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Re: PMTU-D: remember, your load balancer is broken
From: "Stephen Sprunk" <ssprunk () cisco com>
Date: Wed, 14 Jun 2000 17:47:52 -0500

Sez <rdobbins () netmore net>
The 576 value of the MS PPP MTU is merely a default - it can be
changed with a registry hack.

Expecting the tens of millions of novice computer users to set their
systems for a 1500 byte MTU is irrational.  Those who are knowledgeable
enough to do so are generally reducing it due to "speed up your modem"
articles and programs which improve interactive performance at the
expense of throughput.

Forcing excessive fragmentation/defragmentation is an effective DoS.

Effective, but a fairly difficult problem to exploit.

I thought the frame-size limits for Gigabit Ethernet were 64-1518/1522
bytes?  And isn't that the limit on most host IP stacks for Ethernet
Or am I off in left field, here?

IIRC, during development of the GigE spec, several vendors wished to
increase the GigE MTU to ~9000 bytes.  Due to the technical
ramifications of bridging to low-MTU FastE segments, as well as
inter-vendor politics, it didn't make it as part of the GigE spec but
was later published as an optional extension.

There are now several devices on the market that will do Jumbo frames on
GigE.  For instance, the Catalyst 6000 and GSR do:


I know there are several other vendors who support Jumbo frames as well.

Finally, I would say that on any medium, <100% utilization in and of
isn't grounds for fiddling with the MTU.  There are lots of other
things to
look at, first.

I hear consistent requests from server folks for higher MTUs; they claim
the per-frame CPU hit is significant enough to warrant using Jumbo
frames to increase throughput.  The results clearly show that it helps.


     |          |         Stephen Sprunk, K5SSS, CCIE #3723
    :|:        :|:        Network Design Consultant, HCOE
   :|||:      :|||:       14875 Landmark Blvd #400; Dallas, TX
.:|||||||:..:|||||||:.    Email: ssprunk () cisco com

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