MPLS Branches Out

It's more than a non-ATM way to engineer the core.

Sam Masud, Senior Technology Editor

In upstate New York, Westelcom is in the process of firing up its broadband network. A unit of a nearly 100-year-old rural telephone company, Westelcom intends to avoid the pain and death that have befallen some of its CLEC brethren. "We're targeting extremely rural areas where none of the big guys want to do business, and we're not building an expensive network like the first-generation CLECs," said Eric Kreckel, Westelcom's corporate engineer. Westelcom plans to provide data and packet voice services over DSL using a platform from Integral Access.

"We've been testing the system locally and in our offices for four months and there has not been a single hitch. I can be downloading a full-screen video and talking on the phone at the same time," Kreckel said. He can do this because Integral Access' solution, which among other technologies incorporates a DSLAM (DSL access multiplexer) and voice gateway, assures that voice calls always get priority over data, while simultaneously enabling Westelcom to offer differentiated IP services that meet customers' SLAs. Consider Westelcom, in combination with Integral Access, a pioneer, because it uses MPLS (multiprotocol label switching) technology all the way to the customer premises.

Initially cast as a TE (traffic engineering) mechanism for the IP network core, MPLS is emanating downstream as just about every type of equipment manufacturer, from new edge router vendors to Ethernet, ATM, IP service and next-gen switch vendors, embraces it. Its complexities aside, there's a simple reason why so many are chanting the MPLS mantra. In an IP network, MPLS establishes a switched connection between any two points. Instead of making routing decisions at each node, MPLS lets carriers decide which MPLS virtual circuits, or label switched paths (LSPs), IP packets will take as they traverse the MPLS network. MPLS is viewed as a necessary antidote to next-hop routing because that high bandwidth OC-48/OC-192 connection to the next node may not be the most optimal path due to congestion. An underutilized OC-12 link to a node farther away may actually be a better route.

Dual Role for ATM

The beauty of MPLS is that although it's an IETF standard and early implementers were core router vendors such as Cisco and Juniper, it runs on any Layer 2 switch, allowing carriers that use ATM switches for TE to preserve their investments. But equipment vendors offer a dual role for ATM switches. Cable and Wireless has a public IP network that uses ATM switches for TE and another, physically separate one for frame relay and ATM services. The latter network also supports the carrier's private IP network. Cable and Wireless plans to use the Alcatel 7670 RSP (routing switch platform), an MPLS-enabled ATM core switch, not as simply an MPLS switch, but as a mediation device between its MPLS core network and its legacy frame relay/ATM network. Put another way, Cable and Wireless will be using MPLS, sometimes referred to as a Layer 2.5 protocol, to transport legacy frame relay/ATM services while only managing an MPLS-based core.

"We've already decided to migrate our public network from using ATM for TE to MPLS TE. Now we want our existing legacy network to converge on the new, integrated MPLS network. At the same time, we don't want to sacrifice any edge services, namely frame relay and ATM, and the SLAs we have in place," said Adeel Siddiqui, Cable and Wireless' senior manager for data network engineering. To Chad Dunn, director of product marketing for WaveSmith, an emerging multiservice switch vendor, Cable and Wireless' move to an MPLS core makes sense. "I'm not surprised that Cable and Wireless would do this, because it has a very robust Juniper core network that can handle very stringent QoS requirements. But this probably wouldn't work for a typical RBOC, because a majority of its services will be circuit-, frame relay- or ATM-based. At this time it wouldn't make sense to implement an IP core network and then tunnel services across it," Dunn said.

Because a large part of RBOC/ILEC revenues are derived from ATM-based services such as frame relay and private-line circuit emulation, Jarrod Siket, director of product planning in Marconi's network service provider unit (which includes ATM pioneer Fore Systems), doesn't see carriers abandoning ATM any time soon. "The ILECs also have a lot of IP traffic, but it's predominantly transported in the LATA (local access transport area) over ATM," Siket noted. He sees no early demise for ATM but rather a coexistence with IP even though MPLS seems to be negating ATM's role.

The coexistence of ATM and IP is possible because of what the IETF defines as a ships-in-the-night switch: An ATM switch can function simultaneously as an ATM switch in an ATM network and also as an MPLS switch in an IP network. In fact, the BXR-48000 announced by Marconi last November delivers 240 Gbps of bidirectional switching capacity in a single rack and is intended to compete as both a core router and ATM switch. "We want customers to be able to continue deploying ATM to manage the growth of frame relay and [ATM-based] DSL services, but because we don't want to constrain them, we're offering the capability to upgrade their boxes to extend MPLS-based IP services," Siket said.

