Nov

05

What is MPLS?

Posted by admin | Posted in MPLS | Posted on 05-11-2009

MPLS stands for Multiprotocol Label Switching. Multiprotocol because it might be applied with any Layer 3 network protocol, although almost all of the interest is in using MPLS with IP traffic. But that doesn’t actually give us any idea what MPLS does for you (we’ll get to that momentarily).

Depending on which vendor you ask, MPLS is the solution to any problem they might conceivably have. So the question “What is MPLS” could have a lot of right answers. The presentations from this Spring’s MPLS Forum were all over the place on precisely this.

For me, MPLS is about gluing connectionless IP to connection-oriented networks. Six months ago, I’d have said “gluing IP to ATM”, but now there’s a big push on to use MPLS to mate IP to optical networks. The IETF draft documents refer to this as the “shim layer”, the idea that MPLS is something between Layer 2 and Layer 3 that makes them fit better (and perhaps carries a small amount of information to help that better fit).

MPLS started out as Tag Switching. Ipsilon (remember them?) was the company that got the MPLS buzz started. Back then, there were perhaps two key insights. One was that there is no reason an ATM switch can’t have a router inside it (or a router have ATM switch functionality inside it). Another was that once you’ve got a router on top of your ATM switch, you can use dynamic IP routing to trigger virtual circuit (VC) or path setup. In other words, instead of using management software, or human configuration, or (gasp!) even ATM routing (PNNI) to drive circuit setup, dynamic IP routing might actually drive the creation of circuits. You might even have a variety of protocols for different purposes, each driving Label Switch Path establishment.

I’ve been thinking of this as avoiding the hop-by-hop decision making, by setting up a “Layer 2 fast path” using tags (think ATM or Frame Relay addressing) to move things quickly along a pre-established path, without such “deep analysis”. The packet then needs to be examined closely exactly once, at entry to the MPLS network. After that, it is somewhere along the path, and forwarding is based on the simple tagging scheme, not on more complex and variable IP headers. The U.S. postal system seems to work like that: forward mail to a regional center, do handwriting recognition once, apply some sort of infrared or ultraviolet bar code to the bottom edge of the envelope, from there onwards, just use the bar code to route the letter. When you start thinking about fast forwarding with Class of Service (CoS), then incoming interface, source address, port and application information, all might play a role in the forwarding decision. By rolling the results into one label per path the packet might take, subsequent devices do not need to make such complex decisions.

Fairly soon after the basic idea of Tag Switching got publicized, Cisco got visibly involved, and then so did all the other vendors of course. For a couple of years now, Cisco Tag Switching in the 7000 series has allowed using Tag Switching on high-speed IP networks. This is migrating right now, to support the final standardized Label Switching. Other Cisco platforms now supporting MPLS: LS1010, 3600 (the release notes for 12.1(3) T say 2600), 12000 GSR series.

It now looks like optical networking devices will be capable of fast circuit establishment. Lucent has announced an “Optical Router”, using 256 very small mirrors on a chip, steered under electrical control. Agilent (HP) and Texas Instruments have announced liquid or gel-based chips where current turns the fluid to a reflective surface, deflecting light from one waveguide into another. For me, all these devices deserve a title like Optical DACS (Cross-Connect Switch), but who asked me? (The Cisco press release for the Monterey Networks acquisition refers to their optical cross-connect technology). These devices are not routers in the sense of looking into packets and determining path dynamically. They are routers in the sense of figuring out and plumbing a path through multiple Layer 1 devices. I prefer not to call that routing.

MPLS ties to optical by using the idea that when a route to a specific destination or group of destinations is propagated, a light path might also be set up. This light path could then be used by packets going to that destination or group of destinations, getting them there faster (one hopes) than if every router or device along the path examined the Layer 3 header. Actually having a program examine the Layer 3 information would involve converting the light to and from electrical signals at each step along the way.

So we have several media where MPLS is being considered:

  • high-speed IP backbones
  • legacy ATM
  • MPLS-capable ATM
  • optical

Frame Relay MPLS is also receiving some consideration by vendors other than Cisco.

You might be wondering if anyone is actually doing MPLS, or is this cutting-edge stuff? Well, AT&T is offering a service called IP Frame Relay, with IP access via Frame Relay to an MPLS network. This uses the Cisco-driven idea of MPLS-based VPN’s, discussed very lucidly in RFC 2547.  MCI has announced a similar service, called Business Class IP (which I can’t find on their service offering web pages). See also:

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