Label switching pumps up IP speed

Routers and switches use MPLS to communicate quality-of-service requests.

By Tim Greene
Network World, 6/8/98

Users looking to speed traffic across IP networks should start paying closer attention to this acronym: MPLS.

Multi-protocol Label Switching, a technology designed to hurry traffic through routed networks, has been endorsed lately by four major switch vendors as a means to guarantee quality-of-service (QoS) levels across hybrid routed/switched networks.

[MPLS diagram] Cisco Systems, Inc., Ascend Communications, Inc., Hughes Network Systems, Inc. and Lucent Technologies, Inc. have all recently adopted MPLS as a way to support IP intranets and virtual private networks (VPN) that require limited network delay.

In the short term, MPLS in combination with ATM is the only reliable way to translate IP traffic priorities to switches, according to Tom Nolle, president of CIMI Corp., a technology assessment firm in Voorhees, N.J. "It is literally the only way that we know of to efficiently and scalably map IP services to ATM infrastructure," Nolle said.

Using MPLS, designated traffic can demand and receive specific performance characteristics from the network. Whether corporate users want to set up their own private networks with QoS guarantees or buy QoS IP services from a carrier, they should know about the technology.

MPLS simplifies the work a router does to forward packets so the router can work faster. Rather than having every router in a network look up the destination of each packet in a route table and map that destination to a router port, the process is streamlined.

The initial router in an MPLS-enabled network does look up the destination of the packet in its route table, but rather than just forwarding the packet, the router also adds a label to it that indicates where the packet has to go.

When subsequent routers in the network receive that packet, they read just the label and check it against their own label tables. The tables denote which router port the packet should exit to move toward its destination. The label table needs only as many entries as the router has ports, which makes a much shorter list than the one listing all possible destination addresses in a network. Looking up a label requires less processing than finding a destination address in a route table, hence packets move faster across the network.

Different switch vendors use MPLS in different ways to map labels to QoS schemes they have set up in their frame relay and ATM switches.

Hughes employs a policy server called a Director that associates MPLS labels with virtual circuits. Virtual circuits can be engineered to carry traffic with defined delay characteristics. A packet coming from an MPLS-enabled router hits the switch, and the switch checks the label against the Director's label table and gets instructions to set up a switched virtual circuit for that traffic flow. That circuit can be assigned a standardized ATM QoS.

Hughes and Ascend have set up independent schemes for maintaining QoS across networks that include their frame relay as well as their ATM switches. In the Hughes strategy, the only time traffic is switched is when it requires QoS, otherwise traffic is routed, according to Kumar Shah, senior director of marketing for Hughes. "The only time we marry the worlds of routing and switching is when you want to deliver QoS," Shah said.

Maintaining QoS is key to an IP VPN, according to Dave Norton, process owner and team leader for American Standard Corp. in LaCrosse, Wis. American Standard's subsidiary, Trane Co., runs an IP VPN supported by Advanced Network & Services, Inc. Norton said certain client/server applications require responsiveness that pure routed networks - particularly public ones - cannot provide.

And he said that while network designs using MPLS to support QoS make sense, he had doubts about whether such networks could offer fast enough response times for applications such as voice and video. "I think it would be good for data but stay away from video and voice," he said. For those, Norton would recommend dedicated circuits, he said.

Without some plan for mapping IP QoS requests to ATM, there is no way today to offer QoS that has definable delay characteristics, according to Brett Azuma, an analyst with DataQuest in San Jose, Calif.

"ATM is the best game in town right now for absolute quality of service. It is the only game. IP just can't do it yet," Azuma said.