[rbridge] Hop Count processing

[James Carlson]

Joe Touch writes:
> When I get something with hopcount=0, I respond back with an "error
> hopcount exceeded". I need to pick one of the egress tags to use as the
> message source address. I'm presuming I pick the "canonical address" of
> that rbridge, for which there would be only one.

You'd pick the MAC address of the interface over which you received
the frame.

> So how do you know you reached the last hop?

Because frames with higher Hop Count never return.

> Why is it even important to be able to do a traceroute with a regular
> packet? Why not just require something like PING, i.e., a TRILL message
> that, once received at the destination address in the TRILL header,
> responds with a "success" response?

That's good question.  I have exactly the same concern -- basically,
that we're designing the 'traceroute' infrastructure before we know
anything about how the functionality will be implemented -- but Dinesh
convinced me that it doesn't matter.

If we do this as Dinesh has argued, then we'll always have the drop
information from that last hop.  If we don't, then we won't.  That
means that the method proposed provides additional information without
significant expense.

I agree that a decent 'traceroute' feature should use a special packet
that allows us to send back "and here's what I'd do with that once
I've decapsulated" information from the last (destination) TRILL node.

-- 
James Carlson, Solaris Networking              <james.d.carlson at sun.com>
Sun Microsystems / 35 Network Drive        71.232W   Vox +1 781 442 2084
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