Subject: RE: MPLS routing accross AS boundaries

From: "Peter Ashwood-Smith" <[email-masked]>
Date: Wed, 29 Mar 2000 17:04:09 -0600

All of it is correct and you are perfectly right that it is possible to do
local constraint based routing between AS's but not over the AS. This is
called stepwise or piecewise constraint based routing as opposed to
hierarchical constraint based routing which requires that summarized
constraint information cross AS boundaries.

My own opinion, for what it is worth, is that we will probably have to start
with piecewise routing since this is relatively easy to do.  Then we can
look at ways to make better gateway decisions based on constraints, possibly
with more elaborate forms of inter AS feedback. We will have far more
success getting something working reasonably well if we keep the solution
simple and start with stepwise.

My guess is the BGP guys will like this idea and the PNNI guys will prefer
full hierarchy. Like I said though some simulations are probably in order
here to help put things properly in perspective.

This would be a great project for a masters student somewhere, simulate the
two approaches for a set of real AS and come to PA with some results.
Perhaps some of the George Mason Students from the advanced research lab
would be interested in doing this? 

Cheers,

Peter Ashwod-Smith

	-----Original Message-----
	From:	Abes, Andi [SMTP:[email-masked]]
	Sent:	Wednesday, March 29, 2000 5:28 PM
	To:	MPLS mailing list
	Subject:	RE: MPLS routing accross AS boundaries

	Hi,
	 
	I'm probably out of my league here so please correct me.
	 
	One of the rules of BGP, and in general inter-domain routing
protocols
	is to advertise reachability to other AS's, while hiding as much as
possible

	the local AS's topology.
	 
	Such that when route Ax (AS A, route x) needs to send a packet to an
address
	D, the only thing it will learn from BGP is that there's a route to
the
	destination D, that travels through some AS's along the way. It will
also
	learn the egress router in his own AS that needs to get the packet.
	How he needs to get to the egress router is a matter for an IGP to
resolve.
	 
	Any of this correct ?
	 
	Now my question is, why can't this model be followed under traffic
	engineering conditions ?
	 
	Router Ax will learn from BGP what is the local AS egress point
towards D.
	It will then consult the IGP-TE database and construct an explicit
route to
	that router, and tuck at the end a loose route to D.
	It is then the egress router's job to "fix" the explicit route such
that the
	ERO now reflects the route to the next AS. The Ingress router in
transit
	AS's will do just the same - it will change the ERO (that now
contains 2
	entries - one that indicates the current AS, and one for the end
	destination) such that it contains a strict route accross the new
current AS
	and a loose route to the destination.
	and so on till the destination AS.
	 
	Each BGP router can consult what ever TE constraits it has to deceid
what
	BGP next hop to use, but it will only construct a strict route
through the
	local AS and append a loose route to the final destination.
	 
	Please let me know if this makes some sense.

---

From: Tony Li <[email-masked]>
Date: Wed, 29 Mar 2000 23:05:57 -0800

> All of it is correct and you are perfectly right that it is possible
> to do local constraint based routing between AS's but not over the AS.
> This is called stepwise or piecewise constraint based routing as
> opposed to hierarchical constraint based routing which requires that
> summarized constraint information cross AS boundaries.
>
> My own opinion, for what it is worth, is that we will probably have to
> start with piecewise routing since this is relatively easy to do.
> Then we can look at ways to make better gateway decisions based on
> constraints, possibly with more elaborate forms of inter AS feedback.
> We will have far more success getting something working reasonably
> well if we keep the solution simple and start with stepwise.
>
> My guess is the BGP guys will like this idea and the PNNI guys will
> prefer full hierarchy. Like I said though some simulations are
> probably in order here to help put things properly in perspective.

This BGP guy will simply point out that BGP does not and cannot
reasonably propagate anything like the number of possible paths
necessary to perform any reasonable type of inter-domain traffic
engineering, stepwise or global.  This is simply due to the path-vector
nature of the protocol.

We should be directing our efforts to the locations that will maximize
our return:
    - continued experience with intra-domain TE
    - expansion of this technology into the optical plane

Regards,
Tony