[pjsip] Missing ACK,

Martin Umgeher m.umgeher at easymobiz.com
Fri Jun 15 08:24:10 EDT 2012

Hi everyone,

I'm using pjsip 2 on iOS 5. I have a strange issue concerning the connection state of my SIP call.

The problem occurs when I want to accept an incoming call. After I send an OK response to the INVITE, I apparently do not receive an ACK. As a result the call state stays PJSIP_INV_STATE_CONNECTING. I do get on_call_media_state callback and thus can set up the voice connection; but my app gets in a somewhat inconsistent state, as it thinks the connection it not established yet.

This situation only occurs when I call using a specific SIP client software (which unfortunately I have to support with my own pjsip-based client implementation). No other client I tested my own code with so far causes this problem.

Looks a lot like the problem lies in this other client (let's call it X). But I tested X with a lot of other SIP soft phones, and none seem to have any problem with setting up the connection to X.

I read of a similar problem which was due to a NAT problem. I tried to configure both sides of the configuration with the STUN server, but this didn't change anything.

So now to my questions…
1. Is there a strategy for handling a missing ACK that I can follow? (This would imply that such behavior happens more often, probably due to bad SIP implementations; and that the other SIP clients I tested would implement such a strategy)
2. Can this be a NAT problem at all? I assume that if NAT was not configured correctly, I wouldn't get anything (no INVITE, no voice), not just have a missing ACK.

Any hint on what might cause this weirdness would be very much appreciated!


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