), CHAR (any <xref target="USASCII"/> character,
451   excluding NUL), CR (carriage return), CRLF (CR LF), CTL (controls),
452   DIGIT (decimal 0-9), DQUOTE (double quote),
453   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab),
454   LF (line feed), OCTET (any 8-bit sequence of data), SP (space)
455   and WSP (white space).
457<t anchor="rule.CRLF">
458  <x:anchor-alias value="CRLF"/>
459   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
460   protocol elements except the entity-body (see <xref target="tolerant.applications"/> for
461   tolerant applications). The end-of-line marker within an entity-body
462   is defined by its associated media type, as described in &media-types;.
464<t anchor="rule.LWS">
465   All linear white space (LWS) in header field-values has the same semantics as SP. A
466   recipient &MAY; replace any such linear white space with a single SP before
467   interpreting the field value or forwarding the message downstream.
470   Historically, HTTP/1.1 header field values allow linear white space folding across
471   multiple lines. However, this specification deprecates its use; senders &MUST-NOT;
472   produce messages that include LWS folding (i.e., use the obs-fold rule), except
473   within the message/http media type (<xref target=""/>).
474   Receivers &SHOULD; still parse folded linear white space.
477   This specification uses three rules to denote the use of linear white space;
478   BWS ("Bad" White Space), OWS (Optional White Space), and RWS (Required White Space).
481   "Bad" white space is allowed by the BNF, but senders &SHOULD-NOT; produce it in messages.
482   Receivers &MUST; accept it in incoming messages.
485   Required white space is used when at least one linear white space character
486   is required to separate field tokens. In all such cases, a single SP character
487   &SHOULD; be used.
489<t anchor="rule.whitespace">
490  <x:anchor-alias value="BWS"/>
491  <x:anchor-alias value="OWS"/>
492  <x:anchor-alias value="RWS"/>
493  <x:anchor-alias value="obs-fold"/>
495<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="OWS"/><iref primary="true" item="Grammar" subitem="RWS"/><iref primary="true" item="Grammar" subitem="BWS"/>
496  <x:ref>OWS</x:ref>            = *( [ obs-fold ] <x:ref>WSP</x:ref> )
497                 ; "optional" white space
498  <x:ref>RWS</x:ref>            = 1*( [ obs-fold ] <x:ref>WSP</x:ref> )
499                 ; "required" white space
500  <x:ref>BWS</x:ref>            = <x:ref>OWS</x:ref>
501                 ; "bad" white space
502  <x:ref>obs-fold</x:ref>       = <x:ref>CRLF</x:ref>
504<t anchor="rule.TEXT">
505  <x:anchor-alias value="TEXT"/>
506   The TEXT rule is only used for descriptive field contents and values
507   that are not intended to be interpreted by the message parser. Words
508   of *TEXT &MAY; contain characters from character sets other than ISO-8859-1
509   <xref target="ISO-8859-1"/> only when encoded according to the rules of
510   <xref target="RFC2047"/>.
512<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TEXT"/>
513  <x:ref>TEXT</x:ref>           = %x20-7E / %x80-FF / <x:ref>OWS</x:ref>
514                 ; any <x:ref>OCTET</x:ref> except <x:ref>CTL</x:ref>s, but including <x:ref>OWS</x:ref>
517   A CRLF is allowed in the definition of TEXT only as part of a header
518   field continuation. It is expected that the folding LWS will be
519   replaced with a single SP before interpretation of the TEXT value.
521<t anchor="rule.token.separators">
522  <x:anchor-alias value="tchar"/>
523  <x:anchor-alias value="token"/>
524   Many HTTP/1.1 header field values consist of words separated by LWS
525   or special characters. These special characters &MUST; be in a quoted
526   string to be used within a parameter value (as defined in
527   <xref target="transfer.codings"/>).
529<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="token"/><iref primary="true" item="Grammar" subitem="tchar"/>
530  <x:ref>tchar</x:ref>          = "!" / "#" / "$" / "%" / "&amp;" / "'" / "*"
531                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
532                 / <x:ref>DIGIT</x:ref> / <x:ref>ALPHA</x:ref>
534  <x:ref>token</x:ref>          = 1*<x:ref>tchar</x:ref>
536<t anchor="rule.comment">
537  <x:anchor-alias value="comment"/>
538  <x:anchor-alias value="ctext"/>
539   Comments can be included in some HTTP header fields by surrounding
540   the comment text with parentheses. Comments are only allowed in
541   fields containing "comment" as part of their field value definition.
542   In all other fields, parentheses are considered part of the field
543   value.
545<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="comment"/><iref primary="true" item="Grammar" subitem="ctext"/>
546  <x:ref>comment</x:ref>        = "(" *( <x:ref>ctext</x:ref> / <x:ref>quoted-pair</x:ref> / <x:ref>comment</x:ref> ) ")"
547  <x:ref>ctext</x:ref>          = &lt;any <x:ref>TEXT</x:ref> excluding "(" and ")"&gt;
549<t anchor="rule.quoted-string">
550  <x:anchor-alias value="quoted-string"/>
551  <x:anchor-alias value="qdtext"/>
552   A string of text is parsed as a single word if it is quoted using
553   double-quote marks.
555<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-string"/><iref primary="true" item="Grammar" subitem="qdtext"/>
556  <x:ref>quoted-string</x:ref>  = <x:ref>DQUOTE</x:ref> *(<x:ref>qdtext</x:ref> / <x:ref>quoted-pair</x:ref> ) <x:ref>DQUOTE</x:ref>
557  <x:ref>qdtext</x:ref>         = &lt;any <x:ref>TEXT</x:ref> excluding <x:ref>DQUOTE</x:ref> and "\">
559<t anchor="rule.quoted-pair">
560  <x:anchor-alias value="quoted-pair"/>
561  <x:anchor-alias value="quoted-text"/>
562   The backslash character ("\") &MAY; be used as a single-character
563   quoting mechanism only within quoted-string and comment constructs.
565<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="quoted-text"/><iref primary="true" item="Grammar" subitem="quoted-pair"/>
566  <x:ref>quoted-text</x:ref>    = %x01-09 /
567                   %x0B-0C /
568                   %x0E-FF ; Characters excluding NUL, <x:ref>CR</x:ref> and <x:ref>LF</x:ref>
569  <x:ref>quoted-pair</x:ref>    = "\" <x:ref>quoted-text</x:ref>
573<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
574  <x:anchor-alias value="request-header"/>
575  <x:anchor-alias value="response-header"/>
576  <x:anchor-alias value="accept-params"/>
577  <x:anchor-alias value="entity-body"/>
578  <x:anchor-alias value="entity-header"/>
579  <x:anchor-alias value="Cache-Control"/>
580  <x:anchor-alias value="Pragma"/>
581  <x:anchor-alias value="Warning"/>
583  The ABNF rules below are defined in other parts:
585<figure><!-- Part2--><artwork type="abnf2616">
586  <x:ref>request-header</x:ref>  = &lt;request-header, defined in &request-header-fields;&gt;
587  <x:ref>response-header</x:ref> = &lt;response-header, defined in &response-header-fields;&gt;
589<figure><!-- Part3--><artwork type="abnf2616">
590  <x:ref>accept-params</x:ref>   = &lt;accept-params, defined in &header-accept;&gt;
591  <x:ref>entity-body</x:ref>     = &lt;entity-body, defined in &entity-body;&gt;
592  <x:ref>entity-header</x:ref>   = &lt;entity-header, defined in &entity-header-fields;&gt;
594<figure><!-- Part6--><artwork type="abnf2616">
595  <x:ref>Cache-Control</x:ref>   = &lt;Cache-Control, defined in &header-pragma;&gt;
596  <x:ref>Pragma</x:ref>          = &lt;Pragma, defined in &header-pragma;&gt;
597  <x:ref>Warning</x:ref>         = &lt;Warning, defined in &header-warning;&gt;
603<section title="Protocol Parameters" anchor="protocol.parameters">
605<section title="HTTP Version" anchor="http.version">
606  <x:anchor-alias value="HTTP-Version"/>
607  <x:anchor-alias value="HTTP-Prot-Name"/>
609   HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions
610   of the protocol. The protocol versioning policy is intended to allow
611   the sender to indicate the format of a message and its capacity for
612   understanding further HTTP communication, rather than the features
613   obtained via that communication. No change is made to the version
614   number for the addition of message components which do not affect
615   communication behavior or which only add to extensible field values.
616   The &lt;minor&gt; number is incremented when the changes made to the
617   protocol add features which do not change the general message parsing
618   algorithm, but which may add to the message semantics and imply
619   additional capabilities of the sender. The &lt;major&gt; number is
620   incremented when the format of a message within the protocol is
621   changed. See <xref target="RFC2145"/> for a fuller explanation.
624   The version of an HTTP message is indicated by an HTTP-Version field
625   in the first line of the message. HTTP-Version is case-sensitive.
627<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-Version"/><iref primary="true" item="Grammar" subitem="HTTP-Prot-Name"/>
628  <x:ref>HTTP-Version</x:ref>   = <x:ref>HTTP-Prot-Name</x:ref> "/" 1*<x:ref>DIGIT</x:ref> "." 1*<x:ref>DIGIT</x:ref>
629  <x:ref>HTTP-Prot-Name</x:ref> = <x:abnf-char-sequence>"HTTP"</x:abnf-char-sequence> ; "HTTP", case-sensitive
632   Note that the major and minor numbers &MUST; be treated as separate
633   integers and that each &MAY; be incremented higher than a single digit.
634   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
635   lower than HTTP/12.3. Leading zeros &MUST; be ignored by recipients and
636   &MUST-NOT; be sent.
639   An application that sends a request or response message that includes
640   HTTP-Version of "HTTP/1.1" &MUST; be at least conditionally compliant
641   with this specification. Applications that are at least conditionally
642   compliant with this specification &SHOULD; use an HTTP-Version of
643   "HTTP/1.1" in their messages, and &MUST; do so for any message that is
644   not compatible with HTTP/1.0. For more details on when to send
645   specific HTTP-Version values, see <xref target="RFC2145"/>.
648   The HTTP version of an application is the highest HTTP version for
649   which the application is at least conditionally compliant.
652   Proxy and gateway applications need to be careful when forwarding
653   messages in protocol versions different from that of the application.
654   Since the protocol version indicates the protocol capability of the
655   sender, a proxy/gateway &MUST-NOT; send a message with a version
656   indicator which is greater than its actual version. If a higher
657   version request is received, the proxy/gateway &MUST; either downgrade
658   the request version, or respond with an error, or switch to tunnel
659   behavior.
662   Due to interoperability problems with HTTP/1.0 proxies discovered
663   since the publication of <xref target="RFC2068"/>, caching proxies &MUST;, gateways
664   &MAY;, and tunnels &MUST-NOT; upgrade the request to the highest version
665   they support. The proxy/gateway's response to that request &MUST; be in
666   the same major version as the request.
669  <list>
670    <t>
671      <x:h>Note:</x:h> Converting between versions of HTTP may involve modification
672      of header fields required or forbidden by the versions involved.
673    </t>
674  </list>
678<section title="Uniform Resource Identifiers" anchor="uri">
680   Uniform Resource Identifiers (URIs) <xref target="RFC3986"/> are used in HTTP
681   to indicate the target of a request and to identify additional resources related
682   to that resource, the request, or the response. Each protocol element in HTTP
683   that allows a URI reference will indicate in its ABNF whether the element allows
684   only a URI in absolute form, any relative reference, or some limited subset of
685   the URI-reference grammar. Unless otherwise indicated, relative URI references
686   are to be parsed relative to the URI corresponding to the request target
687   (the base URI).
689  <x:anchor-alias value="URI-reference"/>
690  <x:anchor-alias value="absolute-URI"/>
691  <x:anchor-alias value="authority"/>
692  <x:anchor-alias value="fragment"/>
693  <x:anchor-alias value="path-abempty"/>
694  <x:anchor-alias value="path-absolute"/>
695  <x:anchor-alias value="port"/>
696  <x:anchor-alias value="query"/>
697  <x:anchor-alias value="relativeURI"/>
698  <x:anchor-alias value="relative-part"/>
699  <x:anchor-alias value="uri-host"/>
701   This specification adopts the definitions of "URI-reference", "absolute-URI", "fragment", "port",
702   "host", "path-abempty", "path-absolute", "query", and "authority" from <xref target="RFC3986"/>:
704<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="URI-reference"/><iref primary="true" item="Grammar" subitem="absolute-URI"/><iref primary="true" item="Grammar" subitem="authority"/><iref primary="true" item="Grammar" subitem="path-absolute"/><iref primary="true" item="Grammar" subitem="port"/><iref primary="true" item="Grammar" subitem="query"/><iref primary="true" item="Grammar" subitem="uri-host"/><iref primary="true" item="Grammar" subitem="relativeURI"/><iref primary="true" item="Grammar" subitem="relative-part"/>
705  <x:ref>absolute-URI</x:ref>   = &lt;absolute-URI, defined in <xref target="RFC3986" x:fmt="," x:sec="4.3"/>>
706  <x:ref>authority</x:ref>     = &lt;authority, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2"/>>
707  <x:ref>fragment</x:ref>      = &lt;fragment, defined in <xref target="RFC3986" x:fmt="," x:sec="3.5"/>>
708  <x:ref>path-abempty</x:ref>  = &lt;path-abempty, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>>
709  <x:ref>path-absolute</x:ref> = &lt;path-absolute, defined in <xref target="RFC3986" x:fmt="," x:sec="3.3"/>>
710  <x:ref>port</x:ref>          = &lt;port, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.3"/>>
711  <x:ref>query</x:ref>         = &lt;query, defined in <xref target="RFC3986" x:fmt="," x:sec="3.4"/>>
712  <x:ref>uri-host</x:ref>      = &lt;host, defined in <xref target="RFC3986" x:fmt="," x:sec="3.2.2"/>>
714  <x:ref>relative-part</x:ref> = &lt;relative-part, defined in <xref target="RFC3986" x:fmt="," x:sec="4.2"/>>
715  <x:ref>relativeURI</x:ref>   = <x:ref>relative-part</x:ref> [ "?" <x:ref>query</x:ref> ]
718   HTTP does not place an a priori limit on the length of
719   a URI. Servers &MUST; be able to handle the URI of any resource they
720   serve, and &SHOULD; be able to handle URIs of unbounded length if they
721   provide GET-based forms that could generate such URIs. A server
722   &SHOULD; return 414 (Request-URI Too Long) status if a URI is longer
723   than the server can handle (see &status-414;).
726  <list>
727    <t>
728      <x:h>Note:</x:h> Servers ought to be cautious about depending on URI lengths
729      above 255 bytes, because some older client or proxy
730      implementations might not properly support these lengths.
731    </t>
732  </list>
735<section title="http URI scheme" anchor="http.uri">
736  <x:anchor-alias value="http-URI"/>
737  <iref item="http URI scheme" primary="true"/>
738  <iref item="URI scheme" subitem="http" primary="true"/>
740   The "http" scheme is used to locate network resources via the HTTP
741   protocol. This section defines the syntax and semantics for identifiers
742   using the http or https URI schemes.
744<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="http-URI"/>
745  <x:ref>http-URI</x:ref> = "http:" "//" <x:ref>authority</x:ref> <x:ref>path-abempty</x:ref> [ "?" <x:ref>query</x:ref> ]
748   If the port is empty or not given, port 80 is assumed. The semantics
749   are that the identified resource is located at the server listening
750   for TCP connections on that port of that host, and the Request-URI
751   for the resource is path-absolute (<xref target="request-uri"/>). The use of IP addresses
752   in URLs &SHOULD; be avoided whenever possible (see <xref target="RFC1900"/>). If
753   the path-absolute is not present in the URL, it &MUST; be given as "/" when
754   used as a Request-URI for a resource (<xref target="request-uri"/>). If a proxy
755   receives a host name which is not a fully qualified domain name, it
756   &MAY; add its domain to the host name it received. If a proxy receives
757   a fully qualified domain name, the proxy &MUST-NOT; change the host
758   name.
761  <iref item="https URI scheme"/>
762  <iref item="URI scheme" subitem="https"/>
763  <x:h>Note:</x:h> the "https" scheme is defined in <xref target="RFC2818"/>.
767<section title="URI Comparison" anchor="uri.comparison">
769   When comparing two URIs to decide if they match or not, a client
770   &SHOULD; use a case-sensitive octet-by-octet comparison of the entire
771   URIs, with these exceptions:
772  <list style="symbols">
773    <t>A port that is empty or not given is equivalent to the default
774        port for that URI-reference;</t>
775    <t>Comparisons of host names &MUST; be case-insensitive;</t>
776    <t>Comparisons of scheme names &MUST; be case-insensitive;</t>
777    <t>An empty path-absolute is equivalent to an path-absolute of "/".</t>
778  </list>
781   Characters other than those in the "reserved" set (see
782   <xref target="RFC3986" x:fmt="," x:sec="2.2"/>) are equivalent to their
783   ""%" <x:ref>HEXDIG</x:ref> <x:ref>HEXDIG</x:ref>" encoding.
786   For example, the following three URIs are equivalent:
788<figure><artwork type="example">
796<section title="Date/Time Formats" anchor="date.time.formats">
797<section title="Full Date" anchor="">
798  <x:anchor-alias value="HTTP-date"/>
799  <x:anchor-alias value="obsolete-date"/>
800  <x:anchor-alias value="rfc1123-date"/>
801  <x:anchor-alias value="rfc850-date"/>
802  <x:anchor-alias value="asctime-date"/>
803  <x:anchor-alias value="date1"/>
804  <x:anchor-alias value="date2"/>
805  <x:anchor-alias value="date3"/>
806  <x:anchor-alias value="rfc1123-date"/>
807  <x:anchor-alias value="time"/>
808  <x:anchor-alias value="wkday"/>
809  <x:anchor-alias value="weekday"/>
810  <x:anchor-alias value="month"/>
812   HTTP applications have historically allowed three different formats
813   for the representation of date/time stamps:
815<figure><artwork type="example">
816   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
817   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
818   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
821   The first format is preferred as an Internet standard and represents
822   a fixed-length subset of that defined by <xref target="RFC1123"/> (an update to
823   <xref target="RFC822"/>). The other formats are described here only for
824   compatibility with obsolete implementations.
825   HTTP/1.1 clients and servers that parse the date value &MUST; accept
826   all three formats (for compatibility with HTTP/1.0), though they &MUST;
827   only generate the RFC 1123 format for representing HTTP-date values
828   in header fields. See <xref target="tolerant.applications"/> for further information.
831      <x:h>Note:</x:h> Recipients of date values are encouraged to be robust in
832      accepting date values that may have been sent by non-HTTP
833      applications, as is sometimes the case when retrieving or posting
834      messages via proxies/gateways to SMTP or NNTP.
837   All HTTP date/time stamps &MUST; be represented in Greenwich Mean Time
838   (GMT), without exception. For the purposes of HTTP, GMT is exactly
839   equal to UTC (Coordinated Universal Time). This is indicated in the
840   first two formats by the inclusion of "GMT" as the three-letter
841   abbreviation for time zone, and &MUST; be assumed when reading the
842   asctime format. HTTP-date is case sensitive and &MUST-NOT; include
843   additional LWS beyond that specifically included as SP in the
844   grammar.
