Ignore:
Timestamp:
01/10/12 07:39:45 (9 years ago)
Author:
fielding@…
Message:

(editorial) move protocol elements up near their primary use in fields

File:
1 edited

Legend:

Unmodified
Added
Removed

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

    r1922 r1923  
    289289</t>
    290290
    291 <section title="Representation Header Fields" anchor="representation.header.fields">
     291<section title="Representation Metadata" anchor="representation.metadata">
    292292  <x:anchor-alias value="representation-header"/>
    293293<t>
    … …  
    300300</t>
    301301<t>
    302    The following header fields are defined as representation metadata:
     302   The following header fields are defined to convey representation metadata:
    303303</t>
    304304<texttable align="left">
    … …  
    312312  <c>Expires</c> <c>&header-expires;</c>
    313313</texttable>
     314
     315<section title="Data Type" anchor="data.type">
     316
     317<section title="Media Types" anchor="media.types">
     318  <x:anchor-alias value="media-type"/>
     319  <x:anchor-alias value="type"/>
     320  <x:anchor-alias value="subtype"/>
     321<t>
     322   HTTP uses Internet Media Types <xref target="RFC2046"/> in the
     323   <x:ref>Content-Type</x:ref> (<xref target="header.content-type"/>)
     324   and <x:ref>Accept</x:ref> (<xref target="header.accept"/>) header fields in
     325   order to provide open and extensible data typing and type negotiation.
     326</t>
     327<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="media-type"/><iref primary="true" item="Grammar" subitem="type"/><iref primary="true" item="Grammar" subitem="subtype"/>
     328  <x:ref>media-type</x:ref> = <x:ref>type</x:ref> "/" <x:ref>subtype</x:ref> *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>parameter</x:ref> )
     329  <x:ref>type</x:ref>       = <x:ref>token</x:ref>
     330  <x:ref>subtype</x:ref>    = <x:ref>token</x:ref>
     331</artwork></figure>
     332<t anchor="rule.parameter">
     333  <x:anchor-alias value="attribute"/>
     334  <x:anchor-alias value="parameter"/>
     335  <x:anchor-alias value="value"/>
     336   The type/subtype &MAY; be followed by parameters in the form of
     337   attribute/value pairs.
     338</t>
     339<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"/>
     340  <x:ref>parameter</x:ref>      = <x:ref>attribute</x:ref> "=" <x:ref>value</x:ref>
     341  <x:ref>attribute</x:ref>      = <x:ref>token</x:ref>
     342  <x:ref>value</x:ref>          = <x:ref>word</x:ref>
     343</artwork></figure>
     344<t>
     345   The type, subtype, and parameter attribute names are case-insensitive.
     346   Parameter values might or might not be case-sensitive, depending on the
     347   semantics of the parameter name.  The presence or absence of a parameter might
     348   be significant to the processing of a media-type, depending on its
     349   definition within the media type registry.
     350</t>
     351<t>
     352   A parameter value that matches the <x:ref>token</x:ref> production can be
     353   transmitted as either a token or within a quoted-string. The quoted and
     354   unquoted values are equivalent.
     355</t>
     356<t>
     357   Media-type values are registered with the Internet Assigned Number
     358   Authority (IANA). The media type registration process is
     359   outlined in <xref target="RFC4288"/>. Use of non-registered media types is
     360   discouraged.
     361</t>
     362</section>
     363
     364<section title="Character Encodings (charset)" anchor="character.sets">
     365<t>
     366   HTTP uses charset names to indicate the character encoding of a
     367   textual representation.
     368</t>
     369<t anchor="rule.charset">
     370  <x:anchor-alias value="charset"/>
     371   A character encoding is identified by a case-insensitive token. The
     372   complete set of tokens is defined by the IANA Character Set registry
     373   (<eref target="http://www.iana.org/assignments/character-sets"/>).
     374</t>
     375<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="charset"/>
     376  <x:ref>charset</x:ref> = <x:ref>token</x:ref>
     377</artwork></figure>
     378<t>
     379   Although HTTP allows an arbitrary token to be used as a charset
     380   value, any token that has a predefined value within the IANA
     381   Character Set registry &MUST; represent the character encoding defined
     382   by that registry. Applications &SHOULD; limit their use of character
     383   encodings to those defined within the IANA registry.
     384</t>
     385<t>
     386   HTTP uses charset in two contexts: within an <x:ref>Accept-Charset</x:ref>
     387   request header field (in which the charset value is an unquoted token) and
     388   as the value of a parameter in a <x:ref>Content-Type</x:ref> header field
     389   (within a request or response), in which case the parameter value of the
     390   charset parameter can be quoted.
     391</t>
     392<t>
     393   Implementers need to be aware of IETF character set requirements <xref target="RFC3629"/>
     394   <xref target="RFC2277"/>.
     395</t>
     396</section>
     397
     398<section title="Canonicalization and Text Defaults" anchor="canonicalization.and.text.defaults">
     399<t>
     400   Internet media types are registered with a canonical form. A
     401   representation transferred via HTTP messages &MUST; be in the
     402   appropriate canonical form prior to its transmission except for
     403   "text" types, as defined in the next paragraph.
     404</t>
     405<t>
     406   When in canonical form, media subtypes of the "text" type use CRLF as
     407   the text line break. HTTP relaxes this requirement and allows the
     408   transport of text media with plain CR or LF alone representing a line
     409   break when it is done consistently for an entire representation. HTTP
     410   applications &MUST; accept CRLF, bare CR, and bare LF as indicating
     411   a line break in text media received via HTTP. In
     412   addition, if the text is in a character encoding that does not
     413   use octets 13 and 10 for CR and LF respectively, as is the case for
     414   some multi-byte character encodings, HTTP allows the use of whatever octet
     415   sequences are defined by that character encoding to represent the
     416   equivalent of CR and LF for line breaks. This flexibility regarding
     417   line breaks applies only to text media in the payload body; a bare CR
     418   or LF &MUST-NOT; be substituted for CRLF within any of the HTTP control
     419   structures (such as header fields and multipart boundaries).
     420</t>
     421<t>
     422   If a representation is encoded with a content-coding, the underlying
     423   data &MUST; be in a form defined above prior to being encoded.
     424</t>
     425</section>
     426
     427<section title="Multipart Types" anchor="multipart.types">
     428<t>
     429   MIME provides for a number of "multipart" types &mdash; encapsulations of
     430   one or more representations within a single message body. All multipart
     431   types share a common syntax, as defined in <xref target="RFC2046" x:sec="5.1.1" x:fmt="of"/>,
     432   and include a boundary parameter as part of the media type
     433   value. The message body is itself a protocol element; a sender &MUST;
     434   generate only CRLF to represent line breaks between body-parts.
     435</t>
     436<t>
     437   In general, HTTP treats a multipart message body no differently than
     438   any other media type: strictly as payload.  HTTP does not use the
     439   multipart boundary as an indicator of message body length.
     440   <!-- jre: re-insert removed text pointing to caching? -->
     441   In all other respects, an HTTP user agent &SHOULD; follow the same or similar
     442   behavior as a MIME user agent would upon receipt of a multipart type.
     443   The MIME header fields within each body-part of a multipart message body
     444   do not have any significance to HTTP beyond that defined by
     445   their MIME semantics.
     446</t>
     447<t>
     448   A recipient &MUST; treat an unrecognized multipart subtype
     449   as being equivalent to "multipart/mixed".
     450</t>
     451<x:note>
     452  <t>
     453    &Note; The "multipart/form-data" type has been specifically defined
     454    for carrying form data suitable for processing via the POST
     455    request method, as described in <xref target="RFC2388"/>.
     456  </t>
     457</x:note>
     458</section>
    314459
    315460<section title="Content-Type" anchor="header.content-type">
    … …  
    353498   processing semantics.  Implementers are encouraged to provide a means of
    354499   disabling such "content sniffing" when it is used.
     500</t>
     501</section>
     502</section>
     503
     504<section title="Data Encoding" anchor="data.encoding">
     505
     506<section title="Content Codings" anchor="content.codings">
     507  <x:anchor-alias value="content-coding"/>
     508<t>
     509   Content coding values indicate an encoding transformation that has
     510   been or can be applied to a representation. Content codings are primarily
     511   used to allow a representation to be compressed or otherwise usefully
     512   transformed without losing the identity of its underlying media type
     513   and without loss of information. Frequently, the representation is stored in
     514   coded form, transmitted directly, and only decoded by the recipient.
