Re: Comparing an old flow snapshot with some packet size data

"Dorian R. Kim" <dorian@cic.net> Thu, 22 August 1996 01:05 UTC

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Date: Wed, 21 Aug 1996 20:29:36 -0400 (EDT)
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From: "Dorian R. Kim" <dorian@cic.net>
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To: John Hawkinson <jhawk@bbnplanet.com>
Cc: "Kent W. England" <kwe@6sigmanets.com>, asp@partan.com, big-internet@munnari.oz.au
Subject: Re: Comparing an old flow snapshot with some packet size data
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On Wed, 21 Aug 1996, John Hawkinson wrote:

> If you say that "backbones should support up to 9180" then you've just
> disallowed most folks' DS3 interfaces and FDDI, and are mandating that
> folks should all go and use ATM. I think you're well aware there are a
> strong contingent of people who will not throw out their DS3 and FDDI
> infrastructure for ATM :-)

Along with problematic buffering problems with certain vendor equipment for
9180 MTU.

> I think the brunt of asp's question was really "Is it acceptable for
> a backbone service provider to provide a min mtu of 1500 instead of
> a min mtu of of 4352". What this really translates into is:
> 
> 	Is it OK for a backbone service provider to use fast ethernet
> 	as a interconnect medium instead of FDDI.
> 
> Answers seem varied, some folks may have contractual obligations to
> provide that 4k MTU, but most don't.
> 
> Is that 4k MTU worth the trade-offs? It's certainly the case that the
> 1.5k MTU isn't very-well exploited right now, and it's difficult to
> see the 4k MTU being exploited well in the short term, though it
> may be that end-users who check the "4k MTU requirement" checkbox
> are actually folks who have lots of hosts with 4k MTUs on their
> interfaces who exchange significant amounts of traffic with others
> who have similarly-configured  hosts and who implement path MTU
> discovery and "all that good stuff".

It seemed to me intuitively that anything above 1500 MTU doesn't buy one much, 
so I went to my routers and checked, just to be sure.

dgb>sh ip ca flow
IP packet size distribution (6502M total packets):
   1-32   64   96  128  160  192  224  256  288  320  352  384  416  448  480
   .002 .513 .048 .017 .025 .007 .007 .010 .014 .014 .004 .005 .005 .002 .002

    512  544  576 1024 1536 2048 2560 3072 3584 4096 4608
   .006 .006 .174 .000 .076 .052 .000 .000 .000 .000 .000

IP packet size distribution (6283M total packets):
   1-32   64   96  128  160  192  224  256  288  320  352  384  416  448  480
   .002 .515 .049 .016 .024 .007 .007 .016 .011 .013 .007 .006 .007 .002 .002

    512  544  576 1024 1536 2048 2560 3072 3584 4096 4608
   .005 .006 .160 .000 .081 .053 .000 .000 .000 .000 .000

dgd>sh ip ca flow
IP packet size distribution (1182M total packets):
   1-32   64   96  128  160  192  224  256  288  320  352  384  416  448  480
   .001 .480 .062 .015 .011 .008 .007 .012 .011 .014 .004 .009 .007 .003 .002

    512  544  576 1024 1536 2048 2560 3072 3584 4096 4608
   .005 .004 .136 .000 .125 .074 .000 .000 .000 .000 .000

So unless my routers aren't telling the truth, there are significant amount of
> 1500 byte packets floating in my network. I'm somewhat mystified as to where
2048 comes from, but they are there nonetheless, and in our application, it
seems to me that 4K MTU is a good thing.

-dorian