3 Protocol Parameters

3.1 HTTP Version

   HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
   of the protocol. The protocol versioning policy is intended to allow
   the sender to indicate the format of a message and its capacity for
   understanding further HTTP communication, rather than the features
   obtained via that communication. No change is made to the version
   number for the addition of message components which do not affect
   communication behavior or which only add to extensible field values.
   The <minor> number is incremented when the changes made to the
   protocol add features which do not change the general message parsing
   algorithm, but which may add to the message semantics and imply
   additional capabilities of the sender. The <major> number is
   incremented when the format of a message within the protocol is
   changed. See RFC 2145 [36] for a fuller explanation.

   The version of an HTTP message is indicated by an HTTP-Version field
   in the first line of the message.

       HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT

   Note that the major and minor numbers MUST be treated as separate
   integers and that each MAY be incremented higher than a single digit.
   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
   lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and
   MUST NOT be sent.

   An application that sends a request or response message that includes
   HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant
   with this specification. Applications that are at least conditionally
   compliant with this specification SHOULD use an HTTP-Version of
   "HTTP/1.1" in their messages, and MUST do so for any message that is
   not compatible with HTTP/1.0. For more details on when to send
   specific HTTP-Version values, see RFC 2145 [36].

   The HTTP version of an application is the highest HTTP version for
   which the application is at least conditionally compliant.

   Proxy and gateway applications need to be careful when forwarding
   messages in protocol versions different from that of the application.
   Since the protocol version indicates the protocol capability of the
   sender, a proxy/gateway MUST NOT send a message with a version
   indicator which is greater than its actual version. If a higher
   version request is received, the proxy/gateway MUST either downgrade
   the request version, or respond with an error, or switch to tunnel
   behavior.

   Due to interoperability problems with HTTP/1.0 proxies discovered
   since the publication of RFC 2068[33], caching proxies MUST, gateways
   MAY, and tunnels MUST NOT upgrade the request to the highest version
   they support. The proxy/gateway's response to that request MUST be in
   the same major version as the request.

      Note: Converting between versions of HTTP may involve modification
      of header fields required or forbidden by the versions involved.

3.2 Uniform Resource Identifiers

   URIs have been known by many names: WWW addresses, Universal Document
   Identifiers, Universal Resource Identifiers [3], and finally the
   combination of Uniform Resource Locators (URL) [4] and Names (URN)
   [20]. As far as HTTP is concerned, Uniform Resource Identifiers are
   simply formatted strings which identify--via name, location, or any
   other characteristic--a resource.

3.2.1 General Syntax

   URIs in HTTP can be represented in absolute form or relative to some
   known base URI [11], depending upon the context of their use. The two
   forms are differentiated by the fact that absolute URIs always begin
   with a scheme name followed by a colon. For definitive information on
   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
   Generic Syntax and Semantics," RFC 2396 [42] (which replaces RFCs
   1738 [4] and RFC 1808 [11]). This specification adopts the
   definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
   "host","abs_path", "rel_path", and "authority" from that
   specification.

   The HTTP protocol does not place any a priori limit on the length of
   a URI. Servers MUST be able to handle the URI of any resource they
   serve, and SHOULD be able to handle URIs of unbounded length if they
   provide GET-based forms that could generate such URIs. A server
   SHOULD return 414 (Request-URI Too Long) status if a URI is longer
   than the server can handle (see section 10.4.15).

      Note: Servers ought to be cautious about depending on URI lengths
      above 255 bytes, because some older client or proxy
      implementations might not properly support these lengths.

3.2.2 http URL

   The "http" scheme is used to locate network resources via the HTTP
   protocol. This section defines the scheme-specific syntax and
   semantics for http URLs.

   http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]

   If the port is empty or not given, port 80 is assumed. The semantics
   are that the identified resource is located at the server listening
   for TCP connections on that port of that host, and the Request-URI
   for the resource is abs_path (section 5.1.2). The use of IP addresses
   in URLs SHOULD be avoided whenever possible (see RFC 1900 [24]). If
   the abs_path is not present in the URL, it MUST be given as "/" when
   used as a Request-URI for a resource (section 5.1.2). If a proxy
   receives a host name which is not a fully qualified domain name, it
   MAY add its domain to the host name it received. If a proxy receives
   a fully qualified domain name, the proxy MUST NOT change the host
   name.

3.2.3 URI Comparison

   When comparing two URIs to decide if they match or not, a client
   SHOULD use a case-sensitive octet-by-octet comparison of the entire
   URIs, with these exceptions:

      - A port that is empty or not given is equivalent to the default
        port for that URI-reference;

        - Comparisons of host names MUST be case-insensitive;

        - Comparisons of scheme names MUST be case-insensitive;

        - An empty abs_path is equivalent to an abs_path of "/".

   Characters other than those in the "reserved" and "unsafe" sets (see
   RFC 2396 [42]) are equivalent to their ""%" HEX HEX" encoding.

   For example, the following three URIs are equivalent:

      http://abc.com:80/~smith/home.html
      http://ABC.com/%7Esmith/home.html
      http://ABC.com:/%7esmith/home.html

3.3 Date/Time Formats

3.3.1 Full Date

   HTTP applications have historically allowed three different formats
   for the representation of date/time stamps:

      Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
      Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
      Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format

   The first format is preferred as an Internet standard and represents
   a fixed-length subset of that defined by RFC 1123 [8] (an update to
   RFC 822 [9]). The second format is in common use, but is based on the
   obsolete RFC 850 [12] date format and lacks a four-digit year.
   HTTP/1.1 clients and servers that parse the date value MUST accept
   all three formats (for compatibility with HTTP/1.0), though they MUST
   only generate the RFC 1123 format for representing HTTP-date values
   in header fields. See section 19.3 for further information.

      Note: Recipients of date values are encouraged to be robust in
      accepting date values that may have been sent by non-HTTP
      applications, as is sometimes the case when retrieving or posting
      messages via proxies/gateways to SMTP or NNTP.

   All HTTP date/time stamps MUST be represented in Greenwich Mean Time
   (GMT), without exception. For the purposes of HTTP, GMT is exactly
   equal to UTC (Coordinated Universal Time). This is indicated in the
   first two formats by the inclusion of "GMT" as the three-letter
   abbreviation for time zone, and MUST be assumed when reading the
   asctime format. HTTP-date is case sensitive and MUST NOT include
   additional LWS beyond that specifically included as SP in the
   grammar.

       HTTP-date    = rfc1123-date | rfc850-date | asctime-date
       rfc1123-date = wkday "," SP date1 SP time SP "GMT"
       rfc850-date  = weekday "," SP date2 SP time SP "GMT"
       asctime-date = wkday SP date3 SP time SP 4DIGIT
       date1        = 2DIGIT SP month SP 4DIGIT
                      ; day month year (e.g., 02 Jun 1982)
       date2        = 2DIGIT "-" month "-" 2DIGIT
                      ; day-month-year (e.g., 02-Jun-82)
       date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
                      ; month day (e.g., Jun  2)
       time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                      ; 00:00:00 - 23:59:59
       wkday        = "Mon" | "Tue" | "Wed"
                    | "Thu" | "Fri" | "Sat" | "Sun"
       weekday      = "Monday" | "Tuesday" | "Wednesday"
                    | "Thursday" | "Friday" | "Saturday" | "Sunday"
       month        = "Jan" | "Feb" | "Mar" | "Apr"
                    | "May" | "Jun" | "Jul" | "Aug"
                    | "Sep" | "Oct" | "Nov" | "Dec"

      Note: HTTP requirements for the date/time stamp format apply only
      to their usage within the protocol stream. Clients and servers are
      not required to use these formats for user presentation, request
      logging, etc.

3.3.2 Delta Seconds

   Some HTTP header fields allow a time value to be specified as an
   integer number of seconds, represented in decimal, after the time
   that the message was received.

       delta-seconds  = 1*DIGIT

3.4 Character Sets

   HTTP uses the same definition of the term "character set" as that
   described for MIME:

   The term "character set" is used in this document to refer to a
   method used with one or more tables to convert a sequence of octets
   into a sequence of characters. Note that unconditional conversion in
   the other direction is not required, in that not all characters may
   be available in a given character set and a character set may provide
   more than one sequence of octets to represent a particular character.
   This definition is intended to allow various kinds of character
   encoding, from simple single-table mappings such as US-ASCII to
   complex table switching methods such as those that use ISO-2022's
   techniques. However, the definition associated with a MIME character
   set name MUST fully specify the mapping to be performed from octets
   to characters. In particular, use of external profiling information
   to determine the exact mapping is not permitted.

      Note: This use of the term "character set" is more commonly
      referred to as a "character encoding." However, since HTTP and
      MIME share the same registry, it is important that the terminology
      also be shared.

   HTTP character sets are identified by case-insensitive tokens. The
   complete set of tokens is defined by the IANA Character Set registry
   [19].

       charset = token

   Although HTTP allows an arbitrary token to be used as a charset
   value, any token that has a predefined value within the IANA
   Character Set registry [19] MUST represent the character set defined
   by that registry. Applications SHOULD limit their use of character
   sets to those defined by the IANA registry.

   Implementors should be aware of IETF character set requirements [38]
   [41].

3.4.1 Missing Charset

   Some HTTP/1.0 software has interpreted a Content-Type header without
   charset parameter incorrectly to mean "recipient should guess."
   Senders wishing to defeat this behavior MAY include a charset
   parameter even when the charset is ISO-8859-1 and SHOULD do so when
   it is known that it will not confuse the recipient.

   Unfortunately, some older HTTP/1.0 clients did not deal properly with
   an explicit charset parameter. HTTP/1.1 recipients MUST respect the
   charset label provided by the sender; and those user agents that have
   a provision to "guess" a charset MUST use the charset from the

   content-type field if they support that charset, rather than the
   recipient's preference, when initially displaying a document. See
   section 3.7.1.

3.5 Content Codings

   Content coding values indicate an encoding transformation that has
   been or can be applied to an entity. Content codings are primarily
   used to allow a document to be compressed or otherwise usefully
   transformed without losing the identity of its underlying media type
   and without loss of information. Frequently, the entity is stored in
   coded form, transmitted directly, and only decoded by the recipient.

       content-coding   = token

   All content-coding values are case-insensitive. HTTP/1.1 uses
   content-coding values in the Accept-Encoding (section 14.3) and
   Content-Encoding (section 14.11) header fields. Although the value
   describes the content-coding, what is more important is that it
   indicates what decoding mechanism will be required to remove the
   encoding.

   The Internet Assigned Numbers Authority (IANA) acts as a registry for
   content-coding value tokens. Initially, the registry contains the
   following tokens:

   gzip An encoding format produced by the file compression program
        "gzip" (GNU zip) as described in RFC 1952 [25]. This format is a
        Lempel-Ziv coding (LZ77) with a 32 bit CRC.

   compress
        The encoding format produced by the common UNIX file compression
        program "compress". This format is an adaptive Lempel-Ziv-Welch
        coding (LZW).

        Use of program names for the identification of encoding formats
        is not desirable and is discouraged for future encodings. Their
        use here is representative of historical practice, not good
        design. For compatibility with previous implementations of HTTP,
        applications SHOULD consider "x-gzip" and "x-compress" to be
        equivalent to "gzip" and "compress" respectively.

   deflate
        The "zlib" format defined in RFC 1950 [31] in combination with
        the "deflate" compression mechanism described in RFC 1951 [29].

   identity
        The default (identity) encoding; the use of no transformation
        whatsoever. This content-coding is used only in the Accept-
        Encoding header, and SHOULD NOT be used in the Content-Encoding
        header.

   New content-coding value tokens SHOULD be registered; to allow
   interoperability between clients and servers, specifications of the
   content coding algorithms needed to implement a new value SHOULD be
   publicly available and adequate for independent implementation, and
   conform to the purpose of content coding defined in this section.

