Subject: Re: RFC 2529 (6over4)

From: David Borman <[email-masked]>
Date: Sat, 7 Aug 1999 00:39:49 -0500 (CDT)

> Date: Fri, 06 Aug 1999 17:43:28 -0500
> From: Brian E Carpenter <[email-masked]>
> Subject: Re: RFC 2529 (6over4)
>
> Dave, I think the point is that IPv6 will do MTU discovery anyway;
> so there is no need to do it at the IPv4 level too.

Not true.  The IPv6 MTU discovery will not learn anything about
the section of the path that is traversed as 6over4 packets.  Once
the packet is encapsulated, if the DF bit is not set, it will just
go on its way, getting fragmented as needed, and reassembled at
the other IPv4 side.  To the IPv6 code, it looks like the packet
took 1 hop, and did it successfully in one piece.  So, IPv6 can't
discover the smaller MTU along the 6over4 path, unless the DF bit
is set in the outer IPv4 packet.

6over4 is really no different than ESP/AH tunnels with respect to
Path MTU discovery, the same basic issues are involved in both
cases.  See Section 6.1 of RFC 2401.

		-David Borman, [email-masked]
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From: Brian E Carpenter <[email-masked]>
Date: Mon, 09 Aug 1999 10:48:38 -0500

David,

You're causing me second thoughts about this. Rich Draves, as
an implementor, do you have a comment?

   Brian

-- 
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Brian E Carpenter (IAB Chair)
Program Director, Internet Standards & Technology, IBM Internet Div
On assignment for IBM at http://www.iCAIR.org 
Non-IBM email: [email-masked]
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