[Freeswitch-users] Sometimes missing a few seconds of audio when CN is offered in INVITE (and solution)

[Peter Olsson]

Hello everyone! I experience a strange issue for bridged calls from Lync -> FreeSWITCH -> Asterisk, it seems to be related to CN packets. I'm not sure if this a bug or not so I ask the question here first.

The problem is that the audio back from the Asterisk server is not bridged back to the originating client for the first few seconds (how long is different on different calls), instead FS seems to send CN-packets back. I've looked into a wireshark dump, and I see the audio is coming from the Asterisk server (using the correct IP and port), but FS doesn't write the same packet to the other call leg (back to Lync), instead it sends a CN packet every second or so. My guess is that the timestamp is being handled wrong somehow, and FS believes that the real packet is too old (since it has already sent a CN packet with a higher timestamp?), and should not be written to the other leg, then after a while it sends a new CN packet and so on.

The solution is to set  "suppress_cng=true" before bridging the call to the Asterisk server, when this is done, the audio is always bridged correctly, and nothing is missing.

I have pcaps of both working calls (with the variable set) and nonworking calls, so if you believe it might be something that FS should handle differently I'll submit this to Jira.

/Peter
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