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Versions: (RFC 5036) 00 01 02 03 04

Network Working Group                                            X. Chen
Internet-Draft                                              L. Andersson
Intended status: Standards Track                     Huawei Technologies
Expires: October 28, 2017                                     N. Leymann
                                                        Deutsche Telekom
                                                                I. Minei
                                                                  Google
                                                                 K. Raza
                                                     Cisco Systems, Inc.
                                                          April 26, 2017


                           LDP Specification
                   draft-ijln-mpls-rfc5036bis-04.txt

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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 28, 2017.







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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
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   than English.

Table of Contents

   1.  Editors notes - this section will be  removed before
       publication . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Scope of RFC5036bis work  . . . . . . . . . . . . . . . .   5
     1.2.  ToDo  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  LDP Overview  . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  LDP Peers . . . . . . . . . . . . . . . . . . . . . . . .   8
     2.2.  LDP Message Exchange  . . . . . . . . . . . . . . . . . .   8
     2.3.  LDP Message Structure . . . . . . . . . . . . . . . . . .   9
     2.4.  LDP Error Handling  . . . . . . . . . . . . . . . . . . .   9
     2.5.  LDP Extensibility and Future Compatibility  . . . . . . .   9
     2.6.  Specification Language  . . . . . . . . . . . . . . . . .   9
   3.  LDP Operation . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  FECs  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Label Spaces, Identifiers, Sessions, and Transport  . . .  11
       3.2.1.  Label Spaces  . . . . . . . . . . . . . . . . . . . .  11
       3.2.2.  LDP Identifiers . . . . . . . . . . . . . . . . . . .  11
       3.2.3.  LDP Sessions  . . . . . . . . . . . . . . . . . . . .  12
       3.2.4.  LDP Transport . . . . . . . . . . . . . . . . . . . .  12
     3.3.  LDP Sessions between Non-Directly Connected LSRs  . . . .  12



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     3.4.  LDP Discovery . . . . . . . . . . . . . . . . . . . . . .  13
       3.4.1.  Basic Discovery Mechanism . . . . . . . . . . . . . .  13
       3.4.2.  Extended Discovery Mechanism  . . . . . . . . . . . .  13
     3.5.  Establishing and Maintaining LDP Sessions . . . . . . . .  14
       3.5.1.  LDP Session Establishment . . . . . . . . . . . . . .  14
       3.5.2.  Transport Connection Establishment  . . . . . . . . .  14
       3.5.3.  Session Initialization  . . . . . . . . . . . . . . .  16
       3.5.4.  Initialization State Machine  . . . . . . . . . . . .  18
       3.5.5.  Maintaining Hello Adjacencies . . . . . . . . . . . .  20
       3.5.6.  Maintaining LDP Sessions  . . . . . . . . . . . . . .  21
     3.6.  Label Distribution and Management . . . . . . . . . . . .  21
       3.6.1.  Label Distribution Control Mode . . . . . . . . . . .  22
         3.6.1.1.  Independent Label Distribution Control  . . . . .  22
         3.6.1.2.  Ordered Label Distribution Control  . . . . . . .  22
       3.6.2.  Label Retention Mode  . . . . . . . . . . . . . . . .  23
         3.6.2.1.  Conservative Label Retention Mode . . . . . . . .  23
         3.6.2.2.  Liberal Label Retention Mode  . . . . . . . . . .  23
       3.6.3.  Label Advertisement Mode  . . . . . . . . . . . . . .  24
     3.7.  LDP Identifiers and Next Hop Addresses  . . . . . . . . .  24
     3.8.  Loop Detection  . . . . . . . . . . . . . . . . . . . . .  24
       3.8.1.  Label Request Message . . . . . . . . . . . . . . . .  25
       3.8.2.  Label Mapping Message . . . . . . . . . . . . . . . .  26
       3.8.3.  Discussion  . . . . . . . . . . . . . . . . . . . . .  28
     3.9.  Authenticity and Integrity of LDP Messages  . . . . . . .  29
       3.9.1.  TCP MD5 Signature Option  . . . . . . . . . . . . . .  29
       3.9.2.  LDP Use of TCP MD5 Signature Option . . . . . . . . .  31
   4.  Protocol Specification  . . . . . . . . . . . . . . . . . . .  31
     4.1.  LDP PDUs  . . . . . . . . . . . . . . . . . . . . . . . .  31
     4.2.  LDP Procedures  . . . . . . . . . . . . . . . . . . . . .  32
     4.3.  Type-Length-Value Encoding  . . . . . . . . . . . . . . .  33
     4.4.  TLV Encodings for Commonly Used Parameters  . . . . . . .  35
       4.4.1.  FEC TLV . . . . . . . . . . . . . . . . . . . . . . .  35
         4.4.1.1.  FEC Procedures  . . . . . . . . . . . . . . . . .  37
       4.4.2.  Label TLVs  . . . . . . . . . . . . . . . . . . . . .  37
         4.4.2.1.  Generic Label TLV . . . . . . . . . . . . . . . .  37
         4.4.2.2.  ATM Label TLV . . . . . . . . . . . . . . . . . .  38
         4.4.2.3.  Frame Relay Label TLV . . . . . . . . . . . . . .  39
       4.4.3.  Address List TLV  . . . . . . . . . . . . . . . . . .  40
       4.4.4.  Hop Count TLV . . . . . . . . . . . . . . . . . . . .  41
         4.4.4.1.  Hop Count Procedures  . . . . . . . . . . . . . .  42
       4.4.5.  Path Vector TLV . . . . . . . . . . . . . . . . . . .  43
         4.4.5.1.  Path Vector Procedures  . . . . . . . . . . . . .  44
       4.4.6.  Status TLV  . . . . . . . . . . . . . . . . . . . . .  45
     4.5.  LDP Messages  . . . . . . . . . . . . . . . . . . . . . .  47
       4.5.1.  Notification Message  . . . . . . . . . . . . . . . .  49
         4.5.1.1.  Notification Message Procedures . . . . . . . . .  50
         4.5.1.2.  Events Signaled by Notification Messages  . . . .  51
       4.5.2.  Hello Message . . . . . . . . . . . . . . . . . . . .  53