     515</t>
     516<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="content-coding"/>
     517  <x:ref>content-coding</x:ref>   = <x:ref>token</x:ref>
     518</artwork></figure>
     519<t>
     520   All content-coding values are case-insensitive. HTTP/1.1 uses
     521   content-coding values in the <x:ref>Accept-Encoding</x:ref>
     522   (<xref target="header.accept-encoding"/>) and <x:ref>Content-Encoding</x:ref>
     523   (<xref target="header.content-encoding"/>) header fields. Although the value
     524   describes the content-coding, what is more important is that it
     525   indicates what decoding mechanism will be required to remove the
     526   encoding.
     527</t>
     528<t>
     529   compress<iref item="compress (Coding Format)"/>
     530  <list>
     531    <t>
     532      See &compress-coding;.
     533    </t>
     534  </list>
     535</t>
     536<t>
     537   deflate<iref item="deflate (Coding Format)"/>
     538  <list>
     539    <t>
     540      See &deflate-coding;.
     541    </t>
     542  </list>
     543</t>
     544<t>
     545   gzip<iref item="gzip (Coding Format)"/>
     546  <list>
     547    <t>
     548      See &gzip-coding;.
     549    </t>
     550  </list>
    355551</t>
    356552</section>
    … …  
    413609</t>
    414610</section>
     611</section>
     612
     613<section title="Audience Language" anchor="audience.language">
     614
     615<section title="Language Tags" anchor="language.tags">
     616  <x:anchor-alias value="language-tag"/>
     617<t>
     618   A language tag, as defined in <xref target="RFC5646"/>, identifies a
     619   natural language spoken, written, or otherwise conveyed by human beings for
     620   communication of information to other human beings. Computer languages are
     621   explicitly excluded. HTTP uses language tags within the
     622   <x:ref>Accept-Language</x:ref> and <x:ref>Content-Language</x:ref> fields.
     623</t>
     624<t>
     625   In summary, a language tag is composed of one or more parts: A primary
     626   language subtag followed by a possibly empty series of subtags:
     627</t>
     628<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="language-tag"/>
     629  <x:ref>language-tag</x:ref> = &lt;Language-Tag, defined in <xref target="RFC5646" x:sec="2.1"/>&gt;
     630</artwork></figure>
     631<t>
     632   White space is not allowed within the tag and all tags are case-insensitive.
     633   The name space of language subtags is administered by the IANA (see
     634   <eref target="http://www.iana.org/assignments/language-subtag-registry"/>).
     635</t>
     636<figure>
     637  <preamble>Example tags include:</preamble>
     638<artwork type="example">
     639  en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
     640</artwork>
     641</figure>
     642<t>
     643   See <xref target="RFC5646"/> for further information.
     644</t>
     645</section>
    415646
    416647<section title="Content-Language" anchor="header.content-language">
    … …  
    463694</t>
    464695</section>
    465 
    466 <section title="Content-Location" anchor="header.content-location">
    467   <iref primary="true" item="Content-Location header field" x:for-anchor=""/>
    468   <x:anchor-alias value="Content-Location"/>
    469 <t>
    470    The "Content-Location" header field supplies a URI that can be used
    471    as a specific identifier for the representation in this message.
    472    In other words, if one were to perform a GET on this URI at the time
    473    of this message's generation, then a <x:ref>200 (OK)</x:ref> response would
    474    contain the same representation that is enclosed as payload in this message.
    475 </t>
    476 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Location"/>
    477   <x:ref>Content-Location</x:ref> = <x:ref>absolute-URI</x:ref> / <x:ref>partial-URI</x:ref>
    478 </artwork></figure>
    479 <t>
    480    The Content-Location value is not a replacement for the effective
    481    Request URI (&effective-request-uri;).  It is representation metadata.
    482    It has the same syntax and semantics as the header field of the same name
    483    defined for MIME body parts in <xref target="RFC2557" x:fmt="of" x:sec="4"/>.
    484    However, its appearance in an HTTP message has some special implications
    485    for HTTP recipients.
    486 </t>
    487 <t>
    488    If Content-Location is included in a response message and its value
    489    is the same as the effective request URI, then the response payload
    490    &SHOULD; be considered a current representation of that resource.
    491    For a GET or HEAD request, this is the same as the default semantics
    492    when no Content-Location is provided by the server.  For a state-changing
    493    request like PUT or POST, it implies that the server's response contains
    494    the new representation of that resource, thereby distinguishing it from
    495    representations that might only report about the action (e.g., "It worked!").
    496    This allows authoring applications to update their local copies without
    497    the need for a subsequent GET request.
    498 </t>
    499 <t>
    500    If Content-Location is included in a response message and its value
    501    differs from the effective request URI, then the origin server is
    502    informing recipients that this representation has its own, presumably
    503    more specific, identifier.  For a GET or HEAD request, this is an
    504    indication that the effective request URI identifies a resource that
    505    is subject to content negotiation and the selected representation for
    506    this response can also be found at the identified URI.  For other
    507    methods, such a Content-Location indicates that this representation
    508    contains a report on the action's status and the same report is
    509    available (for future access with GET) at the given URI.  For
    510    example, a purchase transaction made via a POST request might
    511    include a receipt document as the payload of the <x:ref>200 (OK)</x:ref>
    512    response; the Content-Location value provides an identifier for retrieving
    513    a copy of that same receipt in the future.
    514 </t>
    515 <t>
    516    If Content-Location is included in a request message, then it &MAY;
    517    be interpreted by the origin server as an indication of where the
    518    user agent originally obtained the content of the enclosed
    519    representation (prior to any subsequent modification of the content
    520    by that user agent).  In other words, the user agent is providing
    521    the same representation metadata that it received with the original
    522    representation.  However, such interpretation &MUST-NOT; be used to
    523    alter the semantics of the method requested by the client.  For
    524    example, if a client makes a PUT request on a negotiated resource
    525    and the origin server accepts that PUT (without redirection), then the
    526    new set of values for that resource is expected to be consistent with
    527    the one representation supplied in that PUT; the Content-Location
    528    cannot be used as a form of reverse content selection that
    529    identifies only one of the negotiated representations to be updated.
    530    If the user agent had wanted the latter semantics, it would have applied
    531    the PUT directly to the Content-Location URI.
    532 </t>
    533 <t>
    534    A Content-Location field received in a request message is transitory
    535    information that &SHOULD-NOT; be saved with other representation
    536    metadata for use in later responses.  The Content-Location's value
    537    might be saved for use in other contexts, such as within source links
    538    or other metadata.
    539 </t>
    540 <t>
    541    A cache cannot assume that a representation with a Content-Location
    542    different from the URI used to retrieve it can be used to respond to
    543    later requests on that Content-Location URI.
    544 </t>
    545 <t>
    546    If the Content-Location value is a partial URI, the partial URI is
    547    interpreted relative to the effective request URI.
    548 </t>
    549 </section>
    550 </section>
    551 
    552 <section title="Representation Data" anchor="representation.data">
    553   <x:anchor-alias value="representation-data"/>
    554 <t>
    555    The representation data associated with an HTTP message is
    556    either provided as the payload body of the message or
    557    referred to by the message semantics and the effective request
    558    URI.  The representation data is in a format and encoding defined by
    559    the representation metadata header fields.
    560 </t>
    561 <t>
    562    The data type of the representation data is determined via the header fields
    563    <x:ref>Content-Type</x:ref> and <x:ref>Content-Encoding</x:ref>.
    564    These define a two-layer, ordered encoding model:
    565 </t>
    566 <figure><artwork type="example">
    567   representation-data := Content-Encoding( Content-Type( bits ) )
    568 </artwork></figure>
    569 </section>
    570 
    571 <section title="Message Payload" anchor="payload">
    572 <iref item="payload"/>
    573 <t>
    574    Some HTTP messages transfer a complete or partial representation as the
    575    message "<x:dfn>payload</x:dfn>".  In some cases, a payload might only
    576    contain the associated representation's header fields (e.g., responses to
    577    HEAD) or only some part(s) of the representation data
    578    (e.g., the <x:ref>206 (Partial Content)</x:ref> status code).
    579 </t>
    580 <t>
    581    The purpose of a payload in a request is defined by the method semantics.
    582    In a response, the payload's purpose is defined by both the request method
    583    and the response status code.
    584 </t>
    585 <t>
    586    For example, a representation in the payload of a PUT request
    587    (<xref target="PUT"/>) represents the desired state of the target resource
    588    if the request is successfully applied, whereas a representation in the
    589    payload of a POST request (<xref target="POST"/>) represents an anonymous
    590    resource for providing data to be processed, such as the information that
    591    a user entered within an HTML form.