3.6 Transfer Codings

   Transfer-coding values are used to indicate an encoding
   transformation that has been, can be, or may need to be applied to an
   entity-body in order to ensure "safe transport" through the network.
   This differs from a content coding in that the transfer-coding is a
   property of the message, not of the original entity.

       transfer-coding         = "chunked" | transfer-extension
       transfer-extension      = token *( ";" parameter )

   Parameters are in  the form of attribute/value pairs.

       parameter               = attribute "=" value
       attribute               = token
       value                   = token | quoted-string

   All transfer-coding values are case-insensitive. HTTP/1.1 uses
   transfer-coding values in the TE header field (section 14.39) and in
   the Transfer-Encoding header field (section 14.41).

   Whenever a transfer-coding is applied to a message-body, the set of
   transfer-codings MUST include "chunked", unless the message is
   terminated by closing the connection. When the "chunked" transfer-
   coding is used, it MUST be the last transfer-coding applied to the
   message-body. The "chunked" transfer-coding MUST NOT be applied more
   than once to a message-body. These rules allow the recipient to
   determine the transfer-length of the message (section 4.4).

   Transfer-codings are analogous to the Content-Transfer-Encoding
   values of MIME [7], which were designed to enable safe transport of
   binary data over a 7-bit transport service. However, safe transport
   has a different focus for an 8bit-clean transfer protocol. In HTTP,
   the only unsafe characteristic of message-bodies is the difficulty in
   determining the exact body length (section 7.2.2), or the desire to
   encrypt data over a shared transport.

   The Internet Assigned Numbers Authority (IANA) acts as a registry for
   transfer-coding value tokens. Initially, the registry contains the
   following tokens: "chunked" (section 3.6.1), "identity" (section
   3.6.2), "gzip" (section 3.5), "compress" (section 3.5), and "deflate"
   (section 3.5).

   New transfer-coding value tokens SHOULD be registered in the same way
   as new content-coding value tokens (section 3.5).

   A server which receives an entity-body with a transfer-coding it does
   not understand SHOULD return 501 (Unimplemented), and close the
   connection. A server MUST NOT send transfer-codings to an HTTP/1.0
   client.

3.6.1 Chunked Transfer Coding

   The chunked encoding modifies the body of a message in order to
   transfer it as a series of chunks, each with its own size indicator,
   followed by an OPTIONAL trailer containing entity-header fields. This
   allows dynamically produced content to be transferred along with the
   information necessary for the recipient to verify that it has
   received the full message.

       Chunked-Body   = *chunk
                        last-chunk
                        trailer
                        CRLF

       chunk          = chunk-size [ chunk-extension ] CRLF
                        chunk-data CRLF
       chunk-size     = 1*HEX
       last-chunk     = 1*("0") [ chunk-extension ] CRLF

       chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
       chunk-ext-name = token
       chunk-ext-val  = token | quoted-string
       chunk-data     = chunk-size(OCTET)
       trailer        = *(entity-header CRLF)

   The chunk-size field is a string of hex digits indicating the size of
   the chunk. The chunked encoding is ended by any chunk whose size is
   zero, followed by the trailer, which is terminated by an empty line.

   The trailer allows the sender to include additional HTTP header
   fields at the end of the message. The Trailer header field can be
   used to indicate which header fields are included in a trailer (see
   section 14.40).

   A server using chunked transfer-coding in a response MUST NOT use the
   trailer for any header fields unless at least one of the following is
   true:

   a)the request included a TE header field that indicates "trailers" is
     acceptable in the transfer-coding of the  response, as described in
     section 14.39; or,

   b)the server is the origin server for the response, the trailer
     fields consist entirely of optional metadata, and the recipient
     could use the message (in a manner acceptable to the origin server)
     without receiving this metadata.  In other words, the origin server
     is willing to accept the possibility that the trailer fields might
     be silently discarded along the path to the client.

   This requirement prevents an interoperability failure when the
   message is being received by an HTTP/1.1 (or later) proxy and
   forwarded to an HTTP/1.0 recipient. It avoids a situation where
   compliance with the protocol would have necessitated a possibly
   infinite buffer on the proxy.

   An example process for decoding a Chunked-Body is presented in
   appendix 19.4.6.

   All HTTP/1.1 applications MUST be able to receive and decode the
   "chunked" transfer-coding, and MUST ignore chunk-extension extensions
   they do not understand.

3.7 Media Types

   HTTP uses Internet Media Types [17] in the Content-Type (section
   14.17) and Accept (section 14.1) header fields in order to provide
   open and extensible data typing and type negotiation.

       media-type     = type "/" subtype *( ";" parameter )
       type           = token
       subtype        = token

   Parameters MAY follow the type/subtype in the form of attribute/value
   pairs (as defined in section 3.6).

   The type, subtype, and parameter attribute names are case-
   insensitive. Parameter values might or might not be case-sensitive,
   depending on the semantics of the parameter name. Linear white space
   (LWS) MUST NOT be used between the type and subtype, nor between an
   attribute and its value. The presence or absence of a parameter might
   be significant to the processing of a media-type, depending on its
   definition within the media type registry.

   Note that some older HTTP applications do not recognize media type
   parameters. When sending data to older HTTP applications,
   implementations SHOULD only use media type parameters when they are
   required by that type/subtype definition.

   Media-type values are registered with the Internet Assigned Number
   Authority (IANA [19]). The media type registration process is
   outlined in RFC 1590 [17]. Use of non-registered media types is
   discouraged.

3.7.1 Canonicalization and Text Defaults

   Internet media types are registered with a canonical form. An
   entity-body transferred via HTTP messages MUST be represented in the
   appropriate canonical form prior to its transmission except for
   "text" types, as defined in the next paragraph.

