Subject: [IPTEL] TRIP tie-breakers

From: Dave Walker <[email-masked]>
Date: Thu, 22 Jun 2000 14:34:06 -0400

In section 10.2.2.1 of draft-ietf-iptel-trip-02.txt, there are
three means of breaking ties among equally preferred routes to 
the same destination.  One is using MED.  The other two use the 
TRIP ID of some LS:

a) If the local LS is configured to use the MultiExitDisc
   attribute to break ties, and the candidate routes differ 
   in the value of the MultiExitDisc attribute, then select the 
   route that has the larger value of MultiExitDisc.     
b) If at least one of the routes was advertised by an LS in a
   neighboring ITAD, then select the route that was advertised 
   by the LS that has the smallest TRIP ID.
c) Otherwise, select the route that was advertised by the internal
   LS that has the lowest TRIP ID.

I have a simple question on (a).  Since this algorithm is pretty
much lifted from BGP, why does it give preference to the largest
value of MED, when BGP uses the smallest?  In other respects, the
usage of MED in TRIP is the same as in BGP (i.e. routes with lower
MED value are preferred).

I don't understand the difference between (b) and (c).  It seems 
to me that they're the same.  In both cases, the lowest TRIP ID 
is selected.  It's possible that (b) intends that the lowest 
TRIP ID be selected from among the external routes, but that's 
not what it says (according to my interpretation).  BGP's algorithm
for breaking phase 2 ties explicitly says that the selection is 
only from among the external routes.  Because of the change in
the use of MED for tie-breaking (see above), I'm uneasy about 
using BGP for what I think is undefined functionality in TRIP.

Now if the intention for (b) is that the preferred route is
the one that was advertised by an external LS (as per BGP), my 
next question is why?  It seems to me that if I were a service 
provider, I'd prefer to use my own gateways (internal routes) 
over someone else's.

Regards,

Dave Walker
SS8 Networks
Ottawa, Canada

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From: "Hussein F. Salama" <[email-masked]>
Date: Fri, 23 Jun 2000 17:03:25 -0700

>    route that has the larger value of MultiExitDisc.

This statement needs to be tightened up, such that only routes from the
same neighboring ITAD are compared with each other in step a).

> MED value are preferred).

Thanks for pointing this out. The purpose of this divergence from BGP
was to make the LocalPreference attribute and the MED attribute
consistent: higher value means higher preference. It seems that the
change was made in this paragraph only. This will be fixed in the next
revision.

> to me that they're the same.

They're different. Step b) eliminates all internal routes and breaks the
tie between the external routes. TRIP gets to Step c) only if the tie is
among internal routes only. Step c) breaks the tie between these
internal routes.

> In both cases, the lowest TRIP ID
> TRIP ID be selected from among the external routes,

Correct.

> but that's

We had a discussion about the use of MED in TRIP before (check iptel
archives), but we decided at the end to use it exactly the same way it
is defined in BGP, with the exception mentioned above on how to
interpret the MED value.

> provider, I'd prefer to use my own gateways (internal routes)

In BGP, when a BGP speaker receives a route from an external peer and
another route from an internal peer, both to the same destination, this
means that the destination must be located outside the local AS. The
route from an external peer is preferred, because it is the shortest
path out of the local AS.

The wording may not be clear in TRIP. However, TRIP's approach is
different from BGP's. A "route advertised by an internal LS" is a route
to a local GW in the same ITAD. A "route advertised by an LS in a
neighboring ITAD" is a route to a GW outside the local ITAD. It doesn't
matter whether the route was received from an internal peer or from an
external peer, what matters is the location of the terminating GW. 

What you're proposing makes sense. I think, there will be more ITADs
which encourage calls to terminate on their local than ITADs which
prefer to terminate calls on external GWs and keep their local GWs as a
last resort only. So it should be preferable to route calls to local GWs
and not to external GWs.

Hussein

-- 
Hussein F. Salama
Cisco Systems
Mail Stop SJC6/3, 170 W. Tasman Drive, San Jose, CA 95134
Voice: +1 (408) 527-7147, Fax: +1 (408) 527-1714