From: [EMAIL PROTECTED]
Any extensions applied to a non-421 response MUST be listed in a
Require header field included in the response. Of course, the server
MUST NOT apply extensions not listed in the Supported header field in
the request.
Let me also add that in a 421 (Extension Required) response, the
Require field is used for a third purpose, to list the extenstions
that the UAC does not list in Supported but that the UAS needs to
invoke in order to usefully process the request.
(421 should not be confused with 420 (Bad Extension, or Extension Not
Implemented), which is used to reject a request with a Require or
Proxy-Require header. 420 uses the Unsupported header to list the
required extensions that it does not support.)
If I recall correctly, there are three extensions or proposed
extensions that use Require headers in responses routinely. One is
the GRUU extension, but I've forgotten the other two.
John Elwell points out that RFC 3262 is another case (for 1xx
responses that invoke PRACK). The third was discussed in Chicago.
Dale
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