    592 </t>
    593 <t>
    594    Likewise, the payload of a <x:ref>200 (OK)</x:ref> response to GET
    595    (<xref target="GET"/>) contains a representation of the target resource,
    596    as observed at the time of the message origination date
    597    (<xref target="header.date"/>), whereas the same status code in a response
    598    to POST might contain either a representation of the processing result or
    599    a current representation of the target resource after applying the
    600    processing. Response messages with an error status code usually contain
    601    a representation that describes the error and what next steps are suggested
    602    for resolving it.
    603 </t>
    604 
    605 <section title="Payload Header Fields" anchor="payload.header.fields">
    606 <t>
    607    Header fields that specifically describe the payload, rather than the
    608    associated representation, are referred to as "payload header fields".
    609    Payload header fields are defined in other parts of this specification,
    610    due to their impact on message parsing.
    611 </t>
    612 <texttable align="left">
    613   <ttcol>Header Field Name</ttcol>
    614   <ttcol>Defined in...</ttcol>
    615 
    616   <c>Content-Length</c> <c>&header-content-length;</c>
    617   <c>Content-Range</c> <c>&header-content-range;</c>
    618   <c>Transfer-Encoding</c> <c>&header-transfer-encoding;</c>
    619 </texttable>
    620 </section>
     696</section>
     697
     698<section title="Identification" anchor="identification">
    621699
    622700<section title="Identifying the Payload" anchor="identifying.payload">
    … …  
    670748</t>
    671749</section>
     750
     751<section title="Content-Location" anchor="header.content-location">
     752  <iref primary="true" item="Content-Location header field" x:for-anchor=""/>
     753  <x:anchor-alias value="Content-Location"/>
     754<t>
     755   The "Content-Location" header field supplies a URI that can be used
     756   as a specific identifier for the representation in this message.
     757   In other words, if one were to perform a GET on this URI at the time
     758   of this message's generation, then a <x:ref>200 (OK)</x:ref> response would
     759   contain the same representation that is enclosed as payload in this message.
     760</t>
     761<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Location"/>
     762  <x:ref>Content-Location</x:ref> = <x:ref>absolute-URI</x:ref> / <x:ref>partial-URI</x:ref>
     763</artwork></figure>
     764<t>
     765   The Content-Location value is not a replacement for the effective
     766   Request URI (&effective-request-uri;).  It is representation metadata.
     767   It has the same syntax and semantics as the header field of the same name
     768   defined for MIME body parts in <xref target="RFC2557" x:fmt="of" x:sec="4"/>.
     769   However, its appearance in an HTTP message has some special implications
     770   for HTTP recipients.
     771</t>
     772<t>
     773   If Content-Location is included in a response message and its value
     774   is the same as the effective request URI, then the response payload
     775   &SHOULD; be considered a current representation of that resource.
     776   For a GET or HEAD request, this is the same as the default semantics
     777   when no Content-Location is provided by the server.  For a state-changing
     778   request like PUT or POST, it implies that the server's response contains
     779   the new representation of that resource, thereby distinguishing it from
     780   representations that might only report about the action (e.g., "It worked!").
     781   This allows authoring applications to update their local copies without
     782   the need for a subsequent GET request.
     783</t>
     784<t>
     785   If Content-Location is included in a response message and its value
     786   differs from the effective request URI, then the origin server is
     787   informing recipients that this representation has its own, presumably
     788   more specific, identifier.  For a GET or HEAD request, this is an
     789   indication that the effective request URI identifies a resource that
     790   is subject to content negotiation and the selected representation for
     791   this response can also be found at the identified URI.  For other
     792   methods, such a Content-Location indicates that this representation
     793   contains a report on the action's status and the same report is
     794   available (for future access with GET) at the given URI.  For
     795   example, a purchase transaction made via a POST request might
     796   include a receipt document as the payload of the <x:ref>200 (OK)</x:ref>
     797   response; the Content-Location value provides an identifier for retrieving
     798   a copy of that same receipt in the future.
     799</t>
     800<t>
     801   If Content-Location is included in a request message, then it &MAY;
     802   be interpreted by the origin server as an indication of where the
     803   user agent originally obtained the content of the enclosed
     804   representation (prior to any subsequent modification of the content
     805   by that user agent).  In other words, the user agent is providing
     806   the same representation metadata that it received with the original
     807   representation.  However, such interpretation &MUST-NOT; be used to
     808   alter the semantics of the method requested by the client.  For
     809   example, if a client makes a PUT request on a negotiated resource
     810   and the origin server accepts that PUT (without redirection), then the
     811   new set of values for that resource is expected to be consistent with
     812   the one representation supplied in that PUT; the Content-Location
     813   cannot be used as a form of reverse content selection that
     814   identifies only one of the negotiated representations to be updated.
     815   If the user agent had wanted the latter semantics, it would have applied
     816   the PUT directly to the Content-Location URI.
     817</t>
     818<t>
     819   A Content-Location field received in a request message is transitory
     820   information that &SHOULD-NOT; be saved with other representation
     821   metadata for use in later responses.  The Content-Location's value
     822   might be saved for use in other contexts, such as within source links
     823   or other metadata.
     824</t>
     825<t>
     826   A cache cannot assume that a representation with a Content-Location
     827   different from the URI used to retrieve it can be used to respond to
     828   later requests on that Content-Location URI.
     829</t>
     830<t>
     831   If the Content-Location value is a partial URI, the partial URI is
     832   interpreted relative to the effective request URI.
     833</t>
     834</section>
     835</section>
     836</section>
     837
     838<section title="Representation Data" anchor="representation.data">
     839  <x:anchor-alias value="representation-data"/>
     840<t>
     841   The representation data associated with an HTTP message is
     842   either provided as the payload body of the message or
     843   referred to by the message semantics and the effective request
     844   URI.  The representation data is in a format and encoding defined by
     845   the representation metadata header fields.
     846</t>
     847<t>
     848   The data type of the representation data is determined via the header fields
     849   <x:ref>Content-Type</x:ref> and <x:ref>Content-Encoding</x:ref>.
     850   These define a two-layer, ordered encoding model:
     851</t>
     852<figure><artwork type="example">
     853  representation-data := Content-Encoding( Content-Type( bits ) )
     854</artwork></figure>
     855</section>
     856
     857<section title="Payload Semantics" anchor="payload">
     858<iref item="payload"/>
     859<t>
     860   Some HTTP messages transfer a complete or partial representation as the
     861   message "<x:dfn>payload</x:dfn>".  In some cases, a payload might only
     862   contain the associated representation's header fields (e.g., responses to
     863   HEAD) or only some part(s) of the representation data
     864   (e.g., the <x:ref>206 (Partial Content)</x:ref> status code).
     865</t>
     866<t>
     867   The purpose of a payload in a request is defined by the method semantics.
     868   In a response, the payload's purpose is defined by both the request method
     869   and the response status code.
     870</t>
     871<t>
     872   For example, a representation in the payload of a PUT request
     873   (<xref target="PUT"/>) represents the desired state of the target resource
     874   if the request is successfully applied, whereas a representation in the
     875   payload of a POST request (<xref target="POST"/>) represents an anonymous
     876   resource for providing data to be processed, such as the information that
     877   a user entered within an HTML form.
     878</t>
     879<t>
     880   Likewise, the payload of a <x:ref>200 (OK)</x:ref> response to GET
     881   (<xref target="GET"/>) contains a representation of the target resource,
     882   as observed at the time of the message origination date
     883   (<xref target="header.date"/>), whereas the same status code in a response
     884   to POST might contain either a representation of the processing result or
     885   a current representation of the target resource after applying the
     886   processing. Response messages with an error status code usually contain
     887   a representation that describes the error and what next steps are suggested
     888   for resolving it.
     889</t>
     890<t>
     891   Header fields that specifically describe the payload, rather than the
     892   associated representation, are referred to as "payload header fields".
     893   Payload header fields are defined in other parts of this specification,
     894   due to their impact on message parsing.
     895</t>
     896<texttable align="left">
     897  <ttcol>Header Field Name</ttcol>
     898  <ttcol>Defined in...</ttcol>
     899
     900  <c>Content-Length</c> <c>&header-content-length;</c>
     901  <c>Content-Range</c> <c>&header-content-range;</c>
     902  <c>Transfer-Encoding</c> <c>&header-transfer-encoding;</c>
     903</texttable>
    672904</section>
    673905
    … …  
    8331065</section>
    8341066</section>
     1067</section>
     1068
     1069<section title="Product Tokens" anchor="product.tokens">
     1070  <x:anchor-alias value="product"/>
     1071  <x:anchor-alias value="product-version"/>
     1072<t>
     1073   Product tokens are used to allow communicating applications to
     1074   identify themselves by software name and version. Most fields using
     1075   product tokens also allow sub-products which form a significant part
     1076   of the application to be listed, separated by whitespace. By
     1077   convention, the products are listed in order of their significance
     1078   for identifying the application.
