TCP congestion control and large router buffers

Tue Dec 14 17:28:25 UTC 2010

> On Tue, 14 Dec 2010, Sam Stickland wrote:
> 
> > But there's no need for AQM, just smaller buffers would make a huge
> > difference.
> 
> Well, yes, buffering packets more than let's say 30-50ms on a 1 meg
> link
> doesn't make much sense. But doing some basic AQM would make things
> even
> better (some packets would see 0 buffering instead of 30ms).
> 
> > Surely buffers that can store seconds worth of data are simply too
> big?
> 
> FIFO with seconds worth of data is just silly, yes.
> 
> --
> Mikael Abrahamsson    

Well, Jim Getty was reporting seeing "tens of seconds" of buffering
(comments in the original LWN link to his first posting) which is just
ludicrous. No TCP stack is going to respond properly to congestion with
that sort of delay. Some form of AQM is probably a good thing as would
be the wider use of ECN.  Finding out that a buffer filled and a packet
(or many packets) was dropped five seconds after the fact, isn't going
to help anyone and you just end up whipsawing the window size (Lawrence
Welk effect http://www.oeta.onenet.net/welk/PM/images/Lawrence.jpg ?).
I would favor seeing more use of ECN so that a sender can be notified to
back off when a buffer is approaching capacity but there is apparently
still a lot of hardware out there that has problems with it.

You need enough buffering to satisfy packets "in flight" for a
connection on the other side of the planet but man, what he has been
reporting is just insane and it would be no wonder performance can be
crap.