> From: pranab sahoo [pranabsahoog...@gmail.com]
>
> suppose A calls B.B's phone start ringing,B did not recieve the
> call.Finally B's phone stops ringing after some time.So what will be
> the error response will go from B to A?
If B's phone decides that the call has rung long enough, or the time
specified in the Expires header has passed, B's phone will respond
with 487. See RFC 3261, section 13.3.1, item 1:
1. If the request is an INVITE that contains an Expires header
field, the UAS core sets a timer for the number of seconds
indicated in the header field value. When the timer fires, the
invitation is considered to be expired. If the invitation
expires before the UAS has generated a final response, a 487
(Request Terminated) response SHOULD be generated.
More commonly, the expiration time is enforced by a proxy which sends
an INVITE to B's phone, and when the expiration time expires, the
proxy sends a CANCEL to B's phone. Upon receiving the CANCEL, B's
phone gives a 487 response to the INVITE. (And a 200 response to the
CANCEL.)
Dale
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