   When in canonical form, media subtypes of the "text" type use CRLF as
   the text line break. HTTP relaxes this requirement and allows the
   transport of text media with plain CR or LF alone representing a line
   break when it is done consistently for an entire entity-body. HTTP
   applications MUST accept CRLF, bare CR, and bare LF as being
   representative of a line break in text media received via HTTP. In
   addition, if the text is represented in a character set that does not
   use octets 13 and 10 for CR and LF respectively, as is the case for
   some multi-byte character sets, HTTP allows the use of whatever octet
   sequences are defined by that character set to represent the
   equivalent of CR and LF for line breaks. This flexibility regarding
   line breaks applies only to text media in the entity-body; a bare CR
   or LF MUST NOT be substituted for CRLF within any of the HTTP control
   structures (such as header fields and multipart boundaries).

   If an entity-body is encoded with a content-coding, the underlying
   data MUST be in a form defined above prior to being encoded.

   The "charset" parameter is used with some media types to define the
   character set (section 3.4) of the data. When no explicit charset
   parameter is provided by the sender, media subtypes of the "text"
   type are defined to have a default charset value of "ISO-8859-1" when
   received via HTTP. Data in character sets other than "ISO-8859-1" or
   its subsets MUST be labeled with an appropriate charset value. See
   section 3.4.1 for compatibility problems.

3.7.2 Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   one or more entities within a single message-body. All multipart
   types share a common syntax, as defined in section 5.1.1 of RFC 2046

   [40], and MUST include a boundary parameter as part of the media type
   value. The message body is itself a protocol element and MUST
   therefore use only CRLF to represent line breaks between body-parts.
   Unlike in RFC 2046, the epilogue of any multipart message MUST be
   empty; HTTP applications MUST NOT transmit the epilogue (even if the
   original multipart contains an epilogue). These restrictions exist in
   order to preserve the self-delimiting nature of a multipart message-
   body, wherein the "end" of the message-body is indicated by the
   ending multipart boundary.

   In general, HTTP treats a multipart message-body no differently than
   any other media type: strictly as payload. The one exception is the
   "multipart/byteranges" type (appendix 19.2) when it appears in a 206
   (Partial Content) response, which will be interpreted by some HTTP
   caching mechanisms as described in sections 13.5.4 and 14.16. In all
   other cases, an HTTP user agent SHOULD follow the same or similar
   behavior as a MIME user agent would upon receipt of a multipart type.
   The MIME header fields within each body-part of a multipart message-
   body do not have any significance to HTTP beyond that defined by
   their MIME semantics.

   In general, an HTTP user agent SHOULD follow the same or similar
   behavior as a MIME user agent would upon receipt of a multipart type.
   If an application receives an unrecognized multipart subtype, the
   application MUST treat it as being equivalent to "multipart/mixed".

      Note: The "multipart/form-data" type has been specifically defined
      for carrying form data suitable for processing via the POST
      request method, as described in RFC 1867 [15].

3.8 Product Tokens

   Product tokens are used to allow communicating applications to
   identify themselves by software name and version. Most fields using
   product tokens also allow sub-products which form a significant part
   of the application to be listed, separated by white space. By
   convention, the products are listed in order of their significance
   for identifying the application.

       product         = token ["/" product-version]
       product-version = token

   Examples:

       User-Agent: CERN-LineMode/2.15 libwww/2.17b3
       Server: Apache/0.8.4

   Product tokens SHOULD be short and to the point. They MUST NOT be
   used for advertising or other non-essential information. Although any
   token character MAY appear in a product-version, this token SHOULD
   only be used for a version identifier (i.e., successive versions of
   the same product SHOULD only differ in the product-version portion of
   the product value).

3.9 Quality Values

   HTTP content negotiation (section 12) uses short "floating point"
   numbers to indicate the relative importance ("weight") of various
   negotiable parameters.  A weight is normalized to a real number in
   the range 0 through 1, where 0 is the minimum and 1 the maximum
   value. If a parameter has a quality value of 0, then content with
   this parameter is `not acceptable' for the client. HTTP/1.1
   applications MUST NOT generate more than three digits after the
   decimal point. User configuration of these values SHOULD also be
   limited in this fashion.

       qvalue         = ( "0" [ "." 0*3DIGIT ] )
                      | ( "1" [ "." 0*3("0") ] )

   "Quality values" is a misnomer, since these values merely represent
   relative degradation in desired quality.

3.10 Language Tags

   A language tag identifies a natural language spoken, written, or
   otherwise conveyed by human beings for communication of information
   to other human beings. Computer languages are explicitly excluded.
   HTTP uses language tags within the Accept-Language and Content-
   Language fields.

   The syntax and registry of HTTP language tags is the same as that
   defined by RFC 1766 [1]. In summary, a language tag is composed of 1
   or more parts: A primary language tag and a possibly empty series of
   subtags:

        language-tag  = primary-tag *( "-" subtag )
        primary-tag   = 1*8ALPHA
        subtag        = 1*8ALPHA

   White space is not allowed within the tag and all tags are case-
   insensitive. The name space of language tags is administered by the
   IANA. Example tags include:

       en, en-US, en-cockney, i-cherokee, x-pig-latin

   where any two-letter primary-tag is an ISO-639 language abbreviation
   and any two-letter initial subtag is an ISO-3166 country code. (The
   last three tags above are not registered tags; all but the last are
   examples of tags which could be registered in future.)

3.11 Entity Tags

   Entity tags are used for comparing two or more entities from the same
   requested resource. HTTP/1.1 uses entity tags in the ETag (section
   14.19), If-Match (section 14.24), If-None-Match (section 14.26), and
   If-Range (section 14.27) header fields. The definition of how they
   are used and compared as cache validators is in section 13.3.3. An
   entity tag consists of an opaque quoted string, possibly prefixed by
   a weakness indicator.

      entity-tag = [ weak ] opaque-tag
      weak       = "W/"
      opaque-tag = quoted-string

   A "strong entity tag" MAY be shared by two entities of a resource
   only if they are equivalent by octet equality.

   A "weak entity tag," indicated by the "W/" prefix, MAY be shared by
   two entities of a resource only if the entities are equivalent and
   could be substituted for each other with no significant change in
   semantics. A weak entity tag can only be used for weak comparison.

