MPLS: Enabling Enhanced IP Services In The Access Network

Mark Veil, Product Manager, Integral Access

The demand for converged IP-based services and applications has created a host of new market opportunities for today's Integrated Communications Provider (ICP). With each new market having its own unique set of demands, the ICP must be capable of creating, delivering, and billing for graded service offerings specifically tailored for these requirements.

As a result, today's providers need tools to effectively and efficiently engineer and manage their networks for class and quality of service (CoS/QoS). Fortunately, new IP-based technologies for supporting enhanced broadband voice and data services over a single access network can provide competitive carriers the opportunity to aggressively attack new markets with broad and richly differentiated service offerings. Multi-Protocol Label Switching (MPLS), in combination with effective traffic management facilities, provides the best approach for managing QoS/CoS in today's access networks and allows ICPs to deliver differentiated IP-based services.

Engineering And Managing Network Traffic For QoS

QoS, which we will define as the collective measure of service levels delivered to the customer premise and characterized by performance criteria such as availability, response time, throughput, error rates, and packet loss, enables certain traffic types to be given priority treatment in the network. This capability is required to support specific latency requirements of individual applications, or to meet customer-contracted Service Level Agreement (SLA) obligations. To support QoS requirements, network facilities must provide traffic management services such as admission control, traffic classification, traffic marking, and traffic conditioning. Once appropriately classified and groomed, traffic must be aggregated and efficiently mapped onto the existing network topology for the purposes of controlling network behavior and optimizing network utilization and traffic performance. This is the domain of traffic engineering and MPLS.

MPLS represents the best alternative for enabling traffic engineering and QoS in the heterogeneous world of network communications. Although originally intended as a means to enhance routing performance, continued improvements in that area have shifted the application focus of MPLS to its inherent capabilities for delivering efficient and scalable traffic engineering and QoS in IP-based networks. MPLS operates at Layer Two-and-a-half (L2.5) and is protocol-agnostic to the layers above and below it. Its architecture is based on the multi-layer switch concept -- which cleanly separates the forwarding and control functions --with the former being the domain of MPLS.

The power of MPLS stems from its ability to associate and allocate any type of user traffic with a particular Forwarding Equivalency Class (FEC). Each FEC represents an aggregation of traffic that will be treated in the same manner as it traverses the network. Typically these traffic classifications are based on the contents of the packet header, such as the L2 and L3 source/destination address. In practice, however, classifications may be derived from (and applied to) a virtually unlimited range, combination, and granularity of packet attributes -- including physical ingress port/interface, application protocol type or Ipv4 type of service (ToS), and Ipv6 CoS markings.

These FECs are then mapped to label switched paths (LSPs) that have been engineered to support specific traffic QoS requirements -- guaranteed bandwidth or low latency for instance. LSPs behave in a similar fashion to the more familiar ATM virtual circuit and frame relay data link connection identifier (DLCI), but do so with much greater efficiencies.

Upon ingress to a particular MPLS domain, all packets are assigned a label that serves to represent its FEC/LSP binding as well as a short-hand reference to the contents of the IP packet header. In sharp contrast to today's "longest-match" routing paradigm, MPLS-equipped routers are able to perform ultra-fast forwarding of IP packets via "exact match" label swapping. Moreover, MPLS overcomes the inherent limitations of traditional destination-based routing by supporting both explicit and constraint-based approaches for establishing LSPs. This capability allows network administrators to bypass potential points of congestion and direct traffic away from the default path selected by today's Interior Gateway Protocol (IGP)-based networks and deliver precise control of network traffic and behavior.

Enabling Enhanced Services With MPLS

For the resource-constrained local access loop, MPLS is best matched with the IETF's Integrated Services (IntServ) architecture to provide explicit QoS controls at the LSP level for delivering enhanced IP-based services. The MPLS-IntServ combination delivers connection-oriented behavior to connectionless access networks. Each LSP is apportioned and allocated access bandwidth in accordance with its traffic contract parameters to ensure the given application receives its appropriate level of QoS. For instance, individual LSPs can be created to support the unique requirements of services such as Voice over IP (VoIP), VoMPLS, or premium data applications such as e-commerce or VPNs. In the core network, where explicit end-to-end resource reservations are not practical and additional packet processing overhead is not advantageous, pairing MPLS with another IETF initiative, differentiated services (DiffServ) provide the required levels of QoS management.

MPLS offers a practical QoS solution for ICPs that want to deliver differentiated, IP-based services. MPLS enables all services -- packet-based voice services, tiered data offerings, and secure VPNs -- to dynamically share the same link, delivering optimum resource utilization for resource-constrained access networks. Moreover, service providers can provision, account, and bill for differentiated services -- and turn previously "flat" minutes into "fat" revenue-generating minutes. Finally, MPLS promises to reduce the complexities and costs associated with operating multiple access networks and provides protection for existing technology investments, while allowing carriers to follow a clear migration path to a converged, IP-dominated world.

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