Re: Fw: Question about Routing Headers

[Lori Napoli <lanapoli@xxxxxxxxxx>]

Roy Brabson/Raleigh/IBM wrote on 04/22/2004 09:46:15
AM:

> > > 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).
> 

That is definitely doable albeit VERY expensive since
you would need to lookup the MTU for ALL the entries in the routing header
for EACH send since they could change.

Lori