[Bloat] better tc support for bittorrent/diffserv

[Juliusz Chroboczek]

[Pro memoria, I'm the author of a few of the networking bits in
Transmission, notably the DHT, the µTP support and large parts of the
IPv6 code.]

> 0) No ecn on transmission, at least.

TCP flows will use ECN if your system is configured to use ECN.  µTP
doesn't do ECN, and there's no easy way to implement it -- there's no
API to set ECN on UDP packets.

> Packets not marked as CS1,

Add

  "peer-socket-tos": "lowcost",

to your settings.json.

> 1) The port number is consistent, but user-settable.

Only for µTP and incoming TCP.  Outgoing TCP uses a random port.
It's a well known flaw of the sockets API that you cannot use the same
local port for incoming and outgoing connections.

> 4)  Syntactically tc is missing some ip6 functionality, notably an
> orthogonal "tos" field to the ipv4 version.

Use ip6tables -j MARK.

-- Juliusz