[Galene] About GCC in Galene

[Juliusz Chroboczek <jch@irif.fr>]

Hi,

Currently, Galene implements the older REMB encapsulation of the GCC
congestion control algorithm, and white it implements full GCC in the
Galene->client direction, it only implements a loss-based controller in
the client->Galene direction.

Since Pion claims to include a full implementation of GCC over the newer
TWCC encapsulation, I set out to implement it in Galene.  In short, it's
not going well.

I started with porting Galene to the v4 branch of Pion.  That went fine,
the results are in the pion-v4 branch, and are ready to be merged.

I then implemented support for RTX tracks, which are not strictly
necessary for GCC, but they cannot harm.  I almost got there, then got
stuck on

  https://github.com/pion/webrtc/issues/2994

Okay, no RTX for me then.  I started looking at TWCC, at which point
I found out that the congestion control API in Pion is almost unbelievably
convoluted, it uses callbacks in order to return the results of
a synchronous operation.  So I tried to fix that:

  https://github.com/pion/interceptor/pull/297
  https://github.com/pion/webrtc/pull/3002
  https://github.com/pion/interceptor/pull/298
  https://github.com/pion/webrtc/pull/3003

At this point, I implemented a branch of Galene with TWCC, only to find
that the GCC implementation in Pion doesn't work correctly:

  https://github.com/pion/interceptor/issues/299

It looks like there are other issues with it:

  https://github.com/pion/interceptor/issues/260
  https://github.com/pion/interceptor/issues/271

Fortunately, there are some good people working on fixing that, so there
is some hope.  In the meantime, I think I'm going to merge my port to
pion-v4, but keep my current homebrew implementation of GCC: it might use
an older packet format, and might be incomplete, but at least it works.

-- Juliusz