RTP FEC + RTX: Using both rtpulpfecenc and rtprtxqueue

[Mathieu Duponchelle]

Worth mentioning http://mathieuduponchelle.github.io/2020-10-09-SMPTE-2022-1-2D-Forward-Error-Correction-in-GStreamer.html,
we can probably use this as the basis for flex fec, I second Olivier's
comment that we want that :)

On 6/8/21 7:04 PM, Olivier Crête via gstreamer-devel wrote:
> Hi,
>
> You must place the rtprtxsend inside rtpbin too, you can do this by
> inserting using the "request-aux-sender" signal that was designed for
> this.
>
> As for the FEC's success, if you do random drops and you drop multiple
> packets in a sequence, it's expected that it won't be that great. For
> those cases, we would need to implement 2D FEC and that's not
> implemented yet for ULP. Ideally, we'd actually implement flexfec is
> which the future of FEC, but that's also in the future.
>
> Olivier
>
> On Tue, 2021-06-08 at 17:42 +0200, Pablo Odorico via gstreamer-devel
> wrote:
>> Hello,
>>
>> Thank you for the awesome project.
>>
>> I'm trying to enable both RTP ULP-based FEC and RTX on a pipeline
>> which is using rtpbin.
>>
>> They are both somewhat working independently*, but RTX breaks down
>> when FEC is enabled. This is most likely due to this comment in the
>> rtpulpfecenc doc:
>>> This element rewrites packets' seqnums, which means that when combined with retransmission elements such as GstRtpRtxSend, it *must* be placed upstream of those, otherwise retransmission requests will request incorrect seqnums.
>> Currently rtpulpfecenc is inside the rtpbin ("request-fec-encoder"
>> signal) and rtprtxqueue is placed right before rtpbin.send_rtp_sink_0
>> which upstream of rtpulpfecenc.
>>
>> Any idea how I can switch this order considering rtpbin is doing the
>> linking of the rtpulpfecenc?
>>
>> * rtpulpfecenc is not working that great so far. Dropping 1% of
>> packets it can only recover <50% of the packets no matter what the FEC
>> percentage I use (even 100%), is this expected?
>>
>> Thank you,
>> Pablo
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