Hi Erik, The first thing is IPv6 MTU discovery is optional and the only requirement is that the Minimum MTU is satisfied.
Also in cases like LLN the Routing paths may change so even after discovery the Packet too big may come anyway. Besides PMTU issues you mention are no different than a lot of cases such as tunnelling and transition mechanisms. For MTU discovery in cases when padding is done, the right MTU will be found as the data packets will undergo the same behavior as the PMTU discovery message. Thanks, Vishwas On Sun, May 30, 2010 at 7:56 AM, Erik Nordmark <erik.nordm...@oracle.com> wrote: > > The draft says: > A RPL Router MAY insert a Type 4 Routing header if one does not > already exist. The conditions for inserting a Type 4 Routing header > are out of scope of this document. > > Having routers insert headers in packets they forward is know to be > problematic in general, since it breaks path MTU discovery. > Taking an example of a host which sends a packet which is 1500 bytes, and > then receives an ICMP packet too big error saying "please limit packets to > 1500 bytes", will not change the host's behavior. But that can happen if > some router grows the packet to be more than 1500 bytes. > > Thus in general, the only safe way to insert headers in a packet is to have > the "inserter" put an extra IP header, with itself as a source, then the > inserted header, and then the original packet. > That extra IP header ensures that any ICMP error go back to the "inserter", > which can then compensate for its insertion before sending the ICMP error > towards the sender of the original packet. > > Thus I think there is a question for 6man whether we think the constrained > environment of ROLL is such that we can relax this. But it does require that > all the routers at the edge of the ROLL network adjust the ICMP errors they > forward to compensate for any inserted RH4 headers. > > Erik > > -------------------------------------------------------------------- > IETF IPv6 working group mailing list > ipv6@ietf.org > Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6 > -------------------------------------------------------------------- > -------------------------------------------------------------------- IETF IPv6 working group mailing list ipv6@ietf.org Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6 --------------------------------------------------------------------
