LDP Applicability
RFC 3037

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Network Working Group                                          B. Thomas
Request for Comments: 3037                           Cisco Systems, Inc.
Category: Informational                                          E. Gray
                                                           Zaffire, Inc.
                                                            January 2001

                           LDP Applicability

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   Multiprotocol Label Switching (MPLS) is a method for forwarding
   packets that uses short, fixed-length values carried by packets,
   called labels, to determine packet nexthops.  A fundamental concept
   in MPLS is that two Label Switching Routers (LSRs) must agree on the
   meaning of the labels used to forward traffic between and through
   them.  This common understanding is achieved by using a set of
   procedures, called a label distribution protocol, by which one LSR
   informs another of label bindings it has made.  This document
   describes the applicability of a set of such procedures called LDP
   (for Label Distribution Protocol) by which LSRs distribute labels to
   support MPLS forwarding along normally routed paths.

1. LDP Applicability

   A label distribution protocol is a set of procedures by which one
   Label Switching Router (LSR) informs another of the meaning of labels
   used to forward traffic between and through them.

   The MPLS architecture allows for the possibility of more than a
   single method for distributing labels, and a number of different
   label distribution protocols are being standardized.  Existing
   protocols have been extended so that label distribution can be
   piggybacked on them, and new protocols have been defined for the
   explicit purpose of distributing labels.

Thomas & Gray                Informational                      [Page 1]
RFC 3037                   LDP Applicability                January 2001

   This document describes the applicability of the Label Distribution
   Protocol (LDP), a new protocol for label distribution designed to
   support label distribution for MPLS forwarding along normally routed
   paths as determined by destination-based routing protocols.  This is
   sometimes called MPLS hop-by-hop forwarding.

   LDP, together with an IP routing plane and software to program ATM
   switch or Frame Relay switch cross-connect tables, can implement IP
   in a network of ATM and/or Frame Relay switches without requiring an
   overlay or the use of ATM-specific or Frame Relay-specific addressing
   or routing.

   LDP is also useful in situations that require efficient hop-by-hop
   routed tunnels, such as MPLS-based VPN architectures [RFC2574] and
   tunneling between BGP border routers.

   In addition, LDP includes a mechanism that makes it possible to
   extend it to support MPLS features that go beyond best effort hop-
   by-hop forwarding.

   As a stand-alone protocol for distributing labels LDP does not rely
   on the presence of specific routing protocols at every hop along an
   LSP path in order to establish an LSP.  Hence LDP is useful in
   situations in which an LSP must traverse nodes which may not all
   support a common piggybacked approach to distributing labels.

   Traffic Engineering [TE] is expected to be an important MPLS
   application.  MPLS support for Traffic Engineering uses explicitly
   routed LSPs, which need not follow normally-routed (hop-by-hop)
   paths.

   Explicitly routed LSPs may be setup by CR-LDP [CRLDP-AS], a set of
   extensions to LDP, or by RSVP-TE [RSVP-TE-AS], a set of extensions to
   RSVP.  There is currently no consensus on which of these protocols is
   technically superior.  Therefore, network administrators should make
   a choice between the two based upon their needs and particular
   situation.

2. Requirement Level

   The "requirement level" [RFC2026] for LDP is:

      Implementation of LDP is recommended for devices that perform MPLS
      forwarding along normally routed paths as determined by
      destination-based routing protocols.

Thomas & Gray                Informational                      [Page 2]
RFC 3037                   LDP Applicability                January 2001

3. Feature Overview

   LDP associates a Forwarding Equivalence Class (FEC) [RFC3031] with
   each label it distributes.  Two LSRs which use LDP to exchange FEC-
   label binding information are known as "LDP Peers", and we speak of
   there being an "LDP Session" between them.

   LDP uses TCP for session communication.  Use of TCP ensures that
   session messages are reliably delivered, and that distributed labels
   and state information associated with LSPs need not be refreshed
   periodically.