Subject: A fundamental question

From: "Rae M. Chang" <[email-masked]>
Date: Wed, 2 May 2001 09:35:25 +0900

Hello,

I have a fundamental doutb on MPLS traffic engineering.
(pls take it account my poor understanding on ATM routing stuffs.)
What I can't figure it out is if a user part connected to the ATM network can dynamically setup a SVC
to the other user part. Can the UNI and PNNI stuffs be used to set up it? 
If that's true, what're the differences with the LSP setup across ATM network using another
signaling mechanism, or MPLS?
Why don't they use existing ATM signalling methods to achieve the traffic engineering of MPLS?
(i know of the ATM's N square complexity issue with PVC setup on the IP over ATM model.
but, in my thought, you can just setup necessary VCs on-demand basis like MPLS TE)

-Rae

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From: David Charlap <[email-masked]>
Date: Wed, 02 May 2001 10:07:44 -0400

"Rae M. Chang" wrote:
> What I can't figure it out is if a user part connected to the ATM
> network can dynamically setup a SVC to the other user part. Can the
> UNI and PNNI stuffs be used to set up it?

No.  ATM signaling and MPLS signaling are two different things.  One is
not used to setup the other.

An ATM switch acting as LSR (a transit router) will ultimately have the
same kind of cell-forwarding installed on its fabric with each protocol,
but that's where the similarities end.

An ATM switch acting as LER (an edge router) has different semantics on
the data plane.  On ingress, it must apply labels to incoming unlabeled
traffic (or push labels if one LSP is being tunneled through another.) 
On egress, it must be able to pop labels and make forwarding decisions
based on the result.

> If that's true, what're the differences with the LSP setup across
> ATM network using another signaling mechanism, or MPLS?

ATM's routing and signaling protocols are closely tied to ATM's
addressing architecture.  These protocols are not based on IP, and they
do not scale up to networks the size of the internet.  Finally, ATM only
works on ATM interfaces - it doesn't work over PPP, Frame Relay,
Ethernet, or other types of interfaces that are commonly found in
networks.

MPLS is intended to solve these problems.  The routing protocols are
well-established IP protocols (OSPF, IS-IS, BGP).  The signaling
protocols are designed around IP.  And MPLS works over all IP-
compatible interfaces.  Routers of all kinds (ATM, Frame, IP, etc.) can
interoperate in a single MPLS cloud.

> (i know of the ATM's N square complexity issue with PVC setup on the
> IP over ATM model.  but, in my thought, you can just setup necessary
> VCs on-demand basis like MPLS TE)

On-demand setup works fine in an enterprise network.  It doesn't work in
a core network.  When you've got hundreds of GB/s of data flowing
through a network, the delays needed to dynamically allocate new VCs (or
LSPs) will almost certainly be unacceptible.

MPLS avoids the N^2 problem in two ways:

1: The LSPs follow the routing protocols' shortest-paths.  Meaning
   you don't necessarily need an LSP to get from everywhere to
   everywhere.  Only along the shortest routed paths.

2: Routers that support label merging can greatly reduce the amount
   of resources required.  Two LSPs arriving at a single transit
   router, that are intended to terminate at the same egress router
   may be merged (assuming there are no QoS concerns, of course.),
   eliminating the need for the second LSP if the downstream rouers.

-- David

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From: "Jeremy" <[email-masked]>
Date: Wed, 2 May 2001 10:38:49 -0400

Is their a table available that compares/contrasts ATM vs. MPLS?
Thanks, Jeremy.

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From: "Rae M. Chang" <[email-masked]>
Date: Thu, 3 May 2001 14:11:14 +0900

David,

>    everywhere.  Only along the shortest routed paths.

That's the point I can't understand yet. For example, if you want a 
partial meshed network consisting of a set of LSPs, then why couldn't you make
the same network with only neccessary ATM VCs (not using MPLS)? 
Aren't they essentially equivalent TE schemes? Why do they make the
N^2 number of VCs?

Rae

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From: David Charlap <[email-masked]>
Date: Thu, 03 May 2001 10:31:41 -0400

"Rae M. Chang" wrote:
> partial meshed network consisting of a set of LSPs, then why
> couldn't you make the same network with only neccessary ATM VCs
> (not using MPLS)?

ATM is not an IP protocol.

There is no way that an ATM-specific setup can know what IP routes
exist.  So if any auto-VC setup is going to happen, it must either be
traffic-driven (which will not perform well in a network core) or a
full-mesh must be pre-established.

Now, if you want to go and invent a new ATM that uses IP routing
protocols in order to fix this, go right ahead.  But that's not what
MPLS is trying to do.

-- David