[rbridge] Setting initial "hop count" value

[Radia Perlman]

We didn't say what the initial value of the "hop count" field should be.

One tempting thing would be to set it to the computing number of hops to
the egress RBridge, since the ingress RBridge knows how long the path is 
(based on
computation from the link state database). It would be easy for an 
RBridge to
include not just the next hop to the egress RBridge, but the hop count 
as well,
so that it would know how to set the hop count field.

However, layer 3 people these days do various clever "detour" things 
when they
can't reach the next hop, so that packets can still get to the 
destination while
the routing algorithm is converging. So the hop count could need to be 
more than
what the ingress guy thinks.

But I think it's a good idea for multicast for the ingress RBridge to 
set the initial
hop count to be equal to the maximum length from that RBridge to any 
other RBridge
(in the selected tree).

We even could be clever and an RBridge in the core that is forwarding on 
various branches
could decrement the hop count by more if it knows that some branch of 
the chosen
distribution tree needs a smaller count in order to reach to the ends of 
that branch.

This would make multicast even safer.

At any rate, it's always seemed a bit weird to have a field like "TTL" 
in IP without
saying in the spec what to set it to. We don't have to make the 
suggestion a MUST, but it would
be nice to give some recommendation in the TRILL spec about what to set 
it to.

Comments?

Radia