   An entity tag MUST be unique across all versions of all entities
   associated with a particular resource. A given entity tag value MAY
   be used for entities obtained by requests on different URIs. The use
   of the same entity tag value in conjunction with entities obtained by
   requests on different URIs does not imply the equivalence of those
   entities.

3.12 Range Units

   HTTP/1.1 allows a client to request that only part (a range of) the
   response entity be included within the response. HTTP/1.1 uses range
   units in the Range (section 14.35) and Content-Range (section 14.16)
   header fields. An entity can be broken down into subranges according
   to various structural units.

      range-unit       = bytes-unit | other-range-unit
      bytes-unit       = "bytes"
      other-range-unit = token

   The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
   implementations MAY ignore ranges specified using other units.

   HTTP/1.1 has been designed to allow implementations of applications
   that do not depend on knowledge of ranges.

4 HTTP Message

4.1 Message Types

   HTTP messages consist of requests from client to server and responses
   from server to client.

       HTTP-message   = Request | Response     ; HTTP/1.1 messages

   Request (section 5) and Response (section 6) messages use the generic
   message format of RFC 822 [9] for transferring entities (the payload
   of the message). Both types of message consist of a start-line, zero
   or more header fields (also known as "headers"), an empty line (i.e.,
   a line with nothing preceding the CRLF) indicating the end of the
   header fields, and possibly a message-body.

        generic-message = start-line
                          *(message-header CRLF)
                          CRLF
                          [ message-body ]
        start-line      = Request-Line | Status-Line

   In the interest of robustness, servers SHOULD ignore any empty
   line(s) received where a Request-Line is expected. In other words, if
   the server is reading the protocol stream at the beginning of a
   message and receives a CRLF first, it should ignore the CRLF.

   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
   after a POST request. To restate what is explicitly forbidden by the
   BNF, an HTTP/1.1 client MUST NOT preface or follow a request with an
   extra CRLF.

4.2 Message Headers

   HTTP header fields, which include general-header (section 4.5),
   request-header (section 5.3), response-header (section 6.2), and
   entity-header (section 7.1) fields, follow the same generic format as
   that given in Section 3.1 of RFC 822 [9]. Each header field consists
   of a name followed by a colon (":") and the field value. Field names
   are case-insensitive. The field value MAY be preceded by any amount
   of LWS, though a single SP is preferred. Header fields can be
   extended over multiple lines by preceding each extra line with at
   least one SP or HT. Applications ought to follow "common form", where
   one is known or indicated, when generating HTTP constructs, since
   there might exist some implementations that fail to accept anything

   beyond the common forms.

       message-header = field-name ":" [ field-value ]
       field-name     = token
       field-value    = *( field-content | LWS )
       field-content  = <the OCTETs making up the field-value
                        and consisting of either *TEXT or combinations
                        of token, separators, and quoted-string>

   The field-content does not include any leading or trailing LWS:
   linear white space occurring before the first non-whitespace
   character of the field-value or after the last non-whitespace
   character of the field-value. Such leading or trailing LWS MAY be
   removed without changing the semantics of the field value. Any LWS
   that occurs between field-content MAY be replaced with a single SP
   before interpreting the field value or forwarding the message
   downstream.

   The order in which header fields with differing field names are
   received is not significant. However, it is "good practice" to send
   general-header fields first, followed by request-header or response-
   header fields, and ending with the entity-header fields.

   Multiple message-header fields with the same field-name MAY be
   present in a message if and only if the entire field-value for that
   header field is defined as a comma-separated list [i.e., #(values)].
   It MUST be possible to combine the multiple header fields into one
   "field-name: field-value" pair, without changing the semantics of the
   message, by appending each subsequent field-value to the first, each
   separated by a comma. The order in which header fields with the same
   field-name are received is therefore significant to the
   interpretation of the combined field value, and thus a proxy MUST NOT
   change the order of these field values when a message is forwarded.

4.3 Message Body

   The message-body (if any) of an HTTP message is used to carry the
   entity-body associated with the request or response. The message-body
   differs from the entity-body only when a transfer-coding has been
   applied, as indicated by the Transfer-Encoding header field (section
   14.41).

       message-body = entity-body
                    | <entity-body encoded as per Transfer-Encoding>

   Transfer-Encoding MUST be used to indicate any transfer-codings
   applied by an application to ensure safe and proper transfer of the
   message. Transfer-Encoding is a property of the message, not of the

   entity, and thus MAY be added or removed by any application along the
   request/response chain. (However, section 3.6 places restrictions on
   when certain transfer-codings may be used.)

   The rules for when a message-body is allowed in a message differ for
   requests and responses.

   The presence of a message-body in a request is signaled by the
   inclusion of a Content-Length or Transfer-Encoding header field in
   the request's message-headers. A message-body MUST NOT be included in
   a request if the specification of the request method (section 5.1.1)
   does not allow sending an entity-body in requests. A server SHOULD
   read and forward a message-body on any request; if the request method
   does not include defined semantics for an entity-body, then the
   message-body SHOULD be ignored when handling the request.

   For response messages, whether or not a message-body is included with
   a message is dependent on both the request method and the response
   status code (section 6.1.1). All responses to the HEAD request method
   MUST NOT include a message-body, even though the presence of entity-
   header fields might lead one to believe they do. All 1xx
   (informational), 204 (no content), and 304 (not modified) responses
   MUST NOT include a message-body. All other responses do include a
   message-body, although it MAY be of zero length.

4.4 Message Length

   The transfer-length of a message is the length of the message-body as
   it appears in the message; that is, after any transfer-codings have
   been applied. When a message-body is included with a message, the
   transfer-length of that body is determined by one of the following
   (in order of precedence):

   1.Any response message which "MUST NOT" include a message-body (such
     as the 1xx, 204, and 304 responses and any response to a HEAD
     request) is always terminated by the first empty line after the
     header fields, regardless of the entity-header fields present in
     the message.

   2.If a Transfer-Encoding header field (section 14.41) is present and
     has any value other than "identity", then the transfer-length is
     defined by use of the "chunked" transfer-coding (section 3.6),
     unless the message is terminated by closing the connection.

   3.If a Content-Length header field (section 14.13) is present, its
     decimal value in OCTETs represents both the entity-length and the
     transfer-length. The Content-Length header field MUST NOT be sent
     if these two lengths are different (i.e., if a Transfer-Encoding

     header field is present). If a message is received with both a
     Transfer-Encoding header field and a Content-Length header field,
     the latter MUST be ignored.

   4.If the message uses the media type "multipart/byteranges", and the
     ransfer-length is not otherwise specified, then this self-
     elimiting media type defines the transfer-length. This media type
     UST NOT be used unless the sender knows that the recipient can arse
     it; the presence in a request of a Range header with ultiple byte-
     range specifiers from a 1.1 client implies that the lient can parse
     multipart/byteranges responses.

       A range header might be forwarded by a 1.0 proxy that does not
       understand multipart/byteranges; in this case the server MUST
       delimit the message using methods defined in items 1,3 or 5 of
       this section.

   5.By the server closing the connection. (Closing the connection
     cannot be used to indicate the end of a request body, since that
     would leave no possibility for the server to send back a response.)

   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
   containing a message-body MUST include a valid Content-Length header
   field unless the server is known to be HTTP/1.1 compliant. If a
   request contains a message-body and a Content-Length is not given,
   the server SHOULD respond with 400 (bad request) if it cannot
   determine the length of the message, or with 411 (length required) if
   it wishes to insist on receiving a valid Content-Length.

   All HTTP/1.1 applications that receive entities MUST accept the
   "chunked" transfer-coding (section 3.6), thus allowing this mechanism
   to be used for messages when the message length cannot be determined
   in advance.

   Messages MUST NOT include both a Content-Length header field and a
   non-identity transfer-coding. If the message does include a non-
   identity transfer-coding, the Content-Length MUST be ignored.

   When a Content-Length is given in a message where a message-body is
   allowed, its field value MUST exactly match the number of OCTETs in
   the message-body. HTTP/1.1 user agents MUST notify the user when an
   invalid length is received and detected.

4.5 General Header Fields

   There are a few header fields which have general applicability for
   both request and response messages, but which do not apply to the
   entity being transferred. These header fields apply only to the

   message being transmitted.

       general-header = Cache-Control            ; Section 14.9
                      | Connection               ; Section 14.10
                      | Date                     ; Section 14.18
                      | Pragma                   ; Section 14.32
                      | Trailer                  ; Section 14.40
                      | Transfer-Encoding        ; Section 14.41
                      | Upgrade                  ; Section 14.42
                      | Via                      ; Section 14.45
                      | Warning                  ; Section 14.46

   General-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields may be given the semantics of general
   header fields if all parties in the communication recognize them to
   be general-header fields. Unrecognized header fields are treated as
   entity-header fields.

5 Request

   A request message from a client to a server includes, within the
   first line of that message, the method to be applied to the resource,
   the identifier of the resource, and the protocol version in use.

        Request       = Request-Line              ; Section 5.1
                        *(( general-header        ; Section 4.5
                         | request-header         ; Section 5.3
                         | entity-header ) CRLF)  ; Section 7.1
                        CRLF
                        [ message-body ]          ; Section 4.3

5.1 Request-Line

   The Request-Line begins with a method token, followed by the
   Request-URI and the protocol version, and ending with CRLF. The
   elements are separated by SP characters. No CR or LF is allowed
   except in the final CRLF sequence.

        Request-Line   = Method SP Request-URI SP HTTP-Version CRLF

5.1.1 Method

   The Method  token indicates the method to be performed on the
   resource identified by the Request-URI. The method is case-sensitive.

       Method         = "OPTIONS"                ; Section 9.2
                      | "GET"                    ; Section 9.3
                      | "HEAD"                   ; Section 9.4
                      | "POST"                   ; Section 9.5
                      | "PUT"                    ; Section 9.6
                      | "DELETE"                 ; Section 9.7
                      | "TRACE"                  ; Section 9.8
                      | "CONNECT"                ; Section 9.9
                      | extension-method
       extension-method = token

   The list of methods allowed by a resource can be specified in an
   Allow header field (section 14.7). The return code of the response
   always notifies the client whether a method is currently allowed on a
   resource, since the set of allowed methods can change dynamically. An
   origin server SHOULD return the status code 405 (Method Not Allowed)
   if the method is known by the origin server but not allowed for the
   requested resource, and 501 (Not Implemented) if the method is
   unrecognized or not implemented by the origin server. The methods GET
   and HEAD MUST be supported by all general-purpose servers. All other
   methods are OPTIONAL; however, if the above methods are implemented,
   they MUST be implemented with the same semantics as those specified
   in section 9.

5.1.2 Request-URI

   The Request-URI is a Uniform Resource Identifier (section 3.2) and
   identifies the resource upon which to apply the request.

       Request-URI    = "*" | absoluteURI | abs_path | authority

   The four options for Request-URI are dependent on the nature of the
   request. The asterisk "*" means that the request does not apply to a
   particular resource, but to the server itself, and is only allowed
   when the method used does not necessarily apply to a resource. One
   example would be

       OPTIONS * HTTP/1.1

   The absoluteURI form is REQUIRED when the request is being made to a
   proxy. The proxy is requested to forward the request or service it
   from a valid cache, and return the response. Note that the proxy MAY
   forward the request on to another proxy or directly to the server

   specified by the absoluteURI. In order to avoid request loops, a
   proxy MUST be able to recognize all of its server names, including
   any aliases, local variations, and the numeric IP address. An example
   Request-Line would be:

       GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1

   To allow for transition to absoluteURIs in all requests in future
   versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI
   form in requests, even though HTTP/1.1 clients will only generate
   them in requests to proxies.

   The authority form is only used by the CONNECT method (section 9.9).

