[rbridge] Hop Count processing

[Joe Touch]

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James Carlson wrote:
> 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.

What if there are multiple such addresses (i.e., MAC overloading)?

>> So how do you know you reached the last hop?
> 
> Because frames with higher Hop Count never return.

That could also mean:

	- the return error message was lost

	- routing had an incomplete path (never reaches the dest)

>> 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. 

But you don't know it's the last hop, as per above.

> If we don't, then we won't.  That
> means that the method proposed provides additional information without
> significant expense.

As in my other message, this doesn't correlate to user packet behavior
anyway. We need to keep sending the same packet repeatedly with
increasing hopcount, which is a separate function that needs to be
implemented. That function has no benefit from using a user packet as
payload, since the TRILL header hides that content anyway.

> 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.

That'd be "PING" ('echo request') (i.e., that's how traceroute works in
IPv4/IPv6).

Joe

Joe
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