846<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="obsolete-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"/>
847  <x:ref>HTTP-date</x:ref>    = <x:ref>rfc1123-date</x:ref> / <x:ref>obsolete-date</x:ref>
848  <x:ref>obsolete-date</x:ref> = <x:ref>rfc850-date</x:ref> / <x:ref>asctime-date</x:ref>
849  <x:ref>rfc1123-date</x:ref> = <x:ref>wkday</x:ref> "," <x:ref>SP</x:ref> date1 <x:ref>SP</x:ref> time <x:ref>SP</x:ref> GMT
850  <x:ref>rfc850-date</x:ref>  = <x:ref>weekday</x:ref> "," <x:ref>SP</x:ref> date2 <x:ref>SP</x:ref> time <x:ref>SP</x:ref> GMT
851  <x:ref>asctime-date</x:ref> = <x:ref>wkday</x:ref> <x:ref>SP</x:ref> <x:ref>date3</x:ref> <x:ref>SP</x:ref> <x:ref>time</x:ref> <x:ref>SP</x:ref> 4<x:ref>DIGIT</x:ref>
852  <x:ref>date1</x:ref>        = 2<x:ref>DIGIT</x:ref> <x:ref>SP</x:ref> <x:ref>month</x:ref> <x:ref>SP</x:ref> 4<x:ref>DIGIT</x:ref>
853                 ; day month year (e.g., 02 Jun 1982)
854  <x:ref>date2</x:ref>        = 2<x:ref>DIGIT</x:ref> "-" <x:ref>month</x:ref> "-" 2<x:ref>DIGIT</x:ref>
855                 ; day-month-year (e.g., 02-Jun-82)
856  <x:ref>date3</x:ref>        = <x:ref>month</x:ref> <x:ref>SP</x:ref> ( 2<x:ref>DIGIT</x:ref> / ( <x:ref>SP</x:ref> 1<x:ref>DIGIT</x:ref> ))
857                 ; month day (e.g., Jun  2)
858  <x:ref>time</x:ref>         = 2<x:ref>DIGIT</x:ref> ":" 2<x:ref>DIGIT</x:ref> ":" 2<x:ref>DIGIT</x:ref>
859                 ; 00:00:00 - 23:59:59
860  <x:ref>wkday</x:ref>        = s-Mon / s-Tue / s-Wed
861               / s-Thu / s-Fri / s-Sat / s-Sun
862  <x:ref>weekday</x:ref>      = l-Mon / l-Tue / l-Wed
863               / l-Thu / l-Fri / l-Sat / l-Sun
864  <x:ref>month</x:ref>        = s-Jan / s-Feb / s-Mar / s-Apr
865               / s-May / s-Jun / s-Jul / s-Aug
866               / s-Sep / s-Oct / s-Nov / s-Dec
868  GMT   = <x:abnf-char-sequence>"GMT"</x:abnf-char-sequence> ; "GMT", case-sensitive
870  s-Mon = <x:abnf-char-sequence>"Mon"</x:abnf-char-sequence> ; "Mon", case-sensitive
871  s-Tue = <x:abnf-char-sequence>"Tue"</x:abnf-char-sequence> ; "Tue", case-sensitive
872  s-Wed = <x:abnf-char-sequence>"Wed"</x:abnf-char-sequence> ; "Wed", case-sensitive
873  s-Thu = <x:abnf-char-sequence>"Thu"</x:abnf-char-sequence> ; "Thu", case-sensitive
874  s-Fri = <x:abnf-char-sequence>"Fri"</x:abnf-char-sequence> ; "Fri", case-sensitive
875  s-Sat = <x:abnf-char-sequence>"Sat"</x:abnf-char-sequence> ; "Sat", case-sensitive
876  s-Sun = <x:abnf-char-sequence>"Sun"</x:abnf-char-sequence> ; "Sun", case-sensitive
878  l-Mon = <x:abnf-char-sequence>"Monday"</x:abnf-char-sequence>          ; "Monday", case-sensitive
879  l-Tue = <x:abnf-char-sequence>"Tuesday"</x:abnf-char-sequence>       ; "Tuesday", case-sensitive
880  l-Wed = <x:abnf-char-sequence>"Wednesday"</x:abnf-char-sequence> ; "Wednesday", case-sensitive
881  l-Thu = <x:abnf-char-sequence>"Thursday"</x:abnf-char-sequence>    ; "Thursday", case-sensitive
882  l-Fri = <x:abnf-char-sequence>"Friday"</x:abnf-char-sequence>          ; "Friday", case-sensitive
883  l-Sat = <x:abnf-char-sequence>"Saturday"</x:abnf-char-sequence>    ; "Saturday", case-sensitive
884  l-Sun = <x:abnf-char-sequence>"Sunday"</x:abnf-char-sequence>          ; "Sunday", case-sensitive
886  s-Jan = <x:abnf-char-sequence>"Jan"</x:abnf-char-sequence> ; "Jan", case-sensitive
887  s-Feb = <x:abnf-char-sequence>"Feb"</x:abnf-char-sequence> ; "Feb", case-sensitive
888  s-Mar = <x:abnf-char-sequence>"Mar"</x:abnf-char-sequence> ; "Mar", case-sensitive
889  s-Apr = <x:abnf-char-sequence>"Apr"</x:abnf-char-sequence> ; "Apr", case-sensitive
890  s-May = <x:abnf-char-sequence>"May"</x:abnf-char-sequence> ; "May", case-sensitive
891  s-Jun = <x:abnf-char-sequence>"Jun"</x:abnf-char-sequence> ; "Jun", case-sensitive
892  s-Jul = <x:abnf-char-sequence>"Jul"</x:abnf-char-sequence> ; "Jul", case-sensitive
893  s-Aug = <x:abnf-char-sequence>"Aug"</x:abnf-char-sequence> ; "Aug", case-sensitive
894  s-Sep = <x:abnf-char-sequence>"Sep"</x:abnf-char-sequence> ; "Sep", case-sensitive
895  s-Oct = <x:abnf-char-sequence>"Oct"</x:abnf-char-sequence> ; "Oct", case-sensitive
896  s-Nov = <x:abnf-char-sequence>"Nov"</x:abnf-char-sequence> ; "Nov", case-sensitive
897  s-Dec = <x:abnf-char-sequence>"Dec"</x:abnf-char-sequence> ; "Dec", case-sensitive
900      <x:h>Note:</x:h> HTTP requirements for the date/time stamp format apply only
901      to their usage within the protocol stream. Clients and servers are
902      not required to use these formats for user presentation, request
903      logging, etc.
908<section title="Transfer Codings" anchor="transfer.codings">
909  <x:anchor-alias value="parameter"/>
910  <x:anchor-alias value="transfer-coding"/>
911  <x:anchor-alias value="transfer-extension"/>
913   Transfer-coding values are used to indicate an encoding
914   transformation that has been, can be, or may need to be applied to an
915   entity-body in order to ensure "safe transport" through the network.
916   This differs from a content coding in that the transfer-coding is a
917   property of the message, not of the original entity.
919<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="transfer-coding"/><iref primary="true" item="Grammar" subitem="transfer-extension"/>
920  <x:ref>transfer-coding</x:ref>         = "chunked" / <x:ref>transfer-extension</x:ref>
921  <x:ref>transfer-extension</x:ref>      = <x:ref>token</x:ref> *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>parameter</x:ref> )
923<t anchor="rule.parameter">
924  <x:anchor-alias value="attribute"/>
925  <x:anchor-alias value="parameter"/>
926  <x:anchor-alias value="value"/>
927   Parameters are in  the form of attribute/value pairs.
929<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"/>
930  <x:ref>parameter</x:ref>               = <x:ref>attribute</x:ref> <x:ref>BWS</x:ref> "=" <x:ref>BWS</x:ref> <x:ref>value</x:ref>
931  <x:ref>attribute</x:ref>               = <x:ref>token</x:ref>
932  <x:ref>value</x:ref>                   = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
935   All transfer-coding values are case-insensitive. HTTP/1.1 uses
936   transfer-coding values in the TE header field (<xref target="header.te"/>) and in
937   the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
940   Whenever a transfer-coding is applied to a message-body, the set of
941   transfer-codings &MUST; include "chunked", unless the message indicates it
942   is terminated by closing the connection. When the "chunked" transfer-coding
943   is used, it &MUST; be the last transfer-coding applied to the
944   message-body. The "chunked" transfer-coding &MUST-NOT; be applied more
945   than once to a message-body. These rules allow the recipient to
946   determine the transfer-length of the message (<xref target="message.length"/>).
949   Transfer-codings are analogous to the Content-Transfer-Encoding
950   values of MIME <xref target="RFC2045"/>, which were designed to enable safe transport of
951   binary data over a 7-bit transport service. However, safe transport
952   has a different focus for an 8bit-clean transfer protocol. In HTTP,
953   the only unsafe characteristic of message-bodies is the difficulty in
954   determining the exact body length (<xref target="message.length"/>), or the desire to
955   encrypt data over a shared transport.
958   The Internet Assigned Numbers Authority (IANA) acts as a registry for
959   transfer-coding value tokens. Initially, the registry contains the
960   following tokens: "chunked" (<xref target="chunked.transfer.encoding"/>),
961   "gzip", "compress", and "deflate" (&content-codings;).
964   New transfer-coding value tokens &SHOULD; be registered in the same way
965   as new content-coding value tokens (&content-codings;).
968   A server which receives an entity-body with a transfer-coding it does
969   not understand &SHOULD; return 501 (Not Implemented), and close the
970   connection. A server &MUST-NOT; send transfer-codings to an HTTP/1.0
971   client.
974<section title="Chunked Transfer Coding" anchor="chunked.transfer.encoding">
975  <x:anchor-alias value="chunk"/>
976  <x:anchor-alias value="Chunked-Body"/>
977  <x:anchor-alias value="chunk-data"/>
978  <x:anchor-alias value="chunk-ext"/>
979  <x:anchor-alias value="chunk-ext-name"/>
980  <x:anchor-alias value="chunk-ext-val"/>
981  <x:anchor-alias value="chunk-size"/>
982  <x:anchor-alias value="last-chunk"/>
983  <x:anchor-alias value="trailer-part"/>
985   The chunked encoding modifies the body of a message in order to
986   transfer it as a series of chunks, each with its own size indicator,
987   followed by an &OPTIONAL; trailer containing entity-header fields. This
988   allows dynamically produced content to be transferred along with the
989   information necessary for the recipient to verify that it has
990   received the full message.
992<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-ext"/><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-part"/>
993  <x:ref>Chunked-Body</x:ref>   = *<x:ref>chunk</x:ref>
994                   <x:ref>last-chunk</x:ref>
995                   <x:ref>trailer-part</x:ref>
996                   <x:ref>CRLF</x:ref>
998  <x:ref>chunk</x:ref>          = <x:ref>chunk-size</x:ref> *WSP [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
999                   <x:ref>chunk-data</x:ref> <x:ref>CRLF</x:ref>
1000  <x:ref>chunk-size</x:ref>     = 1*<x:ref>HEXDIG</x:ref>
1001  <x:ref>last-chunk</x:ref>     = 1*("0") *WSP [ <x:ref>chunk-ext</x:ref> ] <x:ref>CRLF</x:ref>
1003  <x:ref>chunk-ext</x:ref>      = *( ";" *WSP <x:ref>chunk-ext-name</x:ref>
1004                      [ "=" <x:ref>chunk-ext-val</x:ref> ] *WSP )
1005  <x:ref>chunk-ext-name</x:ref> = <x:ref>token</x:ref>
1006  <x:ref>chunk-ext-val</x:ref>  = <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref>
1007  <x:ref>chunk-data</x:ref>     = 1*<x:ref>OCTET</x:ref> ; a sequence of chunk-size octets
1008  <x:ref>trailer-part</x:ref>   = *(<x:ref>entity-header</x:ref> <x:ref>CRLF</x:ref>)
1011   The chunk-size field is a string of hex digits indicating the size of
1012   the chunk-data in octets. The chunked encoding is ended by any chunk whose size is
1013   zero, followed by the trailer, which is terminated by an empty line.
1016   The trailer allows the sender to include additional HTTP header
1017   fields at the end of the message. The Trailer header field can be
1018   used to indicate which header fields are included in a trailer (see
1019   <xref target="header.trailer"/>).
1022   A server using chunked transfer-coding in a response &MUST-NOT; use the
1023   trailer for any header fields unless at least one of the following is
1024   true:
1025  <list style="numbers">
1026    <t>the request included a TE header field that indicates "trailers" is
1027     acceptable in the transfer-coding of the  response, as described in
1028     <xref target="header.te"/>; or,</t>
1030    <t>the server is the origin server for the response, the trailer
1031     fields consist entirely of optional metadata, and the recipient
1032     could use the message (in a manner acceptable to the origin server)
1033     without receiving this metadata.  In other words, the origin server
1034     is willing to accept the possibility that the trailer fields might
1035     be silently discarded along the path to the client.</t>
1036  </list>
1039   This requirement prevents an interoperability failure when the
1040   message is being received by an HTTP/1.1 (or later) proxy and
1041   forwarded to an HTTP/1.0 recipient. It avoids a situation where
1042   compliance with the protocol would have necessitated a possibly
1043   infinite buffer on the proxy.
1046   A process for decoding the "chunked" transfer-coding
1047   can be represented in pseudo-code as:
1049<figure><artwork type="code">
1050  length := 0
1051  read chunk-size, chunk-ext (if any) and CRLF
1052  while (chunk-size &gt; 0) {
1053     read chunk-data and CRLF
1054     append chunk-data to entity-body
1055     length := length + chunk-size
1056     read chunk-size and CRLF
1057  }
1058  read entity-header
1059  while (entity-header not empty) {
1060     append entity-header to existing header fields
1061     read entity-header
1062  }
1063  Content-Length := length
1064  Remove "chunked" from Transfer-Encoding
1067   All HTTP/1.1 applications &MUST; be able to receive and decode the
1068   "chunked" transfer-coding, and &MUST; ignore chunk-ext extensions
1069   they do not understand.
1074<section title="Product Tokens" anchor="product.tokens">
1075  <x:anchor-alias value="product"/>
1076  <x:anchor-alias value="product-version"/>
1078   Product tokens are used to allow communicating applications to
1079   identify themselves by software name and version. Most fields using
1080   product tokens also allow sub-products which form a significant part
1081   of the application to be listed, separated by white space. By
1082   convention, the products are listed in order of their significance
1083   for identifying the application.
1085<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
1086  <x:ref>product</x:ref>         = <x:ref>token</x:ref> ["/" <x:ref>product-version</x:ref>]
1087  <x:ref>product-version</x:ref> = <x:ref>token</x:ref>
1090   Examples:
1092<figure><artwork type="example">
1093    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
1094    Server: Apache/0.8.4
1097   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
1098   used for advertising or other non-essential information. Although any
1099   token character &MAY; appear in a product-version, this token &SHOULD;
1100   only be used for a version identifier (i.e., successive versions of
1101   the same product &SHOULD; only differ in the product-version portion of
1102   the product value).
1108<section title="HTTP Message" anchor="http.message">
1110<section title="Message Types" anchor="message.types">
1111  <x:anchor-alias value="generic-message"/>
1112  <x:anchor-alias value="HTTP-message"/>
1113  <x:anchor-alias value="start-line"/>
1115   HTTP messages consist of requests from client to server and responses
1116   from server to client.
1118<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="HTTP-message"/>
1119  <x:ref>HTTP-message</x:ref>   = <x:ref>Request</x:ref> / <x:ref>Response</x:ref>     ; HTTP/1.1 messages
1122   Request (<xref target="request"/>) and Response (<xref target="response"/>) messages use the generic
1123   message format of <xref target="RFC5322"/> for transferring entities (the payload
1124   of the message). Both types of message consist of a start-line, zero
1125   or more header fields (also known as "headers"), an empty line (i.e.,
1126   a line with nothing preceding the CRLF) indicating the end of the
1127   header fields, and possibly a message-body.
1129<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="generic-message"/><iref primary="true" item="Grammar" subitem="start-line"/>
1130  <x:ref>generic-message</x:ref> = <x:ref>start-line</x:ref>
1131                    *(<x:ref>message-header</x:ref> <x:ref>CRLF</x:ref>)
1132                    <x:ref>CRLF</x:ref>
1133                    [ <x:ref>message-body</x:ref> ]
1134  <x:ref>start-line</x:ref>      = <x:ref>Request-Line</x:ref> / <x:ref>Status-Line</x:ref>
1137   In the interest of robustness, servers &SHOULD; ignore any empty
1138   line(s) received where a Request-Line is expected. In other words, if
1139   the server is reading the protocol stream at the beginning of a
1140   message and receives a CRLF first, it should ignore the CRLF.
1143   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
1144   after a POST request. To restate what is explicitly forbidden by the
1145   BNF, an HTTP/1.1 client &MUST-NOT; preface or follow a request with an
1146   extra CRLF.
1150<section title="Message Headers" anchor="message.headers">
1151  <x:anchor-alias value="field-content"/>
1152  <x:anchor-alias value="field-name"/>
1153  <x:anchor-alias value="field-value"/>
1154  <x:anchor-alias value="message-header"/>
1156   HTTP header fields, which include general-header (<xref target="general.header.fields"/>),
1157   request-header (&request-header-fields;), response-header (&response-header-fields;), and
1158   entity-header (&entity-header-fields;) fields, follow the same generic format as
1159   that given in <xref target="RFC5322" x:fmt="of" x:sec="2.1"/>. Each header field consists
1160   of a name followed by a colon (":") and the field value. Field names
1161   are case-insensitive. The field value &MAY; be preceded by any amount
1162   of LWS, though a single SP is preferred. Header fields can be
1163   extended over multiple lines by preceding each extra line with at
1164   least one SP or HTAB. Applications ought to follow "common form", where
1165   one is known or indicated, when generating HTTP constructs, since
1166   there might exist some implementations that fail to accept anything
1167   beyond the common forms.
1169<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"/>
1170  <x:ref>message-header</x:ref> = <x:ref>field-name</x:ref> ":" [ <x:ref>field-value</x:ref> ]
1171  <x:ref>field-name</x:ref>     = <x:ref>token</x:ref>
1172  <x:ref>field-value</x:ref>    = *( <x:ref>field-content</x:ref> / <x:ref>OWS</x:ref> )
1173  <x:ref>field-content</x:ref>  = &lt;field content&gt;
1176  <cref>whitespace between field-name and colon is an error and MUST NOT be accepted</cref>
1179   The field-content does not include any leading or trailing LWS:
1180   linear white space occurring before the first non-whitespace
1181   character of the field-value or after the last non-whitespace
1182   character of the field-value. Such leading or trailing LWS &MAY; be
1183   removed without changing the semantics of the field value. Any LWS
1184   that occurs between field-content &MAY; be replaced with a single SP
1185   before interpreting the field value or forwarding the message
1186   downstream.
1189   The order in which header fields with differing field names are
1190   received is not significant. However, it is "good practice" to send
1191   general-header fields first, followed by request-header or response-header
1192   fields, and ending with the entity-header fields.
1195   Multiple message-header fields with the same field-name &MAY; be
1196   present in a message if and only if the entire field-value for that
1197   header field is defined as a comma-separated list [i.e., #(values)].
1198   It &MUST; be possible to combine the multiple header fields into one
1199   "field-name: field-value" pair, without changing the semantics of the
1200   message, by appending each subsequent field-value to the first, each
1201   separated by a comma. The order in which header fields with the same
1202   field-name are received is therefore significant to the
1203   interpretation of the combined field value, and thus a proxy &MUST-NOT;
1204   change the order of these field values when a message is forwarded.
1207  <list><t>
1208   <x:h>Note:</x:h> the "Set-Cookie" header as implemented in
1209   practice (as opposed to how it is specified in <xref target="RFC2109"/>)
1210   can occur multiple times, but does not use the list syntax, and thus cannot
1211   be combined into a single line. (See Appendix A.2.3 of <xref target="Kri2001"/>
1212   for details.) Also note that the Set-Cookie2 header specified in
1213   <xref target="RFC2965"/> does not share this problem.
1214  </t></list>
1219<section title="Message Body" anchor="message.body">
1220  <x:anchor-alias value="message-body"/>
1222   The message-body (if any) of an HTTP message is used to carry the
1223   entity-body associated with the request or response. The message-body
1224   differs from the entity-body only when a transfer-coding has been
1225   applied, as indicated by the Transfer-Encoding header field (<xref target="header.transfer-encoding"/>).
