Other proxies may be bypassed, however.
REPLY-TO: Travis Russell
Require This header is used by entities to identify any SIP extensions that must
be supported by the other entities. This is similar to the PROXY-REQUIRE header
discussed previously, with the exception that this is sent by devices rather than
proxies.
This header is needed because SIP extensions may not be supported in every network
(and consequently by every device). If a device is sending a request using specific services
that are enabled through a SIP extension, then those extensions must be identified
to the other endpoint to ensure the session can be supported.
REQUIRE: 100 REL
Retry-After This header is used with the 500 Server Internal Error and 503 Service
Unavailable responses to indicate to the requestor the duration that the server is expected
to be unavailable. It can also be used with 4xx responses to indicate when the
called party expects to be available again.
The parameters would indicate the duration of time and possibly the time when the
calling party could try to reach the called party again. Optionally a message such as ???In
a Meeting??? can be provided as part of the response to identify why the party is rejecting
a call or why the called party is not available.
RETRY-AFTER: 19000; duration=3600
Route This header is used along with the RECORD-ROUTE header when strict routing
is implemented (as is the case in the IMS).
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