[Bloat] Steam's download CDNs - breaking bufferbloat and inbound policers

[cloneman <bufferbloat@flamingpc.com>]

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Hi,

Apologies in advance if this is the wrong place to ask.

I'm looking for any comments on Steam's game distribution download system -
specifically how it defeats any bufferbloat management system I've used.

It seems to push past inbound policers, exceeding them by about 40%. That
is to say, you must police steam traffic to half your line rate, then
enough capacity will remain to avoid packet loss, latency, etc. Obviously
this is too much bandwidth to reserve for practical use.

Without any inbound control, you can expect very heavy packet loss and
jitter. With fq_codel or sfq and taking the usual recommended 15% off the
table, you get improved, but still unacceptable performance in your small
flows / ping etc.

The behavior can be observed by downloading any free game on their
platform. I'm trying to figure out how they've accomplished this and how to
mitigate this behavior. It operates with 20 http connections
simultaneously, which is normally not an issue (20 multiple web downloads
perform well under fq_codel and 15% reserve bandwidth)

Note: in my testing cable and vdsl below 100mbit were vulnerable to this
behavior, while fiber was immune.

Basically there are edge application cases on the internet that like to
push too many bytes down a line. I would like to see some discussion or
testing of this issue.

I haven't tried tweaking any of the parameters / latency targets in
fq_codel.

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