MTU (was Re: ULA and RIR cost-recovery)

Thu Nov 25 14:05:25 UTC 2004

--On 25 November 2004 13:16 +0000 Michael.Dillon at radianz.com wrote:

> In today's network, is there anyone left who uses 1500 byte
> MTUs in their core?

I expect there are quite a few networks who will give you workable
end-to-end MTU's >1500 bytes, either because of the above or because of
peering links.

Given how pMTUd works, this speculation should be relatively easy to test
(take end point on >1500 byte MTU, run traceroute with appropriate MTU to
various points and see where fragmentation required comes back). Of course
I'd have tried this myself before posting, except, urm, I can't find a
single machine I have root on that I can get more than a hop or two from
without running into 1500 byte (or less) MTU.

I am guessing also that a recent netflow sample from a commercial core (not
Internet2), even with jumbo frames enabled, will show <0.01% of packets
will not fit in 1500 byte MTU. Anyone have data?

Alex