Subject: a doubt on MTU discovery in IPv6

From: "Srinivasa Rao Nalluri" <[email-masked]>
Date: Tue, 28 Aug 2001 20:16:46 +0530

hi

in IPv6 networks different IP packets from same source to same distention may travel through different paths.
then how path MTU discovery process can help to find MTU value?
we don't know through how many different paths packets may travel between same source and distention.

can anybody explain how this will happen in IPv6 networks.

regards
Srinivas Nalluri

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From: "Jari Arkko" <[email-masked]>
Date: Tue, 28 Aug 2001 19:14:10 +0300

>in IPv6 networks different IP packets from same source to same
>distention may travel through different paths

I suppose you would eventually get the ICMPv6 Packet Too Bigs
from all the possible paths, and would set the path MTU to the
minimum of the received values.

Jari Arkko

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From: Pekka Savola <[email-masked]>
Date: Tue, 28 Aug 2001 21:17:58 +0300 (EEST)

On Tue, 28 Aug 2001, Srinivasa Rao Nalluri wrote:
> distention may travel through different paths.

This is true with IPv4 too, and exactly equivalent if you put Don't
Fragment bit in IPv4 datagrams.

> we don't know through how many different paths packets may travel between same source and distention.

If the path changes so radically that MTU changes, ICMPv6 "Packet too Big"
message will be sent to the source, MTU will be reduced and the packet
re-sent.

Your confusion may be based on the difference that in IPv4, routers also
may fragment packets.  IPv6 fragmentation is end-to-end, and you either
use minimum MTU 1280, or implement Path MTU Discovery (above).

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords