[Galene] Re: about packet pacers

[Juliusz Chroboczek <jch@irif.fr>]

I'll let Toke (or Dave?) answer the other points, but here's one I think
I understand.

> - I guess the pacer must act before the RTP packetizer so that timing
> information present in RTP headers are correct/not modified by the pacer. Is
> this correct?

As far as I understand, the pacer is applied after the RTP packetiser.
For each frame (or "picture" in VP9) produduced by the codec, the RTP
packetiser will produce a burst of packets with the same timestamp.  If
these packets are sent in quick succession, they will temporarily congest
the sending interface or the bottleneck router.

The role of the pacer, as I undestand it, is to schedule the set of
packets in a single frame so that they go out at regular intervals.  The
effect is a slight increase in delay (since the frame needs to be
reassembled at the receiver before it can be processed), in jitter (since
some packets might be larger than others), but a reduction in short-term
network congestion and therefore in packet loss.

The pacer does not rewrite RTP timestamps: the RTP timestamp describes
where the packets fit in the reconstructed video, it has little to do with
the time at which the packet is actually sent.  (If the actual sending
time is needed, there's the abs-send-time extension, which should be
inserted after the pacer.)

Galene does not use a pacer, since it merely forwards packets that have
already been properly disciplined by the sender.  (There is one exception,
it's when we send a cached keyframe to a new receiver -- we currently send
the keyframe as a burst, we should probably be conditioning it.)

-- Juliusz