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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.
+
+   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
+
+
+
+Thomas & Gray                Informational                      [Page 3]
+
+RFC 3037                   LDP Applicability                January 2001
+
+
+   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.
+
+
+
+
+
+
+Thomas & Gray                Informational                      [Page 4]
+
+RFC 3037                   LDP Applicability                January 2001
+
+
+      -  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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+Thomas & Gray                Informational                      [Page 5]
+
+RFC 3037                   LDP Applicability                January 2001
+
+
+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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+Thomas & Gray                Informational                      [Page 6]
+
+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
+   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Acknowledgement
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
+
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+Thomas & Gray                Informational                      [Page 7]
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