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Re: jumbo frames
From: "Richard A. Steenbergen" <ras () e-gerbil net>
Date: Wed, 30 May 2001 17:50:19 -0400 (EDT)

On Wed, 30 May 2001, Dave Siegel wrote:

There will obviously be different packet handling techniques for the
larger packets, 

Why will there "obviously" be different packet handling techniques? is
there a special routine for packets over 1500 bytes?  4470 bytes? Or
do you mean policy mapping jumbo packets into dedicated queues?

Generally, yes. There are often different memory pools for different sized

and I'm not aware of any performance or stability testing
that has been done for jumbo frames. I'm guessing the people who are
actively using them havn't been putting it in line rate mixed small and
large packets conditions.

Probably not, but if you take a person who is using them and let them
use your backbone as a point to point network, you start to see
that...not at line rate (preferably, anyway) but certainly large
quantities of small packets mixed in with some really big ones.

My gut instinct is that there are very few people who have fully tested
their products at close to line rate speeds with a full mix of small and
jumbo frames, but I'd love for someone to prove me wrong.

Where your possible QoS impact comes in is where you have jumbo
packets sitting in the best effort queue, and small frames sitting in
your priority queue, you have some queue contention.  This doesn't
happen so much as a result of having the large packet there, but
rather because your minimum queue size has to be of sufficient size to
carry the packet.  The linecard is going to spend more time draining
the best effort queue while the priority queue gets stacked.

Even without QoS, what do jumbo frames do to overall buffer
utilization? (I would imagine nothing, since I suspect this is a
function of line speed, not packet size).

True, if you're running a very congested gige pipe with jumbo frames
taking a large amount of the traffic the smaller frames may be delayed.
Many jumbo frame implementations have seperate queues for regular vs jumbo
frames, I know at least the Alteon products do. I'm not sure if anyone is
intelligently using this to provide a WFQ-like service though.

Richard A Steenbergen <ras () e-gerbil net>       http://www.e-gerbil.net/ras
PGP Key ID: 0x138EA177  (67 29 D7 BC E8 18 3E DA  B2 46 B3 D8 14 36 FE B6)

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