Assuming the UAS decides that it is the proper element to process the
request, it examines the Require header field, if present.
The Require header field is used by a UAC to tell a UAS about SIP
extensions that the UAC expects the UAS to support in order to
process the request properly. Its format is described in Section
20.32. If a UAS does not understand an option-tag listed in a
Require header field, it MUST respond by generating a response with
status code 420 (Bad Extension). The UAS MUST add an Unsupported
header field, and list in it those options it does not understand
amongst those in the Require header field of the request.
Note that Require and Proxy-Require MUST NOT be used in a SIP CANCEL
request, or in an ACK request sent for a non-2xx response. These
header fields MUST be ignored if they are present in these requests.
An ACK request for a 2xx response MUST contain only those Require and
Proxy-Require values that were present in the initial request.
Example:
UAC->UAS: INVITE sip:[email protected] SIP/2.0
Require: 100rel
UAS->UAC: SIP/2.0 420 Bad Extension
Unsupported: 100rel
This behavior ensures that the client-server interaction will
proceed without delay when all options are understood by both
sides, and only slow down if options are not understood (as in the
example above). For a well-matched client-server pair, the
interaction proceeds quickly, saving a round-trip often required
by negotiation mechanisms. In addition, it also removes ambiguity
when the client requires features that the server does not
understand. Some features, such as call handling fields, are only
of interest to end systems.
