Subject: Path MTU implementation issue

From: "Revathi Unnava" <[email-masked]>
Date: Mon, 30 Oct 2000 14:11:35 -0800

In RFC1981, Path MTU Discovery for IPv6, page 4, it is mentioned that:

A node MUST NOT reduce its estimate of the Path MTU below the IPv6 minimum
link MTU. 
Note: A node may receive a Packet Too Big message reporting a next-hop MTU
that is less than the IPv6 minimum link MTU. In that case, the node is not
required to reduce the size of subsequent packets sent on the path to less
than the IPv6 minimun link MTU, but rather must include a Fragment header in
those packets [IPv6- SPEC]. 

If a node discovers a path MTU X, which is less than IPv6 minimum link MTU
(1280),
it is not stored (as per RFC) with the local representation of the path.
Now, when the node sends a packet
of the size Y (Y > X), it receives a TooBig ICMP error. But it does not
store the MTU that comes with
that ICMP error. So next time there is a packet for that destination, it
will still send it with the size Y.
How does this work ? At the time we are sending, we have the packet, but
dont have the correct MTU, 
while at the time of receiving TooBig error, we have the correct MTU, but
dont have the packet to 
apply it to. Somebody please explain ...

By the way, what is the purpose of having that restriction (dont store the
PMTU < 1280)  in the RFC ? 

I'll appreciate any help.

Thanks
Revathi

---

From: [email-masked]
Date: Tue, 31 Oct 2000 09:55:26 +0900

>those packets [IPv6- SPEC]. 

	see RFC2460 section 5, very beginning.

itojun

---
5. Packet Size Issues

   IPv6 requires that every link in the internet have an MTU of 1280
   octets or greater.  On any link that cannot convey a 1280-octet
   packet in one piece, link-specific fragmentation and reassembly must
   be provided at a layer below IPv6.
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