rfc2068.txt

穿越防火墙技术代码

   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 encodings, 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.








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   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 MUST represent the character set defined by
   that registry.  Applications SHOULD limit their use of character sets
   to those defined by the IANA registry.

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 14.12) 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).









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RFC 2068                        HTTP/1.1                    January 1997


     Note: Use of program names for the identification of encoding
     formats is not desirable and should be 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].

   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

   All transfer-coding values are case-insensitive. HTTP/1.1 uses
   transfer coding values in the Transfer-Encoding header field (section
   14.40).

   Transfer codings are analogous to the Content-Transfer-Encoding
   values of MIME , 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 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 footer 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.





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       Chunked-Body   = *chunk
                        "0" CRLF
                        footer
                        CRLF

       chunk          = chunk-size [ chunk-ext ] CRLF
                        chunk-data CRLF

       hex-no-zero    = <HEX excluding "0">

       chunk-size     = hex-no-zero *HEX
       chunk-ext      = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
       chunk-ext-name = token
       chunk-ext-val  = token | quoted-string
       chunk-data     = chunk-size(OCTET)

       footer         = *entity-header

   The chunked encoding is ended by a zero-sized chunk followed by the
   footer, which is terminated by an empty line. The purpose of the
   footer is to provide an efficient way to supply information about an
   entity that is generated dynamically; applications MUST NOT send
   header fields in the footer which are not explicitly defined as being
   appropriate for the footer, such as Content-MD5 or future extensions
   to HTTP for digital signatures or other facilities.

   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 transfer coding extensions
   they do not understand. 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.

3.7 Media Types

   HTTP uses Internet Media Types  in the Content-Type (section 14.18)
   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.



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

   The type, subtype, and parameter attribute names are case-
   insensitive.  Parameter values may or may 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. User agents that recognize the media-type
   MUST process (or arrange to be processed by any external applications
   used to process that type/subtype by the user agent) the parameters
   for that MIME type as described by that type/subtype definition to
   the and inform the user of any problems discovered.

     Note: 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). The media type registration process is outlined in
   RFC 2048 [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. In
   general, an entity-body transferred via HTTP messages MUST be
   represented in the appropriate canonical form prior to its
   transmission; the exception is "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-Encoding, the underlying
   data MUST be in a form defined above prior to being encoded.



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   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.

   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.

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  MIME [7], 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 MIME, the
   epilogue of any multipart message MUST be empty; HTTP applications
   MUST NOT transmit the epilogue (even if the original multipart
   contains