Chapter 3 - Overview of MPLS Protocols

Abstract

This chapter provides an overview of the Label Distribution Protocol (LDP), and the Traffic Engineering extensions to Resource Reservation Protocol (RSVP) for Label Switching Paths (LSP) Tunnels (RSVP-TE), the two Multiprotocol Label Switching (MPLS) signaling protocols. The LDP is one of the fundamental approaches to distributing labels between Label Switching Router (LSR). LDP can be used to distribute labels that can be used for traffic that matches an FEC according to specific requests for such a label or in an unsolicited manner as new routes become available. LDP exchanges messages between LDP-capable LSRs packaged into Protocol Data Units (PDUs). Each LDP PDU begins with a header that indicates the length of the whole PDU, and is followed by one or more messages from one LSR to the same partner LSR. A second protocol suitable for label distribution in a traffic engineered MPLS networks is based on the RSVP that is suitable for extension to the MPLS world because it deals with end-to-end reservation of resources for traffic flows, a concept similar to traffic engineered MPLS. RSVP is essentially a request/response protocol with Path messages being used to navigate a path, and request resources for traffic flows, and Resv messages returning along the path to indicate what resources should be reserved. This flow of messages matches the requirements for Downstream-On-Demand label distribution and can be extended easily by adding information to the messages.