[Cake] [Codel] Proposing COBALTLike the issues with streaming video, which are potentially addressed by the fq qdisc. I wonder how much it would help for torrents. (Avoiding bursting the entire congestion window at the start of chunk 2, using pacing).

[Jonathan Morton]

> On 4 Jun, 2016, at 23:04, Alan Jenkins <alan.christopher.jenkins at gmail.com> wrote:
> 
> Am I right in thinking Steam uses a torrent download?

No; though it is a multi-stream system, it’s basically HTTP (with offset/range) plus some minor special sauce.  Several files can be fetched at once in parallel, and these separate connections are usually to different servers.

It differs from torrent swarms most fundamentally in that there is no uploading; all the content comes from official servers.  This also means that fewer connections are needed to fill a given download pipe (usually just one would do the trick).

We’re at a significant disadvantage because we typically have to apply our AQM *after* the bottleneck link in the downstream direction.  This means we have to somehow prevent senders from sending too many packets in order to avoid inducing delay within the ISP’s dumb buffers, rather than being able to buffer them smartly ourselves as a last resort.  We therefore rely on effective congestion control, with flow-isolation being primarily a means to target the signals more precisely.

Servers which defeat our primary method of doing so (by ignoring the copious signals saying “slow down please”) are ultimately unhelpful.  But at least they are still ack-clocked.

 - Jonathan Morton