Finally ready for prime time: multiservice consolidation over MPLS now makes technical and business sense - Network Core

Telecommunications Americas, Nov, 2003 by Riad Hartani

Despite some false starts, network convergence may have arrived. While consolidation efforts with ATM ultimately stalled, multi-service consolidation on IP/MPLS-based solutions are reaching the stage of technical and business viability.

Traditional ATM, frame relay, Ethernet and TDM services are all candidates for consolidation over a common IP/MPLS infrastructure. Over the last few years, the IETF has led the way by specifying data plane and control plane interworking, with standards bodies such as the ATM Forum, the ITU-T, the MPLS Forum and the Frame Relay Forum contributing. Consensus is finally being achieved and interoperability demonstrated. Recent innovations or agreements in QoS, OAM, restoration, control plane interworking, and interworking with Layer 2- and Layer 3-based VPN services have been accomplished.

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On the business side, issues have involved the validity of IP/MPLS as the convergence platform, the timing of migration, various network deployment scenarios and concerns regarding mixing public and private services over the same infrastructure. These questions now have increasingly compelling answers.

Network Convergence Revisited

Defining network convergence depends on the context in which it's used. In the packet world, ATM held promise as the basis for multi-service convergence of IP traffic, frame relay, Ethernet, and in some cases TDM over the backbone, but the exponential growth of IP traffic called for a different IP/MPLS-based convergence strategy.

As a platform for multi-service convergence, MPLS leverages experience gained designing and deploying ATM. For example, features such as label stacking, link layer independence and its coupling with Internet routing and enhanced QoS mechanisms gives MPLS scalability. MPLS, however, is not the only choice for multi-service convergence. Various IP tunneling mechanisms such as L2TP (Layer 2 Tunneling Protocol), GRE (generic routing encapsulation) and IPSec tunneling can be used to carry multiple services over an IP backbone.

A Convergence Platform

For control plane support, LDP (Label Distribution Protocol) is seen as the protocol of choice to carry virtual circuit information from edge to edge, with BGP (Border Gateway Protocol) being considered as an alternative. At the end of 2002, a number of IETF specifications known as the Martini Specifications described the industry compromise related to ATM, frame relay, Ethernet, PPP/HDLC and TDM over MPLS.

* Frame relay over MPLS. Carrying frame relay traffic over MPLS is a natural fit. In fact, in its early days MPLS was considered an evolution of frame relay toward higher-speed interfaces. As such, there was little controversy in defining a scheme to encapsulate frame relay over MPLS

because the payload is carried natively.

* Ethernet and VLANs over MPLS. The frame nature of Ethernet/VLAN (virtual LAN) is also a natural fit over MPLS. In its most basic point-to-point service form, the whole Ethernet frame is carried without preamble and FCS (frame check sequence). Point-to-multipoint Ethernet services over MPLS, as potentially used in VPLS (virtual private LAN service), introduces some additional but tractable complexity in terms of carrying Ethernet/VLAN traffic. The major controversies regarding Ethernet over MPLS involves the model used to deliver VPLS and defining the Layer 2 VPN service model and use of either LDP or BGP as a basis for VPN discovery and signaling.

* ATM over MPLS, ATM over MPLS has been by far the most controversial specification. The ATM Forum, IETF and ITU-T have been working on different encapsulation schemes, but are now compromising in a subset of the specification. The variety of encapsulation schemes for ATM over MPLS and the stringent ATM service emulation requirements for OAM and QoS contributed to the problem's complexity. Overall, three major encapsulation modes are emerging: ATM cell mode, ATM concatenated cell mode and AAL5 (ATM Adaptation Layer 5) frame mode, with additional ones related to the ATM virtual path and ATM port modes also being considered.

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* TDM over MPLS. Encapsulation of TDM is being defined, but specification is less mature than for ATM/Ethernet/frame relay. This is mainly due to the nature of TDM service and its stringent requirements on the emulating scheme regarding packetization, reassembly and carriage over the backbone, while ensuring that appropriate synchronization, clocking and timing constraints are not violated.

The various specifications for transporting multiple services over MPLS are slowly getting to a stage where interoperability can be verified. More encapsulation schemes are being developed to carry other services over MPLS, native voice being the most notable.

The Road Ahead

A number of vendors have announced implementation of multiservice over MPLS, but deployments service over MPLS, but deployments have been slow. Reasons include lack of proven interoperability and slow deployment of complementary MPLS-centric features required for overall service emulation. This includes: