Re: Fw: Question about Routing Headers

[Roy Brabson <rbrabson@xxxxxxxxxx>]

> > 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