Subject: RE: [manet] MPLS

From: "Manfredi, Albert E" <[email-masked]>
Date: Mon, 12 Aug 2002 09:49:21 -0400

John Tapsell wrote:

> Hi,
>   Could someone explain in just a few lines, what MPLS is?
> I read bits and pieces on
> http://www.ietf.org/html.charters/mpls-charter.html
> but can't understand what on earth it actually is..

MPLS is the IETF's answer to ATM.

One good idea ATM had was to use short and sweet VPI/VCI "labels" to switch packets through the nework. These labels are not globally unique, not capable by themselves of finding a route to a destination, but they are an efficient means of switching packets through once the route from source to destination has been determined. With MPLS, you add "labels" to IP packets. Routers use those labels instead of the entire IP header to switch packets through, once the "labeled path" has been set up.

In ATM, it was PNNI (Private Network Network Interface, a link state routing protocol similar to OSPF) that could dynamically find routes and, in conjunction with UNI (user network interface), would specify the VPI/VCI labels that a source would use, and that the destination would see. These labels can change at each hop. When ATM uses switched virtual circuits (SVCs), it assigns these VPI/VCI labels in real time.

MPLS can't do this, however Generalized MPLS (GMPLS), being developed in the IPO wg of the IETF, aims at doing just that. Allowing network users to signal the core network somehow, to set up a labeled path when necessary. In fact, IPO even uses the "UNI" terminology, reminiscent of ATM. It is, after all, much the same problem. How best to use a circuit-switched (or circuit-based) infrastructure to carry packet-switched IP traffic. In the case of IPO, the circuits are either SONET "pipes" or wavelengths in a wave-divided optical network. Or a combination of the two.

Bert

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