Subject: [Isis-wg] What is the largest MTU we can assume that won't get fragmented?

From: Radia Perlman - Boston Center for Networking <[email-masked]>
Date: Fri, 30 Aug 2002 14:40:14 -0400 (EDT)

I vaguely remember in the dark ages it was 576, not necessarily because
of link technology that required <576, but because of not wanting to
require routers to have such large buffers perhaps?

And I vaguely remember that more recently than the dark ages, people decided
that all links can handle at least 1500, minus fudge factors for various
headers (802.3, then VLANs, etc.)

Anyway, I thought IS-IS had LSP fragments of 1500 because it was assumed
that would be safely carried on all links. (Do I remember correctly?).

And if I remember correctly, can we assume 1500 for data as well?

Thanks,

Radia

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From: Jeff Learman <[email-masked]>
Date: Thu, 05 Sep 2002 15:05:01 -0400

WAY back in the dark ages, 512 bytes (layer 3) was a minimum, because
that's the minimum "max LSDU size" required by CLNP specs.  576 rings
a faint bell, but I'm not sure why or what the extra 64 would be about.

Support for mixed MTU sizes was never handled properly by ISIS, and
that's discussed in the interop draft.

But unfortunately (IMHO) Bellcore in GR-253-CORE mandated 512
(or was it 576?) as the default LSDU size for SONET SDCC, where
CLNP & ISIS are used to provide an embedded management channel.
(SDCC, or Section Data Communications Channel, is a 192 kbit/s
channel in the SONET frame overhead.)  This was deployed and
used in some telco SP's networks, and is probably still in use
and needs to be supported.

To complicate matters, the SONET community is working towards specs
to allow IP-based management communications on these same channels,
using IS-IS.  I don't know whether the default max LSDU size was
raised to 1500 as a part of this effort, but I suspect not.

Not important for mainstream IP, but something to keep in mind.

Regards,
Jeff