19.6.2 Additional Header Field Definitions

19.6.2.1 Alternates

   The Alternates response-header field has been proposed as a means for
   the origin server to inform the client about other available
   representations of the requested resource, along with their
   distinguishing attributes, and thus providing a more reliable means
   for a user agent to perform subsequent selection of another
   representation which better fits the desires of its user (described
   as agent-driven negotiation in section 12).













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RFC 2068                        HTTP/1.1                    January 1997


   The Alternates header field is orthogonal to the Vary header field in
   that both may coexist in a message without affecting the
   interpretation of the response or the available representations. It
   is expected that Alternates will provide a significant improvement
   over the server-driven negotiation provided by the Vary field for
   those resources that vary over common dimensions like type and
   language.

   The Alternates header field will be defined in a future
   specification.

19.6.2.2 Content-Version

   The Content-Version entity-header field defines the version tag
   associated with a rendition of an evolving entity. Together with the
   Derived-From field described in section 19.6.2.3, it allows a group
   of people to work simultaneously on the creation of a work as an
   iterative process. The field should be used to allow evolution of a
   particular work along a single path rather than derived works or
   renditions in different representations.

          Content-Version = "Content-Version" ":" quoted-string

   Examples of the Content-Version field include:

          Content-Version: "2.1.2"
          Content-Version: "Fred 19950116-12:26:48"
          Content-Version: "2.5a4-omega7"

19.6.2.3 Derived-From

   The Derived-From entity-header field can be used to indicate the
   version tag of the resource from which the enclosed entity was
   derived before modifications were made by the sender. This field is
   used to help manage the process of merging successive changes to a
   resource, particularly when such changes are being made in parallel
   and from multiple sources.

          Derived-From   = "Derived-From" ":" quoted-string

   An example use of the field is:

          Derived-From: "2.1.1"

   The Derived-From field is required for PUT and PATCH requests if the
   entity being sent was previously retrieved from the same URI and a
   Content-Version header was included with the entity when it was last
   retrieved.



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