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

John Hawkinson <jhawk@bbnplanet.com> Wed, 21 August 1996 21:13 UTC

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Subject: Re: Comparing an old flow snapshot with some packet size data
To: "Kent W. England" <kwe@6sigmanets.com>
Date: Wed, 21 Aug 1996 16:50:14 -0400 (EDT)
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From: John Hawkinson <jhawk@bbnplanet.com>
Cc: asp@partan.com, big-internet@munnari.oz.au
In-Reply-To: <2.2.32.19960821182334.00740718@mail.cts.com> from "Kent W. England" at Aug 21, 96 11:23:34 am
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> Backbones must support at least 1500 bytes and should support up to 9180.
> The only difference is how the routers use memory for buffering. If each
> packet is allocated the router's MTU, then there is a lot of wasted buffer
> memory between 500 bytes typical and 9180. If a router vendor has a problem
> with that much memory wastage, then I'd say it was up to them to allocate,
> say, 1500 bytes per packet, and handle an exception for anything beyond that
> MTU.

Kent, I think you're slightly confused.

Part of this is what defines "the mtu a backbone can support".

There are two ways (at least!) to look at that:

	1.	The minimum MTU of all the interfaces in the
	backbone.

	2.	Various buffering considerations on all of the
	equipment involved in the backbone.

Without the former, the latter seems kind of irrelevent, since path
MTU discovery will never let you go above that minimum (and rightly
so).

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 :-)

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".

--jhawk