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

[Dmitriy Ageev]

Thank you for quick response.

Maybe it’s not clearly related but we have another question related to the situation described below. Probably partially due to the spoken problem with sequence numbers we clearly observe audio drops in previous situation.

But we have another use case when we make a direct client to client call with Opus codec through Freesiwtch with invoking media bugs. And in such case we don’t notice any audio drops.

So from this there are questions in which we would be thankful to have answer:

-          Maybe there are some difference in how Freesiwtch handles packet out of order or lost packets when media bug is involved? Why there is a clear difference in audio for these two situations because in both cases we don’t enable Freeswitch Jitter Buffer?

-           I assume that the reason is that for media bug it performs opus decoding so mod_opus performs accurate FEC based on the sequence number and maybe some other opus internal work.  Also looking into mod_opus code we see that even jitter buffer is used there. Is that correct? Is there something we are missing?

-          Would you recommend to enable jitter buffer at Freeswitch for the use case below?

Thanks a lot for all you efforts in Freeswitch and making it clear to us on how it works.
___________
Best Regards,
Dmitriy Ageev

From: freeswitch-users-bounces at lists.freeswitch.org [mailto:freeswitch-users-bounces at lists.freeswitch.org] On Behalf Of Anthony Minessale
Sent: Monday, January 30, 2017 7:54 PM
To: FreeSWITCH Users Help <freeswitch-users at lists.freeswitch.org>
Subject: Re: [Freeswitch-users] Sequence numbers are created on outgoing call leg without taking into account lost sequence number on incoming call leg

This is why audio jitter buffers need to look at timestamp and not seq numbers to determine lost packets.
Its required by the RFC to always write new seq numbers when generating a new stream.



On Mon, Jan 30, 2017 at 10:12 AM, Dmitriy Ageev <dmitriy.a at soniccloud.com<mailto:dmitriy.a at soniccloud.com>> wrote:
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


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