A common misconception - the ack for2xx is directed towards the dialog and not the transaction. If the ack is not received in time, the UAS is supposed to delete the dialog. Note that if the UAC is doing things right the ack will match the dialog of the UAS because it must use the tags of the 200 ok and the dialog will transition to established.

Ranga.

Markus Hofmann wrote:

Hi,

I don't understand following situation:

UAC ----------- INVITE --------------> UAS
state = CALLING state = PROCEEDING
UAC <-----------200 OK -------------- UAS state = TERMINATED state = TERMINATED
UAC -------- ACK ----- X UAS


The UAC sends an INVITE to the UAS. The UAC is able to accept this call and sends back 
a 200 OK. Its state machine is going into the state TERMINATED. The UAC receives the 
200 OK and changes its state in TERMINATED too. Then the UAC sents an ACK with a new 
via branch parameter because a new transaction. But this ACK will never reach its 
target. What is to do?
I know a dialog is established and the core will handle this ACK on both sides. But no 
timer is running to find out that the ACK is lost. No response will send to the UAC. 
UAC will never no that the ACK is lost. What will the UAS do, which waits for this 
ACK? Will an RTP Session start?
If the ACK has an Message Body with sdp information how would be the behaviour of the 
UAS? Will the UAS sends immediately a BYE if they don't have negotiate the right 
things. The receiver side doesn't know that an ACK was send?

Thank you in advance.

Markus
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