LDP Specification
RFC 5036

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Network Working Group                                  L. Andersson, Ed.
Request for Comments: 5036                                      Acreo AB
Obsoletes: 3036                                            I. Minei, Ed.
Category: Standards Track                               Juniper Networks
                                                          B. Thomas, Ed.
                                                     Cisco Systems, Inc.
                                                            October 2007

                           LDP Specification

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   The architecture for Multiprotocol Label Switching (MPLS) is
   described in RFC 3031.  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 defines a set of such
   procedures called LDP (for Label Distribution Protocol) by which LSRs
   distribute labels to support MPLS forwarding along normally routed
   paths.

Table of Contents

   1. LDP Overview ....................................................5
      1.1. LDP Peers ..................................................6
      1.2. LDP Message Exchange .......................................6
      1.3. LDP Message Structure ......................................7
      1.4. LDP Error Handling .........................................7
      1.5. LDP Extensibility and Future Compatibility .................7
      1.6. Specification Language .....................................7
   2. LDP Operation ...................................................8
      2.1. FECs .......................................................8
      2.2. Label Spaces, Identifiers, Sessions, and Transport .........9
           2.2.1. Label Spaces ........................................9
           2.2.2. LDP Identifiers .....................................9
           2.2.3. LDP Sessions .......................................10
           2.2.4. LDP Transport ......................................10

Andersson, et al.           Standards Track                     [Page 1]
RFC 5036                   LDP Specification                October 2007

      2.3. LDP Sessions between Non-Directly Connected LSRs ..........10
      2.4. LDP Discovery .............................................11
           2.4.1. Basic Discovery Mechanism ..........................11
           2.4.2. Extended Discovery Mechanism .......................11
      2.5. Establishing and Maintaining LDP Sessions .................12
           2.5.1. LDP Session Establishment ..........................12
           2.5.2. Transport Connection Establishment .................12
           2.5.3. Session Initialization .............................14
           2.5.4. Initialization State Machine .......................16
           2.5.5. Maintaining Hello Adjacencies ......................19
           2.5.6. Maintaining LDP Sessions ...........................19
      2.6. Label Distribution and Management .........................20
           2.6.1. Label Distribution Control Mode ....................20
                  2.6.1.1. Independent Label Distribution Control ....20
                  2.6.1.2. Ordered Label Distribution Control ........20
           2.6.2. Label Retention Mode ...............................21
                  2.6.2.1. Conservative Label Retention Mode .........21
                  2.6.2.2. Liberal Label Retention Mode ..............21
           2.6.3. Label Advertisement Mode ...........................22
      2.7. LDP Identifiers and Next Hop Addresses ....................22
      2.8. Loop Detection ............................................23
           2.8.1. Label Request Message ..............................23
           2.8.2. Label Mapping Message ..............................25
           2.8.3. Discussion .........................................26
      2.9. Authenticity and Integrity of LDP Messages ................27
           2.9.1. TCP MD5 Signature Option ...........................27
           2.9.2. LDP Use of TCP MD5 Signature Option ................29
      2.10. Label Distribution for Explicitly Routed LSPs ............29
   3. Protocol Specification .........................................30
      3.1. LDP PDUs ..................................................30
      3.2. LDP Procedures ............................................31