[asterisk-dev] SDP version

Dmitry Andrianov dimas at dataart.com
Mon Jul 7 15:38:27 CDT 2008

Hello list. (And I believe, Raj is the right person to send this question to)

After addition of SIP session timers support Asterisk ignores SDP with the already seen version number. The comment says:

        /* Store the SDP version number of remote UA. This will allow us to
        distinguish between session modifications and session refreshes. If
        the remote UA does not send an incremented SDP version number in a
        subsequent RE-INVITE then that means its not changing media session.
        The RE-INVITE may have been sent to update connected party, remote
        target or to refresh the session (Session-Timers).  Asterisk must not
        change media session and increment its own version number in answer
        SDP in this case. */

The problem I'm having is that Kapanga softphone does not increment version number when it actually changes the media (when it responds with OK to Asterisk's T38 re-INVITE). As result, Asterisk ignores that OK and T38 switchover does not occur.

The question is: does some SIP/SDP specification state that client MUST always change SDP version on T38 switchover? In this case it would be clearly a Kapanga bug which I will report to them. However I cannot find documentation which states that.

Dmitry Andrianov
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