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MPLS: Status and Concepts

The Multiprotocol Label Switching (MPLS) Protocol is published by the Internet Engineering Task Force as RFC 3031.

MPLS is a label swapping (mapping) and forwarding technology, but it integrates label swapping with network layer routing. Label swapping, or mapping, means the changing of the label value in the packet header as the packet moves from one node to another; the rationale for this operation is explained in Chapters 2 and 4.

The idea of MPLS is to improve the performance of network layer routing and the scalability of the network layer. An additional goal is to provide greater flexibility in the delivery of routing services (by allowing new routing services to be added without a change to the forwarding paradigm).

MPLS does not make a forwarding decision with each L_3 datagram but uses a concept called the forwarding equivalence class (FEC). An FEC is associated with a class of datagrams; the class depends on a number of factors, at a minimum the destination address and perhaps the type of traffic in the datagram (voice, data, fax, etc.). Based on the FEC, a label is then negotiated between neighbor LSRs from the ingress to the egress of a routing domain. As we showed earlier, the label is then used to relay the traffic through the network.

The initial MPLS efforts of the IETF focus on IPv4 and IPv6. The core technology can be extended to multiple network layer protocols, such as IPX, and SNA. However, there is little interest in expanding MPLS to other network layer protocols, since IP is by far the most pervasive.

The basic idea is not to restrict MPLS to any specific link layer technology, such as ATM or Frame Relay. Most of the efforts so far are directed to the interworking of MPLS and ATM, but in the future, MPLS will operate directly with IP over the physical layer and not use ATM at all.

In addition, MPLS does not require one specific label distribution protocol (agreeing on the use of label values between neighbor LSRs). It assumes there may be more than one, such as the Resource Reservation Protocol (RSVP), Border Gateway Protocol (BGP), or the Label Distribution Protocol (LDP). Considerable attention is on LDP, since it is being designed from scratch for MPLS networks. Other protocols, such as BGP and RSVP, are also good methods for label distribution.

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