Subject: Re: RFC 2529 (6over4)

From: David Borman <[email-masked]>
Date: Fri, 6 Aug 1999 17:03:04 -0500 (CDT)

> Date: Fri, 06 Aug 1999 11:48:59 -0700
> From: Cyndi Jung <[email-masked]>
> Subject: Re: RFC 2529 (6over4)
>
> I really think that the black hole caused by an IPv4 segment of
> the path with a smaller MTU than ethernet would be a major reason
> to make the use of DF a MUST NOT.

I think the setting of the DF bit should be at least SHOULD NOT,
not MUST NOT.

Why would a segment with less than an ethernet MTU be a black
hole?  The whole point of setting the DF bit is to allow you to
discover the MTU of the path.  In the case of 6over4, if you
discover a link with an MTU less than 1300 (1280 minimum MTU
for IPv6 plus 20 bytes for IPv6 header), then for that host
you have discovered that the Path MTU is too small, so you turn
off the setting of the DF bit for that host, and let it fragment
the packet and send it on.

The point is that you only set the DF bit in the IPv4 header if
you are planning on doing IPv4 Path MTU Discovery.  If you find
that the discovered MTU doesn't give you a usable path, then you
stop setting the DF bit.  Also, it might be that you only set the
DF bit for TCP packets.

As for real black holes (gateways that discard packets due to the
DF bit being set, but don't generate an appropriate ICMP message),
we shouldn't be limiting the design of a protocol based on bad
implementations, the bad implementations should be fixed.

> >>From [email-masked] Fri Aug  6 12:28:08 1999
> >>...
> >>Can you come up with a case where setting DF would be a good
> >>thing to do?

If you have two IPv6 hosts, each connected to an fddi ring, using
6over4 between them other over a path that has ethernet in the middle,
it would be good to know that so that they don't keep sending fddi
size packets that have to be fragmented/reassembled.  If you can't
set the DF bit, you can't do Path MTU discovery and hence you can't
find that out.

Sure, it takes some thinking and extra code to do it right, but
if the implementor is willing to do the work, why should the
protocol prohibit it?

			-David Borman, [email-masked]
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From: Brian E Carpenter <[email-masked]>
Date: Fri, 06 Aug 1999 17:43:28 -0500

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.

   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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