Label Switching Aids Scalability, QoS

FRAMINGHAM (03/06/2000) - Mainly defined as a set of Internet Engineering Task Force drafts, Multi-protocol Label Switching (MPLS) is a technology that can address ever-increasing network requirements.

The benefits of MPLS include improved network scalability, quality of service (QoS) and traffic engineering. MPLS QoS supports sensitive data such as video by incorporating mechanisms that can provide such traffic with preferential treatment. MPLS traffic engineering allows manipulation of data flow across networks, so that new services can be offered. Additionally, unlike other technologies, MPLS won't consume your budget because many existing routers and WAN switches support the standard.

MPLS presents a simple way to forward information through networks by installing and then examining short, fixed-length ID markings called labels in packets. By relying on labels to forward information, MPLS usually doesn't use a packet's IP header unless it is entering or leaving MPLS. Because labels reduce dependency on IP data, new and larger network designs become possible.

MPLS-enabled devices - called MPLS nodes or Label Switch Routers (LSR) - process labels at Layer 2. Therefore, slower network devices may work more quickly when enabled for MPLS. In some scenarios, the MPLS label can be directly integrated with other Layer 2 protocol headers. Because MPLS nodes can find integrated labels more rapidly than IP data or regular MPLS labels, network performance may be further improved.

As a packet arrives on an MPLS node, its label - provided there is one - is compared to an onboard database called the label information base (LIB). The LIB contains a table that is used to add a label to a packet, or change/remove the existing label, while determining the outgoing interface to which the data will be sent. After consulting the LIB, the MPLS node forwards the packet toward its destination. The LIB can simplify forwarding and increase scalability by tying many incoming labels to the same outgoing label information, providing a new type of integrated summary routing.

When a set of MPLS nodes have information in their LIBs related to the same destination, a label switched path (LSP) has been created. LSPs may be built automatically by the label distribution protocol (LDP) or other protocols.

In the simplest form of LDP, each MPLS node informs upstream (that is farther away from a destination) nodes of which label they should apply to packets that are heading to a destination downstream. Label distribution is normally based on routing protocols such as Intermediate System to Intermediate System, Border Gateway Protocol or Open Shortest Path First. Because all MPLS nodes use a routing protocol, the inherent benefits of these protocols, such as fault-tolerance, are retained or expanded. When enabled in WAN switches, routing protocols can simplify routing and increase network scalability.

Scalability can also be increased by deploying MPLS tunnels, which are LSPs that carry routing information and/or other LSPs. Tunneling allows network devices to be shielded from unneeded network information, thereby operating more efficiently.

QoS and traffic engineering components round out MPLS' core features. As part of LDP, forwarding equivalence classes specify selected data that should use a particular LSP. MPLS nodes can then provide basic QoS to that LSP and the chosen data within. Providing a basis for traffic engineering, explicitly routed LSPs are LSPs whose route through an MPLS network has been granted a priority, avoiding the automatic setup described earlier. Explicitly routed LSPs allow traffic to be engineered to travel through the net and avoid congested areas, according to QoS requirement, security need or other criteria.

Considering the increasing expectations being placed on networks, the benefits provided by MPLS make it a promising technology. MPLS can enhance network scalability and performance while providing advanced services in a simple package. If you're upgrading an existing network or planning a next-generation network, MPLS is a concept to bear in mind.

Giacalone is a managing consultant at Predictive Systems, an international network consulting firm. He can be reached at Spencer.Giacalone@

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