option 212 for 6RD

Tassos Chatzithomaoglou achatz at forthnetgroup.gr
Sat Jan 19 15:27:55 CET 2013

Mikael Abrahamsson wrote on 19/1/2013 12:45:
> Also, the references to the RFCs, that talks about MTU advertised in RA. I see no reason
> why a host system can't have a per-route MTU and treat each RA received and the
> connected routes created from it, have a different MTU.
> I'm going to test this someday, I don't see a reason for it not to work.


I don't know if technically MTU is prohibited to be used for per prefix assigment, but i
don't see a problem having something (called "MTU", or whatever else) that pre-defines at
the source the maximum packet size allowed to be used when ipv6 packets are sent towards a
specific destination.

In my mind the best scenario is that if the CPE knows the next link's "MTU" where the
majority of prefixes are (WAN side), why not inform the hosts (LAN side) about it and
prevent them from discovering it through PMTUD or any other "intrusive" way. I understand
the possibility for the "MTU" to be further reduced in other hops, but we're comparing
something with 100% probability (everything passing through the WAN must be lower than x)
vs something with <100% probability (in further hops).
In an old email of mine i had also proposed the idea of supplying an "MTU" per prefix
through mif's dhcpv6 route option. Since some operating systems already store the PMTUD
result in a per destination cache, why not pre-fill a part of that table?

Having fought for years with IPv4 PMTUD issues, i don't see any real possibility to fix
IPv6 ones, unless something of a MUST is introduced into the IPv6/ICMPv6 standard itself
that enables this to work by default. And maybe it's too late for that too.


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