Re: [RAM] Re: Tunneling overheads and fragmentation

David Meyer <dmm@1-4-5.net> Thu, 19 July 2007 16:30 UTC

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Date: Thu, 19 Jul 2007 09:30:33 -0700
From: David Meyer <dmm@1-4-5.net>
To: Stephane Bortzmeyer <bortzmeyer@nic.fr>
Subject: Re: [RAM] Re: Tunneling overheads and fragmentation
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On Thu, Jul 19, 2007 at 06:01:25PM +0200, Stephane Bortzmeyer wrote:
> On Fri, Jul 20, 2007 at 12:34:27AM +1000,
>  Robin Whittle <rw@firstpr.com.au> wrote 
>  a message of 54 lines which said:
> 
> > Can anyone help me understand the likely problems with tunneling
> > packets and what MTU limits might be exceeded if LISP, eFIT-APT or
> > Ivip is used with IPv4?
> 
> Executive summary: tunnels create a lot of problems (see the RFC
> indicated by Fred Templin plus RFC 2923).

	The whole thing with PMTU discovery continues to come up
	whenever we debate tunneling. We spent a lot of time
	trying to document that particular issue in RFC 2784 (GRE). 

> > For instance, if all provider and transit routers happily handle
> > packets significantly longer than whatever hosts normally produce
> > (say 1500 bytes) then adding the encapsulation won't lead to any
> > fragmentation.  Is this a reasonable assumption?
> 
> I do not think so.

	Agreed. As mentioned above, there is also the interaction
	with PMTU discovery; basically, it can be nontrivial to
	find the original source of the packet so you can send a 
	a "fragmentation needed and DF set" back to the
	source. If indeed you can't find the source (multiple
	encaps or whatever), then the source's packets (those
	that have the DF bit set, i.e., doing PMTU discovery) get
	black-holed. Not sure if that was your question, but in
	any event...

	Dave
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