1227<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="message-body"/>
1228  <x:ref>message-body</x:ref> = <x:ref>entity-body</x:ref>
1229               / &lt;entity-body encoded as per <x:ref>Transfer-Encoding</x:ref>&gt;
1232   Transfer-Encoding &MUST; be used to indicate any transfer-codings
1233   applied by an application to ensure safe and proper transfer of the
1234   message. Transfer-Encoding is a property of the message, not of the
1235   entity, and thus &MAY; be added or removed by any application along the
1236   request/response chain. (However, <xref target="transfer.codings"/> places restrictions on
1237   when certain transfer-codings may be used.)
1240   The rules for when a message-body is allowed in a message differ for
1241   requests and responses.
1244   The presence of a message-body in a request is signaled by the
1245   inclusion of a Content-Length or Transfer-Encoding header field in
1246   the request's message-headers. A message-body &MUST-NOT; be included in
1247   a request if the specification of the request method (&method;)
1248   explicitly disallows an entity-body in requests.
1249   When a request message contains both a message-body of non-zero
1250   length and a method that does not define any semantics for that
1251   request message-body, then an origin server &SHOULD; either ignore
1252   the message-body or respond with an appropriate error message
1253   (e.g., 413).  A proxy or gateway, when presented the same request,
1254   &SHOULD; either forward the request inbound with the message-body or
1255   ignore the message-body when determining a response.
1258   For response messages, whether or not a message-body is included with
1259   a message is dependent on both the request method and the response
1260   status code (<xref target="status.code.and.reason.phrase"/>). All responses to the HEAD request method
1261   &MUST-NOT; include a message-body, even though the presence of entity-header
1262   fields might lead one to believe they do. All 1xx
1263   (informational), 204 (No Content), and 304 (Not Modified) responses
1264   &MUST-NOT; include a message-body. All other responses do include a
1265   message-body, although it &MAY; be of zero length.
1269<section title="Message Length" anchor="message.length">
1271   The transfer-length of a message is the length of the message-body as
1272   it appears in the message; that is, after any transfer-codings have
1273   been applied. When a message-body is included with a message, the
1274   transfer-length of that body is determined by one of the following
1275   (in order of precedence):
1278  <list style="numbers">
1279    <x:lt><t>
1280     Any response message which "&MUST-NOT;" include a message-body (such
1281     as the 1xx, 204, and 304 responses and any response to a HEAD
1282     request) is always terminated by the first empty line after the
1283     header fields, regardless of the entity-header fields present in
1284     the message.
1285    </t></x:lt>
1286    <x:lt><t>
1287     If a Transfer-Encoding header field (<xref target="header.transfer-encoding"/>)
1288     is present and the "chunked" transfer-coding (<xref target="transfer.codings"/>)
1289     is used, the transfer-length is defined by the use of this transfer-coding.
1290     If a Transfer-Encoding header field is present and the "chunked" transfer-coding
1291     is not present, the transfer-length is defined by the sender closing the connection.
1292    </t></x:lt>
1293    <x:lt><t>
1294     If a Content-Length header field (<xref target="header.content-length"/>) is present, its
1295     decimal value in OCTETs represents both the entity-length and the
1296     transfer-length. The Content-Length header field &MUST-NOT; be sent
1297     if these two lengths are different (i.e., if a Transfer-Encoding
1298     header field is present). If a message is received with both a
1299     Transfer-Encoding header field and a Content-Length header field,
1300     the latter &MUST; be ignored.
1301    </t></x:lt>
1302    <x:lt><t>
1303     If the message uses the media type "multipart/byteranges", and the
1304     transfer-length is not otherwise specified, then this self-delimiting
1305     media type defines the transfer-length. This media type
1306     &MUST-NOT; be used unless the sender knows that the recipient can parse
1307     it; the presence in a request of a Range header with multiple byte-range
1308     specifiers from a 1.1 client implies that the client can parse
1309     multipart/byteranges responses.
1310    <list style="empty"><t>
1311       A range header might be forwarded by a 1.0 proxy that does not
1312       understand multipart/byteranges; in this case the server &MUST;
1313       delimit the message using methods defined in items 1, 3 or 5 of
1314       this section.
1315    </t></list>
1316    </t></x:lt>
1317    <x:lt><t>
1318     By the server closing the connection. (Closing the connection
1319     cannot be used to indicate the end of a request body, since that
1320     would leave no possibility for the server to send back a response.)
1321    </t></x:lt>
1322  </list>
1325   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
1326   containing a message-body &MUST; include a valid Content-Length header
1327   field unless the server is known to be HTTP/1.1 compliant. If a
1328   request contains a message-body and a Content-Length is not given,
1329   the server &SHOULD; respond with 400 (Bad Request) if it cannot
1330   determine the length of the message, or with 411 (Length Required) if
1331   it wishes to insist on receiving a valid Content-Length.
1334   All HTTP/1.1 applications that receive entities &MUST; accept the
1335   "chunked" transfer-coding (<xref target="transfer.codings"/>), thus allowing this mechanism
1336   to be used for messages when the message length cannot be determined
1337   in advance.
1340   Messages &MUST-NOT; include both a Content-Length header field and a
1341   transfer-coding. If the message does include a
1342   transfer-coding, the Content-Length &MUST; be ignored.
1345   When a Content-Length is given in a message where a message-body is
1346   allowed, its field value &MUST; exactly match the number of OCTETs in
1347   the message-body. HTTP/1.1 user agents &MUST; notify the user when an
1348   invalid length is received and detected.
1352<section title="General Header Fields" anchor="general.header.fields">
1353  <x:anchor-alias value="general-header"/>
1355   There are a few header fields which have general applicability for
1356   both request and response messages, but which do not apply to the
1357   entity being transferred. These header fields apply only to the
1358   message being transmitted.
1360<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="general-header"/>
1361  <x:ref>general-header</x:ref> = <x:ref>Cache-Control</x:ref>            ; &header-cache-control;
1362                 / <x:ref>Connection</x:ref>               ; <xref target="header.connection"/>
1363                 / <x:ref>Date</x:ref>                     ; <xref target=""/>
1364                 / <x:ref>Pragma</x:ref>                   ; &header-pragma;
1365                 / <x:ref>Trailer</x:ref>                  ; <xref target="header.trailer"/>
1366                 / <x:ref>Transfer-Encoding</x:ref>        ; <xref target="header.transfer-encoding"/>
1367                 / <x:ref>Upgrade</x:ref>                  ; <xref target="header.upgrade"/>
1368                 / <x:ref>Via</x:ref>                      ; <xref target="header.via"/>
1369                 / <x:ref>Warning</x:ref>                  ; &header-warning;
1372   General-header field names can be extended reliably only in
1373   combination with a change in the protocol version. However, new or
1374   experimental header fields may be given the semantics of general
1375   header fields if all parties in the communication recognize them to
1376   be general-header fields. Unrecognized header fields are treated as
1377   entity-header fields.
1382<section title="Request" anchor="request">
1383  <x:anchor-alias value="Request"/>
1385   A request message from a client to a server includes, within the
1386   first line of that message, the method to be applied to the resource,
1387   the identifier of the resource, and the protocol version in use.
1389<!--                 Host                      ; should be moved here eventually -->
1390<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request"/>
1391  <x:ref>Request</x:ref>       = <x:ref>Request-Line</x:ref>              ; <xref target="request-line"/>
1392                  *(( <x:ref>general-header</x:ref>        ; <xref target="general.header.fields"/>
1393                   / <x:ref>request-header</x:ref>         ; &request-header-fields;
1394                   / <x:ref>entity-header</x:ref> ) <x:ref>CRLF</x:ref>)  ; &entity-header-fields;
1395                  <x:ref>CRLF</x:ref>
1396                  [ <x:ref>message-body</x:ref> ]          ; <xref target="message.body"/>
1399<section title="Request-Line" anchor="request-line">
1400  <x:anchor-alias value="Request-Line"/>
1402   The Request-Line begins with a method token, followed by the
1403   Request-URI and the protocol version, and ending with CRLF. The
1404   elements are separated by SP characters. No CR or LF is allowed
1405   except in the final CRLF sequence.
1407<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-Line"/>
1408  <x:ref>Request-Line</x:ref>   = <x:ref>Method</x:ref> <x:ref>SP</x:ref> <x:ref>Request-URI</x:ref> <x:ref>SP</x:ref> <x:ref>HTTP-Version</x:ref> <x:ref>CRLF</x:ref>
1411<section title="Method" anchor="method">
1412  <x:anchor-alias value="Method"/>
1414   The Method  token indicates the method to be performed on the
1415   resource identified by the Request-URI. The method is case-sensitive.
1417<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/>
1418  <x:ref>Method</x:ref>         = <x:ref>token</x:ref>
1422<section title="Request-URI" anchor="request-uri">
1423  <x:anchor-alias value="Request-URI"/>
1425   The Request-URI is a Uniform Resource Identifier (<xref target="uri"/>) and
1426   identifies the resource upon which to apply the request.
1428<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Request-URI"/>
1429  <x:ref>Request-URI</x:ref>    = "*"
1430                 / <x:ref>absolute-URI</x:ref>
1431                 / ( <x:ref>path-absolute</x:ref> [ "?" <x:ref>query</x:ref> ] )
1432                 / <x:ref>authority</x:ref>
1435   The four options for Request-URI are dependent on the nature of the
1436   request. The asterisk "*" means that the request does not apply to a
1437   particular resource, but to the server itself, and is only allowed
1438   when the method used does not necessarily apply to a resource. One
1439   example would be
1441<figure><artwork type="example">
1442    OPTIONS * HTTP/1.1
1445   The absolute-URI form is &REQUIRED; when the request is being made to a
1446   proxy. The proxy is requested to forward the request or service it
1447   from a valid cache, and return the response. Note that the proxy &MAY;
1448   forward the request on to another proxy or directly to the server
1449   specified by the absolute-URI. In order to avoid request loops, a
1450   proxy &MUST; be able to recognize all of its server names, including
1451   any aliases, local variations, and the numeric IP address. An example
1452   Request-Line would be:
1454<figure><artwork type="example">
1455    GET HTTP/1.1
1458   To allow for transition to absolute-URIs in all requests in future
1459   versions of HTTP, all HTTP/1.1 servers &MUST; accept the absolute-URI
1460   form in requests, even though HTTP/1.1 clients will only generate
1461   them in requests to proxies.
1464   The authority form is only used by the CONNECT method (&CONNECT;).
1467   The most common form of Request-URI is that used to identify a
1468   resource on an origin server or gateway. In this case the absolute
1469   path of the URI &MUST; be transmitted (see <xref target="http.uri"/>, path-absolute) as
1470   the Request-URI, and the network location of the URI (authority) &MUST;
1471   be transmitted in a Host header field. For example, a client wishing
1472   to retrieve the resource above directly from the origin server would
1473   create a TCP connection to port 80 of the host "" and send
1474   the lines:
1476<figure><artwork type="example">
1477    GET /pub/WWW/TheProject.html HTTP/1.1
1478    Host:
1481   followed by the remainder of the Request. Note that the absolute path
1482   cannot be empty; if none is present in the original URI, it &MUST; be
1483   given as "/" (the server root).
1486   The Request-URI is transmitted in the format specified in
1487   <xref target="http.uri"/>. If the Request-URI is encoded using the
1488   "% <x:ref>HEXDIG</x:ref> <x:ref>HEXDIG</x:ref>" encoding
1489   (<xref target="RFC3986" x:fmt="," x:sec="2.4"/>), the origin server
1490   &MUST; decode the Request-URI in order to
1491   properly interpret the request. Servers &SHOULD; respond to invalid
1492   Request-URIs with an appropriate status code.
1495   A transparent proxy &MUST-NOT; rewrite the "path-absolute" part of the
1496   received Request-URI when forwarding it to the next inbound server,
1497   except as noted above to replace a null path-absolute with "/".
1500  <list><t>
1501      <x:h>Note:</x:h> The "no rewrite" rule prevents the proxy from changing the
1502      meaning of the request when the origin server is improperly using
1503      a non-reserved URI character for a reserved purpose.  Implementors
1504      should be aware that some pre-HTTP/1.1 proxies have been known to
1505      rewrite the Request-URI.
1506  </t></list>
1511<section title="The Resource Identified by a Request" anchor="">
1513   The exact resource identified by an Internet request is determined by
1514   examining both the Request-URI and the Host header field.
1517   An origin server that does not allow resources to differ by the
1518   requested host &MAY; ignore the Host header field value when
1519   determining the resource identified by an HTTP/1.1 request. (But see
1520   <xref target=""/>
1521   for other requirements on Host support in HTTP/1.1.)
1524   An origin server that does differentiate resources based on the host
1525   requested (sometimes referred to as virtual hosts or vanity host
1526   names) &MUST; use the following rules for determining the requested
1527   resource on an HTTP/1.1 request:
1528  <list style="numbers">
1529    <t>If Request-URI is an absolute-URI, the host is part of the
1530     Request-URI. Any Host header field value in the request &MUST; be
1531     ignored.</t>
1532    <t>If the Request-URI is not an absolute-URI, and the request includes
1533     a Host header field, the host is determined by the Host header
1534     field value.</t>
1535    <t>If the host as determined by rule 1 or 2 is not a valid host on
1536     the server, the response &MUST; be a 400 (Bad Request) error message.</t>
1537  </list>
1540   Recipients of an HTTP/1.0 request that lacks a Host header field &MAY;
1541   attempt to use heuristics (e.g., examination of the URI path for
1542   something unique to a particular host) in order to determine what
1543   exact resource is being requested.
1550<section title="Response" anchor="response">
1551  <x:anchor-alias value="Response"/>
1553   After receiving and interpreting a request message, a server responds
1554   with an HTTP response message.
1556<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Response"/>
1557  <x:ref>Response</x:ref>      = <x:ref>Status-Line</x:ref>               ; <xref target="status-line"/>
1558                  *(( <x:ref>general-header</x:ref>        ; <xref target="general.header.fields"/>
1559                   / <x:ref>response-header</x:ref>        ; &response-header-fields;
1560                   / <x:ref>entity-header</x:ref> ) <x:ref>CRLF</x:ref>)  ; &entity-header-fields;
1561                  <x:ref>CRLF</x:ref>
1562                  [ <x:ref>message-body</x:ref> ]          ; <xref target="message.body"/>
1565<section title="Status-Line" anchor="status-line">
1566  <x:anchor-alias value="Status-Line"/>
1568   The first line of a Response message is the Status-Line, consisting
1569   of the protocol version followed by a numeric status code and its
1570   associated textual phrase, with each element separated by SP
1571   characters. No CR or LF is allowed except in the final CRLF sequence.
1573<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Line"/>
1574  <x:ref>Status-Line</x:ref> = <x:ref>HTTP-Version</x:ref> <x:ref>SP</x:ref> <x:ref>Status-Code</x:ref> <x:ref>SP</x:ref> <x:ref>Reason-Phrase</x:ref> <x:ref>CRLF</x:ref>
1577<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
1578  <x:anchor-alias value="Reason-Phrase"/>
1579  <x:anchor-alias value="Status-Code"/>
1581   The Status-Code element is a 3-digit integer result code of the
1582   attempt to understand and satisfy the request. These codes are fully
1583   defined in &status-codes;.  The Reason Phrase exists for the sole
1584   purpose of providing a textual description associated with the numeric
1585   status code, out of deference to earlier Internet application protocols
1586   that were more frequently used with interactive text clients.
1587   A client &SHOULD; ignore the content of the Reason Phrase.
1590   The first digit of the Status-Code defines the class of response. The
1591   last two digits do not have any categorization role. There are 5
1592   values for the first digit:
1593  <list style="symbols">
1594    <t>
1595      1xx: Informational - Request received, continuing process
1596    </t>
1597    <t>
1598      2xx: Success - The action was successfully received,
1599        understood, and accepted
1600    </t>
1601    <t>
1602      3xx: Redirection - Further action must be taken in order to
1603        complete the request
1604    </t>
1605    <t>
1606      4xx: Client Error - The request contains bad syntax or cannot
1607        be fulfilled
1608    </t>
1609    <t>
1610      5xx: Server Error - The server failed to fulfill an apparently
1611        valid request
1612    </t>
1613  </list>
1615<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Status-Code"/><iref primary="true" item="Grammar" subitem="extension-code"/><iref primary="true" item="Grammar" subitem="Reason-Phrase"/>
1616  <x:ref>Status-Code</x:ref>    = 3<x:ref>DIGIT</x:ref>
1617  <x:ref>Reason-Phrase</x:ref>  = *&lt;<x:ref>TEXT</x:ref>, excluding <x:ref>CR</x:ref>, <x:ref>LF</x:ref>&gt;
1625<section title="Connections" anchor="connections">
1627<section title="Persistent Connections" anchor="persistent.connections">
1629<section title="Purpose" anchor="persistent.purpose">
1631   Prior to persistent connections, a separate TCP connection was
1632   established to fetch each URL, increasing the load on HTTP servers
1633   and causing congestion on the Internet. The use of inline images and
1634   other associated data often require a client to make multiple
1635   requests of the same server in a short amount of time. Analysis of
1636   these performance problems and results from a prototype
1637   implementation are available <xref target="Pad1995"/> <xref target="Spe"/>. Implementation experience and
1638   measurements of actual HTTP/1.1 (<xref target="RFC2068" x:fmt="none">RFC 2068</xref>) implementations show good
1639   results <xref target="Nie1997"/>. Alternatives have also been explored, for example,
1640   T/TCP <xref target="Tou1998"/>.
1643   Persistent HTTP connections have a number of advantages:
1644  <list style="symbols">
1645      <t>
1646        By opening and closing fewer TCP connections, CPU time is saved
1647        in routers and hosts (clients, servers, proxies, gateways,
1648        tunnels, or caches), and memory used for TCP protocol control
1649        blocks can be saved in hosts.
1650      </t>
1651      <t>
1652        HTTP requests and responses can be pipelined on a connection.
1653        Pipelining allows a client to make multiple requests without
1654        waiting for each response, allowing a single TCP connection to
1655        be used much more efficiently, with much lower elapsed time.
1656      </t>
1657      <t>
1658        Network congestion is reduced by reducing the number of packets
1659        caused by TCP opens, and by allowing TCP sufficient time to
1660        determine the congestion state of the network.
1661      </t>
1662      <t>
1663        Latency on subsequent requests is reduced since there is no time
1664        spent in TCP's connection opening handshake.
1665      </t>
1666      <t>
1667        HTTP can evolve more gracefully, since errors can be reported
1668        without the penalty of closing the TCP connection. Clients using
1669        future versions of HTTP might optimistically try a new feature,
1670        but if communicating with an older server, retry with old
1671        semantics after an error is reported.
1672      </t>
1673    </list>
1676   HTTP implementations &SHOULD; implement persistent connections.
1680<section title="Overall Operation" anchor="persistent.overall">
1682   A significant difference between HTTP/1.1 and earlier versions of
1683   HTTP is that persistent connections are the default behavior of any
1684   HTTP connection. That is, unless otherwise indicated, the client
1685   &SHOULD; assume that the server will maintain a persistent connection,
1686   even after error responses from the server.
1689   Persistent connections provide a mechanism by which a client and a
1690   server can signal the close of a TCP connection. This signaling takes
1691   place using the Connection header field (<xref target="header.connection"/>). Once a close
1692   has been signaled, the client &MUST-NOT; send any more requests on that
1693   connection.
1696<section title="Negotiation" anchor="persistent.negotiation">
1698   An HTTP/1.1 server &MAY; assume that a HTTP/1.1 client intends to
1699   maintain a persistent connection unless a Connection header including
1700   the connection-token "close" was sent in the request. If the server
1701   chooses to close the connection immediately after sending the
1702   response, it &SHOULD; send a Connection header including the
1703   connection-token close.
1706   An HTTP/1.1 client &MAY; expect a connection to remain open, but would
1707   decide to keep it open based on whether the response from a server
1708   contains a Connection header with the connection-token close. In case
1709   the client does not want to maintain a connection for more than that
1710   request, it &SHOULD; send a Connection header including the
1711   connection-token close.
1714   If either the client or the server sends the close token in the
1715   Connection header, that request becomes the last one for the
1716   connection.
