12.2.2
If the remote sequence number is empty, it MUST be set to the value
of the sequence number in the CSeq header field value in the request.
If the remote sequence number was not empty, but the sequence number
of the request is lower than the remote sequence number, the request
is out of order and MUST be rejected with a 500 (Server Internal
Error) response. If the remote sequence number was not empty, and
the sequence number of the request is greater than the remote
sequence number, the request is in order. It is possible for the
CSeq sequence number to be higher than the remote sequence number by
more than one. This is not an error condition, and a UAS SHOULD be
prepared to receive and process requests with CSeq values more than
one higher than the previous received request. The UAS MUST then set
the remote sequence number to the value of the sequence number in the
CSeq header field value in the request.
If a proxy challenges a request generated by the UAC, the UAC has
to resubmit the request with credentials. The resubmitted request
will have a new CSeq number. The UAS will never see the first
request, and thus, it will notice a gap in the CSeq number space.
Such a gap does not represent any error condition.
o Daniel-Constantin Mierla on 04/24/2012 02:10 PM:
Hello,
I was wondering if someone here can point quickly where specs mention
what is the right reply code to send when a request within dialog is
received with lower cseq value than the previous request. I couldn't
spot the part in the RFC yet, if any related exists.
Cheers,
Daniel
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