[15158] in North American Network Operators' Group
Re: MTU of the Internet?
daemon@ATHENA.MIT.EDU (Frank Kastenholz)
Mon Feb 9 10:58:21 1998
Date: Mon, 09 Feb 1998 10:42:46 -0500
To: "Sean M. Doran" <smd@clock.org>, Paul A Vixie <paul@vix.com>
From: Frank Kastenholz <kasten@argon.com>
Cc: nanog@merit.edu
In-Reply-To: <ytoh0iznnb.fsf@cesium.clock.org>
At 09:11 PM 2/7/98 -0800, Sean M. Doran wrote:
>Tli was just pointing out n messages ago that no matter
>how well you do in terms of aggregating data traffic into
>bigger chunks, you still will see an enormous number of
>small packets around (ACKs). You have to be prepared to
>switch those at line rate; engineering for some
>statistical mix of big and small packets is asking for a
>disaster when someone suddenly goes simplex.
some of the histograms i've seen show close to 50%
of the packets being 40 bytes long. the 'desired'
tcp behavior is to have no more than 2 data packets
for every ack (since congestion control uses ack-reception
to pace the transmission of data and try to
quickly detect losses).
>There is, however, the spectre of there being so many SYNs
>flying around that they alone might cause congestion
>collapse. I dunno if I should be frightened of that or
>not
you should be.
not because of the packet-load it causes (as tony pointed out,
you have to be able to move 40-byte packets at 'fiber speed')
but because it's a symptom of lots of short-lived tcp connections.
these connections never get out of slowstart. when there is only
a small number of them, it's not important. when there is a large
number of them, you have large, non-congestion-controlled, data flows.
it's called being nibbled to death by mice.
