Network Working Group B. Thomas
Request for Comments: 3037 Cisco Systems, Inc.
Category: Informational E. Gray
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 (C) The Internet Society (2001). All Rights Reserved.
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
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-
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)
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
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
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