     1079</t>
     1080<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
     1081  <x:ref>product</x:ref>         = <x:ref>token</x:ref> ["/" <x:ref>product-version</x:ref>]
     1082  <x:ref>product-version</x:ref> = <x:ref>token</x:ref>
     1083</artwork></figure>
     1084<t>
     1085   Examples:
     1086</t>
     1087<figure><artwork type="example">
     1088  User-Agent: CERN-LineMode/2.15 libwww/2.17b3
     1089  Server: Apache/0.8.4
     1090</artwork></figure>
     1091<t>
     1092   Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
     1093   used for advertising or other non-essential information. Although any
     1094   token octet &MAY; appear in a product-version, this token &SHOULD;
     1095   only be used for a version identifier (i.e., successive versions of
     1096   the same product &SHOULD; only differ in the product-version portion of
     1097   the product value).
     1098</t>
    8351099</section>
    8361100
    … …  
    20912355</section>
    20922356
    2093 <section title="Authentication" anchor="request.auth">
     2357<section title="Authentication Credentials" anchor="request.auth">
    20942358<texttable align="left">
    20952359  <ttcol>Header Field Name</ttcol>
    … …  
    32333497</texttable>
    32343498
    3235 <section title="Date" anchor="header.date">
    3236   <iref primary="true" item="Date header field" x:for-anchor=""/>
    3237   <x:anchor-alias value="Date"/>
    3238 <t>
    3239    The "Date" header field represents the date and time at which
    3240    the message was originated, having the same semantics as the Origination
    3241    Date Field (orig-date) defined in <xref target="RFC5322" x:fmt="of" x:sec="3.6.1"/>.
    3242    The field value is an HTTP-date, as defined in <xref target="http.date"/>;
    3243    it &MUST; be sent in rfc1123-date format.
    3244 </t>
    3245 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
    3246   <x:ref>Date</x:ref> = <x:ref>HTTP-date</x:ref>
    3247 </artwork></figure>
    3248 <t>
    3249    An example is
    3250 </t>
    3251 <figure><artwork type="example">
    3252   Date: Tue, 15 Nov 1994 08:12:31 GMT
    3253 </artwork></figure>
    3254 <t>
    3255    Origin servers &MUST; include a Date header field in all responses,
    3256    except in these cases:
    3257   <list style="numbers">
    3258       <t>If the response status code is <x:ref>100 (Continue)</x:ref> or
    3259          <x:ref>101 (Switching Protocols)</x:ref>, the response &MAY; include a
    3260          Date header field, at the server's option.</t>
    3261 
    3262       <t>If the response status code conveys a server error, e.g., <x:ref>500
    3263          (Internal Server Error)</x:ref> or <x:ref>503 (Service Unavailable)</x:ref>,
    3264          and it is inconvenient or impossible to generate a valid Date.</t>
    3265 
    3266       <t>If the server does not have a clock that can provide a
    3267          reasonable approximation of the current time, its responses
    3268          &MUST-NOT; include a Date header field.</t>
    3269   </list>
    3270 </t>
    3271 <t>
    3272    A received message that does not have a Date header field &MUST; be
    3273    assigned one by the recipient if the message will be cached by that
    3274    recipient.
    3275 </t>
    3276 <t>
    3277    Clients can use the Date header field as well; in order to keep request
    3278    messages small, they are advised not to include it when it doesn't convey
    3279    any useful information (as is usually the case for requests that do not
    3280    contain a payload).
    3281 </t>
    3282 <t>
    3283    The HTTP-date sent in a Date header field &SHOULD-NOT; represent a date and
    3284    time subsequent to the generation of the message. It &SHOULD; represent
    3285    the best available approximation of the date and time of message
    3286    generation, unless the implementation has no means of generating a
    3287    reasonably accurate date and time. In theory, the date ought to
    3288    represent the moment just before the payload is generated. In
    3289    practice, the date can be generated at any time during the message
    3290    origination without affecting its semantic value.
    3291 </t>
    3292 </section>
    3293 
    3294 <section title="Location" anchor="header.location">
    3295   <iref primary="true" item="Location header field" x:for-anchor=""/>
    3296   <x:anchor-alias value="Location"/>
    3297 <t>
    3298    The "Location" header field &MAY; be sent in responses to refer to
    3299    a specific resource in accordance with the semantics of the status
    3300    code.
    3301 </t>
    3302 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Location"/>
    3303   <x:ref>Location</x:ref> = <x:ref>URI-reference</x:ref>
    3304 </artwork></figure>
    3305 <t>
    3306    For <x:ref>201 (Created)</x:ref> responses, the Location is the URI of the
    3307    new resource which was created by the request. For <x:ref>3xx (Redirection)</x:ref>
    3308    responses, the location &SHOULD; indicate the server's preferred URI for
    3309    automatic redirection to the resource.
    3310 </t>
    3311 <t>
    3312    The field value consists of a single URI-reference. When it has the form
    3313    of a relative reference (<xref target="RFC3986" x:fmt="," x:sec="4.2"/>),
    3314    the final value is computed by resolving it against the effective request
    3315    URI (<xref target="RFC3986" x:fmt="," x:sec="5"/>). If the original URI, as
    3316    navigated to by the user agent, did contain a fragment identifier, and the
    3317    final value does not, then the original URI's fragment identifier is added
    3318    to the final value.
    3319 </t>
    3320 <figure>
    3321 <preamble>For example, the original URI "http://www.example.org/~tim", combined with a field value given as:</preamble><!--DO NOT DARE changing the vertical spacing below, it's necessary this way for xml2rfc-->
    3322 <artwork type="example">
    3323   Location: /pub/WWW/People.html#tim
    3324 </artwork>
    3325 <postamble>would result in a final value of "http://www.example.org/pub/WWW/People.html#tim"</postamble>
    3326 </figure>
    3327 <figure>
    3328 <preamble>An original URI "http://www.example.org/index.html#larry", combined with a field value given as:</preamble><!--DO NOT DARE changing the vertical spacing below, it's necessary this way for xml2rfc-->
    3329 <artwork type="example">
    3330   Location: http://www.example.net/index.html
    3331 </artwork>
    3332 <postamble>would result in a final value of "http://www.example.net/index.html#larry", preserving the original fragment identifier.</postamble>
    3333 </figure>
    3334 <x:note>
    3335   <t>
    3336     &Note; Some recipients attempt to recover from Location fields
    3337     that are not valid URI references. This specification does not mandate or
    3338     define such processing, but does allow it.
    3339   </t>
    3340 </x:note>
    3341 <t>
    3342    There are circumstances in which a fragment identifier in a Location URI
    3343    would not be appropriate. For instance, when it appears in a <x:ref>201
    3344    (Created)</x:ref> response, where the Location header field specifies the
    3345    URI for the entire created resource.
    3346 </t>
    3347 <x:note>
    3348   <t>
    3349     &Note; The <x:ref>Content-Location</x:ref> header field
    3350     (&header-content-location;) differs from Location in that the
    3351     Content-Location identifies the most specific resource corresponding to the
    3352     enclosed representation. It is therefore possible for a response to contain
    3353     header fields for both Location and Content-Location.
    3354   </t>
    3355 </x:note>
    3356 </section>
    3357 
    3358 <section title="Retry-After" anchor="header.retry-after">
    3359   <iref primary="true" item="Retry-After header field" x:for-anchor=""/>
    3360   <x:anchor-alias value="Retry-After"/>
    3361 <t>
    3362    The header "Retry-After" field can be used with a <x:ref>503 (Service
    3363    Unavailable)</x:ref> response to indicate how long the service is expected to
    3364    be unavailable to the requesting client. This field &MAY; also be used
    3365    with any <x:ref>3xx (Redirection)</x:ref> response to indicate the minimum time the
    3366    user-agent is asked to wait before issuing the redirected request.
    3367 </t>
    3368 <t>
    3369    The value of this field can be either an HTTP-date or an integer number
    3370    of seconds (in decimal) after the time of the response.
