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.









 
RFC 2068                        HTTP/1.1                    January 1997


   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).










 
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.