presentation, request logging, etc.
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.
HTTP character sets are identified by case-insensitive tokens. The
complete set of tokens are defined by the IANA Character Set registry
. However, because that registry does not define a single,
consistent token for each character set, we define here the preferred
names for those character sets most likely to be used with HTTP
entities. These character sets include those registered by RFC 1521
 -- the US-ASCII  and ISO-8859  character sets -- and
other names specifically recommended for use within MIME charset
charset = "US-ASCII"
| "ISO-8859-1" | "ISO-8859-2" | "ISO-8859-3"
| "ISO-8859-4" | "ISO-8859-5" | "ISO-8859-6"
| "ISO-8859-7" | "ISO-8859-8" | "ISO-8859-9"
| "ISO-2022-JP" | "ISO-2022-JP-2" | "ISO-2022-KR"
| "UNICODE-1-1" | "UNICODE-1-1-UTF-7" | "UNICODE-1-1-UTF-8"
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
RFC 1945 HTTP/1.0 May 1996
by that registry. Applications should limit their use of character
sets to those defined by the IANA registry.
The character set of an entity body should be labelled as the lowest
common denominator of the character codes used within that body, with
the exception that no label is preferred over the labels US-ASCII or
3.5 Content Codings
Content coding values are used to indicate an encoding transformation
that has been applied to a resource. Content codings are primarily
used to allow a document to be compressed or encrypted without losing
the identity of its underlying media type. Typically, the resource is
stored in this encoding and only decoded before rendering or
content-coding = "x-gzip" | "x-compress" | token
Note: For future compatibility, HTTP/1.0 applications should
consider "gzip" and "compress" to be equivalent to "x-gzip"
and "x-compress", respectively.
All content-coding values are case-insensitive. HTTP/1.0 uses
content-coding values in the Content-Encoding (Section 10.3) header
field. 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. Note that a single program may be
capable of decoding multiple content-coding formats. Two values are
defined by this specification:
An encoding format produced by the file compression program
"gzip" (GNU zip) developed by Jean-loup Gailly. This format is
typically a Lempel-Ziv coding (LZ77) with a 32 bit CRC.
The encoding format produced by the file compression program
"compress". This format is an adaptive Lempel-Ziv-Welch coding
Note: Use of program names for the identification of