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