    3371 </t>
    3372 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Retry-After"/>
    3373   <x:ref>Retry-After</x:ref> = <x:ref>HTTP-date</x:ref> / <x:ref>delta-seconds</x:ref>
    3374 </artwork></figure>
    3375 <t anchor="rule.delta-seconds">
    3376   <x:anchor-alias value="delta-seconds"/>
    3377    Time spans are non-negative decimal integers, representing time in
    3378    seconds.
    3379 </t>
    3380 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
    3381   <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
    3382 </artwork></figure>
    3383 <t>
    3384    Two examples of its use are
    3385 </t>
    3386 <figure><artwork type="example">
    3387   Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
    3388   Retry-After: 120
    3389 </artwork></figure>
    3390 <t>
    3391    In the latter example, the delay is 2 minutes.
    3392 </t>
    3393 </section>
    3394 </section>
    3395 
    3396 <section title="Selected Representation Header Fields" anchor="selected.representation">
    3397 <t><iref primary="true" item="selected representation"/>
    3398    We use the term "<x:dfn>selected representation</x:dfn>" to refer to the
    3399    the current representation of a target resource that would have been
    3400    selected in a successful response if the same request had used the
    3401    method GET and excluded any conditional request header fields.
    3402 </t>
    3403 <t>
    3404    Additional header fields define metadata about the selected
    3405    representation, which might differ from the representation included
    3406    in the message for responses to some state-changing methods.
    3407    The following header fields are defined as selected representation
    3408    metadata:
    3409 </t>
    3410 <texttable align="left">
    3411   <ttcol>Header Field Name</ttcol>
    3412   <ttcol>Defined in...</ttcol>
    3413 
    3414   <c>ETag</c> <c>&header-etag;</c>
    3415   <c>Last-Modified</c> <c>&header-last-modified;</c>
    3416   <c>Vary</c> <c><xref target="header.vary"/></c>
    3417 </texttable>
    3418 
    3419 <section anchor="header.vary" title="Vary">
    3420    <iref item="Vary header field" primary="true" x:for-anchor="" />
    3421    <x:anchor-alias value="Vary"/>
    3422 <t>
    3423    The "Vary" header field conveys the set of header fields
    3424    that were used to select the representation.
    3425 </t>
    3426 <t>
    3427    Caches use this information as part of determining whether a stored
    3428    response can be used to satisfy a given request (&caching-neg-resp;).
    3429 </t>
    3430 <t>
    3431    In uncacheable or stale responses, the Vary field value advises the user
    3432    agent about the criteria that were used to select the representation.
    3433 </t>
    3434 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
    3435   <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
    3436 </artwork></figure>
    3437 <t>
    3438    The set of header fields named by the Vary field value is known as the
    3439    selecting header fields.
    3440 </t>
    3441 <t>
    3442    A server &SHOULD; include a Vary header field with any cacheable response
    3443    that is subject to proactive negotiation. Doing so allows a cache to
    3444    properly interpret future requests on that resource and informs the user
    3445    agent about the presence of negotiation on that resource. A server &MAY;
    3446    include a Vary header field with a non-cacheable response that is subject
    3447    to proactive negotiation, since this might provide the user agent with
    3448    useful information about the dimensions over which the response varies at
    3449    the time of the response.
    3450 </t>
    3451 <t>
    3452    A Vary field value of "*" signals that unspecified parameters not limited
    3453    to the header fields (e.g., the network address of the client), play a
    3454    role in the selection of the response representation; therefore, a cache
    3455    cannot determine whether this response is appropriate. A proxy &MUST-NOT;
    3456    generate the "*" value.
    3457 </t>
    3458 <t>
    3459    The field-names given are not limited to the set of standard header
    3460    fields defined by this specification. Field names are case-insensitive.
    3461 </t>
    3462 </section>
    3463 </section>
    3464 
    3465 <section title="Authentication Challenges" anchor="response.auth">
    3466 <t>
    3467    Authentication challenges indicate what mechanisms are available for the
    3468    client to provide authentication credentials in future requests.
    3469 </t>
    3470 <texttable align="left">
    3471   <ttcol>Header Field Name</ttcol><ttcol>Defined in...</ttcol>
    3472 
    3473   <c>WWW-Authenticate</c> <c>&header-www-authenticate;</c>
    3474   <c>Proxy-Authenticate</c> <c>&header-proxy-authenticate;</c>
    3475 </texttable>
    3476 </section>
    3477 
    3478 <section title="Informative" anchor="response.inform">
    3479 <t>
    3480    The remaining response header fields provide more information about
    3481    the target resource for potential use in later requests.
    3482 </t>
    3483 <texttable align="left">
    3484   <ttcol>Header Field Name</ttcol><ttcol>Defined in...</ttcol>
    3485 
    3486   <c>Accept-Ranges</c> <c>&header-accept-ranges;</c>
    3487   <c>Allow</c> <c><xref target="header.allow"/></c>
    3488   <c>Server</c> <c><xref target="header.server"/></c>
    3489 </texttable>
    3490 
    3491 <section title="Allow" anchor="header.allow">
    3492   <iref primary="true" item="Allow header field" x:for-anchor=""/>
    3493   <x:anchor-alias value="Allow"/>
    3494 <t>
    3495    The "Allow" header field lists the set of methods advertised as
    3496    supported by the target resource. The purpose of this field is strictly to
    3497    inform the recipient of valid request methods associated with the resource.
    3498 </t>
    3499 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Allow"/>
    3500   <x:ref>Allow</x:ref> = #<x:ref>method</x:ref>
    3501 </artwork></figure>
    3502 <t>
    3503    Example of use:
    3504 </t>
    3505 <figure><artwork type="example">
    3506   Allow: GET, HEAD, PUT
    3507 </artwork></figure>
    3508 <t>
    3509    The actual set of allowed methods is defined by the origin server at the
    3510    time of each request.
    3511 </t>
    3512 <t>
    3513    A proxy &MUST-NOT; modify the Allow header field &mdash; it does not need to
    3514    understand all the methods specified in order to handle them according to
    3515    the generic message handling rules.
    3516 </t>
    3517 </section>
    3518 
    3519 <section title="Server" anchor="header.server">
    3520   <iref primary="true" item="Server header field" x:for-anchor=""/>
    3521   <x:anchor-alias value="Server"/>
    3522 <t>
    3523    The "Server" header field contains information about the
    3524    software used by the origin server to handle the request.
    3525 </t>
    3526 <t>
    3527    The field can contain multiple
    3528    product tokens (<xref target="product.tokens"/>) and
    3529    comments (&header-fields;) identifying the server and any significant
    3530    subproducts. The product tokens are listed in order of their significance
    3531    for identifying the application.
    3532 </t>
    3533 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Server"/>
    3534   <x:ref>Server</x:ref> = <x:ref>product</x:ref> *( <x:ref>RWS</x:ref> ( <x:ref>product</x:ref> / <x:ref>comment</x:ref> ) )
    3535 </artwork></figure>
    3536 <t>
    3537    Example:
    3538 </t>
    3539 <figure><artwork type="example">
    3540   Server: CERN/3.0 libwww/2.17
    3541 </artwork></figure>
    3542 <t>
    3543    If the response is being forwarded through a proxy, the proxy application
    3544    &MUST-NOT; modify the <x:ref>Server</x:ref> header field. Instead, it
    3545    &MUST; include a <x:ref>Via</x:ref> field (as described in &header-via;).
    3546 </t>
    3547 <x:note>
    3548   <t>
    3549     &Note; Revealing the specific software version of the server might
    3550     allow the server machine to become more vulnerable to attacks
    3551     against software that is known to contain security holes. Server
    3552     implementers are encouraged to make this field a configurable
    3553     option.
    3554   </t>
    3555 </x:note>
    3556 </section>
    3557 </section>
    3558 </section>
    3559 
    3560 <section title="Protocol Parameters" anchor="protocol.parameters">
     3499<section title="Origination Date" anchor="origination.date">
     3500
    35613501<section title="Date/Time Formats" anchor="http.date">
    35623502  <x:anchor-alias value="HTTP-date"/>
    … …  
    37043644</section>
    37053645
    3706 <section title="Product Tokens" anchor="product.tokens">
    3707   <x:anchor-alias value="product"/>
    3708   <x:anchor-alias value="product-version"/>
    3709 <t>
    3710    Product tokens are used to allow communicating applications to
    3711    identify themselves by software name and version. Most fields using
    3712    product tokens also allow sub-products which form a significant part
    3713    of the application to be listed, separated by whitespace. By
    3714    convention, the products are listed in order of their significance
    3715    for identifying the application.
    3716 </t>
    3717 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="product"/><iref primary="true" item="Grammar" subitem="product-version"/>
    3718   <x:ref>product</x:ref>         = <x:ref>token</x:ref> ["/" <x:ref>product-version</x:ref>]
    3719   <x:ref>product-version</x:ref> = <x:ref>token</x:ref>
     3646<section title="Date" anchor="header.date">
     3647  <iref primary="true" item="Date header field" x:for-anchor=""/>
     3648  <x:anchor-alias value="Date"/>
     3649<t>
     3650   The "Date" header field represents the date and time at which
     3651   the message was originated, having the same semantics as the Origination
     3652   Date Field (orig-date) defined in <xref target="RFC5322" x:fmt="of" x:sec="3.6.1"/>.
     3653   The field value is an HTTP-date, as defined in <xref target="http.date"/>;
     3654   it &MUST; be sent in rfc1123-date format.
     3655</t>
     3656<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Date"/>
     3657  <x:ref>Date</x:ref> = <x:ref>HTTP-date</x:ref>
    37203658</artwork></figure>
    37213659<t>
    3722    Examples:
     3660   An example is
    37233661</t>
    37243662<figure><artwork type="example">
    3725   User-Agent: CERN-LineMode/2.15 libwww/2.17b3
    3726   Server: Apache/0.8.4
     3663  Date: Tue, 15 Nov 1994 08:12:31 GMT
    37273664</artwork></figure>
    37283665<t>
    3729    Product tokens &SHOULD; be short and to the point. They &MUST-NOT; be
    3730    used for advertising or other non-essential information. Although any
    3731    token octet &MAY; appear in a product-version, this token &SHOULD;
    3732    only be used for a version identifier (i.e., successive versions of
    3733    the same product &SHOULD; only differ in the product-version portion of
    3734    the product value).
    3735 </t>
    3736 </section>
    3737 
    3738 <section title="Character Encodings (charset)" anchor="character.sets">
    3739 <t>
    3740    HTTP uses charset names to indicate the character encoding of a
    3741    textual representation.
    3742 </t>
    3743 <t anchor="rule.charset">
    3744   <x:anchor-alias value="charset"/>
    3745    A character encoding is identified by a case-insensitive token. The
    3746    complete set of tokens is defined by the IANA Character Set registry
    3747    (<eref target="http://www.iana.org/assignments/character-sets"/>).
    3748 </t>
    3749 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="charset"/>
    3750   <x:ref>charset</x:ref> = <x:ref>token</x:ref>
     3666   Origin servers &MUST; include a Date header field in all responses,
     3667   except in these cases:
     3668  <list style="numbers">
     3669      <t>If the response status code is <x:ref>100 (Continue)</x:ref> or
     3670         <x:ref>101 (Switching Protocols)</x:ref>, the response &MAY; include a
     3671         Date header field, at the server's option.</t>
     3672
     3673      <t>If the response status code conveys a server error, e.g., <x:ref>500
     3674         (Internal Server Error)</x:ref> or <x:ref>503 (Service Unavailable)</x:ref>,
     3675         and it is inconvenient or impossible to generate a valid Date.</t>
     3676
     3677      <t>If the server does not have a clock that can provide a
     3678         reasonable approximation of the current time, its responses
     3679         &MUST-NOT; include a Date header field.</t>
     3680  </list>
     3681</t>
     3682<t>
     3683   A received message that does not have a Date header field &MUST; be
     3684   assigned one by the recipient if the message will be cached by that
     3685   recipient.
     3686</t>
     3687<t>
     3688   Clients can use the Date header field as well; in order to keep request
     3689   messages small, they are advised not to include it when it doesn't convey
     3690   any useful information (as is usually the case for requests that do not
     3691   contain a payload).
     3692</t>
     3693<t>
     3694   The HTTP-date sent in a Date header field &SHOULD-NOT; represent a date and
     3695   time subsequent to the generation of the message. It &SHOULD; represent
     3696   the best available approximation of the date and time of message
     3697   generation, unless the implementation has no means of generating a
     3698   reasonably accurate date and time. In theory, the date ought to
     3699   represent the moment just before the payload is generated. In
     3700   practice, the date can be generated at any time during the message
     3701   origination without affecting its semantic value.
     3702</t>
     3703</section>
     3704</section>
     3705
     3706<section title="Location" anchor="header.location">
     3707  <iref primary="true" item="Location header field" x:for-anchor=""/>
     3708  <x:anchor-alias value="Location"/>
     3709<t>
     3710   The "Location" header field &MAY; be sent in responses to refer to
     3711   a specific resource in accordance with the semantics of the status
     3712   code.
     3713</t>
     3714<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Location"/>
     3715  <x:ref>Location</x:ref> = <x:ref>URI-reference</x:ref>
    37513716</artwork></figure>
    37523717<t>
    3753    Although HTTP allows an arbitrary token to be used as a charset
    3754    value, any token that has a predefined value within the IANA
    3755    Character Set registry &MUST; represent the character encoding defined
    3756    by that registry. Applications &SHOULD; limit their use of character
    3757    encodings to those defined within the IANA registry.
    3758 </t>
    3759 <t>
    3760    HTTP uses charset in two contexts: within an <x:ref>Accept-Charset</x:ref>
    3761    request header field (in which the charset value is an unquoted token) and
    3762    as the value of a parameter in a <x:ref>Content-Type</x:ref> header field
    3763    (within a request or response), in which case the parameter value of the
    3764    charset parameter can be quoted.
    3765 </t>
    3766 <t>
    3767    Implementers need to be aware of IETF character set requirements <xref target="RFC3629"/>
    3768    <xref target="RFC2277"/>.
    3769 </t>
    3770 </section>
    3771 
    3772 <section title="Content Codings" anchor="content.codings">
    3773   <x:anchor-alias value="content-coding"/>
    3774 <t>
    3775    Content coding values indicate an encoding transformation that has
    3776    been or can be applied to a representation. Content codings are primarily
    3777    used to allow a representation to be compressed or otherwise usefully
    3778    transformed without losing the identity of its underlying media type
    3779    and without loss of information. Frequently, the representation is stored in
    3780    coded form, transmitted directly, and only decoded by the recipient.
    3781 </t>
    3782 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="content-coding"/>
    3783   <x:ref>content-coding</x:ref>   = <x:ref>token</x:ref>
    3784 </artwork></figure>
    3785 <t>
    3786    All content-coding values are case-insensitive. HTTP/1.1 uses
    3787    content-coding values in the <x:ref>Accept-Encoding</x:ref>
    3788    (<xref target="header.accept-encoding"/>) and <x:ref>Content-Encoding</x:ref>
    3789    (<xref target="header.content-encoding"/>) header fields. Although the value
    3790    describes the content-coding, what is more important is that it
    3791    indicates what decoding mechanism will be required to remove the
    3792    encoding.
    3793 </t>
    3794 <t>
    3795    compress<iref item="compress (Coding Format)"/>
    3796   <list>
    3797     <t>
    3798       See &compress-coding;.
    3799     </t>
    3800   </list>
    3801 </t>
    3802 <t>
    3803    deflate<iref item="deflate (Coding Format)"/>
    3804   <list>
    3805     <t>
    3806       See &deflate-coding;.
    3807     </t>
    3808   </list>
    3809 </t>
    3810 <t>
    3811    gzip<iref item="gzip (Coding Format)"/>
    3812   <list>
    3813     <t>
    3814       See &gzip-coding;.
    3815     </t>
    3816   </list>
    3817 </t>
    3818 </section>
    3819 
    3820 <section title="Media Types" anchor="media.types">
    3821   <x:anchor-alias value="media-type"/>
    3822   <x:anchor-alias value="type"/>
    3823   <x:anchor-alias value="subtype"/>
    3824 <t>
    3825    HTTP uses Internet Media Types <xref target="RFC2046"/> in the
    3826    <x:ref>Content-Type</x:ref> (<xref target="header.content-type"/>)
    3827    and <x:ref>Accept</x:ref> (<xref target="header.accept"/>) header fields in
    3828    order to provide open and extensible data typing and type negotiation.
