[Galene] Re: about packet pacers

[Guillaume Denis <gdenispro@gmail.com>]

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Toke, Juliusz, thanks for your input.

As a followup and even if it's off topic for Galene users I'll share this
implementation of the GCC algorithm based on RTCP packets:
https://github.com/creamlab/ducksoup/blob/main/sfu/mixer.go#L203
(with a resulting periodical update of GStreamer encoder bitrates)

Using pacers is a bit too networky for me right now, I'll see how much I
need it.
Thanks, Guillaume

Le sam. 7 août 2021 à 01:01, Juliusz Chroboczek <jch@irif.fr> a écrit :

> >> Toke, would you have some input regarding this:
> >> - do you think that in this use case monitoring a pacer with the target
> >> bitrate would have real added value?
>
> > Hmm, so the idea here would be that we know the bitrate of the stream
> > from the codec setting, and from that we can compute the packet-level
> > bitrate (by adding the packet overhead) and pace out the packets at that
> > rate? [...]
> > One question is whether the codec can be relied upon to stay within a
> > certain bitrate and over what timescales?
>
> No, but we know where the current frame ends, and we know when the next
> frame is due.  So we can spread the current frame smoothly over the
> interval between the current frame and the next one.  That will break when
> we get variable frame-rate codecs, but we'll cross that bridge when we get
> there.
>
> This is not what the GCC draft recommends[1]: what they suggest is to
> consider the target bitrate (the one that the congestion controller yields)
> and send a burst of up to 5ms*rate every 5ms.
>
> [1]
> https://datatracker.ietf.org/doc/html/draft-ietf-rmcat-gcc-02#section-4
>
> > On Linux you can use the SO_TXTIME socket option to get the kernel to
> > pace the packets for you; but that only works if there's a qdisc
> > installed that can handle the pacing (such as sch_fq), and it's hardly
> > cross-platform, so dunno if that will work?
>
> I'd rather we did the pacing in the application, since that will allow us
> to drop packets more selectively: if we drop a packet due to congestion
> control, then we want to continue dropping until the end of the frame.
> What's more, both the VP8 and VP9 codecs can be configured in a mode where
> they regularly schedule a "droppable" frame that is not used for
> intra-prediction (it's part of the simulcast implementation).
>
> -- Juliusz
>

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