Re: Fw: Question about Routing Headers

[Roy Brabson]

> > The only problematic case, as far as I can see, would be ICMPv6 too
> > big messages for path MTU discovery.  In this case, however, we can
> > still update the MTU information gradually; we first update the MTU
> > information to the intermediate destination stored in the destination
> > address field of the inner IPv6 packet.  Then succeeding packets will
> > go farther.  And, eventually, we can at least reach at the
> > intermediate destination without seeing an ICMPv6 too big error.  Then
> > the IPv6 destination address will be updated to the next intermediate
> > destination (which might be the final one), and we can still reach
> > there by the similar steps.
>
> Thanks for your reply however I still think there is an issue with Path
> MTU.  If I send a packet outbound, typically I use the MTU associated
> with the destination to which I am sending the packet.  In the case of
> routing headers this will be the first hop (call it A).  Say that works
> fine, but now that node (A) sends the packet to another router (call it
> B) (so the destination address in the IP header has changed).  When this
> node tries to send the packet it out would need to fragment it so it
> generates an ICMPv6 msg back to the originator.  The inner IP header has
> the original source and some intermediate destination address (node B
> would have changed the destination address in the IP header when it
> processed the routing header before it realized it needed to fragment
> the packet so the destination address now would be C).  So when the
> originating host receives this it will adjust it's MTU to get to this
> intermediate node (node C).  But I don't think we will look at this
> information unless we are sending a packet to this node (node C) -- and
> in the case of routing headers we aren't - we're sending to some other
> node (in this case node A).  

When using a routing header, why not use the minimum of the PMTU for the initial first-hop and every hop in the routing header?  You are, after all, sending the packet to each of the intermediate destinations, albeit in a circuitous manner.  Doing that, you use the minimum of the PMTU to (A), (B) and (C).

Roy