Shady areas of TCP window autotuning?

Mikael Abrahamsson swmike at swm.pp.se
Tue Mar 17 02:46:50 CDT 2009


On Mon, 16 Mar 2009, Leo Bicknell wrote:

> What appears to happen is vendors don't auto-size queues.  Something

In my mind, the problem is that they tend to use FIFO, not that the queues 
are too large.

This is most likely due to the enormous price competition in the market, 
where you might lose a DSL CPE deal because you charged $1 per unit more 
than the competition.

What we need is ~100ms of buffer and fair-queue or equivalent, at both 
ends of the end-user link (unless it's 100 meg or more, where 5ms buffers 
and FIFO tail-drop seems to work just fine), because 1 meg uplink (ADSL) 
and 200ms buffer is just bad for the customer experience, and if they 
can't figure out how to do fair-queue properly, they might as well just to 
WRED 30 ms 50 ms (100% drop probability at 50ms) or even taildrop at 50ms.
It's very rare today that an end user is helped by anything buffering 
their packet more than 50ms.

I've done some testing with fairly fast links with big buffers (T3/OC3 and 
real routers) and doing FIFO and tuned TCP windows (single session) it's 
easy to get 100ms buffering, which is just pointless.

So either smaller buffers and FIFO, or large buffers and some kind of 
intelligent queue handling.

-- 
Mikael Abrahamsson    email: swmike at swm.pp.se




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