    3829 </t>
    3830 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="media-type"/><iref primary="true" item="Grammar" subitem="type"/><iref primary="true" item="Grammar" subitem="subtype"/>
    3831   <x:ref>media-type</x:ref> = <x:ref>type</x:ref> "/" <x:ref>subtype</x:ref> *( <x:ref>OWS</x:ref> ";" <x:ref>OWS</x:ref> <x:ref>parameter</x:ref> )
    3832   <x:ref>type</x:ref>       = <x:ref>token</x:ref>
    3833   <x:ref>subtype</x:ref>    = <x:ref>token</x:ref>
    3834 </artwork></figure>
    3835 <t anchor="rule.parameter">
    3836   <x:anchor-alias value="attribute"/>
    3837   <x:anchor-alias value="parameter"/>
    3838   <x:anchor-alias value="value"/>
    3839    The type/subtype &MAY; be followed by parameters in the form of
    3840    attribute/value pairs.
    3841 </t>
    3842 <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"/>
    3843   <x:ref>parameter</x:ref>      = <x:ref>attribute</x:ref> "=" <x:ref>value</x:ref>
    3844   <x:ref>attribute</x:ref>      = <x:ref>token</x:ref>
    3845   <x:ref>value</x:ref>          = <x:ref>word</x:ref>
    3846 </artwork></figure>
    3847 <t>
    3848    The type, subtype, and parameter attribute names are case-insensitive.
    3849    Parameter values might or might not be case-sensitive, depending on the
    3850    semantics of the parameter name.  The presence or absence of a parameter might
    3851    be significant to the processing of a media-type, depending on its
    3852    definition within the media type registry.
    3853 </t>
    3854 <t>
    3855    A parameter value that matches the <x:ref>token</x:ref> production can be
    3856    transmitted as either a token or within a quoted-string. The quoted and
    3857    unquoted values are equivalent.
    3858 </t>
    3859 <t>
    3860    Media-type values are registered with the Internet Assigned Number
    3861    Authority (IANA). The media type registration process is
    3862    outlined in <xref target="RFC4288"/>. Use of non-registered media types is
    3863    discouraged.
    3864 </t>
    3865 
    3866 <section title="Canonicalization and Text Defaults" anchor="canonicalization.and.text.defaults">
    3867 <t>
    3868    Internet media types are registered with a canonical form. A
    3869    representation transferred via HTTP messages &MUST; be in the
    3870    appropriate canonical form prior to its transmission except for
    3871    "text" types, as defined in the next paragraph.
    3872 </t>
    3873 <t>
    3874    When in canonical form, media subtypes of the "text" type use CRLF as
    3875    the text line break. HTTP relaxes this requirement and allows the
    3876    transport of text media with plain CR or LF alone representing a line
    3877    break when it is done consistently for an entire representation. HTTP
    3878    applications &MUST; accept CRLF, bare CR, and bare LF as indicating
    3879    a line break in text media received via HTTP. In
    3880    addition, if the text is in a character encoding that does not
    3881    use octets 13 and 10 for CR and LF respectively, as is the case for
    3882    some multi-byte character encodings, HTTP allows the use of whatever octet
    3883    sequences are defined by that character encoding to represent the
    3884    equivalent of CR and LF for line breaks. This flexibility regarding
    3885    line breaks applies only to text media in the payload body; a bare CR
    3886    or LF &MUST-NOT; be substituted for CRLF within any of the HTTP control
    3887    structures (such as header fields and multipart boundaries).
    3888 </t>
    3889 <t>
    3890    If a representation is encoded with a content-coding, the underlying
    3891    data &MUST; be in a form defined above prior to being encoded.
    3892 </t>
    3893 </section>
    3894 
    3895 <section title="Multipart Types" anchor="multipart.types">
    3896 <t>
    3897    MIME provides for a number of "multipart" types &mdash; encapsulations of
    3898    one or more representations within a single message body. All multipart
    3899    types share a common syntax, as defined in <xref target="RFC2046" x:sec="5.1.1" x:fmt="of"/>,
    3900    and include a boundary parameter as part of the media type
    3901    value. The message body is itself a protocol element; a sender &MUST;
    3902    generate only CRLF to represent line breaks between body-parts.
    3903 </t>
    3904 <t>
    3905    In general, HTTP treats a multipart message body no differently than
    3906    any other media type: strictly as payload.  HTTP does not use the
    3907    multipart boundary as an indicator of message body length.
    3908    <!-- jre: re-insert removed text pointing to caching? -->
    3909    In all other respects, an HTTP user agent &SHOULD; follow the same or similar
    3910    behavior as a MIME user agent would upon receipt of a multipart type.
    3911    The MIME header fields within each body-part of a multipart message body
    3912    do not have any significance to HTTP beyond that defined by
    3913    their MIME semantics.
    3914 </t>
    3915 <t>
    3916    A recipient &MUST; treat an unrecognized multipart subtype
    3917    as being equivalent to "multipart/mixed".
    3918 </t>
     3718   For <x:ref>201 (Created)</x:ref> responses, the Location is the URI of the
     3719   new resource which was created by the request. For <x:ref>3xx (Redirection)</x:ref>
     3720   responses, the location &SHOULD; indicate the server's preferred URI for
     3721   automatic redirection to the resource.
     3722</t>
     3723<t>
     3724   The field value consists of a single URI-reference. When it has the form
     3725   of a relative reference (<xref target="RFC3986" x:fmt="," x:sec="4.2"/>),
     3726   the final value is computed by resolving it against the effective request
     3727   URI (<xref target="RFC3986" x:fmt="," x:sec="5"/>). If the original URI, as
     3728   navigated to by the user agent, did contain a fragment identifier, and the
     3729   final value does not, then the original URI's fragment identifier is added
     3730   to the final value.
     3731</t>
     3732<figure>
     3733<preamble>For example, the original URI "http://www.example.org/~tim", combined with a field value given as:</preamble><!--DO NOT DARE changing the vertical spacing below, it's necessary this way for xml2rfc-->
     3734<artwork type="example">
     3735  Location: /pub/WWW/People.html#tim
     3736</artwork>
     3737<postamble>would result in a final value of "http://www.example.org/pub/WWW/People.html#tim"</postamble>
     3738</figure>
     3739<figure>
     3740<preamble>An original URI "http://www.example.org/index.html#larry", combined with a field value given as:</preamble><!--DO NOT DARE changing the vertical spacing below, it's necessary this way for xml2rfc-->
     3741<artwork type="example">
     3742  Location: http://www.example.net/index.html
     3743</artwork>
     3744<postamble>would result in a final value of "http://www.example.net/index.html#larry", preserving the original fragment identifier.</postamble>
     3745</figure>
    39193746<x:note>
    39203747  <t>
    3921     &Note; The "multipart/form-data" type has been specifically defined
    3922     for carrying form data suitable for processing via the POST
    3923     request method, as described in <xref target="RFC2388"/>.
     3748    &Note; Some recipients attempt to recover from Location fields
     3749    that are not valid URI references. This specification does not mandate or
     3750    define such processing, but does allow it.
    39243751  </t>
    39253752</x:note>
    3926 </section>
    3927 </section>
    3928 
    3929 <section title="Language Tags" anchor="language.tags">
    3930   <x:anchor-alias value="language-tag"/>
    3931 <t>
    3932    A language tag, as defined in <xref target="RFC5646"/>, identifies a
    3933    natural language spoken, written, or otherwise conveyed by human beings for
    3934    communication of information to other human beings. Computer languages are
    3935    explicitly excluded. HTTP uses language tags within the
    3936    <x:ref>Accept-Language</x:ref> and <x:ref>Content-Language</x:ref> fields.
    3937 </t>
    3938 <t>
    3939    In summary, a language tag is composed of one or more parts: A primary
    3940    language subtag followed by a possibly empty series of subtags:
    3941 </t>
    3942 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="language-tag"/>
    3943   <x:ref>language-tag</x:ref> = &lt;Language-Tag, defined in <xref target="RFC5646" x:sec="2.1"/>&gt;
     3753<t>
     3754   There are circumstances in which a fragment identifier in a Location URI
     3755   would not be appropriate. For instance, when it appears in a <x:ref>201
     3756   (Created)</x:ref> response, where the Location header field specifies the
     3757   URI for the entire created resource.
     3758</t>
     3759<x:note>
     3760  <t>
     3761    &Note; The <x:ref>Content-Location</x:ref> header field
     3762    (&header-content-location;) differs from Location in that the
     3763    Content-Location identifies the most specific resource corresponding to the
     3764    enclosed representation. It is therefore possible for a response to contain
     3765    header fields for both Location and Content-Location.
