    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.

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    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.

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    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.

       LDP includes a mechanism by which an LSR can discover potential LDP
       peers.  The discovery mechanism makes it unnecessary for operators to
       explicitly configure each LSR with its LDP peers.

       When an LSR discovers another LSR it follows the LDP session setup
       procedure to establish an LDP session.  By means of this procedure
       the LSRs establish a session TCP connection and use it to negotiate
       parameters for the session, such as the label distribution method to
       be used (see below).  After the LSRs agree on the parameters, the
       session is operational and the LSRs use the TCP connection for label
       distribution.

       LDP supports two different methods for label distribution.  An LSR
       using Downstream Unsolicited distribution advertises FEC-label
       bindings to its peers when it is ready to forward packets in the FEC
       by means of MPLS.  An LSR using Downstream on Demand distribution
       provides FEC-label bindings to a peer in response to specific
       requests from the peer for a label for the FEC.

       LDP allows LSRs flexibility in strategies for retaining learned
       labels.  An LSR using liberal label retention stores all labels
       learned from peers regardless of whether it currently needs them for
       forwarding, whereas one using conservative label retention stores
       only labels for which it has immediate use and releases unneeded
       labels to the peer that advertised them.

       In addition, LDP allows flexibility in strategies for when to
       advertise FEC-label bindings.  An LSR using independent control mode
       advertises FEC-label bindings to peers whenever it sees fit, whereas
       one using ordered control advertises bindings only when it has
       previously received a label for the FEC from the FEC nexthop or it is
       an MPLS egress for the FEC.

       Downstream on Demand distribution with conservative label retention
       and ordered control is appropriate in situations where labels are a
       relatively scarce resource that must be conserved, and Downstream

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       Unsolicited distribution with liberal label retention and independent
       control is appropriate where labels are plentiful and need not be
       carefully conserved.  However, the protocol permits other
       combinations of distribution method, label retention mode and control
       mode, including hybrid variants of them.

       LDP defines a mechanism for loop detection to protect against
       forwarding loops in LSPs that traverse non-TTL MPLS clouds; see
       [RFC3031] for discussion of situations which may benefit from this
       mechanism.  The loop detection mechanism is optional in the sense
       that it may be disabled by LSR configuration.  However, an LDP-
       compliant LSR must implement it.

       LDP includes an extension mechanism which supports the development of
       vendor-private and experimental features.  This mechanism defines
       procedures for introducing new types of messages and TLVs, methods an
       LSR can use for detecting such messages and TLVs, and procedures an
       LSR must follow when it receives a message or TLV it does not
       implement.  While it is not possible to make every future enhancement
       backwards compatible, these procedures facilitate the introduction of
       new capabilities in MPLS networks that include older implementations
       that do not recognize them.

    4. Scalability Considerations

       The following factors may influence the scalability of LDP
       implementations:

          -  LDP label distribution is incremental, requiring no periodic
             refresh of FEC-label bindings.

          -  In situations were label resources may be scarce (ATM and Frame
             Relay links) the use of the Downstream on Demand distribution
             method with conservative label retention ensures that only
             those labels required to support normally-routed paths are
             allocated and distributed.

          -  In situations where label resources are not scarce, the use of
             the Downstream Unsolicited method with liberal label retention
             ensures that changes in FEC nexthop from one LDP peer to
             another require no distribution action to update previously
             distributed labels.

          -  Limitations on the number of TCP connections an LSR supports
             limit the number of LDP peers the LSR can support.

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          -  Use of the optional path vector based loop detection mechanism
             imposes additional memory and processing requirements on an
             LSR, as well as additional LDP traffic.  Both impact
             scalability.

    5. Security Considerations

       LDP defines the optional use of the TCP MD5 Signature Option to
       protect against the introduction of spoofed TCP segments into LDP
       session connection streams.  LDP use of the TCP MD5 Signature Option
       is similar to BGP [RFC1771] use of the option specified in [RFC2385].

    6. References

       [CRLDP-AS]   J. Ash, M. Girish, E. Gray, B. Jamoussi, G. Wright,
                    "Applicability Statement for CR-LDP", Work in Progress,
                    September 1999.

       [RFC1771]    Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
                    (BGP-4)", RFC 1771, March 1995.

       [RFC2026]    Bradner, S., "The Internet Standards Process -- Revision
                    3", BCP 9, RFC 2026, October 1996.

       [RFC2385]    Heffernan, A., "Protection of BGP Sessions via the TCP
                    MD5 Signature Option", RFC 2385, August 1998.

       [RFC2547]    Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547,
                    March 1999.

       [RFC3036]    Andersson, L., Doolan, P., Feldman, N., Fredette, A. and
                    B. Thomas, "LDP Specification", RFC 3036, January 2001.

       [RFC3031]    Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol
                    Label Switching Architecture", RFC 3031, January 2001.

       [RSVP-TE-AS] Awduche, D., Hannan, A. and X. Xiao, "Applicability
                    State for Extensions to RSVP for LSP-Tunnels", Work in
                    Progress, April 2000.

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    7. Authors' Addresses

       Eric Gray
       Zaffire, Inc
       2630 Orchard Parkway,
       San Jose, CA 95134-2020

       Phone:  408-894-7362
       EMail: ewgray@mindspring.com

       Bob Thomas
       Cisco Systems, Inc.
       250 Apollo Dr.
       Chelmsford, MA 01824

       Phone:  978-244-8078
       EMail: rhthomas@cisco.com

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    RFC 3037                   LDP Applicability                January 2001

    8. Full Copyright Statement

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

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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