   The most common form of Request-URI is that used to identify a
   resource on an origin server or gateway. In this case the absolute
   path of the URI MUST be transmitted (see section 3.2.1, abs_path) as
   the Request-URI, and the network location of the URI (authority) MUST
   be transmitted in a Host header field. For example, a client wishing
   to retrieve the resource above directly from the origin server would
   create a TCP connection to port 80 of the host "www.w3.org" and send
   the lines:

       GET /pub/WWW/TheProject.html HTTP/1.1
       Host: www.w3.org

   followed by the remainder of the Request. Note that the absolute path
   cannot be empty; if none is present in the original URI, it MUST be
   given as "/" (the server root).

   The Request-URI is transmitted in the format specified in section
   3.2.1. If the Request-URI is encoded using the "% HEX HEX" encoding
   [42], the origin server MUST decode the Request-URI in order to
   properly interpret the request. Servers SHOULD respond to invalid
   Request-URIs with an appropriate status code.

   A transparent proxy MUST NOT rewrite the "abs_path" part of the
   received Request-URI when forwarding it to the next inbound server,
   except as noted above to replace a null abs_path with "/".

      Note: The "no rewrite" rule prevents the proxy from changing the
      meaning of the request when the origin server is improperly using
      a non-reserved URI character for a reserved purpose.  Implementors
      should be aware that some pre-HTTP/1.1 proxies have been known to
      rewrite the Request-URI.

5.2 The Resource Identified by a Request

   The exact resource identified by an Internet request is determined by
   examining both the Request-URI and the Host header field.

   An origin server that does not allow resources to differ by the
   requested host MAY ignore the Host header field value when
   determining the resource identified by an HTTP/1.1 request. (But see
   section 19.6.1.1 for other requirements on Host support in HTTP/1.1.)

   An origin server that does differentiate resources based on the host
   requested (sometimes referred to as virtual hosts or vanity host
   names) MUST use the following rules for determining the requested
   resource on an HTTP/1.1 request:

   1. If Request-URI is an absoluteURI, the host is part of the
     Request-URI. Any Host header field value in the request MUST be
     ignored.

   2. If the Request-URI is not an absoluteURI, and the request includes
     a Host header field, the host is determined by the Host header
     field value.

   3. If the host as determined by rule 1 or 2 is not a valid host on
     the server, the response MUST be a 400 (Bad Request) error message.

   Recipients of an HTTP/1.0 request that lacks a Host header field MAY
   attempt to use heuristics (e.g., examination of the URI path for
   something unique to a particular host) in order to determine what
   exact resource is being requested.

5.3 Request Header Fields

   The request-header fields allow the client to pass additional
   information about the request, and about the client itself, to the
   server. These fields act as request modifiers, with semantics
   equivalent to the parameters on a programming language method
   invocation.

       request-header = Accept                   ; Section 14.1
                      | Accept-Charset           ; Section 14.2
                      | Accept-Encoding          ; Section 14.3
                      | Accept-Language          ; Section 14.4
                      | Authorization            ; Section 14.8
                      | Expect                   ; Section 14.20
                      | From                     ; Section 14.22
                      | Host                     ; Section 14.23
                      | If-Match                 ; Section 14.24

                      | If-Modified-Since        ; Section 14.25
                      | If-None-Match            ; Section 14.26
                      | If-Range                 ; Section 14.27
                      | If-Unmodified-Since      ; Section 14.28
                      | Max-Forwards             ; Section 14.31
                      | Proxy-Authorization      ; Section 14.34
                      | Range                    ; Section 14.35
                      | Referer                  ; Section 14.36
                      | TE                       ; Section 14.39
                      | User-Agent               ; Section 14.43

   Request-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of request-
   header fields if all parties in the communication recognize them to
   be request-header fields. Unrecognized header fields are treated as
   entity-header fields.

6 Response

   After receiving and interpreting a request message, a server responds
   with an HTTP response message.

       Response      = Status-Line               ; Section 6.1
                       *(( general-header        ; Section 4.5
                        | response-header        ; Section 6.2
                        | entity-header ) CRLF)  ; Section 7.1
                       CRLF
                       [ message-body ]          ; Section 7.2

6.1 Status-Line

   The first line of a Response message is the Status-Line, consisting
   of the protocol version followed by a numeric status code and its
   associated textual phrase, with each element separated by SP
   characters. No CR or LF is allowed except in the final CRLF sequence.

       Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF

6.1.1 Status Code and Reason Phrase

   The Status-Code element is a 3-digit integer result code of the
   attempt to understand and satisfy the request. These codes are fully
   defined in section 10. The Reason-Phrase is intended to give a short
   textual description of the Status-Code. The Status-Code is intended
   for use by automata and the Reason-Phrase is intended for the human
   user. The client is not required to examine or display the Reason-
   Phrase.

   The first digit of the Status-Code defines the class of response. The
   last two digits do not have any categorization role. There are 5
   values for the first digit:

      - 1xx: Informational - Request received, continuing process

      - 2xx: Success - The action was successfully received,
        understood, and accepted

      - 3xx: Redirection - Further action must be taken in order to
        complete the request

      - 4xx: Client Error - The request contains bad syntax or cannot
        be fulfilled

      - 5xx: Server Error - The server failed to fulfill an apparently
        valid request

   The individual values of the numeric status codes defined for
   HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
   presented below. The reason phrases listed here are only
   recommendations -- they MAY be replaced by local equivalents without
   affecting the protocol.

