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Shady areas of TCP window autotuning?

It was my understanding that (most) cable modems are L2 devices -- how it is
that they have a buffer, other than what the network processor needs to
switch it?


-----Original Message-----
From: Leo Bicknell [mailto:bicknell at ufp.org] 
Sent: Monday, March 16, 2009 9:10 AM
To: nanog at nanog.org
Subject: Re: Shady areas of TCP window autotuning?


What appears to happen is vendors don't auto-size queues.  Something
like a cable or DSL modem may be designed for a maximum speed of
10Mbps, and the vendor sizes the queue appropriately.  The service
provider then deploys the device at 2.5Mbps, which means roughly
(as it can be more complex) the queue should be 1/4th the size.
However the software doesn't auto-size the buffer to the link speed,
and the operator doesn't adjust the buffer size in their config.


My wish is for the vendors to step up.  I would love to be able to
configure my router/cable modem/dsl box with "queue-size 50ms" and
have it compute, for the current link speed, 50ms of buffer.  Sure,
I can do that by hand and turn it into "queue 20 packets", but that
is very manual and must be done for every different link speed (at
least, at slower speeds).  Operators don't adjust because it is too
much work.


       Leo Bicknell - bicknell at ufp.org - CCIE 3440
        PGP keys at http://www.ufp.org/~bicknell/