Router vendors are pushing MPLS technology because the IP-over-ATM overlay model poses an adjacency problem, since each router appears to be directly connected to every other router in the network via ATM VCs (virtual circuits). The addition of each new router leads to a disproportionate increase in the number of VCs-what the industry calls the N² problem. But with the introduction of MPLS in the network, a router only has an adjacency with the router to which it is directly connected. However, Siket, who is also vice chairman of the MPLS Forum's technical committee, noted that there is a second scalability issue with any connection-oriented technology (e.g., ATM and MPLS)-the ability of the switch to provide all the VCs or LSPs a large network might need.

If TE was intended to be the first benefit MPLS brings to an IP network, then MPLS-based TE in combination with DiffServ, which specifies how a given flow of packets is treated in the network, provides the QoS for service providers to offer SLAs to customers. For Telica, a vendor of next-gen Class 4/5 switches, the DiffServ/MPLS combination is important for predicting how the network treats VoIP traffic. "Today MPLS is largely in network cores, so we take advantage of that by supporting DiffServ on our Plexus platform. But we will support MPLS in the first half of 2002, because MPLS as an enabler of DiffServ lets you control the path the packets travel," said Ali Kafel, Telica's vice president of marketing.

VPNs

Beyond the QoS issue, vendors increasingly tout Layer 2 and Layer 3 VPNs supported by MPLS. While RFC 2547 defines a method for providing Layer 3 VPNs, Layer 2 VPNs are still essentially in the concept stage. Layer 2 VPNs enable access to the MPLS network via any Layer 2 technology (e.g., frame relay, ATM, Ethernet, L2TP) with the LSPs in the MPLS network assuming the role traditionally played by frame/ATM PVCs (permanent virtual circuits)-in effect tunneling the Layer 2 technology across an MPLS core. These VPNs restrict customers to using the same data link technology at either end of the connection.

Layer 3 VPNs, also called BGP/MPLS VPNs, are transparent to the access technology and ensure that the IP traffic between sites is assigned to a particular LSP, but allow a particular customer site to have membership in more than one VPN. While only IP traffic is supported by Layer 3 VPNs, Layer 2 VPNs are not limited to a specific Layer 2 or 3 technology, a key difference. Layer 2 VPNs require a full mesh of connections between customers sites, either configured manually or through an element management system. The RFC 2547 approach permits the service provider's edge routers to discover their VPN peers and exchange routing information for that specific VPN. Traffic between those VPN sites is then transported through LSPs that can be set up dynamically across the network.

Edge router vendor Laurel Networks says MPLS will enable Ethernet providers such as Yipes, Telseon and Cogent to offer Ethernet private-line services. "We've co-authored a draft RFC that defines a technique for utilizing MPLS to carry service-specific traffic. So we're talking about the concept of using an Ethernet interface to a customer and then mapping that onto an MPLS connection," said Steve Vogelsang, Laurel's co-founder and vice president of marketing. Extreme Networks, which views MPLS-based Ethernet metro networks as capable of offering QoS, transparent LAN and differentiated IP services, is among the Ethernet switch makers that embrace transporting Layer 2 traffic over MPLS.

For new Motorola acquisition RiverDelta Networks, whose product is at once a policy router, switch and CMTS (cable modem termination system), MPLS enables cable operators to generate revenues through partnerships with ISPs, ASPs, and content, voice or VPN providers. Using the RiverDelta platform, a cable operator could deploy MPLS VPNs within the metro network-regardless of whether it uses Ethernet, ATM or POS (packet-over-SONET) technology-to connect subscribers to the appropriate service providers. Also, by using its platform's per-flow queuing on the HFC (hybrid fiber coax) side of the network together with MPLS in the metro network, RiverDelta believes it has a solution for end-to-end QoS (i.e., from the customer to the partnering service provider's POP).

So strong is the MPLS bandwagon, that it would be hard to find a router/switch vendor that isn't embracing the technology. CoSine Communications, whose switch allows ISPs to offer network-enabled services such as firewall, virus detection and VPNs using technologies such as PPTP, L2TP and IPSec, believes that its support for Layer 3 VPNs makes its IPSX platform especially attractive for enterprise customers. "L2TP, PPTP and IPSec are good dial VPN solutions, whereas MPLS will give you site-to-site VPNs," said David Messina, CoSine's director of product marketing.

At this time, the din about MPLS is ahead of technology deployment. Many vendors that include MPLS in their product demonstrations aren't yet shipping it. Important standards work remains to be done, and it will be some time before the power of MPLS is used to create interprovider MPLS networks that deliver edge-to-edge QoS and VPNs-which many call the killer MPLS application-that cut across multiple provider boundaries. While the MPLS incubation process will take time, it would be unwise for ISPs to delay evaluating a technology that the industry anticipates will have a faster take-rate in networks than frame relay/ATM and will be a defining step toward an everything-over-IP network.

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