Light at the end of the L2TPv3 tunnel

Companies and carriers have been looking for ways to maximize the efficiency and cost of their infrastructures and simplify management by transporting multiple Layer 2 services across a common IP backbone. Unlike IP-based VPNs, Layer 2 VPNs are multiprotocol, allowing the transport of IP and non-IP traffic across a common router infrastructure. With Layer 2 VPNs, complexity is reduced by eliminating the need for edge routers to support every enterprise VPN routing table and Layer 3 routing environment.

Several Layer 2 VPN techniques, such as the IETF Martini draft and Circuit Cross Connect, have been developed to let packet-switched traffic, such as frame relay, ATM and Ethernet, and time-division multiplexed traffic, such as voice and leased line, be transported across a Multi-protocol Label Switching (MPLS)-enabled network.

Another technology being developed within the IETF is the Layer 2 Tunneling Protocol Version 3 (L2TPv3). Currently an IETF draft on the standards track, L2TPv3 is emerging as a lightweight yet robust alternative to creating Layer 2 VPNs across MPLS and pure IP backbones.

L2TPv3, an extension of the L2TP, is a stateless protocol with no inherent signaling or keep-alive mechanism. L2TP, originally defined in RFC 2661, was designed to provide dynamic tunneling for multiple Layer 2 circuits across packet-oriented data networks. It describes a standard method of tunneling that lets circuitlike connections across one or many Layer 3 networks appear as point-to-point or point-to-multipoint links between customer locations. The base L2TP protocol consists of a control protocol for dynamic creation, maintenance and tear-down of L2TP sessions; and data encapsulation to multiplex and demultiplex Layer 2 datastreams between IP-connected nodes.

L2TP has been focused on narrowband dial-up protocols. L2TPv3 extends L2TP by letting it run on higher-speed devices such as routers because of reduced overhead and the related decrease in processing chores. It also adds important new features such as increasing the session and tunnel ID space from 16 to 32 bits, which dramatically increases the number of tunnels from 65,000 to more than 4 billion.

With L2TPv3, the physical interface connecting to a customer's network becomes the tunnel ingress/egress interface. Consequently, traffic does not need to be routed into the tunnel by the provider's router. As packets arrive at the interface, they are encapsulated and forwarded directly toward the remote tunnel endpoint. Once received and de-encapsulated, the original packet can be forwarded out of the egress interface if the tunnel identifier is recognized by the router. If it isn't, the packet is discarded.

With L2TPv3, companies reap lower-cost services because carriers can offer frame relay, ATM and Ethernet over a common IP backbone - radically lowering capital and operational costs. And because L2TPv3 adds no new requirements to the IP transport infrastructure, it is inherently easier and simpler to implement and support, because network staff is familiar with IP.

Driving the technology are a number of new applications, such as the ability to offer transparent Ethernet LAN services across the wide area, scaling frame relay networks to higher speeds, and infrastructure optimization by collapsing multiple networks onto one IP backbone.

Meanwhile, corporations have a single connection that provides a secure Layer 2 VPN to remote sites and general Internet access, as opposed to different connections for multiple services, such as a connection for Internet access and discrete private lines for intranet access - a common enterprise problem.

But there is always a downside. While L2TPv3 makes better use of a shared resource (the Internet and IP backbones), resource sharing is always a compromise and lacks true predictability and guarantees. This is not only true of logical circuits but also of physical assets such as routers where schemes such as virtual routing have been proposed.

Ginsburg is vice president of marketing and product management at Allegro Networks. He can be reached at gins@allegronetworks.com.

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The Edge, 03/04/02.