1719   Clients and servers &SHOULD-NOT;  assume that a persistent connection is
1720   maintained for HTTP versions less than 1.1 unless it is explicitly
1721   signaled. See <xref target="compatibility.with.http.1.0.persistent.connections"/> for more information on backward
1722   compatibility with HTTP/1.0 clients.
1725   In order to remain persistent, all messages on the connection &MUST;
1726   have a self-defined message length (i.e., one not defined by closure
1727   of the connection), as described in <xref target="message.length"/>.
1731<section title="Pipelining" anchor="pipelining">
1733   A client that supports persistent connections &MAY; "pipeline" its
1734   requests (i.e., send multiple requests without waiting for each
1735   response). A server &MUST; send its responses to those requests in the
1736   same order that the requests were received.
1739   Clients which assume persistent connections and pipeline immediately
1740   after connection establishment &SHOULD; be prepared to retry their
1741   connection if the first pipelined attempt fails. If a client does
1742   such a retry, it &MUST-NOT; pipeline before it knows the connection is
1743   persistent. Clients &MUST; also be prepared to resend their requests if
1744   the server closes the connection before sending all of the
1745   corresponding responses.
1748   Clients &SHOULD-NOT;  pipeline requests using non-idempotent methods or
1749   non-idempotent sequences of methods (see &idempotent-methods;). Otherwise, a
1750   premature termination of the transport connection could lead to
1751   indeterminate results. A client wishing to send a non-idempotent
1752   request &SHOULD; wait to send that request until it has received the
1753   response status for the previous request.
1758<section title="Proxy Servers" anchor="persistent.proxy">
1760   It is especially important that proxies correctly implement the
1761   properties of the Connection header field as specified in <xref target="header.connection"/>.
1764   The proxy server &MUST; signal persistent connections separately with
1765   its clients and the origin servers (or other proxy servers) that it
1766   connects to. Each persistent connection applies to only one transport
1767   link.
1770   A proxy server &MUST-NOT; establish a HTTP/1.1 persistent connection
1771   with an HTTP/1.0 client (but see <xref target="RFC2068"/> for information and
1772   discussion of the problems with the Keep-Alive header implemented by
1773   many HTTP/1.0 clients).
1777<section title="Practical Considerations" anchor="persistent.practical">
1779   Servers will usually have some time-out value beyond which they will
1780   no longer maintain an inactive connection. Proxy servers might make
1781   this a higher value since it is likely that the client will be making
1782   more connections through the same server. The use of persistent
1783   connections places no requirements on the length (or existence) of
1784   this time-out for either the client or the server.
1787   When a client or server wishes to time-out it &SHOULD; issue a graceful
1788   close on the transport connection. Clients and servers &SHOULD; both
1789   constantly watch for the other side of the transport close, and
1790   respond to it as appropriate. If a client or server does not detect
1791   the other side's close promptly it could cause unnecessary resource
1792   drain on the network.
1795   A client, server, or proxy &MAY; close the transport connection at any
1796   time. For example, a client might have started to send a new request
1797   at the same time that the server has decided to close the "idle"
1798   connection. From the server's point of view, the connection is being
1799   closed while it was idle, but from the client's point of view, a
1800   request is in progress.
1803   This means that clients, servers, and proxies &MUST; be able to recover
1804   from asynchronous close events. Client software &SHOULD; reopen the
1805   transport connection and retransmit the aborted sequence of requests
1806   without user interaction so long as the request sequence is
1807   idempotent (see &idempotent-methods;). Non-idempotent methods or sequences
1808   &MUST-NOT; be automatically retried, although user agents &MAY; offer a
1809   human operator the choice of retrying the request(s). Confirmation by
1810   user-agent software with semantic understanding of the application
1811   &MAY; substitute for user confirmation. The automatic retry &SHOULD-NOT;
1812   be repeated if the second sequence of requests fails.
1815   Servers &SHOULD; always respond to at least one request per connection,
1816   if at all possible. Servers &SHOULD-NOT;  close a connection in the
1817   middle of transmitting a response, unless a network or client failure
1818   is suspected.
1821   Clients that use persistent connections &SHOULD; limit the number of
1822   simultaneous connections that they maintain to a given server. A
1823   single-user client &SHOULD-NOT; maintain more than 2 connections with
1824   any server or proxy. A proxy &SHOULD; use up to 2*N connections to
1825   another server or proxy, where N is the number of simultaneously
1826   active users. These guidelines are intended to improve HTTP response
1827   times and avoid congestion.
1832<section title="Message Transmission Requirements" anchor="message.transmission.requirements">
1834<section title="Persistent Connections and Flow Control" anchor="persistent.flow">
1836   HTTP/1.1 servers &SHOULD; maintain persistent connections and use TCP's
1837   flow control mechanisms to resolve temporary overloads, rather than
1838   terminating connections with the expectation that clients will retry.
1839   The latter technique can exacerbate network congestion.
1843<section title="Monitoring Connections for Error Status Messages" anchor="persistent.monitor">
1845   An HTTP/1.1 (or later) client sending a message-body &SHOULD; monitor
1846   the network connection for an error status while it is transmitting
1847   the request. If the client sees an error status, it &SHOULD;
1848   immediately cease transmitting the body. If the body is being sent
1849   using a "chunked" encoding (<xref target="transfer.codings"/>), a zero length chunk and
1850   empty trailer &MAY; be used to prematurely mark the end of the message.
1851   If the body was preceded by a Content-Length header, the client &MUST;
1852   close the connection.
1856<section title="Use of the 100 (Continue) Status" anchor="use.of.the.100.status">
1858   The purpose of the 100 (Continue) status (see &status-100;) is to
1859   allow a client that is sending a request message with a request body
1860   to determine if the origin server is willing to accept the request
1861   (based on the request headers) before the client sends the request
1862   body. In some cases, it might either be inappropriate or highly
1863   inefficient for the client to send the body if the server will reject
1864   the message without looking at the body.
1867   Requirements for HTTP/1.1 clients:
1868  <list style="symbols">
1869    <t>
1870        If a client will wait for a 100 (Continue) response before
1871        sending the request body, it &MUST; send an Expect request-header
1872        field (&header-expect;) with the "100-continue" expectation.
1873    </t>
1874    <t>
1875        A client &MUST-NOT; send an Expect request-header field (&header-expect;)
1876        with the "100-continue" expectation if it does not intend
1877        to send a request body.
1878    </t>
1879  </list>
1882   Because of the presence of older implementations, the protocol allows
1883   ambiguous situations in which a client may send "Expect: 100-continue"
1884   without receiving either a 417 (Expectation Failed) status
1885   or a 100 (Continue) status. Therefore, when a client sends this
1886   header field to an origin server (possibly via a proxy) from which it
1887   has never seen a 100 (Continue) status, the client &SHOULD-NOT;  wait
1888   for an indefinite period before sending the request body.
1891   Requirements for HTTP/1.1 origin servers:
1892  <list style="symbols">
1893    <t> Upon receiving a request which includes an Expect request-header
1894        field with the "100-continue" expectation, an origin server &MUST;
1895        either respond with 100 (Continue) status and continue to read
1896        from the input stream, or respond with a final status code. The
1897        origin server &MUST-NOT; wait for the request body before sending
1898        the 100 (Continue) response. If it responds with a final status
1899        code, it &MAY; close the transport connection or it &MAY; continue
1900        to read and discard the rest of the request.  It &MUST-NOT;
1901        perform the requested method if it returns a final status code.
1902    </t>
1903    <t> An origin server &SHOULD-NOT;  send a 100 (Continue) response if
1904        the request message does not include an Expect request-header
1905        field with the "100-continue" expectation, and &MUST-NOT; send a
1906        100 (Continue) response if such a request comes from an HTTP/1.0
1907        (or earlier) client. There is an exception to this rule: for
1908        compatibility with <xref target="RFC2068"/>, a server &MAY; send a 100 (Continue)
1909        status in response to an HTTP/1.1 PUT or POST request that does
1910        not include an Expect request-header field with the "100-continue"
1911        expectation. This exception, the purpose of which is
1912        to minimize any client processing delays associated with an
1913        undeclared wait for 100 (Continue) status, applies only to
1914        HTTP/1.1 requests, and not to requests with any other HTTP-version
1915        value.
1916    </t>
1917    <t> An origin server &MAY; omit a 100 (Continue) response if it has
1918        already received some or all of the request body for the
1919        corresponding request.
1920    </t>
1921    <t> An origin server that sends a 100 (Continue) response &MUST;
1922    ultimately send a final status code, once the request body is
1923        received and processed, unless it terminates the transport
1924        connection prematurely.
1925    </t>
1926    <t> If an origin server receives a request that does not include an
1927        Expect request-header field with the "100-continue" expectation,
1928        the request includes a request body, and the server responds
1929        with a final status code before reading the entire request body
1930        from the transport connection, then the server &SHOULD-NOT;  close
1931        the transport connection until it has read the entire request,
1932        or until the client closes the connection. Otherwise, the client
1933        might not reliably receive the response message. However, this
1934        requirement is not be construed as preventing a server from
1935        defending itself against denial-of-service attacks, or from
1936        badly broken client implementations.
1937      </t>
1938    </list>
1941   Requirements for HTTP/1.1 proxies:
1942  <list style="symbols">
1943    <t> If a proxy receives a request that includes an Expect request-header
1944        field with the "100-continue" expectation, and the proxy
1945        either knows that the next-hop server complies with HTTP/1.1 or
1946        higher, or does not know the HTTP version of the next-hop
1947        server, it &MUST; forward the request, including the Expect header
1948        field.
1949    </t>
1950    <t> If the proxy knows that the version of the next-hop server is
1951        HTTP/1.0 or lower, it &MUST-NOT; forward the request, and it &MUST;
1952        respond with a 417 (Expectation Failed) status.
1953    </t>
1954    <t> Proxies &SHOULD; maintain a cache recording the HTTP version
1955        numbers received from recently-referenced next-hop servers.
1956    </t>
1957    <t> A proxy &MUST-NOT; forward a 100 (Continue) response if the
1958        request message was received from an HTTP/1.0 (or earlier)
1959        client and did not include an Expect request-header field with
1960        the "100-continue" expectation. This requirement overrides the
1961        general rule for forwarding of 1xx responses (see &status-1xx;).
1962    </t>
1963  </list>
1967<section title="Client Behavior if Server Prematurely Closes Connection" anchor="connection.premature">
1969   If an HTTP/1.1 client sends a request which includes a request body,
1970   but which does not include an Expect request-header field with the
1971   "100-continue" expectation, and if the client is not directly
1972   connected to an HTTP/1.1 origin server, and if the client sees the
1973   connection close before receiving any status from the server, the
1974   client &SHOULD; retry the request.  If the client does retry this
1975   request, it &MAY; use the following "binary exponential backoff"
1976   algorithm to be assured of obtaining a reliable response:
1977  <list style="numbers">
1978    <t>
1979      Initiate a new connection to the server
1980    </t>
1981    <t>
1982      Transmit the request-headers
1983    </t>
1984    <t>
1985      Initialize a variable R to the estimated round-trip time to the
1986         server (e.g., based on the time it took to establish the
1987         connection), or to a constant value of 5 seconds if the round-trip
1988         time is not available.
1989    </t>
1990    <t>
1991       Compute T = R * (2**N), where N is the number of previous
1992         retries of this request.
1993    </t>
1994    <t>
1995       Wait either for an error response from the server, or for T
1996         seconds (whichever comes first)
1997    </t>
1998    <t>
1999       If no error response is received, after T seconds transmit the
2000         body of the request.
2001    </t>
2002    <t>
2003       If client sees that the connection is closed prematurely,
2004         repeat from step 1 until the request is accepted, an error
2005         response is received, or the user becomes impatient and
2006         terminates the retry process.
2007    </t>
2008  </list>
2011   If at any point an error status is received, the client
2012  <list style="symbols">
2013      <t>&SHOULD-NOT;  continue and</t>
2015      <t>&SHOULD; close the connection if it has not completed sending the
2016        request message.</t>
2017    </list>
2024<section title="Header Field Definitions" anchor="header.fields">
2026   This section defines the syntax and semantics of HTTP/1.1 header fields
2027   related to message framing and transport protocols.
2030   For entity-header fields, both sender and recipient refer to either the
2031   client or the server, depending on who sends and who receives the entity.
2034<section title="Connection" anchor="header.connection">
2035  <iref primary="true" item="Connection header" x:for-anchor=""/>
2036  <iref primary="true" item="Headers" subitem="Connection" x:for-anchor=""/>
2037  <x:anchor-alias value="Connection"/>
2038  <x:anchor-alias value="connection-token"/>
2039  <x:anchor-alias value="Connection-v"/>
2041   The general-header field "Connection" allows the sender to specify
2042   options that are desired for that particular connection and &MUST-NOT;
2043   be communicated by proxies over further connections.
2046   The Connection header's value has the following grammar:
2048<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Connection"/><iref primary="true" item="Grammar" subitem="Connection-v"/><iref primary="true" item="Grammar" subitem="connection-token"/>
2049  <x:ref>Connection</x:ref>       = "Connection" ":" <x:ref>OWS</x:ref> <x:ref>Connection-v</x:ref>
2050  <x:ref>Connection-v</x:ref>     = 1#<x:ref>connection-token</x:ref>
2051  <x:ref>connection-token</x:ref> = <x:ref>token</x:ref>
2054   HTTP/1.1 proxies &MUST; parse the Connection header field before a
2055   message is forwarded and, for each connection-token in this field,
2056   remove any header field(s) from the message with the same name as the
2057   connection-token. Connection options are signaled by the presence of
2058   a connection-token in the Connection header field, not by any
2059   corresponding additional header field(s), since the additional header
2060   field may not be sent if there are no parameters associated with that
2061   connection option.
2064   Message headers listed in the Connection header &MUST-NOT; include
2065   end-to-end headers, such as Cache-Control.
2068   HTTP/1.1 defines the "close" connection option for the sender to
2069   signal that the connection will be closed after completion of the
2070   response. For example,
2072<figure><artwork type="example">
2073  Connection: close
2076   in either the request or the response header fields indicates that
2077   the connection &SHOULD-NOT;  be considered `persistent' (<xref target="persistent.connections"/>)
2078   after the current request/response is complete.
2081   An HTTP/1.1 client that does not support persistent connections &MUST;
2082   include the "close" connection option in every request message.
2085   An HTTP/1.1 server that does not support persistent connections &MUST;
2086   include the "close" connection option in every response message that
2087   does not have a 1xx (informational) status code.
2090   A system receiving an HTTP/1.0 (or lower-version) message that
2091   includes a Connection header &MUST;, for each connection-token in this
2092   field, remove and ignore any header field(s) from the message with
2093   the same name as the connection-token. This protects against mistaken
2094   forwarding of such header fields by pre-HTTP/1.1 proxies. See <xref target="compatibility.with.http.1.0.persistent.connections"/>.
2098<section title="Content-Length" anchor="header.content-length">
2099  <iref primary="true" item="Content-Length header" x:for-anchor=""/>
2100  <iref primary="true" item="Headers" subitem="Content-Length" x:for-anchor=""/>
2101  <x:anchor-alias value="Content-Length"/>
2102  <x:anchor-alias value="Content-Length-v"/>
2104   The entity-header field "Content-Length" indicates the size of the
2105   entity-body, in decimal number of OCTETs, sent to the recipient or,
2106   in the case of the HEAD method, the size of the entity-body that
2107   would have been sent had the request been a GET.
2109<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Length"/><iref primary="true" item="Grammar" subitem="Content-Length-v"/>
2110  <x:ref>Content-Length</x:ref>   = "Content-Length" ":" <x:ref>OWS</x:ref> 1*<x:ref>Content-Length-v</x:ref>
2111  <x:ref>Content-Length-v</x:ref> = 1*<x:ref>DIGIT</x:ref>
2114   An example is
2116<figure><artwork type="example">
2117  Content-Length: 3495
2120   Applications &SHOULD; use this field to indicate the transfer-length of
2121   the message-body, unless this is prohibited by the rules in <xref target="message.length"/>.
2124   Any Content-Length greater than or equal to zero is a valid value.
2125   <xref target="message.length"/> describes how to determine the length of a message-body
2126   if a Content-Length is not given.
2129   Note that the meaning of this field is significantly different from
2130   the corresponding definition in MIME, where it is an optional field
2131   used within the "message/external-body" content-type. In HTTP, it
2132   &SHOULD; be sent whenever the message's length can be determined prior
2133   to being transferred, unless this is prohibited by the rules in
2134   <xref target="message.length"/>.
2138<section title="Date" anchor="">
2139  <iref primary="true" item="Date header" x:for-anchor=""/>
2140  <iref primary="true" item="Headers" subitem="Date" x:for-anchor=""/>
2141  <x:anchor-alias value="Date"/>
2142  <x:anchor-alias value="Date-v"/>
2144   The general-header field "Date" represents the date and time at which
2145   the message was originated, having the same semantics as orig-date in
2146   <xref target="RFC5322" x:fmt="of" x:sec="3.6.1"/>. The field value is an
2147   HTTP-date, as described in <xref target=""/>;
2148   it &MUST; be sent in rfc1123-date format.
2150<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/><iref primary="true" item="Grammar" subitem="Date-v"/>
2151  <x:ref>Date</x:ref>   = "Date" ":" <x:ref>OWS</x:ref> <x:ref>Date-v</x:ref>
2152  <x:ref>Date-v</x:ref> = <x:ref>HTTP-date</x:ref>
2155   An example is
2157<figure><artwork type="example">
2158  Date: Tue, 15 Nov 1994 08:12:31 GMT
2161   Origin servers &MUST; include a Date header field in all responses,
2162   except in these cases:
2163  <list style="numbers">
2164      <t>If the response status code is 100 (Continue) or 101 (Switching
2165         Protocols), the response &MAY; include a Date header field, at
2166         the server's option.</t>
2168      <t>If the response status code conveys a server error, e.g. 500
2169         (Internal Server Error) or 503 (Service Unavailable), and it is
2170         inconvenient or impossible to generate a valid Date.</t>
2172      <t>If the server does not have a clock that can provide a
2173         reasonable approximation of the current time, its responses
2174         &MUST-NOT; include a Date header field. In this case, the rules
2175         in <xref target="clockless.origin.server.operation"/> &MUST; be followed.</t>
2176  </list>
2179   A received message that does not have a Date header field &MUST; be
2180   assigned one by the recipient if the message will be cached by that
2181   recipient or gatewayed via a protocol which requires a Date. An HTTP
2182   implementation without a clock &MUST-NOT; cache responses without
2183   revalidating them on every use. An HTTP cache, especially a shared
2184   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to synchronize its
2185   clock with a reliable external standard.
2188   Clients &SHOULD; only send a Date header field in messages that include
2189   an entity-body, as in the case of the PUT and POST requests, and even
2190   then it is optional. A client without a clock &MUST-NOT; send a Date
2191   header field in a request.
2194   The HTTP-date sent in a Date header &SHOULD-NOT;  represent a date and
2195   time subsequent to the generation of the message. It &SHOULD; represent
2196   the best available approximation of the date and time of message
2197   generation, unless the implementation has no means of generating a
2198   reasonably accurate date and time. In theory, the date ought to
2199   represent the moment just before the entity is generated. In
2200   practice, the date can be generated at any time during the message
2201   origination without affecting its semantic value.
2204<section title="Clockless Origin Server Operation" anchor="clockless.origin.server.operation">
2206   Some origin server implementations might not have a clock available.
2207   An origin server without a clock &MUST-NOT; assign Expires or Last-Modified
2208   values to a response, unless these values were associated
2209   with the resource by a system or user with a reliable clock. It &MAY;
2210   assign an Expires value that is known, at or before server
2211   configuration time, to be in the past (this allows "pre-expiration"
2212   of responses without storing separate Expires values for each
2213   resource).
