option 212 for 6RD

Tassos Chatzithomaoglou achatz at forthnetgroup.gr
Sat Jan 19 16:22:23 CET 2013

Tassos Chatzithomaoglou wrote on 19/1/2013 16:27:
> 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.
> Agree...
> 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.
> --
> Tassos
Just to add that Cisco's MRU in MPLS is actually MTU per FEC or prefix, because it informs
the LSR how big a packet can be for a particular FEC (i.e destination prefix) that can
still be forwarded out without being fragmented (the MRU changes per prefix depending on
the label operation performed on the prefix).


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