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





 
RFC 2616                        HTTP/1.1                       June 1999


   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.




 
RFC 2616                        HTTP/1.1                       June 1999


   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