An UAC sends an INVITE to an UAS. The UAS responds with a 200 (no 1xx was
sent, or the 1xx was lost), but the 200 is lost in the network. In this
case the UAC will retransmit the INVITE until a 200 is received, and the
UAS will retransmit the 200 until an ACK is received. What is not clear is
what the UAS must do with the retransmitted INVITE received. The
transaction is gone, so the INVITE is received directly by the TU.
Bug654[1] says that a 500 must be sent, but it looks like a bad idea, as
the 200 can still be retransmitted and received by the UAC. A better
alternative would be to simply discard the INVITE that have a sequence
number equals to the remote sequence number; the 200 will be retransmitted
and hopefully the UAC will send an ACK to the UAS.
Another related problem is when there is a stateful proxy between the UAC
and the UAS. If the 200 is lost between the UAC and the proxy, then what
the proxy must do with the retransmitted INVITE?
Thank you for your responses.
[1] http://www.sipwg.org/sipwg/show_bug.cgi?id=654
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