[Bloat] better tc support for bittorrent/diffserv

[Juliusz Chroboczek]

> I think, but am not certain, jch is referring to the problem of
> negotiating ecn over uTP.

No, I was actually unaware that Linux allows one to set the ECN bits
from userspace.

OTOH, you're right that µTP doesn't currently support ECN.  Not that it
matters much, ECN is useful for interactive connections (where it can
avoid a one-RTT delay due to a packet loss) but pretty useless for bulk
transfers[1].

On a related note, the µTP framing is pretty horrible IMHO, and LEDBAT
is better implemented within TCP.  Unfortunately, this is not possible
under Windows (yeah, I got one of the µTorrent guys to contact Microsoft
on this subject).

On a note related to the related note, Transmission already has the
ability to make use of a non-default congestion controller for TCP:

  http://www.pps.jussieu.fr/~jch/software/bittorrent/tcp-congestion-control.html

-- Juliusz

[1] Marek Malowidzki, Simulation-based Study of ECN Performance in RED
    Networks, In Proc. SPECTS'03. 2003.