Subject: [Int-area] Path MTU discovery when having varying MTU

From: Magnus Westerlund <[email-masked]>
Date: Mon, 18 May 2009 15:33:51 +0200

This email is BCC to a number of list. Please respond to the TSV-area list.

In the AD review of the documents for defining Robust Header Compression
(ROHC) over IPsec: http://tools.ietf.org/wg/rohc/draft-ietf-rohc-hcoipsec/

I picked up on a issue that has been mentioned before but not really
been dealt with. Namely the varying MTU for packets entering this tunnel.

So this work is to enable so that ROHC can compress the inner IP,
transport, etc headers inside an IPsec tunnel to save bandwidth that
way. ROHC is a stateful compression technology which can result in that
the headers being compressed can be both slightly bigger as well as much
smaller. Thus the effective MTU for a particular packet inside an IPsec
tunnel with ROHC varies depending on which packet in a sequence it is.
This will create some issues for any path MTU discovery mechanism, where
a smaller packet may result in a ICMP TOO BIG while a slightly larger
packet doesn't.

So I am interested in what issues you see arising with this technology
and what you think should be done about it.

Cheers

Magnus Westerlund

IETF Transport Area Director & TSVWG Chair
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