     3766  </t>
     3767</x:note>
     3768</section>
     3769
     3770<section title="Retry-After" anchor="header.retry-after">
     3771  <iref primary="true" item="Retry-After header field" x:for-anchor=""/>
     3772  <x:anchor-alias value="Retry-After"/>
     3773<t>
     3774   The header "Retry-After" field can be used with a <x:ref>503 (Service
     3775   Unavailable)</x:ref> response to indicate how long the service is expected to
     3776   be unavailable to the requesting client. This field &MAY; also be used
     3777   with any <x:ref>3xx (Redirection)</x:ref> response to indicate the minimum time the
     3778   user-agent is asked to wait before issuing the redirected request.
     3779</t>
     3780<t>
     3781   The value of this field can be either an HTTP-date or an integer number
     3782   of seconds (in decimal) after the time of the response.
     3783</t>
     3784<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Retry-After"/>
     3785  <x:ref>Retry-After</x:ref> = <x:ref>HTTP-date</x:ref> / <x:ref>delta-seconds</x:ref>
    39443786</artwork></figure>
    3945 <t>
    3946    White space is not allowed within the tag and all tags are case-insensitive.
    3947    The name space of language subtags is administered by the IANA (see
    3948    <eref target="http://www.iana.org/assignments/language-subtag-registry"/>).
    3949 </t>
    3950 <figure>
    3951   <preamble>Example tags include:</preamble>
    3952 <artwork type="example">
    3953   en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
    3954 </artwork>
    3955 </figure>
    3956 <t>
    3957    See <xref target="RFC5646"/> for further information.
    3958 </t>
     3787<t anchor="rule.delta-seconds">
     3788  <x:anchor-alias value="delta-seconds"/>
     3789   Time spans are non-negative decimal integers, representing time in
     3790   seconds.
     3791</t>
     3792<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
     3793  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
     3794</artwork></figure>
     3795<t>
     3796   Two examples of its use are
     3797</t>
     3798<figure><artwork type="example">
     3799  Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
     3800  Retry-After: 120
     3801</artwork></figure>
     3802<t>
     3803   In the latter example, the delay is 2 minutes.
     3804</t>
     3805</section>
     3806</section>
     3807
     3808<section title="Selected Representation Header Fields" anchor="selected.representation">
     3809<t><iref primary="true" item="selected representation"/>
     3810   We use the term "<x:dfn>selected representation</x:dfn>" to refer to the
     3811   the current representation of a target resource that would have been
     3812   selected in a successful response if the same request had used the
     3813   method GET and excluded any conditional request header fields.
     3814</t>
     3815<t>
     3816   Additional header fields define metadata about the selected
     3817   representation, which might differ from the representation included
     3818   in the message for responses to some state-changing methods.
     3819   The following header fields are defined as selected representation
     3820   metadata:
     3821</t>
     3822<texttable align="left">
     3823  <ttcol>Header Field Name</ttcol>
     3824  <ttcol>Defined in...</ttcol>
     3825
     3826  <c>ETag</c> <c>&header-etag;</c>
     3827  <c>Last-Modified</c> <c>&header-last-modified;</c>
     3828  <c>Vary</c> <c><xref target="header.vary"/></c>
     3829</texttable>
     3830
     3831<section anchor="header.vary" title="Vary">
     3832   <iref item="Vary header field" primary="true" x:for-anchor="" />
     3833   <x:anchor-alias value="Vary"/>
     3834<t>
     3835   The "Vary" header field conveys the set of header fields
     3836   that were used to select the representation.
     3837</t>
     3838<t>
     3839   Caches use this information as part of determining whether a stored
     3840   response can be used to satisfy a given request (&caching-neg-resp;).
     3841</t>
     3842<t>
     3843   In uncacheable or stale responses, the Vary field value advises the user
     3844   agent about the criteria that were used to select the representation.
     3845</t>
     3846<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
     3847  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
     3848</artwork></figure>
     3849<t>
     3850   The set of header fields named by the Vary field value is known as the
     3851   selecting header fields.
     3852</t>
     3853<t>
     3854   A server &SHOULD; include a Vary header field with any cacheable response
     3855   that is subject to proactive negotiation. Doing so allows a cache to
     3856   properly interpret future requests on that resource and informs the user
     3857   agent about the presence of negotiation on that resource. A server &MAY;
     3858   include a Vary header field with a non-cacheable response that is subject
     3859   to proactive negotiation, since this might provide the user agent with
     3860   useful information about the dimensions over which the response varies at
     3861   the time of the response.
     3862</t>
     3863<t>
     3864   A Vary field value of "*" signals that unspecified parameters not limited
     3865   to the header fields (e.g., the network address of the client), play a
     3866   role in the selection of the response representation; therefore, a cache
     3867   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
     3868   generate the "*" value.
     3869</t>
     3870<t>
     3871   The field-names given are not limited to the set of standard header
     3872   fields defined by this specification. Field names are case-insensitive.
     3873</t>
     3874</section>
     3875</section>
     3876
     3877<section title="Authentication Challenges" anchor="response.auth">
     3878<t>
     3879   Authentication challenges indicate what mechanisms are available for the
     3880   client to provide authentication credentials in future requests.
     3881</t>
     3882<texttable align="left">
     3883  <ttcol>Header Field Name</ttcol><ttcol>Defined in...</ttcol>
     3884
     3885  <c>WWW-Authenticate</c> <c>&header-www-authenticate;</c>
     3886  <c>Proxy-Authenticate</c> <c>&header-proxy-authenticate;</c>
     3887</texttable>
     3888</section>
     3889
     3890<section title="Informative" anchor="response.inform">
     3891<t>
     3892   The remaining response header fields provide more information about
     3893   the target resource for potential use in later requests.
     3894</t>
     3895<texttable align="left">
     3896  <ttcol>Header Field Name</ttcol><ttcol>Defined in...</ttcol>
     3897
     3898  <c>Accept-Ranges</c> <c>&header-accept-ranges;</c>
     3899  <c>Allow</c> <c><xref target="header.allow"/></c>
     3900  <c>Server</c> <c><xref target="header.server"/></c>
     3901</texttable>
     3902
     3903<section title="Allow" anchor="header.allow">
     3904  <iref primary="true" item="Allow header field" x:for-anchor=""/>
     3905  <x:anchor-alias value="Allow"/>
     3906<t>
     3907   The "Allow" header field lists the set of methods advertised as
     3908   supported by the target resource. The purpose of this field is strictly to
     3909   inform the recipient of valid request methods associated with the resource.
     3910</t>
     3911<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Allow"/>
     3912  <x:ref>Allow</x:ref> = #<x:ref>method</x:ref>
     3913</artwork></figure>
     3914<t>
     3915   Example of use:
     3916</t>
     3917<figure><artwork type="example">
     3918  Allow: GET, HEAD, PUT
     3919</artwork></figure>
     3920<t>
     3921   The actual set of allowed methods is defined by the origin server at the
     3922   time of each request.
     3923</t>
     3924<t>
     3925   A proxy &MUST-NOT; modify the Allow header field &mdash; it does not need to
     3926   understand all the methods specified in order to handle them according to
     3927   the generic message handling rules.
     3928</t>
     3929</section>
     3930
     3931<section title="Server" anchor="header.server">
     3932  <iref primary="true" item="Server header field" x:for-anchor=""/>
     3933  <x:anchor-alias value="Server"/>
     3934<t>
     3935   The "Server" header field contains information about the
     3936   software used by the origin server to handle the request.
     3937</t>
     3938<t>
     3939   The field can contain multiple
     3940   product tokens (<xref target="product.tokens"/>) and
     3941   comments (&header-fields;) identifying the server and any significant
     3942   subproducts. The product tokens are listed in order of their significance
     3943   for identifying the application.
     3944</t>
     3945<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Server"/>
     3946  <x:ref>Server</x:ref> = <x:ref>product</x:ref> *( <x:ref>RWS</x:ref> ( <x:ref>product</x:ref> / <x:ref>comment</x:ref> ) )
     3947</artwork></figure>
     3948<t>
     3949   Example:
     3950</t>
     3951<figure><artwork type="example">
     3952  Server: CERN/3.0 libwww/2.17
     3953</artwork></figure>
     3954<t>
     3955   If the response is being forwarded through a proxy, the proxy application
     3956   &MUST-NOT; modify the <x:ref>Server</x:ref> header field. Instead, it
     3957   &MUST; include a <x:ref>Via</x:ref> field (as described in &header-via;).
     3958</t>
     3959<x:note>
     3960  <t>
     3961    &Note; Revealing the specific software version of the server might
     3962    allow the server machine to become more vulnerable to attacks
     3963    against software that is known to contain security holes. Server
     3964    implementers are encouraged to make this field a configurable
     3965    option.
     3966  </t>
     3967</x:note>
     3968</section>
    39593969</section>
    39603970</section>
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