2218<section title="Host" anchor="">
2219  <iref primary="true" item="Host header" x:for-anchor=""/>
2220  <iref primary="true" item="Headers" subitem="Host" x:for-anchor=""/>
2221  <x:anchor-alias value="Host"/>
2222  <x:anchor-alias value="Host-v"/>
2224   The request-header field "Host" specifies the Internet host and port
2225   number of the resource being requested, as obtained from the original
2226   URI given by the user or referring resource (generally an HTTP URL,
2227   as described in <xref target="http.uri"/>). The Host field value &MUST; represent
2228   the naming authority of the origin server or gateway given by the
2229   original URL. This allows the origin server or gateway to
2230   differentiate between internally-ambiguous URLs, such as the root "/"
2231   URL of a server for multiple host names on a single IP address.
2233<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Host"/><iref primary="true" item="Grammar" subitem="Host-v"/>
2234  <x:ref>Host</x:ref>   = "Host" ":" <x:ref>OWS</x:ref> <x:ref>Host-v</x:ref>
2235  <x:ref>Host-v</x:ref> = <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ; <xref target="http.uri"/>
2238   A "host" without any trailing port information implies the default
2239   port for the service requested (e.g., "80" for an HTTP URL). For
2240   example, a request on the origin server for
2241   &lt;; would properly include:
2243<figure><artwork type="example">
2244  GET /pub/WWW/ HTTP/1.1
2245  Host:
2248   A client &MUST; include a Host header field in all HTTP/1.1 request
2249   messages. If the requested URI does not include an Internet host
2250   name for the service being requested, then the Host header field &MUST;
2251   be given with an empty value. An HTTP/1.1 proxy &MUST; ensure that any
2252   request message it forwards does contain an appropriate Host header
2253   field that identifies the service being requested by the proxy. All
2254   Internet-based HTTP/1.1 servers &MUST; respond with a 400 (Bad Request)
2255   status code to any HTTP/1.1 request message which lacks a Host header
2256   field.
2259   See Sections <xref target="" format="counter"/>
2260   and <xref target="" format="counter"/>
2261   for other requirements relating to Host.
2265<section title="TE" anchor="header.te">
2266  <iref primary="true" item="TE header" x:for-anchor=""/>
2267  <iref primary="true" item="Headers" subitem="TE" x:for-anchor=""/>
2268  <x:anchor-alias value="TE"/>
2269  <x:anchor-alias value="TE-v"/>
2270  <x:anchor-alias value="t-codings"/>
2272   The request-header field "TE" indicates what extension transfer-codings
2273   it is willing to accept in the response and whether or not it is
2274   willing to accept trailer fields in a chunked transfer-coding. Its
2275   value may consist of the keyword "trailers" and/or a comma-separated
2276   list of extension transfer-coding names with optional accept
2277   parameters (as described in <xref target="transfer.codings"/>).
2279<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="TE"/><iref primary="true" item="Grammar" subitem="TE-v"/><iref primary="true" item="Grammar" subitem="t-codings"/>
2280  <x:ref>TE</x:ref>        = "TE" ":" <x:ref>OWS</x:ref> <x:ref>TE-v</x:ref>
2281  <x:ref>TE-v</x:ref>      = #<x:ref>t-codings</x:ref>
2282  <x:ref>t-codings</x:ref> = "trailers" / ( <x:ref>transfer-extension</x:ref> [ <x:ref>accept-params</x:ref> ] )
2285   The presence of the keyword "trailers" indicates that the client is
2286   willing to accept trailer fields in a chunked transfer-coding, as
2287   defined in <xref target="chunked.transfer.encoding"/>. This keyword is reserved for use with
2288   transfer-coding values even though it does not itself represent a
2289   transfer-coding.
2292   Examples of its use are:
2294<figure><artwork type="example">
2295  TE: deflate
2296  TE:
2297  TE: trailers, deflate;q=0.5
2300   The TE header field only applies to the immediate connection.
2301   Therefore, the keyword &MUST; be supplied within a Connection header
2302   field (<xref target="header.connection"/>) whenever TE is present in an HTTP/1.1 message.
2305   A server tests whether a transfer-coding is acceptable, according to
2306   a TE field, using these rules:
2307  <list style="numbers">
2308    <x:lt>
2309      <t>The "chunked" transfer-coding is always acceptable. If the
2310         keyword "trailers" is listed, the client indicates that it is
2311         willing to accept trailer fields in the chunked response on
2312         behalf of itself and any downstream clients. The implication is
2313         that, if given, the client is stating that either all
2314         downstream clients are willing to accept trailer fields in the
2315         forwarded response, or that it will attempt to buffer the
2316         response on behalf of downstream recipients.
2317      </t><t>
2318         <x:h>Note:</x:h> HTTP/1.1 does not define any means to limit the size of a
2319         chunked response such that a client can be assured of buffering
2320         the entire response.</t>
2321    </x:lt>
2322    <x:lt>
2323      <t>If the transfer-coding being tested is one of the transfer-codings
2324         listed in the TE field, then it is acceptable unless it
2325         is accompanied by a qvalue of 0. (As defined in &qvalue;, a
2326         qvalue of 0 means "not acceptable.")</t>
2327    </x:lt>
2328    <x:lt>
2329      <t>If multiple transfer-codings are acceptable, then the
2330         acceptable transfer-coding with the highest non-zero qvalue is
2331         preferred.  The "chunked" transfer-coding always has a qvalue
2332         of 1.</t>
2333    </x:lt>
2334  </list>
2337   If the TE field-value is empty or if no TE field is present, the only
2338   transfer-coding  is "chunked". A message with no transfer-coding is
2339   always acceptable.
2343<section title="Trailer" anchor="header.trailer">
2344  <iref primary="true" item="Trailer header" x:for-anchor=""/>
2345  <iref primary="true" item="Headers" subitem="Trailer" x:for-anchor=""/>
2346  <x:anchor-alias value="Trailer"/>
2347  <x:anchor-alias value="Trailer-v"/>
2349   The general field "Trailer" indicates that the given set of
2350   header fields is present in the trailer of a message encoded with
2351   chunked transfer-coding.
2353<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Trailer"/><iref primary="true" item="Grammar" subitem="Trailer-v"/>
2354  <x:ref>Trailer</x:ref>   = "Trailer" ":" <x:ref>OWS</x:ref> <x:ref>Trailer-v</x:ref>
2355  <x:ref>Trailer-v</x:ref> = 1#<x:ref>field-name</x:ref>
2358   An HTTP/1.1 message &SHOULD; include a Trailer header field in a
2359   message using chunked transfer-coding with a non-empty trailer. Doing
2360   so allows the recipient to know which header fields to expect in the
2361   trailer.
2364   If no Trailer header field is present, the trailer &SHOULD-NOT;  include
2365   any header fields. See <xref target="chunked.transfer.encoding"/> for restrictions on the use of
2366   trailer fields in a "chunked" transfer-coding.
2369   Message header fields listed in the Trailer header field &MUST-NOT;
2370   include the following header fields:
2371  <list style="symbols">
2372    <t>Transfer-Encoding</t>
2373    <t>Content-Length</t>
2374    <t>Trailer</t>
2375  </list>
2379<section title="Transfer-Encoding" anchor="header.transfer-encoding">
2380  <iref primary="true" item="Transfer-Encoding header" x:for-anchor=""/>
2381  <iref primary="true" item="Headers" subitem="Transfer-Encoding" x:for-anchor=""/>
2382  <x:anchor-alias value="Transfer-Encoding"/>
2383  <x:anchor-alias value="Transfer-Encoding-v"/>
2385   The general-header "Transfer-Encoding" field indicates what (if any)
2386   type of transformation has been applied to the message body in order
2387   to safely transfer it between the sender and the recipient. This
2388   differs from the content-coding in that the transfer-coding is a
2389   property of the message, not of the entity.
2391<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Transfer-Encoding"/><iref primary="true" item="Grammar" subitem="Transfer-Encoding-v"/>
2392  <x:ref>Transfer-Encoding</x:ref>   = "Transfer-Encoding" ":" <x:ref>OWS</x:ref>
2393                        <x:ref>Transfer-Encoding-v</x:ref>
2394  <x:ref>Transfer-Encoding-v</x:ref> = 1#<x:ref>transfer-coding</x:ref>
2397   Transfer-codings are defined in <xref target="transfer.codings"/>. An example is:
2399<figure><artwork type="example">
2400  Transfer-Encoding: chunked
2403   If multiple encodings have been applied to an entity, the transfer-codings
2404   &MUST; be listed in the order in which they were applied.
2405   Additional information about the encoding parameters &MAY; be provided
2406   by other entity-header fields not defined by this specification.
2409   Many older HTTP/1.0 applications do not understand the Transfer-Encoding
2410   header.
2414<section title="Upgrade" anchor="header.upgrade">
2415  <iref primary="true" item="Upgrade header" x:for-anchor=""/>
2416  <iref primary="true" item="Headers" subitem="Upgrade" x:for-anchor=""/>
2417  <x:anchor-alias value="Upgrade"/>
2418  <x:anchor-alias value="Upgrade-v"/>
2420   The general-header "Upgrade" allows the client to specify what
2421   additional communication protocols it supports and would like to use
2422   if the server finds it appropriate to switch protocols. The server
2423   &MUST; use the Upgrade header field within a 101 (Switching Protocols)
2424   response to indicate which protocol(s) are being switched.
2426<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Upgrade"/><iref primary="true" item="Grammar" subitem="Upgrade-v"/>
2427  <x:ref>Upgrade</x:ref>   = "Upgrade" ":" <x:ref>OWS</x:ref> <x:ref>Upgrade-v</x:ref>
2428  <x:ref>Upgrade-v</x:ref> = 1#<x:ref>product</x:ref>
2431   For example,
2433<figure><artwork type="example">
2434  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
2437   The Upgrade header field is intended to provide a simple mechanism
2438   for transition from HTTP/1.1 to some other, incompatible protocol. It
2439   does so by allowing the client to advertise its desire to use another
2440   protocol, such as a later version of HTTP with a higher major version
2441   number, even though the current request has been made using HTTP/1.1.
2442   This eases the difficult transition between incompatible protocols by
2443   allowing the client to initiate a request in the more commonly
2444   supported protocol while indicating to the server that it would like
2445   to use a "better" protocol if available (where "better" is determined
2446   by the server, possibly according to the nature of the method and/or
2447   resource being requested).
2450   The Upgrade header field only applies to switching application-layer
2451   protocols upon the existing transport-layer connection. Upgrade
2452   cannot be used to insist on a protocol change; its acceptance and use
2453   by the server is optional. The capabilities and nature of the
2454   application-layer communication after the protocol change is entirely
2455   dependent upon the new protocol chosen, although the first action
2456   after changing the protocol &MUST; be a response to the initial HTTP
2457   request containing the Upgrade header field.
2460   The Upgrade header field only applies to the immediate connection.
2461   Therefore, the upgrade keyword &MUST; be supplied within a Connection
2462   header field (<xref target="header.connection"/>) whenever Upgrade is present in an
2463   HTTP/1.1 message.
2466   The Upgrade header field cannot be used to indicate a switch to a
2467   protocol on a different connection. For that purpose, it is more
2468   appropriate to use a 301, 302, 303, or 305 redirection response.
2471   This specification only defines the protocol name "HTTP" for use by
2472   the family of Hypertext Transfer Protocols, as defined by the HTTP
2473   version rules of <xref target="http.version"/> and future updates to this
2474   specification. Any token can be used as a protocol name; however, it
2475   will only be useful if both the client and server associate the name
2476   with the same protocol.
2480<section title="Via" anchor="header.via">
2481  <iref primary="true" item="Via header" x:for-anchor=""/>
2482  <iref primary="true" item="Headers" subitem="Via" x:for-anchor=""/>
2483  <x:anchor-alias value="protocol-name"/>
2484  <x:anchor-alias value="protocol-version"/>
2485  <x:anchor-alias value="pseudonym"/>
2486  <x:anchor-alias value="received-by"/>
2487  <x:anchor-alias value="received-protocol"/>
2488  <x:anchor-alias value="Via"/>
2489  <x:anchor-alias value="Via-v"/>
2491   The general-header field "Via" &MUST; be used by gateways and proxies to
2492   indicate the intermediate protocols and recipients between the user
2493   agent and the server on requests, and between the origin server and
2494   the client on responses. It is analogous to the "Received" field defined in
2495   <xref target="RFC5322" x:fmt="of" x:sec="3.6.7"/> and is intended to be used for tracking message forwards,
2496   avoiding request loops, and identifying the protocol capabilities of
2497   all senders along the request/response chain.
2499<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Via"/><iref primary="true" item="Grammar" subitem="Via-v"/><iref primary="true" item="Grammar" subitem="received-protocol"/><iref primary="true" item="Grammar" subitem="protocol-name"/><iref primary="true" item="Grammar" subitem="protocol-version"/><iref primary="true" item="Grammar" subitem="received-by"/><iref primary="true" item="Grammar" subitem="pseudonym"/>
2500  <x:ref>Via</x:ref>               = "Via" ":" <x:ref>OWS</x:ref> <x:ref>Via-v</x:ref>
2501  <x:ref>Via-v</x:ref>             = 1#( <x:ref>received-protocol</x:ref> <x:ref>RWS</x:ref> <x:ref>received-by</x:ref>
2502                          [ <x:ref>RWS</x:ref> <x:ref>comment</x:ref> ] )
2503  <x:ref>received-protocol</x:ref> = [ <x:ref>protocol-name</x:ref> "/" ] <x:ref>protocol-version</x:ref>
2504  <x:ref>protocol-name</x:ref>     = <x:ref>token</x:ref>
2505  <x:ref>protocol-version</x:ref>  = <x:ref>token</x:ref>
2506  <x:ref>received-by</x:ref>       = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
2507  <x:ref>pseudonym</x:ref>         = <x:ref>token</x:ref>
2510   The received-protocol indicates the protocol version of the message
2511   received by the server or client along each segment of the
2512   request/response chain. The received-protocol version is appended to
2513   the Via field value when the message is forwarded so that information
2514   about the protocol capabilities of upstream applications remains
2515   visible to all recipients.
2518   The protocol-name is optional if and only if it would be "HTTP". The
2519   received-by field is normally the host and optional port number of a
2520   recipient server or client that subsequently forwarded the message.
2521   However, if the real host is considered to be sensitive information,
2522   it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
2523   be assumed to be the default port of the received-protocol.
2526   Multiple Via field values represents each proxy or gateway that has
2527   forwarded the message. Each recipient &MUST; append its information
2528   such that the end result is ordered according to the sequence of
2529   forwarding applications.
2532   Comments &MAY; be used in the Via header field to identify the software
2533   of the recipient proxy or gateway, analogous to the User-Agent and
2534   Server header fields. However, all comments in the Via field are
2535   optional and &MAY; be removed by any recipient prior to forwarding the
2536   message.
2539   For example, a request message could be sent from an HTTP/1.0 user
2540   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
2541   forward the request to a public proxy at, which completes
2542   the request by forwarding it to the origin server at
2543   The request received by would then have the following
2544   Via header field:
2546<figure><artwork type="example">
2547  Via: 1.0 fred, 1.1 (Apache/1.1)
2550   Proxies and gateways used as a portal through a network firewall
2551   &SHOULD-NOT;, by default, forward the names and ports of hosts within
2552   the firewall region. This information &SHOULD; only be propagated if
2553   explicitly enabled. If not enabled, the received-by host of any host
2554   behind the firewall &SHOULD; be replaced by an appropriate pseudonym
2555   for that host.
2558   For organizations that have strong privacy requirements for hiding
2559   internal structures, a proxy &MAY; combine an ordered subsequence of
2560   Via header field entries with identical received-protocol values into
2561   a single such entry. For example,
2563<figure><artwork type="example">
2564  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
2567        could be collapsed to
2569<figure><artwork type="example">
2570  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
2573   Applications &SHOULD-NOT;  combine multiple entries unless they are all
2574   under the same organizational control and the hosts have already been
2575   replaced by pseudonyms. Applications &MUST-NOT; combine entries which
2576   have different received-protocol values.
2582<section title="IANA Considerations" anchor="IANA.considerations">
2583<section title="Message Header Registration" anchor="message.header.registration">
2585   The Message Header Registry located at <eref target=""/> should be updated
2586   with the permanent registrations below (see <xref target="RFC3864"/>):
2588<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2589<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2590   <ttcol>Header Field Name</ttcol>
2591   <ttcol>Protocol</ttcol>
2592   <ttcol>Status</ttcol>
2593   <ttcol>Reference</ttcol>
2595   <c>Connection</c>
2596   <c>http</c>
2597   <c>standard</c>
2598   <c>
2599      <xref target="header.connection"/>
2600   </c>
2601   <c>Content-Length</c>
2602   <c>http</c>
2603   <c>standard</c>
2604   <c>
2605      <xref target="header.content-length"/>
2606   </c>
2607   <c>Date</c>
2608   <c>http</c>
2609   <c>standard</c>
2610   <c>
2611      <xref target=""/>
2612   </c>
2613   <c>Host</c>
2614   <c>http</c>
2615   <c>standard</c>
2616   <c>
2617      <xref target=""/>
2618   </c>
2619   <c>TE</c>
2620   <c>http</c>
2621   <c>standard</c>
2622   <c>
2623      <xref target="header.te"/>
2624   </c>
2625   <c>Trailer</c>
2626   <c>http</c>
2627   <c>standard</c>
2628   <c>
2629      <xref target="header.trailer"/>
2630   </c>
2631   <c>Transfer-Encoding</c>
2632   <c>http</c>
2633   <c>standard</c>
2634   <c>
2635      <xref target="header.transfer-encoding"/>
2636   </c>
2637   <c>Upgrade</c>
2638   <c>http</c>
2639   <c>standard</c>
2640   <c>
2641      <xref target="header.upgrade"/>
2642   </c>
2643   <c>Via</c>
2644   <c>http</c>
2645   <c>standard</c>
2646   <c>
2647      <xref target="header.via"/>
2648   </c>
2652   The change controller is: "IETF ( - Internet Engineering Task Force".
2656<section title="URI Scheme Registration" anchor="uri.scheme.registration">
2658   The entry for the "http" URI Scheme in the registry located at
2659   <eref target=""/>
2660   should be updated to point to <xref target="http.uri"/> of this document
2661   (see <xref target="RFC4395"/>).
2665<section title="Internet Media Type Registrations" anchor="">
2667   This document serves as the specification for the Internet media types
2668   "message/http" and "application/http". The following is to be registered with
2669   IANA (see <xref target="RFC4288"/>).
2671<section title="Internet Media Type message/http" anchor="">
2672<iref item="Media Type" subitem="message/http" primary="true"/>
2673<iref item="message/http Media Type" primary="true"/>
2675   The message/http type can be used to enclose a single HTTP request or
2676   response message, provided that it obeys the MIME restrictions for all
2677   "message" types regarding line length and encodings.
2680  <list style="hanging" x:indent="12em">
2681    <t hangText="Type name:">
2682      message
2683    </t>
2684    <t hangText="Subtype name:">
2685      http
2686    </t>
2687    <t hangText="Required parameters:">
2688      none
2689    </t>
2690    <t hangText="Optional parameters:">
2691      version, msgtype
2692      <list style="hanging">
2693        <t hangText="version:">
2694          The HTTP-Version number of the enclosed message
2695          (e.g., "1.1"). If not present, the version can be
2696          determined from the first line of the body.
2697        </t>
2698        <t hangText="msgtype:">
2699          The message type -- "request" or "response". If not
2700          present, the type can be determined from the first
2701          line of the body.
