MPLS and QoS, Part 1

Last time, we promised to begin a discussion of Multi-protocol Label Switching mechanisms for providing quality of service. In order to proceed, however, it's necessary to spend a bit of time discussing (and defining) the " S " in MPLS - switching. In particular, we need to differentiate between the S-word and the R-word - routing.

Over the past few years, the distinction between a switch and a router has become very blurred. At one time, it could be argued that switching occurred at Layer 2 for ATM and frame relay, and routing occurred at Layer 3 for IP. With the advent of IP switching, however, that distinction got chucked out the window. The terminology soon became totally useless, as switches and routers began performing both switching and routing functions.

To us, the primary difference between a switch and a router is whether the packet forwarding decisions are made on a per-packet basis or on a predetermined basis. In the case of a pure IP router, a packet forwarding decision is made for every individual packet based on the IP address. In the case of a frame relay or ATM switch, a virtual circuit is established (either a PVC or an SVC), and packets are forwarded by looking up an entry for a given frame relay data link connection identifier or ATM virtual path identifier/virtual connection identifier in the ... well ... routing table. See the problem?

Fundamentally, it comes back to the connection-oriented vs. connectionless discussion. If the architecture, like IP, is connectionless, then there is no pre-established path through the network. In this case, the packets are forwarded/routed/switched on a packet by packet basis by a procedure we'll call " routing. " If the architecture is connectionless, then a path is set through the network at the time the call is established. All packets follow this same path through the network (until something happens to change the path) and individual packets are simply forwarded/routed/switched along the preset path by looking at the tag/label through a procedure that we'll call " switching. "

Which brings us back full circle to the place we'll take up next time. The key to the power of MPLS is that it is a connection-oriented switching, not routing, technology.

Steve Taylor is President of Distributed Networking Associates and Publisher/Editor-in-Chief of Webtorials.Com. For more detailed information on most of the topics discussed in this newsletter, connect to Webtorials.Com, the first Web site dedicated exclusively to market studies and technology tutorials in the Broadband Packet areas of Frame Relay, ATM, and IP.

Larry Hettick is an independent consultant, with 19 years of experience in telecommunications and data communications marketing and product management for service providers and equipment vendors. He can be reached at larry@larryhettick.com

You can reach the authors at taylor@webtorials.com or larry@larryhettick.com.

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