      Status-Code    =
            "100"  ; Section 10.1.1: Continue
          | "101"  ; Section 10.1.2: Switching Protocols
          | "200"  ; Section 10.2.1: OK
          | "201"  ; Section 10.2.2: Created
          | "202"  ; Section 10.2.3: Accepted
          | "203"  ; Section 10.2.4: Non-Authoritative Information
          | "204"  ; Section 10.2.5: No Content
          | "205"  ; Section 10.2.6: Reset Content
          | "206"  ; Section 10.2.7: Partial Content
          | "300"  ; Section 10.3.1: Multiple Choices
          | "301"  ; Section 10.3.2: Moved Permanently
          | "302"  ; Section 10.3.3: Found
          | "303"  ; Section 10.3.4: See Other
          | "304"  ; Section 10.3.5: Not Modified
          | "305"  ; Section 10.3.6: Use Proxy
          | "307"  ; Section 10.3.8: Temporary Redirect
          | "400"  ; Section 10.4.1: Bad Request
          | "401"  ; Section 10.4.2: Unauthorized
          | "402"  ; Section 10.4.3: Payment Required
          | "403"  ; Section 10.4.4: Forbidden
          | "404"  ; Section 10.4.5: Not Found
          | "405"  ; Section 10.4.6: Method Not Allowed
          | "406"  ; Section 10.4.7: Not Acceptable

          | "407"  ; Section 10.4.8: Proxy Authentication Required
          | "408"  ; Section 10.4.9: Request Time-out
          | "409"  ; Section 10.4.10: Conflict
          | "410"  ; Section 10.4.11: Gone
          | "411"  ; Section 10.4.12: Length Required
          | "412"  ; Section 10.4.13: Precondition Failed
          | "413"  ; Section 10.4.14: Request Entity Too Large
          | "414"  ; Section 10.4.15: Request-URI Too Large
          | "415"  ; Section 10.4.16: Unsupported Media Type
          | "416"  ; Section 10.4.17: Requested range not satisfiable
          | "417"  ; Section 10.4.18: Expectation Failed
          | "500"  ; Section 10.5.1: Internal Server Error
          | "501"  ; Section 10.5.2: Not Implemented
          | "502"  ; Section 10.5.3: Bad Gateway
          | "503"  ; Section 10.5.4: Service Unavailable
          | "504"  ; Section 10.5.5: Gateway Time-out
          | "505"  ; Section 10.5.6: HTTP Version not supported
          | extension-code

      extension-code = 3DIGIT
      Reason-Phrase  = *<TEXT, excluding CR, LF>

   HTTP status codes are extensible. HTTP applications are not required
   to understand the meaning of all registered status codes, though such
   understanding is obviously desirable. However, applications MUST
   understand the class of any status code, as indicated by the first
   digit, and treat any unrecognized response as being equivalent to the
   x00 status code of that class, with the exception that an
   unrecognized response MUST NOT be cached. For example, if an
   unrecognized status code of 431 is received by the client, it can
   safely assume that there was something wrong with its request and
   treat the response as if it had received a 400 status code. In such
   cases, user agents SHOULD present to the user the entity returned
   with the response, since that entity is likely to include human-
   readable information which will explain the unusual status.

6.2 Response Header Fields

   The response-header fields allow the server to pass additional
   information about the response which cannot be placed in the Status-
   Line. These header fields give information about the server and about
   further access to the resource identified by the Request-URI.

       response-header = Accept-Ranges           ; Section 14.5
                       | Age                     ; Section 14.6
                       | ETag                    ; Section 14.19
                       | Location                ; Section 14.30
                       | Proxy-Authenticate      ; Section 14.33

                       | Retry-After             ; Section 14.37
                       | Server                  ; Section 14.38
                       | Vary                    ; Section 14.44
                       | WWW-Authenticate        ; Section 14.47

   Response-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of response-
   header fields if all parties in the communication recognize them to
   be response-header fields. Unrecognized header fields are treated as
   entity-header fields.

7 Entity

   Request and Response messages MAY transfer an entity if not otherwise
   restricted by the request method or response status code. An entity
   consists of entity-header fields and an entity-body, although some
   responses will only include the entity-headers.

   In this section, both sender and recipient refer to either the client
   or the server, depending on who sends and who receives the entity.

7.1 Entity Header Fields

   Entity-header fields define metainformation about the entity-body or,
   if no body is present, about the resource identified by the request.
   Some of this metainformation is OPTIONAL; some might be REQUIRED by
   portions of this specification.

       entity-header  = Allow                    ; Section 14.7
                      | Content-Encoding         ; Section 14.11
                      | Content-Language         ; Section 14.12
                      | Content-Length           ; Section 14.13
                      | Content-Location         ; Section 14.14
                      | Content-MD5              ; Section 14.15
                      | Content-Range            ; Section 14.16
                      | Content-Type             ; Section 14.17
                      | Expires                  ; Section 14.21
                      | Last-Modified            ; Section 14.29
                      | extension-header

       extension-header = message-header

   The extension-header mechanism allows additional entity-header fields
   to be defined without changing the protocol, but these fields cannot
   be assumed to be recognizable by the recipient. Unrecognized header
   fields SHOULD be ignored by the recipient and MUST be forwarded by
   transparent proxies.

7.2 Entity Body

   The entity-body (if any) sent with an HTTP request or response is in
   a format and encoding defined by the entity-header fields.

       entity-body    = *OCTET

   An entity-body is only present in a message when a message-body is
   present, as described in section 4.3. The entity-body is obtained
   from the message-body by decoding any Transfer-Encoding that might
   have been applied to ensure safe and proper transfer of the message.

7.2.1 Type

   When an entity-body is included with a message, the data type of that
   body is determined via the header fields Content-Type and Content-
   Encoding. These define a two-layer, ordered encoding model:

       entity-body := Content-Encoding( Content-Type( data ) )

   Content-Type specifies the media type of the underlying data.
   Content-Encoding may be used to indicate any additional content
   codings applied to the data, usually for the purpose of data
   compression, that are a property of the requested resource. There is
   no default encoding.

   Any HTTP/1.1 message containing an entity-body SHOULD include a
   Content-Type header field defining the media type of that body. If
   and only if the media type is not given by a Content-Type field, the
   recipient MAY attempt to guess the media type via inspection of its
   content and/or the name extension(s) of the URI used to identify the
   resource. If the media type remains unknown, the recipient SHOULD
   treat it as type "application/octet-stream".

7.2.2 Entity Length

   The entity-length of a message is the length of the message-body
   before any transfer-codings have been applied. Section 4.4 defines
   how the transfer-length of a message-body is determined.