Subject: RE: RFC 2529 (6over4)

From: Richard Draves <[email-masked]>
Date: Wed, 11 Aug 1999 11:45:14 -0700

> 1) PMTU is done on a per-host basis, not on a network basis.  So,
>    I don't care if from the IPv6 viewpoint the next host appears to
>    be locally attached, if somehow you get PMTU information for that
>    host, you adjust the MTU for just that host, not for the 
> whole link!
>    The LinkMTU argument is a red-herring.

Hmm, that could work. It's a little strange since normally I would expect to
use the link MTU to reach any neighbor on the link, and could only have a
path MTU smaller than the link MTU if the packet is traversing multiple
links.

You mentioned HIPPI - does it have the property that some link neighbors are
not reachable using the link MTU? The ND spec does not anticipate these
kinds of links.

>    I create a 6over4 packet, set the DF bit in the outer packet
>    and send it.  If I get back an ICMPv4 Unreachable: Too Big message,
>    I create an ICMPv6 Too Big message and pass it up, and the IPv6
>    code should do the right thing.

Synthesizing an ICMPv6 Packet Too Big message would be awkward - what IPv6
source address would you use for the message? Maybe a v4-mapped address :-)

In any case, the conclusion seems to be that one can imagine implementations
that set the DF bit in the encapsulating v4 header. I don't see a reason for
6over4 to specify anything wrt DF bit.

Rich
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From: Steve Deering <[email-masked]>
Date: Fri, 13 Aug 1999 11:16:16 -0700

At 11:45 AM -0700 8/11/99, Richard Draves wrote:
>                ...It's a little strange since normally I would expect to

Rich,

The IPv6 Path MTU Discovery spec (RFC 1981), last paragraph of section 3,
says:

   Note that Path MTU Discovery must be performed even in cases where a
   node "thinks" a destination is attached to the same link as itself.
   In a situation such as when a neighboring router acts as proxy [ND]
   for some destination, the destination can to appear to be directly
   connected but is in fact more than one hop away.

The 6over4 discussion has pointed out another situation that justifies
the above requirement .

Steve