e an LSP. There was an early decision in MPLS not to specify one protocol to do this but to allow any number of reasonable protocols to flourish as required. In practice the following four main protocols were standardized: Label Distribution Protocol (LDP), specified in RFC 5036 BGP-4 extended to carry labels, specified in RFC 3107 RSVP with Traffic Engineering Extensions (RSVP-TE), specified in RFC 3209 Constraint-Routed Ldp (CR-LDP), specified in RFC 3212 In this section only the first will be considered in detail. The last two are concerned with traffic engineering, which is described in the next section. It should be noted that although Chapter 13 Multiprotocol Label Switching the MPLS working group set out to allow a number of protocols to distribute labels, by February 2003 they effectively withdrew CR-LDP (which was a competitor to RSVP-TE), SO we are in reality left with three main protocols that have different purposes. Label Distribution Protocol LDP is a protocol used to establish label mappings for MPLS networks that do not use traffic engineering. The main FEC used for label mappings is the longest prefix match in the IP routing table of each LSR. An LSR can act in a number of different modes with respect to how labels are determined using LDP (see RFC 3031 and RFC 5036 for more detail). Here the implementation with the most common mode settings is described. LDP defines four types of protocol messages, all of which are sent between immediate peers and use TCP/IP unless stated otherwise: Discovery messages are used to announce the presence of an LSR and use UDP/IP. Session messages are used to establish communication channels between LSRs. Notification messages allow alarms to be sent in case of errors. Advertisement messages create, delete, and modify label mappings for a particular FEC. Only advertisement messages will be considered here. Advertisement messages allow a variety of modes to obtain (or request) labels. Here we restrict the description to the case where labels are advertised by an LSR 