2702        </t>
2703      </list>
2704    </t>
2705    <t hangText="Encoding considerations:">
2706      only "7bit", "8bit", or "binary" are permitted
2707    </t>
2708    <t hangText="Security considerations:">
2709      none
2710    </t>
2711    <t hangText="Interoperability considerations:">
2712      none
2713    </t>
2714    <t hangText="Published specification:">
2715      This specification (see <xref target=""/>).
2716    </t>
2717    <t hangText="Applications that use this media type:">
2718    </t>
2719    <t hangText="Additional information:">
2720      <list style="hanging">
2721        <t hangText="Magic number(s):">none</t>
2722        <t hangText="File extension(s):">none</t>
2723        <t hangText="Macintosh file type code(s):">none</t>
2724      </list>
2725    </t>
2726    <t hangText="Person and email address to contact for further information:">
2727      See Authors Section.
2728    </t>
2729                <t hangText="Intended usage:">
2730                  COMMON
2731    </t>
2732                <t hangText="Restrictions on usage:">
2733                  none
2734    </t>
2735    <t hangText="Author/Change controller:">
2736      IESG
2737    </t>
2738  </list>
2741<section title="Internet Media Type application/http" anchor="">
2742<iref item="Media Type" subitem="application/http" primary="true"/>
2743<iref item="application/http Media Type" primary="true"/>
2745   The application/http type can be used to enclose a pipeline of one or more
2746   HTTP request or response messages (not intermixed).
2749  <list style="hanging" x:indent="12em">
2750    <t hangText="Type name:">
2751      application
2752    </t>
2753    <t hangText="Subtype name:">
2754      http
2755    </t>
2756    <t hangText="Required parameters:">
2757      none
2758    </t>
2759    <t hangText="Optional parameters:">
2760      version, msgtype
2761      <list style="hanging">
2762        <t hangText="version:">
2763          The HTTP-Version number of the enclosed messages
2764          (e.g., "1.1"). If not present, the version can be
2765          determined from the first line of the body.
2766        </t>
2767        <t hangText="msgtype:">
2768          The message type -- "request" or "response". If not
2769          present, the type can be determined from the first
2770          line of the body.
2771        </t>
2772      </list>
2773    </t>
2774    <t hangText="Encoding considerations:">
2775      HTTP messages enclosed by this type
2776      are in "binary" format; use of an appropriate
2777      Content-Transfer-Encoding is required when
2778      transmitted via E-mail.
2779    </t>
2780    <t hangText="Security considerations:">
2781      none
2782    </t>
2783    <t hangText="Interoperability considerations:">
2784      none
2785    </t>
2786    <t hangText="Published specification:">
2787      This specification (see <xref target=""/>).
2788    </t>
2789    <t hangText="Applications that use this media type:">
2790    </t>
2791    <t hangText="Additional information:">
2792      <list style="hanging">
2793        <t hangText="Magic number(s):">none</t>
2794        <t hangText="File extension(s):">none</t>
2795        <t hangText="Macintosh file type code(s):">none</t>
2796      </list>
2797    </t>
2798    <t hangText="Person and email address to contact for further information:">
2799      See Authors Section.
2800    </t>
2801                <t hangText="Intended usage:">
2802                  COMMON
2803    </t>
2804                <t hangText="Restrictions on usage:">
2805                  none
2806    </t>
2807    <t hangText="Author/Change controller:">
2808      IESG
2809    </t>
2810  </list>
2817<section title="Security Considerations" anchor="security.considerations">
2819   This section is meant to inform application developers, information
2820   providers, and users of the security limitations in HTTP/1.1 as
2821   described by this document. The discussion does not include
2822   definitive solutions to the problems revealed, though it does make
2823   some suggestions for reducing security risks.
2826<section title="Personal Information" anchor="personal.information">
2828   HTTP clients are often privy to large amounts of personal information
2829   (e.g. the user's name, location, mail address, passwords, encryption
2830   keys, etc.), and &SHOULD; be very careful to prevent unintentional
2831   leakage of this information.
2832   We very strongly recommend that a convenient interface be provided
2833   for the user to control dissemination of such information, and that
2834   designers and implementors be particularly careful in this area.
2835   History shows that errors in this area often create serious security
2836   and/or privacy problems and generate highly adverse publicity for the
2837   implementor's company.
2841<section title="Abuse of Server Log Information" anchor="abuse.of.server.log.information">
2843   A server is in the position to save personal data about a user's
2844   requests which might identify their reading patterns or subjects of
2845   interest. This information is clearly confidential in nature and its
2846   handling can be constrained by law in certain countries. People using
2847   HTTP to provide data are responsible for ensuring that
2848   such material is not distributed without the permission of any
2849   individuals that are identifiable by the published results.
2853<section title="Attacks Based On File and Path Names" anchor="attack.pathname">
2855   Implementations of HTTP origin servers &SHOULD; be careful to restrict
2856   the documents returned by HTTP requests to be only those that were
2857   intended by the server administrators. If an HTTP server translates
2858   HTTP URIs directly into file system calls, the server &MUST; take
2859   special care not to serve files that were not intended to be
2860   delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
2861   other operating systems use ".." as a path component to indicate a
2862   directory level above the current one. On such a system, an HTTP
2863   server &MUST; disallow any such construct in the Request-URI if it
2864   would otherwise allow access to a resource outside those intended to
2865   be accessible via the HTTP server. Similarly, files intended for
2866   reference only internally to the server (such as access control
2867   files, configuration files, and script code) &MUST; be protected from
2868   inappropriate retrieval, since they might contain sensitive
2869   information. Experience has shown that minor bugs in such HTTP server
2870   implementations have turned into security risks.
2874<section title="DNS Spoofing" anchor="dns.spoofing">
2876   Clients using HTTP rely heavily on the Domain Name Service, and are
2877   thus generally prone to security attacks based on the deliberate
2878   mis-association of IP addresses and DNS names. Clients need to be
2879   cautious in assuming the continuing validity of an IP number/DNS name
2880   association.
2883   In particular, HTTP clients &SHOULD; rely on their name resolver for
2884   confirmation of an IP number/DNS name association, rather than
2885   caching the result of previous host name lookups. Many platforms
2886   already can cache host name lookups locally when appropriate, and
2887   they &SHOULD; be configured to do so. It is proper for these lookups to
2888   be cached, however, only when the TTL (Time To Live) information
2889   reported by the name server makes it likely that the cached
2890   information will remain useful.
2893   If HTTP clients cache the results of host name lookups in order to
2894   achieve a performance improvement, they &MUST; observe the TTL
2895   information reported by DNS.
2898   If HTTP clients do not observe this rule, they could be spoofed when
2899   a previously-accessed server's IP address changes. As network
2900   renumbering is expected to become increasingly common <xref target="RFC1900"/>, the
2901   possibility of this form of attack will grow. Observing this
2902   requirement thus reduces this potential security vulnerability.
2905   This requirement also improves the load-balancing behavior of clients
2906   for replicated servers using the same DNS name and reduces the
2907   likelihood of a user's experiencing failure in accessing sites which
2908   use that strategy.
2912<section title="Proxies and Caching" anchor="attack.proxies">
2914   By their very nature, HTTP proxies are men-in-the-middle, and
2915   represent an opportunity for man-in-the-middle attacks. Compromise of
2916   the systems on which the proxies run can result in serious security
2917   and privacy problems. Proxies have access to security-related
2918   information, personal information about individual users and
2919   organizations, and proprietary information belonging to users and
2920   content providers. A compromised proxy, or a proxy implemented or
2921   configured without regard to security and privacy considerations,
2922   might be used in the commission of a wide range of potential attacks.
2925   Proxy operators should protect the systems on which proxies run as
2926   they would protect any system that contains or transports sensitive
2927   information. In particular, log information gathered at proxies often
2928   contains highly sensitive personal information, and/or information
2929   about organizations. Log information should be carefully guarded, and
2930   appropriate guidelines for use developed and followed. (<xref target="abuse.of.server.log.information"/>).
2933   Proxy implementors should consider the privacy and security
2934   implications of their design and coding decisions, and of the
2935   configuration options they provide to proxy operators (especially the
2936   default configuration).
2939   Users of a proxy need to be aware that they are no trustworthier than
2940   the people who run the proxy; HTTP itself cannot solve this problem.
2943   The judicious use of cryptography, when appropriate, may suffice to
2944   protect against a broad range of security and privacy attacks. Such
2945   cryptography is beyond the scope of the HTTP/1.1 specification.
2949<section title="Denial of Service Attacks on Proxies" anchor="attack.DoS">
2951   They exist. They are hard to defend against. Research continues.
2952   Beware.
2957<section title="Acknowledgments" anchor="ack">
2959   HTTP has evolved considerably over the years. It has
2960   benefited from a large and active developer community--the many
2961   people who have participated on the www-talk mailing list--and it is
2962   that community which has been most responsible for the success of
2963   HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
2964   Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
2965   Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
2966   McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
2967   VanHeyningen deserve special recognition for their efforts in
2968   defining early aspects of the protocol.
2971   This document has benefited greatly from the comments of all those
2972   participating in the HTTP-WG. In addition to those already mentioned,
2973   the following individuals have contributed to this specification:
2976   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,
2977   Paul Burchard, Maurizio Codogno, Mike Cowlishaw, Roman Czyborra,
2978   Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan Freier, Marc Hedlund, Greg Herlihy,
2979   Koen Holtman, Alex Hopmann, Bob Jernigan, Shel Kaphan, Rohit Khare,
2980   John Klensin, Martijn Koster, Alexei Kosut, David M. Kristol,
2981   Daniel LaLiberte, Ben Laurie, Paul J. Leach, Albert Lunde,
2982   John C. Mallery, Jean-Philippe Martin-Flatin, Mitra, David Morris,
2983   Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey Perry, Scott Powers, Owen Rees,
2984   Luigi Rizzo, David Robinson, Marc Salomon, Rich Salz,
2985   Allan M. Schiffman, Jim Seidman, Chuck Shotton, Eric W. Sink,
2986   Simon E. Spero, Richard N. Taylor, Robert S. Thau,
2987   Bill (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko,
2988   Josh Cohen.
2991   Thanks to the "cave men" of Palo Alto. You know who you are.
2994   Jim Gettys (the editor of <xref target="RFC2616"/>) wishes particularly
2995   to thank Roy Fielding, the editor of <xref target="RFC2068"/>, along
2996   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
2997   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
2998   Larry Masinter for their help. And thanks go particularly to Jeff
2999   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
3002   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
3003   Frystyk implemented RFC 2068 early, and we wish to thank them for the
3004   discovery of many of the problems that this document attempts to
3005   rectify.
3008   This specification makes heavy use of the augmented BNF and generic
3009   constructs defined by David H. Crocker for <xref target="RFC5234"/>. Similarly, it
3010   reuses many of the definitions provided by Nathaniel Borenstein and
3011   Ned Freed for MIME <xref target="RFC2045"/>. We hope that their inclusion in this
3012   specification will help reduce past confusion over the relationship
3013   between HTTP and Internet mail message formats.
3020<references title="Normative References">
3022<reference anchor="ISO-8859-1">
3023  <front>
3024    <title>
3025     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
3026    </title>
3027    <author>
3028      <organization>International Organization for Standardization</organization>
3029    </author>
3030    <date year="1998"/>
3031  </front>
3032  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
3035<reference anchor="Part2">
3036  <front>
3037    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
3038    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3039      <organization abbrev="Day Software">Day Software</organization>
3040      <address><email></email></address>
3041    </author>
3042    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3043      <organization>One Laptop per Child</organization>
3044      <address><email></email></address>
3045    </author>
3046    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3047      <organization abbrev="HP">Hewlett-Packard Company</organization>
3048      <address><email></email></address>
3049    </author>
3050    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3051      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3052      <address><email></email></address>
3053    </author>
3054    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3055      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3056      <address><email></email></address>
3057    </author>
3058    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3059      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3060      <address><email></email></address>
3061    </author>
3062    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3063      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3064      <address><email></email></address>
3065    </author>
3066    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3067      <organization abbrev="W3C">World Wide Web Consortium</organization>
3068      <address><email></email></address>
3069    </author>
3070    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3071      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3072      <address><email></email></address>
3073    </author>
3074    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3075  </front>
3076  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
3077  <x:source href="p2-semantics.xml" basename="p2-semantics"/>
3080<reference anchor="Part3">
3081  <front>
3082    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
3083    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3084      <organization abbrev="Day Software">Day Software</organization>
3085      <address><email></email></address>
3086    </author>
3087    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3088      <organization>One Laptop per Child</organization>
3089      <address><email></email></address>
3090    </author>
3091    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3092      <organization abbrev="HP">Hewlett-Packard Company</organization>
3093      <address><email></email></address>
3094    </author>
3095    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3096      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3097      <address><email></email></address>
3098    </author>
3099    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3100      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3101      <address><email></email></address>
3102    </author>
3103    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3104      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3105      <address><email></email></address>
3106    </author>
3107    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3108      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3109      <address><email></email></address>
3110    </author>
3111    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3112      <organization abbrev="W3C">World Wide Web Consortium</organization>
3113      <address><email></email></address>
3114    </author>
3115    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3116      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3117      <address><email></email></address>
3118    </author>
3119    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3120  </front>
3121  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
3122  <x:source href="p3-payload.xml" basename="p3-payload"/>
3125<reference anchor="Part5">
3126  <front>
3127    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
3128    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3129      <organization abbrev="Day Software">Day Software</organization>
3130      <address><email></email></address>
3131    </author>
3132    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3133      <organization>One Laptop per Child</organization>
3134      <address><email></email></address>
3135    </author>
3136    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3137      <organization abbrev="HP">Hewlett-Packard Company</organization>
3138      <address><email></email></address>
3139    </author>
3140    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3141      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3142      <address><email></email></address>
3143    </author>
3144    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3145      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3146      <address><email></email></address>
3147    </author>
3148    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3149      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3150      <address><email></email></address>
3151    </author>
3152    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3153      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3154      <address><email></email></address>
3155    </author>
3156    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3157      <organization abbrev="W3C">World Wide Web Consortium</organization>
3158      <address><email></email></address>
3159    </author>
3160    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3161      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3162      <address><email></email></address>
3163    </author>
3164    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3165  </front>
3166  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
3167  <x:source href="p5-range.xml" basename="p5-range"/>
3170<reference anchor="Part6">
3171  <front>
3172    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
3173    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
3174      <organization abbrev="Day Software">Day Software</organization>
3175      <address><email></email></address>
3176    </author>
3177    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3178      <organization>One Laptop per Child</organization>
3179      <address><email></email></address>
3180    </author>
3181    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3182      <organization abbrev="HP">Hewlett-Packard Company</organization>
3183      <address><email></email></address>
3184    </author>
3185    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
3186      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3187      <address><email></email></address>
3188    </author>
3189    <author initials="L." surname="Masinter" fullname="Larry Masinter">
3190      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
3191      <address><email></email></address>
3192    </author>
3193    <author initials="P." surname="Leach" fullname="Paul J. Leach">
3194      <organization abbrev="Microsoft">Microsoft Corporation</organization>
3195      <address><email></email></address>
3196    </author>
3197    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3198      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3199      <address><email></email></address>
3200    </author>
3201    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
3202      <organization abbrev="W3C">World Wide Web Consortium</organization>
3203      <address><email></email></address>
3204    </author>
3205    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
3206      <organization abbrev="greenbytes">greenbytes GmbH</organization>
3207      <address><email></email></address>
3208    </author>
3209    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
3210  </front>
3211  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
3212  <x:source href="p6-cache.xml" basename="p6-cache"/>
3215<reference anchor="RFC5234">
3216  <front>
3217    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
3218    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
3219      <organization>Brandenburg InternetWorking</organization>
3220      <address>
3221      <postal>
3222      <street>675 Spruce Dr.</street>
3223      <city>Sunnyvale</city>
3224      <region>CA</region>
3225      <code>94086</code>
3226      <country>US</country></postal>
3227      <phone>+1.408.246.8253</phone>
3228      <email></email></address> 
3229    </author>
3230    <author initials="P." surname="Overell" fullname="Paul Overell">
3231      <organization>THUS plc.</organization>
3232      <address>
3233      <postal>
3234      <street>1/2 Berkeley Square</street>
3235      <street>99 Berkely Street</street>
3236      <city>Glasgow</city>
3237      <code>G3 7HR</code>
3238      <country>UK</country></postal>
3239      <email></email></address>
3240    </author>
3241    <date month="January" year="2008"/>
3242  </front>
3243  <seriesInfo name="STD" value="68"/>
3244  <seriesInfo name="RFC" value="5234"/>
3247<reference anchor="RFC2045">
3248  <front>
3249    <title abbrev="Internet Message Bodies">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
3250    <author initials="N." surname="Freed" fullname="Ned Freed">
3251      <organization>Innosoft International, Inc.</organization>
3252      <address><email></email></address>
3253    </author>
3254    <author initials="N.S." surname="Borenstein" fullname="Nathaniel S. Borenstein">
3255      <organization>First Virtual Holdings</organization>
3256      <address><email></email></address>
3257    </author>
3258    <date month="November" year="1996"/>
3259  </front>
3260  <seriesInfo name="RFC" value="2045"/>
3263<reference anchor="RFC2047">
3264  <front>
3265    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
3266    <author initials="K." surname="Moore" fullname="Keith Moore">
3267      <organization>University of Tennessee</organization>
3268      <address><email></email></address>
3269    </author>
3270    <date month="November" year="1996"/>
3271  </front>
3272  <seriesInfo name="RFC" value="2047"/>
3275<reference anchor="RFC2119">
3276  <front>
3277    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
3278    <author initials="S." surname="Bradner" fullname="Scott Bradner">
3279      <organization>Harvard University</organization>
3280      <address><email></email></address>
3281    </author>
3282    <date month="March" year="1997"/>
3283  </front>
3284  <seriesInfo name="BCP" value="14"/>
3285  <seriesInfo name="RFC" value="2119"/>
3288<reference anchor="RFC3986">
3289 <front>
3290  <title abbrev='URI Generic Syntax'>Uniform Resource Identifier (URI): Generic Syntax</title>
3291  <author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
3292    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
3293    <address>
3294       <email></email>
3295       <uri></uri>
3296    </address>
3297  </author>
