Re: [LARTC] Making tcp start transfers slow

[Andy Furniss <andy.furniss@dsl.pipex.com>]

Patrick Petersen wrote:
> Hey list
> I have almost gotten my shaping setup up and running as planned. The
> last barrier seems to be tcp overshooting availible bandwidth when its
> starting a transfer, and thereby bursting the line, so ping rises for a
> moment. At least this is my best guess at the problem :)

I sortof workaround this using the connbytes netfilter patch to make the 
first 80k of new connections go into a short queue limited to 1/3 - 1/2 
of my downstream bandwidth. It works well in the case where the link is 
empty apart from a gamestream and someone is browsing "heavy .jpg" type 
web paged. It also helps a bit if there is other traffic - but if there 
are enough tcp connections on the go there will be higher latency bursts
caused by new connections as HTB can't throttle until it's a bit too late.


> There is a possibility that its just plain old traffic being bursty for
> some reason.. I am using bittorrent to test this, as it seems to be what
> stresses the line the most.

Ahh bittorrent - this is a special case. It uses full duplex tcp - so 
may break some upstream shapers, you can assume that a fair number of 
your peers have flooded modem buffers - so there could be quite a few 
"unstoppable" packets headed your way. The worse thing about it, though 
is that it runs it's own algorithm on allready open tcps - so existing 
connections may go back into slowstart.

> Would it be possible to lower the default window size, and thereby
> making tcp start up slower, or would this just lower the overall speed?

It could help, but may also give you less of a share of a given 
uploaders bandwidth. Reducing MTU may also help (if you run BT on linux 
it may reduce rwin for you aswell).

> My efforts so far can be seen here:
> http://tc.schmakk.dk/betashaper

Had a quick look - Some thoughts:

I don't think you can catch all BT traffic by marking  the BT ports, I 
see ipp2p - can you do it with this or maybe do per IP fairness for bulk 
traffic?

Be carefull about priorotising acks - don't all TCP packets after syn 
have ack set. Being lazy on a home setup I get away with giving small 
packets priority - saves alot of marking :-)

For ingress shaping - I find it nicer to shape per IP with htb and use 
esfq classic to get per tcp fairness rather than esfq on dst which is 
going to effectively make many bittorrent connections go into a FIFO, 
which could make for more burstiness.

Andy.




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