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Multiprotocol label switching moves to VPN duties

Posted: 15 Oct 2002 ?? ?Print Version ?Bookmark and Share

Keywords:multiprotocol label switching standard? virtual private network? LANs? Cisco Systems? IP switching technology?

As extensions to the multiprotocol label switching standard make MPLS viable for virtual private network transport, experts are warning against taking the protocol too far, too fast.

MPLS is increasingly being touted as the ideal Layer 2 transport structure, as well as an IP traffic-engineering tool at the network core. Its proponents say its method of tagging IP packets saves costs, which sits well with cash-strapped carriers. But speakers at the MPLScon Fall conference said that MPLS advocates touting the standard for VPNs and transparent LANs must avoid the hyperbole and runaway development that brought down asynchronous transfer mode's backers.

MPLS is an Internet Engineering Task Force standard that grew out of Cisco Systems Inc.'s "tag switching" concept, which Cisco fielded to challenge IP switching technology from Ipsilon Networks Inc. The idea was to make routers run faster: Each packet would be assigned a label summarizing key information, so that routers and other equipment could simply read the label rather than the entire packet header.

Standardized in 1997-98, MPLS found favor as a way to characterize IP flows in the network core. But, now, hardware companies are proposing use of the protocol as a forwarding vehicle for IP Versions 4 and 6 and for private framed-data services such as Ethernet and frame relay. An IETF draft extension of MPLS called Draft-Martini adds point-to-point status signaling to allow MPLS to be used in a virtual-circuit environment. Draft-Martini has won early support from semiconductor vendors in the network processor and switching fabric realms.

The draft's author, Level 3 Communications LLC senior architect Luca Martini, delivered the introductory keynote speech at MPLScon. The extensions will allow MPLS to be used in such applications as VPN and transparent LAN services, Martini said. That will require developers to take maximum advantage of the simplification of the control plane and data-forwarding plane allowed by Draft-Martini. Additional traffic aggregation features will be allowed through the "stacking" of MPLS labels, though standards for stacking the labels must still be defined.

For the ideal mix of edge and core information, many developers are focusing on the use of the multiprotocol Boundary Gateway Protocol (BGP) in edge devices, combined with MPLS in the core. For QoS parameters, the Resource Reservation Protocol (RSVP) can be used at the network core for traffic engineering, with the MPLS Label Distribution Protocol used at the edge. Only by carefully controlling the route information available at edge routers, speakers said, can a network manager avoid an explosion of routing tables at the edge of the network.

Yakov Rekhter, who helped develop MPLS while at Cisco and who now is a distinguished engineer at Juniper Networks Inc., elevated the link between MPLS and BGP to the status of a full-service tool suite in his MPLScon speech. BGP can be used to carry a set of stacked VPN labels for MPLS, making the latter protocol appropriate for Layer 2 VPNs, Layer 3 IP VPNs, and virtual private LAN service, Rekhter said.

Cisco technical fellow Bruce Davie quipped that Rekhter's pitch for the universal applicability of BGP "had the flavor of saying it is a dessert topping and a floor wax, all in one."

Complex path?

Speaker after speaker acknowledged that carriers are embracing the call to deploy MPLS as an enabler for VPNs. But they warned that MPLS' evolution is beginning to look like the ATM LAN Emulation (LANE) standards push of the mid-1990s. LANE grew increasingly complex as it moved closer to the desktop, and it finally collapsed under its own weight when desktop users showed no interest in moving from Ethernet to ATM network interface cards. Indeed, for the past decade, Ethernet's natural affinity for IP has helped relegate ATM largely to the telecom backbone.

Martini said many of the problems with ATM LANE stemmed from the fact that ATM was a Layer 2 protocol trying to operate in a multipoint world. MPLS Draft-Martini is aimed at point-to-point applications, with the understanding that multipoint protocols should operate only at Layer 3 and above, he said.

The IETF has created three working groups to deal with MPLS' wider applications base. VPN functions are handled in the Provider-Provisioned VPN Working Group. Transport of frame-based data protocols like Ethernet is handled in the Pseudo-Wire Emulation End-to-End Working Group. Most functions for optical-control-plane signaling fall under the purview of the Common Control and Measurement Protocol Working Group.

Elizabeth Hache, director of product strategy at Nortel Networks, said she is confident carriers will respond to the rich new mix of MPLS services, once they realize that most of the services can be added without forklifting new equipment into the network. One advantage of the infrastructure overbuild in the late 1990s is that core capacity in the network can be used to provide encapsulated Ethernet or ATM services over MPLS, she said.

Jay Gill, director of product management at Ethernet carrier Yipes Inc., said the Metropolitan Ethernet Forum supports both the Draft-Martini extension of MPLS and the stacking of MPLS labels. But customers using metropolitan Ethernet still must be convinced that MPLS VPNs are preferable to simpler, bridged VPNs based on the Layer 2 Transport Protocol. MPLS may move slowly into Ethernet-based carriers, Gill said, but only on the basis of its cost advantage.

Magued Barsoum, systems architect at multiservice switch vendor Quarry Technologies Inc., said VPNs based on ATM or frame relay afford customers a sense of control that MPLS does not yet provide. "When you use the Internet as a transport vehicle, you take away the sense of control," Barsoum said.

Security checks

To create truly secure VPNs, Quarry advocates the use of MPLS/BGP in association with IP-secure (IPsec) encryption and tunneling.

If MPLS turns into the Swiss army knife of Layer 2 services, standards development issues will follow. The proliferation of industry coalitions dealing with Layer 2 transport standards is grating on network equipment OEMs that are trying to cut costs, Vivace Networks Inc.'s chief technologist Andrew Malis said in his conference speech. He said vendors are pushing for the formation of a "Layer 2 Forum" that would combine the work of the ATM Forum, the Metropolitan Ethernet Forum, the Frame Relay Forum, the MPLS Forum, and perhaps other groups.

While informal discussions about forming an umbrella forum have taken place for nearly a year, Malis said companies that participate in multiple coalitions have turned up the heat on the issue in the past two months. A common coalition to discuss packet transport at the data Link layer makes sense, Malis said, particularly since many carrier networks are beginning to combine traffic types in a common service network in order to reduce operational expenses. But a shotgun marriage of transport coalitions might be tough to realize in practice, he said.

"There are some basic issues to be settled," Malis said. "What would the forum's vision be? What would its goal in life be? Why would it exist in the first place? I expect some very interesting discussions in the coming months."

- Loring Wirbel

EE Times

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