3298  <author initials='R.' surname='Fielding' fullname='Roy T. Fielding'>
3299    <organization abbrev="Day Software">Day Software</organization>
3300    <address>
3301      <email></email>
3302      <uri></uri>
3303    </address>
3304  </author>
3305  <author initials='L.' surname='Masinter' fullname='Larry Masinter'>
3306    <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
3307    <address>
3308      <email></email>
3309      <uri></uri>
3310    </address>
3311  </author>
3312  <date month='January' year='2005'></date>
3313 </front>
3314 <seriesInfo name="RFC" value="3986"/>
3315 <seriesInfo name="STD" value="66"/>
3318<reference anchor="USASCII">
3319  <front>
3320    <title>Coded Character Set -- 7-bit American Standard Code for Information Interchange</title>
3321    <author>
3322      <organization>American National Standards Institute</organization>
3323    </author>
3324    <date year="1986"/>
3325  </front>
3326  <seriesInfo name="ANSI" value="X3.4"/>
3331<references title="Informative References">
3333<reference anchor="Nie1997" target="">
3334  <front>
3335    <title>Network Performance Effects of HTTP/1.1, CSS1, and PNG</title>
3336    <author initials="H.F.." surname="Nielsen" fullname="H.F. Nielsen">
3337      <organization/>
3338    </author>
3339    <author initials="J." surname="Gettys" fullname="J. Gettys">
3340      <organization/>
3341    </author>
3342    <author initials="E." surname="Prud'hommeaux" fullname="E. Prud'hommeaux">
3343      <organization/>
3344    </author>
3345    <author initials="H." surname="Lie" fullname="H. Lie">
3346      <organization/>
3347    </author>
3348    <author initials="C." surname="Lilley" fullname="C. Lilley">
3349      <organization/>
3350    </author>
3351    <date year="1997" month="September"/>
3352  </front>
3353  <seriesInfo name="ACM" value="Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97"/>
3356<reference anchor="Pad1995" target="">
3357  <front>
3358    <title>Improving HTTP Latency</title>
3359    <author initials="V.N." surname="Padmanabhan" fullname="Venkata N. Padmanabhan">
3360      <organization/>
3361    </author>
3362    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3363      <organization/>
3364    </author>
3365    <date year="1995" month="December"/>
3366  </front>
3367  <seriesInfo name="Computer Networks and ISDN Systems" value="v. 28, pp. 25-35"/>
3370<reference anchor="RFC822">
3371  <front>
3372    <title abbrev="Standard for ARPA Internet Text Messages">Standard for the format of ARPA Internet text messages</title>
3373    <author initials="D.H." surname="Crocker" fullname="David H. Crocker">
3374      <organization>University of Delaware, Dept. of Electrical Engineering</organization>
3375      <address><email>DCrocker@UDel-Relay</email></address>
3376    </author>
3377    <date month="August" day="13" year="1982"/>
3378  </front>
3379  <seriesInfo name="STD" value="11"/>
3380  <seriesInfo name="RFC" value="822"/>
3383<reference anchor="RFC959">
3384  <front>
3385    <title abbrev="File Transfer Protocol">File Transfer Protocol</title>
3386    <author initials="J." surname="Postel" fullname="J. Postel">
3387      <organization>Information Sciences Institute (ISI)</organization>
3388    </author>
3389    <author initials="J." surname="Reynolds" fullname="J. Reynolds">
3390      <organization/>
3391    </author>
3392    <date month="October" year="1985"/>
3393  </front>
3394  <seriesInfo name="STD" value="9"/>
3395  <seriesInfo name="RFC" value="959"/>
3398<reference anchor="RFC1123">
3399  <front>
3400    <title>Requirements for Internet Hosts - Application and Support</title>
3401    <author initials="R." surname="Braden" fullname="Robert Braden">
3402      <organization>University of Southern California (USC), Information Sciences Institute</organization>
3403      <address><email>Braden@ISI.EDU</email></address>
3404    </author>
3405    <date month="October" year="1989"/>
3406  </front>
3407  <seriesInfo name="STD" value="3"/>
3408  <seriesInfo name="RFC" value="1123"/>
3411<reference anchor="RFC1305">
3412  <front>
3413    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
3414    <author initials="D." surname="Mills" fullname="David L. Mills">
3415      <organization>University of Delaware, Electrical Engineering Department</organization>
3416      <address><email></email></address>
3417    </author>
3418    <date month="March" year="1992"/>
3419  </front>
3420  <seriesInfo name="RFC" value="1305"/>
3423<reference anchor="RFC1436">
3424  <front>
3425    <title abbrev="Gopher">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</title>
3426    <author initials="F." surname="Anklesaria" fullname="Farhad Anklesaria">
3427      <organization>University of Minnesota, Computer and Information Services</organization>
3428      <address><email></email></address>
3429    </author>
3430    <author initials="M." surname="McCahill" fullname="Mark McCahill">
3431      <organization>University of Minnesota, Computer and Information Services</organization>
3432      <address><email></email></address>
3433    </author>
3434    <author initials="P." surname="Lindner" fullname="Paul Lindner">
3435      <organization>University of Minnesota, Computer and Information Services</organization>
3436      <address><email></email></address>
3437    </author>
3438    <author initials="D." surname="Johnson" fullname="David Johnson">
3439      <organization>University of Minnesota, Computer and Information Services</organization>
3440      <address><email></email></address>
3441    </author>
3442    <author initials="D." surname="Torrey" fullname="Daniel Torrey">
3443      <organization>University of Minnesota, Computer and Information Services</organization>
3444      <address><email></email></address>
3445    </author>
3446    <author initials="B." surname="Alberti" fullname="Bob Alberti">
3447      <organization>University of Minnesota, Computer and Information Services</organization>
3448      <address><email></email></address>
3449    </author>
3450    <date month="March" year="1993"/>
3451  </front>
3452  <seriesInfo name="RFC" value="1436"/>
3455<reference anchor="RFC1900">
3456  <front>
3457    <title>Renumbering Needs Work</title>
3458    <author initials="B." surname="Carpenter" fullname="Brian E. Carpenter">
3459      <organization>CERN, Computing and Networks Division</organization>
3460      <address><email></email></address>
3461    </author>
3462    <author initials="Y." surname="Rekhter" fullname="Yakov Rekhter">
3463      <organization>cisco Systems</organization>
3464      <address><email></email></address>
3465    </author>
3466    <date month="February" year="1996"/>
3467  </front>
3468  <seriesInfo name="RFC" value="1900"/>
3471<reference anchor="RFC1945">
3472  <front>
3473    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
3474    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3475      <organization>MIT, Laboratory for Computer Science</organization>
3476      <address><email></email></address>
3477    </author>
3478    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3479      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3480      <address><email></email></address>
3481    </author>
3482    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3483      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
3484      <address><email></email></address>
3485    </author>
3486    <date month="May" year="1996"/>
3487  </front>
3488  <seriesInfo name="RFC" value="1945"/>
3491<reference anchor="RFC2068">
3492  <front>
3493    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
3494    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
3495      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
3496      <address><email></email></address>
3497    </author>
3498    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3499      <organization>MIT Laboratory for Computer Science</organization>
3500      <address><email></email></address>
3501    </author>
3502    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
3503      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
3504      <address><email></email></address>
3505    </author>
3506    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3507      <organization>MIT Laboratory for Computer Science</organization>
3508      <address><email></email></address>
3509    </author>
3510    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
3511      <organization>MIT Laboratory for Computer Science</organization>
3512      <address><email></email></address>
3513    </author>
3514    <date month="January" year="1997"/>
3515  </front>
3516  <seriesInfo name="RFC" value="2068"/>
3519<reference anchor='RFC2109'>
3520  <front>
3521    <title>HTTP State Management Mechanism</title>
3522    <author initials='D.M.' surname='Kristol' fullname='David M. Kristol'>
3523      <organization>Bell Laboratories, Lucent Technologies</organization>
3524      <address><email></email></address>
3525    </author>
3526    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
3527      <organization>Netscape Communications Corp.</organization>
3528      <address><email></email></address>
3529    </author>
3530    <date year='1997' month='February' />
3531  </front>
3532  <seriesInfo name='RFC' value='2109' />
3535<reference anchor="RFC2145">
3536  <front>
3537    <title abbrev="HTTP Version Numbers">Use and Interpretation of HTTP Version Numbers</title>
3538    <author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
3539      <organization>Western Research Laboratory</organization>
3540      <address><email></email></address>
3541    </author>
3542    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
3543      <organization>Department of Information and Computer Science</organization>
3544      <address><email></email></address>
3545    </author>
3546    <author initials="J." surname="Gettys" fullname="Jim Gettys">
3547      <organization>MIT Laboratory for Computer Science</organization>
3548      <address><email></email></address>
3549    </author>
3550    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
3551      <organization>W3 Consortium</organization>
3552      <address><email></email></address>
3553    </author>
3554    <date month="May" year="1997"/>
3555  </front>
3556  <seriesInfo name="RFC" value="2145"/>
3559<reference anchor="RFC2616">
3560  <front>
3561    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
3562    <author initials="R." surname="Fielding" fullname="R. Fielding">
3563      <organization>University of California, Irvine</organization>
3564      <address><email></email></address>
3565    </author>
3566    <author initials="J." surname="Gettys" fullname="J. Gettys">
3567      <organization>W3C</organization>
3568      <address><email></email></address>
3569    </author>
3570    <author initials="J." surname="Mogul" fullname="J. Mogul">
3571      <organization>Compaq Computer Corporation</organization>
3572      <address><email></email></address>
3573    </author>
3574    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
3575      <organization>MIT Laboratory for Computer Science</organization>
3576      <address><email></email></address>
3577    </author>
3578    <author initials="L." surname="Masinter" fullname="L. Masinter">
3579      <organization>Xerox Corporation</organization>
3580      <address><email></email></address>
3581    </author>
3582    <author initials="P." surname="Leach" fullname="P. Leach">
3583      <organization>Microsoft Corporation</organization>
3584      <address><email></email></address>
3585    </author>
3586    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
3587      <organization>W3C</organization>
3588      <address><email></email></address>
3589    </author>
3590    <date month="June" year="1999"/>
3591  </front>
3592  <seriesInfo name="RFC" value="2616"/>
3595<reference anchor='RFC2818'>
3596  <front>
3597    <title>HTTP Over TLS</title>
3598    <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
3599      <organization>RTFM, Inc.</organization>
3600      <address><email></email></address>
3601    </author>
3602    <date year='2000' month='May' />
3603  </front>
3604  <seriesInfo name='RFC' value='2818' />
3607<reference anchor='RFC2965'>
3608  <front>
3609    <title>HTTP State Management Mechanism</title>
3610    <author initials='D. M.' surname='Kristol' fullname='David M. Kristol'>
3611      <organization>Bell Laboratories, Lucent Technologies</organization>
3612      <address><email></email></address>
3613    </author>
3614    <author initials='L.' surname='Montulli' fullname='Lou Montulli'>
3615      <organization>, Inc.</organization>
3616      <address><email></email></address>
3617    </author>
3618    <date year='2000' month='October' />
3619  </front>
3620  <seriesInfo name='RFC' value='2965' />
3623<reference anchor='RFC3864'>
3624  <front>
3625    <title>Registration Procedures for Message Header Fields</title>
3626    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
3627      <organization>Nine by Nine</organization>
3628      <address><email></email></address>
3629    </author>
3630    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
3631      <organization>BEA Systems</organization>
3632      <address><email></email></address>
3633    </author>
3634    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
3635      <organization>HP Labs</organization>
3636      <address><email></email></address>
3637    </author>
3638    <date year='2004' month='September' />
3639  </front>
3640  <seriesInfo name='BCP' value='90' />
3641  <seriesInfo name='RFC' value='3864' />
3644<reference anchor='RFC3977'>
3645  <front>
3646    <title>Network News Transfer Protocol (NNTP)</title>
3647    <author initials='C.' surname='Feather' fullname='C. Feather'>
3648      <organization>THUS plc</organization>
3649      <address><email></email></address>
3650    </author>
3651    <date year='2006' month='October' />
3652  </front>
3653  <seriesInfo name="RFC" value="3977"/>
3656<reference anchor="RFC4288">
3657  <front>
3658    <title>Media Type Specifications and Registration Procedures</title>
3659    <author initials="N." surname="Freed" fullname="N. Freed">
3660      <organization>Sun Microsystems</organization>
3661      <address>
3662        <email></email>
3663      </address>
3664    </author>
3665    <author initials="J." surname="Klensin" fullname="J. Klensin">
3666      <organization/>
3667      <address>
3668        <email></email>
3669      </address>
3670    </author>
3671    <date year="2005" month="December"/>
3672  </front>
3673  <seriesInfo name="BCP" value="13"/>
3674  <seriesInfo name="RFC" value="4288"/>
3677<reference anchor='RFC4395'>
3678  <front>
3679    <title>Guidelines and Registration Procedures for New URI Schemes</title>
3680    <author initials='T.' surname='Hansen' fullname='T. Hansen'>
3681      <organization>AT&amp;T Laboratories</organization>
3682      <address>
3683        <email></email>
3684      </address>
3685    </author>
3686    <author initials='T.' surname='Hardie' fullname='T. Hardie'>
3687      <organization>Qualcomm, Inc.</organization>
3688      <address>
3689        <email></email>
3690      </address>
3691    </author>
3692    <author initials='L.' surname='Masinter' fullname='L. Masinter'>
3693      <organization>Adobe Systems</organization>
3694      <address>
3695        <email></email>
3696      </address>
3697    </author>
3698    <date year='2006' month='February' />
3699  </front>
3700  <seriesInfo name='BCP' value='115' />
3701  <seriesInfo name='RFC' value='4395' />
3704<reference anchor="RFC5322">
3705  <front>
3706    <title>Internet Message Format</title>
3707    <author initials="P." surname="Resnick" fullname="P. Resnick">
3708      <organization>Qualcomm Incorporated</organization>
3709    </author>
3710    <date year="2008" month="October"/>
3711  </front>
3712  <seriesInfo name="RFC" value="5322"/>
3715<reference anchor="Kri2001" target="">
3716  <front>
3717    <title>HTTP Cookies: Standards, Privacy, and Politics</title>
3718    <author initials="D." surname="Kristol" fullname="David M. Kristol">
3719      <organization/>
3720    </author>
3721    <date year="2001" month="November"/>
3722  </front>
3723  <seriesInfo name="ACM Transactions on Internet Technology" value="Vol. 1, #2"/>
3726<reference anchor="Spe" target="">
3727  <front>
3728  <title>Analysis of HTTP Performance Problems</title>
3729  <author initials="S." surname="Spero" fullname="Simon E. Spero">
3730    <organization/>
3731  </author>
3732  <date/>
3733  </front>
3736<reference anchor="Tou1998" target="">
3737  <front>
3738  <title>Analysis of HTTP Performance</title>
3739  <author initials="J." surname="Touch" fullname="Joe Touch">
3740    <organization>USC/Information Sciences Institute</organization>
3741    <address><email></email></address>
3742  </author>
3743  <author initials="J." surname="Heidemann" fullname="John Heidemann">
3744    <organization>USC/Information Sciences Institute</organization>
3745    <address><email></email></address>
3746  </author>
3747  <author initials="K." surname="Obraczka" fullname="Katia Obraczka">
3748    <organization>USC/Information Sciences Institute</organization>
3749    <address><email></email></address>
3750  </author>
3751  <date year="1998" month="Aug"/>
3752  </front>
3753  <seriesInfo name="ISI Research Report" value="ISI/RR-98-463"/>
3754  <annotation>(original report dated Aug. 1996)</annotation>
3757<reference anchor="WAIS">
3758  <front>
3759    <title>WAIS Interface Protocol Prototype Functional Specification (v1.5)</title>
3760    <author initials="F." surname="Davis" fullname="F. Davis">
3761      <organization>Thinking Machines Corporation</organization>
3762    </author>
3763    <author initials="B." surname="Kahle" fullname="B. Kahle">
3764      <organization>Thinking Machines Corporation</organization>
3765    </author>
3766    <author initials="H." surname="Morris" fullname="H. Morris">
3767      <organization>Thinking Machines Corporation</organization>
3768    </author>
3769    <author initials="J." surname="Salem" fullname="J. Salem">
3770      <organization>Thinking Machines Corporation</organization>
3771    </author>
3772    <author initials="T." surname="Shen" fullname="T. Shen">
3773      <organization>Thinking Machines Corporation</organization>
3774    </author>
3775    <author initials="R." surname="Wang" fullname="R. Wang">
3776      <organization>Thinking Machines Corporation</organization>
3777    </author>
3778    <author initials="J." surname="Sui" fullname="J. Sui">
3779      <organization>Thinking Machines Corporation</organization>
3780    </author>
3781    <author initials="M." surname="Grinbaum" fullname="M. Grinbaum">
3782      <organization>Thinking Machines Corporation</organization>
3783    </author>
3784    <date month="April" year="1990"/>
3785  </front>
3786  <seriesInfo name="Thinking Machines Corporation" value=""/>
3792<section title="Tolerant Applications" anchor="tolerant.applications">
3794   Although this document specifies the requirements for the generation
3795   of HTTP/1.1 messages, not all applications will be correct in their
3796   implementation. We therefore recommend that operational applications
3797   be tolerant of deviations whenever those deviations can be
3798   interpreted unambiguously.
3801   Clients &SHOULD; be tolerant in parsing the Status-Line and servers
3802   tolerant when parsing the Request-Line. In particular, they &SHOULD;
3803   accept any amount of SP or HTAB characters between fields, even though
3804   only a single SP is required.
3807   The line terminator for message-header fields is the sequence CRLF.
3808   However, we recommend that applications, when parsing such headers,
3809   recognize a single LF as a line terminator and ignore the leading CR.
3812   The character set of an entity-body &SHOULD; be labeled as the lowest
3813   common denominator of the character codes used within that body, with
3814   the exception that not labeling the entity is preferred over labeling
3815   the entity with the labels US-ASCII or ISO-8859-1. See &payload;.
3818   Additional rules for requirements on parsing and encoding of dates
3819   and other potential problems with date encodings include:
3822  <list style="symbols">
3823     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
3824        which appears to be more than 50 years in the future is in fact
3825        in the past (this helps solve the "year 2000" problem).</t>
3827     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
3828        Expires date as earlier than the proper value, but &MUST-NOT;
3829        internally represent a parsed Expires date as later than the
3830        proper value.</t>
3832     <t>All expiration-related calculations &MUST; be done in GMT. The
3833        local time zone &MUST-NOT; influence the calculation or comparison
3834        of an age or expiration time.</t>
3836     <t>If an HTTP header incorrectly carries a date value with a time
3837        zone other than GMT, it &MUST; be converted into GMT using the
3838        most conservative possible conversion.</t>
3839  </list>
3843<section title="Conversion of Date Formats" anchor="">
3845   HTTP/1.1 uses a restricted set of date formats (<xref target=""/>) to
3846   simplify the process of date comparison. Proxies and gateways from
3847   other protocols &SHOULD; ensure that any Date header field present in a
3848   message conforms to one of the HTTP/1.1 formats and rewrite the date
3849   if necessary.
3853<section title="Compatibility with Previous Versions" anchor="compatibility">
3855   HTTP has been in use by the World-Wide Web global information initiative
3856   since 1990. The first version of HTTP, later referred to as HTTP/0.9,
3857   was a simple protocol for hypertext data transfer across the Internet
3858   with only a single method and no metadata.
3859   HTTP/1.0, as defined by <xref target="RFC1945"/>, added a range of request
3860   methods and MIME-like messaging that could include metadata about the data
3861   transferred and modifiers on the request/response semantics. However,
3862   HTTP/1.0 did not sufficiently take into consideration the effects of
3863   hierarchical proxies, caching, the need for persistent connections, or
3864   name-based virtual hosts. The proliferation of incompletely-implemented
3865   applications calling themselves "HTTP/1.0" further necessitated a
3866   protocol version change in order for two communicating applications
3867   to determine each other's true capabilities.
3870   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
3871   requirements that enable reliable implementations, adding only
3872   those new features that will either be safely ignored by an HTTP/1.0
3873   recipient or only sent when communicating with a party advertising
3874   compliance with HTTP/1.1.
3877   It is beyond the scope of a protocol specification to mandate
3878   compliance with previous versions. HTTP/1.1 was deliberately
3879   designed, however, to make supporting previous versions easy. It is
3880   worth noting that, at the time of composing this specification
3881   (1996), we would expect commercial HTTP/1.1 servers to:
3882  <list style="symbols">
3883     <t>recognize the format of the Request-Line for HTTP/0.9, 1.0, and
3884        1.1 requests;</t>
3886     <t>understand any valid request in the format of HTTP/0.9, 1.0, or
3887        1.1;</t>
3889     <t>respond appropriately with a message in the same major version
3890        used by the client.</t>
3891  </list>
3894   And we would expect HTTP/1.1 clients to:
3895  <list style="symbols">
3896     <t>recognize the format of the Status-Line for HTTP/1.0 and 1.1
3897        responses;</t>
3899     <t>understand any valid response in the format of HTTP/0.9, 1.0, or
3900        1.1.</t>
3901  </list>
3904   For most implementations of HTTP/1.0, each connection is established
3905   by the client prior to the request and closed by the server after
3906   sending the response. Some implementations implement the Keep-Alive
3907   version of persistent connections described in <xref x:sec="19.7.1" x:fmt="of" target="RFC2068"/>.
3910<section title="Changes from HTTP/1.0" anchor="changes.from.1.0">
3912   This section summarizes major differences between versions HTTP/1.0
3913   and HTTP/1.1.
3916<section title="Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses" anchor="">
3918   The requirements that clients and servers support the Host request-header,
3919   report an error if the Host request-header (<xref target=""/>) is
3920   missing from an HTTP/1.1 request, and accept absolute URIs (<xref target="request-uri"/>)
3921   are among the most important changes defined by this
3922   specification.
