Subject: [rbridge] RE: rbridge Digest, Vol 1, Issue 4

From: Konrad.Roeder at T-Mobile.com (Roeder, Konrad)
Date: Fri May 7 16:42:27 2004

Joe,
my comments inside yours... // //

Roeder, Konrad wrote:

> I agree with Joe that MPLS switches are L2 devices with a special
> L2.5
> shim layer. As part of their behavior, MPLS switches do/can decrement
> TTL. So there is precedence here in L2 (L2.5?) devices meddling with
> TTL. I believe that RBridges could decrement TTL to prevent loops,
> although doing so is not "a clean L2 implementation" because
> decrementing TTL indicates either a second of time went away or that a
> router was traversed.

The difference is that MPLS isn't trying to be an L2. Rbridge is.

//I agree//

If we relax that constraint, and allow rbridges to do what MPLS is, 
there's certainly precedent. And a very large and deep can of worms 
awaiting, IMO ;-)

//Ditto//

> Another point to bring up: Do we want the rbridge to behave
> differently depending on what protocol is carried in L3? If it's IP it
> uses TTL, if it's not it's decrementing a hop counter in its
> encapsulation. Do we really want to add this complexity?

I can't answer whether it's wanted, but it would certainly need to be
sensitive to the L3 protocol if the TTL were decremented.

Perhaps that's another good reason not to decrement the TTL? :-)
I.e., it makes rbridge a true L2 system, which is compatible
with - and independent of - the protocol carried therin (at the edge).

//Ditto. //

> Going back briefly to the subject of the traceroute kludge... if TTL
> gets decremented from 1 to 0, does an Rbridge send an ICMP Timeout
> message? Doing so would make an RBridge appear on a traceroute. Not
> doing so, would make an RBridge appear as a *. I suppose this is an
> implementation detail, but it might be worthwhile specifying the
> interaction. (traceroute is a nice debugging tool)

IMO, anything that decrements the TTL is a router; that means RFC1812 
applies.
RFC2003 points this out with respect to tunnels - wherever TTL is 
decremented, there is the possibility of a zero and thus the need for 
ICMP TTL exceeded messages.

//Since RBridge is a Layer 2 device, it **SHOULD NOT** be meddling with TTL.  I think it's a bad idea due to the fact that there are loads of interactions that I don't think we are ready to deal with.  This means that we should only rely on encapsulation.  Furthermore, this renders the traceroute question a moot point.//

Joe