Subject: Re: [Softwires] MTU issues with tunneling

From: Joe Abley <[email-masked]>
Date: Tue, 4 Apr 2006 23:00:07 -0400

On 31-Mar-2006, at 11:08, Laurent Toutain wrote:

> If there a simple solution to discover pMTU larger than 1500 why not
> otherwise, in phase one, we should keep the 1500 limit.

If the proposed simple solution turns out to be, in fact, path MTU  
discovery, then be aware (if you're not already) that pMTU is  
frequently crippled across the Internet by virtue of the fact that  
its signalling relies on ICMP not being filtered and dropped by  
middleboxes for "reasons of security".

Joe

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From: David Meyer <[email-masked]>
Date: Tue, 4 Apr 2006 20:21:46 -0700

> middleboxes for "reasons of security".

	Or the fact that PMTU doesn't work, in general, though 
	tunnels.

	Dave

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From: Ole Troan <[email-masked]>
Date: Wed, 05 Apr 2006 13:19:18 +0900

> 	Or the fact that PMTU doesn't work, in general, though 

huh?

PMTU would run in the transport. the tunnel headend will have to do
ICMP relaying e.g described in RFC2473. there is the problem of not
enough of the packet carried in IPv4 ICMP messages if that is used as
transport. what else makes "PMTU not work" in the tunnel case?

/ot

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From: "Templin, Fred L" <[email-masked]>
Date: Wed, 5 Apr 2006 15:06:18 -0700

Ole,

I agree with Joe and Dave that there are PMTU problems for
tunnels; please see below for comments:

> PMTU would run in the transport.

It is true that the method specified in 'draft-ietf-pmtud-method-06'
would run in the transport. But, it would have to "see" the tunnel
as a link that provides an assured MTU to upper layers.

> the tunnel headend will have to do

The common tunneling protocols I am familiar with do not rely on
ICMP relaying due to the issue you mentioned and also for the
reasons Joe described and as documented in RFC2923. Instead,
common tunneling protocols set the DF bit in the IPv4 packet
header to 0 and allow the network to fragment the tunneled
packets if it wants to. But, problems with IPv4 fragmentation
are documented in "Fragmentation Considered Harmful" and more
recently: "Fragmentation Considered Very Harmful":

http://www.ietf.org/internet-drafts/draft-heffner-frag-harmful-00.txt

The following solutions-category documents propose solutions
to the tunnel MTU issue (note that there may be others):

http://www.ietf.org/internet-drafts/draft-ietf-pwe3-fragmentation-10.txt
http://www.ietf.org/internet-drafts/draft-templin-linkadapt-02.txt

Fred
[email-masked]

/ot