3925   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
3926   addresses and servers; there was no other established mechanism for
3927   distinguishing the intended server of a request than the IP address
3928   to which that request was directed. The changes outlined above will
3929   allow the Internet, once older HTTP clients are no longer common, to
3930   support multiple Web sites from a single IP address, greatly
3931   simplifying large operational Web servers, where allocation of many
3932   IP addresses to a single host has created serious problems. The
3933   Internet will also be able to recover the IP addresses that have been
3934   allocated for the sole purpose of allowing special-purpose domain
3935   names to be used in root-level HTTP URLs. Given the rate of growth of
3936   the Web, and the number of servers already deployed, it is extremely
3937   important that all implementations of HTTP (including updates to
3938   existing HTTP/1.0 applications) correctly implement these
3939   requirements:
3940  <list style="symbols">
3941     <t>Both clients and servers &MUST; support the Host request-header.</t>
3943     <t>A client that sends an HTTP/1.1 request &MUST; send a Host header.</t>
3945     <t>Servers &MUST; report a 400 (Bad Request) error if an HTTP/1.1
3946        request does not include a Host request-header.</t>
3948     <t>Servers &MUST; accept absolute URIs.</t>
3949  </list>
3954<section title="Compatibility with HTTP/1.0 Persistent Connections" anchor="compatibility.with.http.1.0.persistent.connections">
3956   Some clients and servers might wish to be compatible with some
3957   previous implementations of persistent connections in HTTP/1.0
3958   clients and servers. Persistent connections in HTTP/1.0 are
3959   explicitly negotiated as they are not the default behavior. HTTP/1.0
3960   experimental implementations of persistent connections are faulty,
3961   and the new facilities in HTTP/1.1 are designed to rectify these
3962   problems. The problem was that some existing 1.0 clients may be
3963   sending Keep-Alive to a proxy server that doesn't understand
3964   Connection, which would then erroneously forward it to the next
3965   inbound server, which would establish the Keep-Alive connection and
3966   result in a hung HTTP/1.0 proxy waiting for the close on the
3967   response. The result is that HTTP/1.0 clients must be prevented from
3968   using Keep-Alive when talking to proxies.
3971   However, talking to proxies is the most important use of persistent
3972   connections, so that prohibition is clearly unacceptable. Therefore,
3973   we need some other mechanism for indicating a persistent connection
3974   is desired, which is safe to use even when talking to an old proxy
3975   that ignores Connection. Persistent connections are the default for
3976   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
3977   declaring non-persistence. See <xref target="header.connection"/>.
3980   The original HTTP/1.0 form of persistent connections (the Connection:
3981   Keep-Alive and Keep-Alive header) is documented in <xref target="RFC2068"/>.
3985<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
3987   This specification has been carefully audited to correct and
3988   disambiguate key word usage; RFC 2068 had many problems in respect to
3989   the conventions laid out in <xref target="RFC2119"/>.
3992   Transfer-coding and message lengths all interact in ways that
3993   required fixing exactly when chunked encoding is used (to allow for
3994   transfer encoding that may not be self delimiting); it was important
3995   to straighten out exactly how message lengths are computed. (Sections
3996   <xref target="transfer.codings" format="counter"/>, <xref target="message.length" format="counter"/>,
3997   <xref target="header.content-length" format="counter"/>,
3998   see also <xref target="Part3"/>, <xref target="Part5"/> and <xref target="Part6"/>)
4001   The use and interpretation of HTTP version numbers has been clarified
4002   by <xref target="RFC2145"/>. Require proxies to upgrade requests to highest protocol
4003   version they support to deal with problems discovered in HTTP/1.0
4004   implementations (<xref target="http.version"/>)
4007   Transfer-coding had significant problems, particularly with
4008   interactions with chunked encoding. The solution is that transfer-codings
4009   become as full fledged as content-codings. This involves
4010   adding an IANA registry for transfer-codings (separate from content
4011   codings), a new header field (TE) and enabling trailer headers in the
4012   future. Transfer encoding is a major performance benefit, so it was
4013   worth fixing <xref target="Nie1997"/>. TE also solves another, obscure, downward
4014   interoperability problem that could have occurred due to interactions
4015   between authentication trailers, chunked encoding and HTTP/1.0
4016   clients.(Section <xref target="transfer.codings" format="counter"/>, <xref target="chunked.transfer.encoding" format="counter"/>,
4017   and <xref target="header.te" format="counter"/>)
4021<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
4023  Rules about implicit linear white space between certain grammar productions
4024  have been removed; now it's only allowed when specifically pointed out
4025  in the ABNF.
4026  The CHAR rule does not allow the NUL character anymore (this affects
4027  the comment and quoted-string rules).  Furthermore, the quoted-pair
4028  rule does not allow escaping NUL, CR or LF anymore.
4029  (<xref target="basic.rules"/>)
4032  Clarify that HTTP-Version is case sensitive.
4033  (<xref target="http.version"/>)
4036  Remove reference to non-existant identity transfer-coding value tokens.
4037  (Sections <xref format="counter" target="transfer.codings"/> and
4038  <xref format="counter" target="message.length"/>)
4041  Clarification that the chunk length does not include
4042  the count of the octets in the chunk header and trailer.
4043  (<xref target="chunked.transfer.encoding"/>)
4046  Update use of abs_path production from RFC1808 to the path-absolute + query
4047  components of RFC3986.
4048  (<xref target="request-uri"/>)
4051  Clarify exactly when close connection options must be sent.
4052  (<xref target="header.connection"/>)
4057<section title="Terminology" anchor="terminology">
4059   This specification uses a number of terms to refer to the roles
4060   played by participants in, and objects of, the HTTP communication.
4063  <iref item="connection"/>
4064  <x:dfn>connection</x:dfn>
4065  <list>
4066    <t>
4067      A transport layer virtual circuit established between two programs
4068      for the purpose of communication.
4069    </t>
4070  </list>
4073  <iref item="message"/>
4074  <x:dfn>message</x:dfn>
4075  <list>
4076    <t>
4077      The basic unit of HTTP communication, consisting of a structured
4078      sequence of octets matching the syntax defined in <xref target="http.message"/> and
4079      transmitted via the connection.
4080    </t>
4081  </list>
4084  <iref item="request"/>
4085  <x:dfn>request</x:dfn>
4086  <list>
4087    <t>
4088      An HTTP request message, as defined in <xref target="request"/>.
4089    </t>
4090  </list>
4093  <iref item="response"/>
4094  <x:dfn>response</x:dfn>
4095  <list>
4096    <t>
4097      An HTTP response message, as defined in <xref target="response"/>.
4098    </t>
4099  </list>
4102  <iref item="resource"/>
4103  <x:dfn>resource</x:dfn>
4104  <list>
4105    <t>
4106      A network data object or service that can be identified by a URI,
4107      as defined in <xref target="uri"/>. Resources may be available in multiple
4108      representations (e.g. multiple languages, data formats, size, and
4109      resolutions) or vary in other ways.
4110    </t>
4111  </list>
4114  <iref item="entity"/>
4115  <x:dfn>entity</x:dfn>
4116  <list>
4117    <t>
4118      The information transferred as the payload of a request or
4119      response. An entity consists of metainformation in the form of
4120      entity-header fields and content in the form of an entity-body, as
4121      described in &entity;.
4122    </t>
4123  </list>
4126  <iref item="representation"/>
4127  <x:dfn>representation</x:dfn>
4128  <list>
4129    <t>
4130      An entity included with a response that is subject to content
4131      negotiation, as described in &content.negotiation;. There may exist multiple
4132      representations associated with a particular response status.
4133    </t>
4134  </list>
4137  <iref item="content negotiation"/>
4138  <x:dfn>content negotiation</x:dfn>
4139  <list>
4140    <t>
4141      The mechanism for selecting the appropriate representation when
4142      servicing a request, as described in &content.negotiation;. The
4143      representation of entities in any response can be negotiated
4144      (including error responses).
4145    </t>
4146  </list>
4149  <iref item="variant"/>
4150  <x:dfn>variant</x:dfn>
4151  <list>
4152    <t>
4153      A resource may have one, or more than one, representation(s)
4154      associated with it at any given instant. Each of these
4155      representations is termed a `variant'.  Use of the term `variant'
4156      does not necessarily imply that the resource is subject to content
4157      negotiation.
4158    </t>
4159  </list>
4162  <iref item="client"/>
4163  <x:dfn>client</x:dfn>
4164  <list>
4165    <t>
4166      A program that establishes connections for the purpose of sending
4167      requests.
4168    </t>
4169  </list>
4172  <iref item="user agent"/>
4173  <x:dfn>user agent</x:dfn>
4174  <list>
4175    <t>
4176      The client which initiates a request. These are often browsers,
4177      editors, spiders (web-traversing robots), or other end user tools.
4178    </t>
4179  </list>
4182  <iref item="server"/>
4183  <x:dfn>server</x:dfn>
4184  <list>
4185    <t>
4186      An application program that accepts connections in order to
4187      service requests by sending back responses. Any given program may
4188      be capable of being both a client and a server; our use of these
4189      terms refers only to the role being performed by the program for a
4190      particular connection, rather than to the program's capabilities
4191      in general. Likewise, any server may act as an origin server,
4192      proxy, gateway, or tunnel, switching behavior based on the nature
4193      of each request.
4194    </t>
4195  </list>
4198  <iref item="origin server"/>
4199  <x:dfn>origin server</x:dfn>
4200  <list>
4201    <t>
4202      The server on which a given resource resides or is to be created.
4203    </t>
4204  </list>
4207  <iref item="proxy"/>
4208  <x:dfn>proxy</x:dfn>
4209  <list>
4210    <t>
4211      An intermediary program which acts as both a server and a client
4212      for the purpose of making requests on behalf of other clients.
4213      Requests are serviced internally or by passing them on, with
4214      possible translation, to other servers. A proxy &MUST; implement
4215      both the client and server requirements of this specification. A
4216      "transparent proxy" is a proxy that does not modify the request or
4217      response beyond what is required for proxy authentication and
4218      identification. A "non-transparent proxy" is a proxy that modifies
4219      the request or response in order to provide some added service to
4220      the user agent, such as group annotation services, media type
4221      transformation, protocol reduction, or anonymity filtering. Except
4222      where either transparent or non-transparent behavior is explicitly
4223      stated, the HTTP proxy requirements apply to both types of
4224      proxies.
4225    </t>
4226  </list>
4229  <iref item="gateway"/>
4230  <x:dfn>gateway</x:dfn>
4231  <list>
4232    <t>
4233      A server which acts as an intermediary for some other server.
4234      Unlike a proxy, a gateway receives requests as if it were the
4235      origin server for the requested resource; the requesting client
4236      may not be aware that it is communicating with a gateway.
4237    </t>
4238  </list>
4241  <iref item="tunnel"/>
4242  <x:dfn>tunnel</x:dfn>
4243  <list>
4244    <t>
4245      An intermediary program which is acting as a blind relay between
4246      two connections. Once active, a tunnel is not considered a party
4247      to the HTTP communication, though the tunnel may have been
4248      initiated by an HTTP request. The tunnel ceases to exist when both
4249      ends of the relayed connections are closed.
4250    </t>
4251  </list>
4254  <iref item="cache"/>
4255  <x:dfn>cache</x:dfn>
4256  <list>
4257    <t>
4258      A program's local store of response messages and the subsystem
4259      that controls its message storage, retrieval, and deletion. A
4260      cache stores cacheable responses in order to reduce the response
4261      time and network bandwidth consumption on future, equivalent
4262      requests. Any client or server may include a cache, though a cache
4263      cannot be used by a server that is acting as a tunnel.
4264    </t>
4265  </list>
4268  <iref item="cacheable"/>
4269  <x:dfn>cacheable</x:dfn>
4270  <list>
4271    <t>
4272      A response is cacheable if a cache is allowed to store a copy of
4273      the response message for use in answering subsequent requests. The
4274      rules for determining the cacheability of HTTP responses are
4275      defined in &caching;. Even if a resource is cacheable, there may
4276      be additional constraints on whether a cache can use the cached
4277      copy for a particular request.
4278    </t>
4279  </list>
4282  <iref item="upstream"/>
4283  <iref item="downstream"/>
4284  <x:dfn>upstream</x:dfn>/<x:dfn>downstream</x:dfn>
4285  <list>
4286    <t>
4287      Upstream and downstream describe the flow of a message: all
4288      messages flow from upstream to downstream.
4289    </t>
4290  </list>
4293  <iref item="inbound"/>
4294  <iref item="outbound"/>
4295  <x:dfn>inbound</x:dfn>/<x:dfn>outbound</x:dfn>
4296  <list>
4297    <t>
4298      Inbound and outbound refer to the request and response paths for
4299      messages: "inbound" means "traveling toward the origin server",
4300      and "outbound" means "traveling toward the user agent"
4301    </t>
4302  </list>
4306<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
4308<section title="Since RFC2616">
4310  Extracted relevant partitions from <xref target="RFC2616"/>.
4314<section title="Since draft-ietf-httpbis-p1-messaging-00">
4316  Closed issues:
4317  <list style="symbols">
4318    <t>
4319      <eref target=""/>:
4320      "HTTP Version should be case sensitive"
4321      (<eref target=""/>)
4322    </t>
4323    <t>
4324      <eref target=""/>:
4325      "'unsafe' characters"
4326      (<eref target=""/>)
4327    </t>
4328    <t>
4329      <eref target=""/>:
4330      "Chunk Size Definition"
4331      (<eref target=""/>)
4332    </t>
4333    <t>
4334      <eref target=""/>:
4335      "Message Length"
4336      (<eref target=""/>)
4337    </t>
4338    <t>
4339      <eref target=""/>:
4340      "Media Type Registrations"
4341      (<eref target=""/>)
4342    </t>
4343    <t>
4344      <eref target=""/>:
4345      "URI includes query"
4346      (<eref target=""/>)
4347    </t>
4348    <t>
4349      <eref target=""/>:
4350      "No close on 1xx responses"
4351      (<eref target=""/>)
4352    </t>
4353    <t>
4354      <eref target=""/>:
4355      "Remove 'identity' token references"
4356      (<eref target=""/>)
4357    </t>
4358    <t>
4359      <eref target=""/>:
4360      "Import query BNF"
4361    </t>
4362    <t>
4363      <eref target=""/>:
4364      "qdtext BNF"
4365    </t>
4366    <t>
4367      <eref target=""/>:
4368      "Normative and Informative references"
4369    </t>
4370    <t>
4371      <eref target=""/>:
4372      "RFC2606 Compliance"
4373    </t>
4374    <t>
4375      <eref target=""/>:
4376      "RFC977 reference"
4377    </t>
4378    <t>
4379      <eref target=""/>:
4380      "RFC1700 references"
4381    </t>
4382    <t>
4383      <eref target=""/>:
4384      "inconsistency in date format explanation"
4385    </t>
4386    <t>
4387      <eref target=""/>:
4388      "Date reference typo"
4389    </t>
4390    <t>
4391      <eref target=""/>:
4392      "Informative references"
4393    </t>
4394    <t>
4395      <eref target=""/>:
4396      "ISO-8859-1 Reference"
4397    </t>
4398    <t>
4399      <eref target=""/>:
4400      "Normative up-to-date references"
4401    </t>
4402  </list>
4405  Other changes:
4406  <list style="symbols">
4407    <t>
4408      Update media type registrations to use RFC4288 template.
4409    </t>
4410    <t>
4411      Use names of RFC4234 core rules DQUOTE and HTAB,
4412      fix broken ABNF for chunk-data
4413      (work in progress on <eref target=""/>)
4414    </t>
4415  </list>
4419<section title="Since draft-ietf-httpbis-p1-messaging-01">
4421  Closed issues:
4422  <list style="symbols">
4423    <t>
4424      <eref target=""/>:
4425      "Bodies on GET (and other) requests"
4426    </t>
4427    <t>
4428      <eref target=""/>:
4429      "Updating to RFC4288"
4430    </t>
4431    <t>
4432      <eref target=""/>:
4433      "Status Code and Reason Phrase"
4434    </t>
4435    <t>
4436      <eref target=""/>:
4437      "rel_path not used"
4438    </t>
4439  </list>
4442  Ongoing work on ABNF conversion (<eref target=""/>):
4443  <list style="symbols">
4444    <t>
4445      Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" ->
4446      "trailer-part").
4447    </t>
4448    <t>
4449      Avoid underscore character in rule names ("http_URL" ->
4450      "http-URL", "abs_path" -> "path-absolute").
4451    </t>
4452    <t>
4453      Add rules for terms imported from URI spec ("absoluteURI", "authority",
4454      "path-absolute", "port", "query", "relativeURI", "host) -- these will
4455      have to be updated when switching over to RFC3986.
4456    </t>
4457    <t>
4458      Synchronize core rules with RFC5234 (this includes a change to CHAR
4459      which now excludes NUL).
4460    </t>
4461    <t>
4462      Get rid of prose rules that span multiple lines.
4463    </t>
4464    <t>
4465      Get rid of unused rules LOALPHA and UPALPHA.
4466    </t>
4467    <t>
4468      Move "Product Tokens" section (back) into Part 1, as "token" is used
4469      in the definition of the Upgrade header.
4470    </t>
4471    <t>
4472      Add explicit references to BNF syntax and rules imported from other parts of the specification.
4473    </t>
4474    <t>
4475      Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".
4476    </t>
4477  </list>
4481<section title="Since draft-ietf-httpbis-p1-messaging-02" anchor="changes.since.02">
4483  Closed issues:
4484  <list style="symbols">
4485    <t>
4486      <eref target=""/>:
4487      "HTTP-date vs. rfc1123-date"
4488    </t>
4489    <t>
4490      <eref target=""/>:
4491      "WS in quoted-pair"
4492    </t>
4493  </list>
4496  Ongoing work on IANA Message Header Registration (<eref target=""/>):
4497  <list style="symbols">
4498    <t>
4499      Reference RFC 3984, and update header registrations for headers defined
4500      in this document.
4501    </t>
4502  </list>
4505  Ongoing work on ABNF conversion (<eref target=""/>):
4506  <list style="symbols">
4507    <t>
4508      Replace string literals when the string really is case-sensitive (HTTP-Version).
4509    </t>
4510  </list>
4514<section title="Since draft-ietf-httpbis-p1-messaging-03" anchor="changes.since.03">
4516  Closed issues:
4517  <list style="symbols">
4518    <t>
4519      <eref target=""/>:
4520      "Connection closing"
4521    </t>
4522    <t>
4523      <eref target=""/>:
4524      "Move registrations and registry information to IANA Considerations"
4525    </t>
4526    <t>
4527      <eref target=""/>:
4528      "need new URL for PAD1995 reference"
4529    </t>
4530    <t>
4531      <eref target=""/>:
4532      "IANA Considerations: update HTTP URI scheme registration"
4533    </t>
4534    <t>
4535      <eref target=""/>:
4536      "Cite HTTPS URI scheme definition"
4537    </t>
4538    <t>
4539      <eref target=""/>:
4540      "List-type headers vs Set-Cookie"
4541    </t>
4542  </list>
4545  Ongoing work on ABNF conversion (<eref target=""/>):
4546  <list style="symbols">
4547    <t>
4548      Replace string literals when the string really is case-sensitive (HTTP-Date).
4549    </t>
4550    <t>
4551      Replace HEX by HEXDIG for future consistence with RFC 5234's core rules.
4552    </t>
4553  </list>
4557<section title="Since draft-ietf-httpbis-p1-messaging-04" anchor="changes.since.04">
4559  Closed issues:
4560  <list style="symbols">
4561    <t>
4562      <eref target=""/>:
4563      "Out-of-date reference for URIs"
4564    </t>
4565    <t>
4566      <eref target=""/>:
4567      "RFC 2822 is updated by RFC 5322"
4568    </t>
4569  </list>
4572  Ongoing work on ABNF conversion (<eref target=""/>):
4573  <list style="symbols">
4574    <t>
4575      Use "/" instead of "|" for alternatives.
4576    </t>
4577    <t>
4578      Get rid of RFC822 dependency; use RFC5234 plus extensions instead.
4579    </t>
4580    <t>
4581      Only reference RFC 5234's core rules.
4582    </t>
4583    <t>
4584      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
4585      whitespace ("OWS") and required whitespace ("RWS").
4586    </t>
4587    <t>
4588      Rewrite ABNFs to spell out whitespace rules, factor out
4589      header value format definitions.
4590    </t>
4591  </list>
4595<section title="Since draft-ietf-httpbis-p1-messaging-05" anchor="changes.since.05">
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