14.26 If-None-Match

   The If-None-Match request-header field is used with a method to make
   it conditional. A client that has one or more entities previously
   obtained from the resource can verify that none of those entities is
   current by including a list of their associated entity tags in the
   If-None-Match header field. The purpose of this feature is to allow
   efficient updates of cached information with a minimum amount of
   transaction overhead. It is also used, on updating requests, to
   prevent inadvertent modification of a resource which was not known to
   exist.

   As a special case, the value "*" matches any current entity of the
   resource.

          If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )

   If any of the entity tags match the entity tag of the entity that
   would have been returned in the response to a similar GET request
   (without the If-None-Match header) on that resource, or if "*" is
   given and any current entity exists for that resource, then the
   server MUST NOT perform the requested method. Instead, if the request
   method was GET or HEAD, the server SHOULD respond with a 304 (Not
   Modified) response, including the cache-related entity-header fields
   (particularly ETag) of one of the entities that matched. For all
   other request methods, the server MUST respond with a status of 412
   (Precondition Failed).

   See section 13.3.3 for rules on how to determine if two entity tags
   match. The weak comparison function can only be used with GET or HEAD
   requests.

   If none of the entity tags match, or if "*" is given and no current
   entity exists, then the server MAY perform the requested method as if
   the If-None-Match header field did not exist.




Fielding, et. al.           Standards Track                   [Page 122]

RFC 2068                        HTTP/1.1                    January 1997


   If the request would, without the If-None-Match header field, result
   in anything other than a 2xx status, then the If-None-Match header
   MUST be ignored.

   The meaning of "If-None-Match: *" is that the method MUST NOT be
   performed if the representation selected by the origin server (or by
   a cache, possibly using the Vary mechanism, see section 14.43)
   exists, and SHOULD be performed if the representation does not exist.
   This feature may be useful in preventing races between PUT
   operations.

   Examples:

          If-None-Match: "xyzzy"
          If-None-Match: W/"xyzzy"
          If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
          If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
          If-None-Match: *