[Freeswitch-users] Sequence numbers are created on outgoing call leg without taking into account lost sequence number on incoming call leg

[Dmitriy Ageev]

Hello,

It would be much appreciated to have some advice on following use case.

We establish a call on a Freeswitch by creating two call legs separately and then bridged them together:
Client <--Call Leg A--> Freeswtich <--Call Leg B--> Gstreamer

During a call there are packet losses on Call Leg A between Client and Freeswitch and we have appropriate notifications in a Freeswitch debug logs. As well as it's clear from the pcap logs taken from a Freeswitch that RTP packets are missing with certain sequence number.

Since Client and Gstreamer using same Opus codec Freeswitch just sends packets straightly with same payload and timestamp to Gstreamer. The only thing is changed in RTP packets are SSRC and Sequence Numbers.

In a case when RTP packet is lost sequence numbers are not preserved between Call Leg A and Call Leg B. So Call Leg B just got packets which are make it up to Freeswitch. That means that Call Leg B gets for all received on a Freeswitch RTP packets consecutive sequence numbers (unfortunately lost packets are not count) and so Gstreamer is not aware of the packet loss which took precedence on a previous Call Leg A.

Due to this since all RTP packets are consecutive on Call Leg B Gstreamer have no notion of a lost packet. So it is not starting a procedure of Forward Error Correction (FEC) on a lost packet and eventually we got audio drops which are not handled. As well as Freeswitch is not decoding payload so it doesn't do FEC on Opus side.

That's why we are wondering what could be a solution for that? From first glance into RTP RFC it seems ok to have such behavior, i.e. not take into account previous sequence numbers for a packet loss. But maybe it's just not considering such cases. Is there other RFCs which cover such situations?

We thinking maybe it make sense to have a patch to FS so that we would receive if not the same but at least have the same gaps in the sequence numbers as on Call Leg A?

Or maybe some other solution to that problem which is available on a Freeswitch?

If you need us to share any FS logs will be happy to provide.

I'll attach pcap logs for the time being.

188.130.168.205 - Client
10.240.0.12 - Freeswitch
10.240.0.7 - Gstreamer

Please take a look at following filter. Where 24632 is missing packet on Call leg A:
rtp.seq== 24631 || rtp.seq== 24632 || rtp.seq== 24633 || rtp.seq == 56583 || rtp.seq == 56582
___________
Best Regards,
Dmitriy Ageev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.freeswitch.org/pipermail/freeswitch-users/attachments/20170130/f744da84/attachment-0001.html 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: log_fs.pcap
Type: application/octet-stream
Size: 4624161 bytes
Desc: log_fs.pcap
Url : http://lists.freeswitch.org/pipermail/freeswitch-users/attachments/20170130/f744da84/attachment-0001.obj