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Good day All,

I understand that when I send a SIP INVITE and do not receive a 200 OK, it will keep sending INVITES at regular duration (until sip timeout).

However, If I have received a 100 Trying for the 1st INVITE and no 200 OK (still waiting for it), then will SIP Server send INVITEs as above regular duration?

Or no matter reliable/unreliable(1xx messages), since SIP Server has got a response it will just sit and wait for the 200 OK?

RFC3261 just talks about series of INVITEs sent due to no "200 OK" received from destination.

Please help to understand.

Bashab
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1 Answers1

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After receiving a provisional response (1xx), retransmissions of the INVITE message will end.

The relevant part from RFC3261 is in bold:

17.1.1.1 Overview of INVITE Transaction

The INVITE transaction consists of a three-way handshake. The client transaction sends an INVITE, the server transaction sends responses, and the client transaction sends an ACK. For unreliable transports (such as UDP), the client transaction retransmits requests at an interval that starts at T1 seconds and doubles after every retransmission. T1 is an estimate of the round-trip time (RTT), and it defaults to 500 ms. Nearly all of the transaction timers described here scale with T1, and changing T1 adjusts their values. The request is not retransmitted over reliable transports. After receiving a 1xx response, any retransmissions cease altogether, and the client waits for further responses. The server transaction can send additional 1xx responses, which are not transmitted reliably by the server transaction. Eventually, the server transaction decides to send a final response. For unreliable transports, that response is retransmitted periodically, and for reliable transports, it is sent once. For each final response that is received at the client transaction, the client transaction sends an ACK, the purpose of which is to quench retransmissions of the response.

Pierre-Luc Pineault
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