Greetings,
Consider a case where the response to a REGISTER gets lost, UA resends the
identical request.
This means the callid and cseq remain the same.
RFC 3261 10.3 (step 7) states that if callid is identical:
... the registrar compares the CSeq value. If the value
is higher than that of the existing binding, it MUST update or
remove the binding as above. If not, the update MUST be
aborted and the request fails.
This algorithm ensures that out-of-order requests from the same
UA are ignored.
​Since CSeq is not higher than that of the existing binding, this also
results in the retries to get ignored as well.
Is the the client expected to increment CSeq if a response is not received,
or is the server expected to detect re-transmissions and resend the
previously sent response prior to applying the algorithm described in 10.3.
Thank You
Chaim Geretz
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