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         4.5.2.1.  Hello Message Procedures  . . . . . . . . . . . .  56
       4.5.3.  Initialization Message  . . . . . . . . . . . . . . .  57
         4.5.3.1.  Initialization Message Procedures . . . . . . . .  66
       4.5.4.  KeepAlive Message . . . . . . . . . . . . . . . . . .  67
         4.5.4.1.  KeepAlive Message Procedures  . . . . . . . . . .  67
       4.5.5.  Address Message . . . . . . . . . . . . . . . . . . .  67
         4.5.5.1.  Address Message Procedures  . . . . . . . . . . .  68
       4.5.6.  Address Withdraw Message  . . . . . . . . . . . . . .  69
         4.5.6.1.  Address Withdraw Message Procedures . . . . . . .  70
       4.5.7.  Label Mapping Message . . . . . . . . . . . . . . . .  70
         4.5.7.1.  Label Mapping Message Procedures  . . . . . . . .  71
       4.5.8.  Label Request Message . . . . . . . . . . . . . . . .  74
         4.5.8.1.  Label Request Message Procedures  . . . . . . . .  75
       4.5.9.  Label Abort Request Message . . . . . . . . . . . . .  77
         4.5.9.1.  Label Abort Request Message Procedures  . . . . .  77
       4.5.10. Label Withdraw Message  . . . . . . . . . . . . . . .  79
         4.5.10.1.  Label Withdraw Message Procedures  . . . . . . .  80
       4.5.11. Label Release Message . . . . . . . . . . . . . . . .  81
         4.5.11.1.  Label Release Message Procedures . . . . . . . .  82
     4.6.  Messages and TLVs for Extensibility . . . . . . . . . . .  82
       4.6.1.  LDP Vendor-Private Extensions . . . . . . . . . . . .  83
         4.6.1.1.  LDP Vendor-Private TLVs . . . . . . . . . . . . .  83
         4.6.1.2.  LDP Vendor-Private Messages . . . . . . . . . . .  84
       4.6.2.  LDP Experimental Extensions . . . . . . . . . . . . .  86
     4.7.  Message Summary . . . . . . . . . . . . . . . . . . . . .  86
     4.8.  TLV Summary . . . . . . . . . . . . . . . . . . . . . . .  87
     4.9.  Status Code Summary . . . . . . . . . . . . . . . . . . .  89
     4.10. Well-Known Numbers  . . . . . . . . . . . . . . . . . . .  90
       4.10.1.  UDP and TCP Ports  . . . . . . . . . . . . . . . . .  90
       4.10.2.  Implicit NULL Label  . . . . . . . . . . . . . . . .  90
   5.  RFC 5036 IANA Considerations  . . . . . . . . . . . . . . . .  90
     5.1.  Message Type Name Space . . . . . . . . . . . . . . . . .  91
     5.2.  TLV Type Name Space . . . . . . . . . . . . . . . . . . .  92
     5.3.  FEC Type Name Space . . . . . . . . . . . . . . . . . . .  92
     5.4.  Status Code Name Space  . . . . . . . . . . . . . . . . .  92
     5.5.  Experiment ID Name Space  . . . . . . . . . . . . . . . .  93
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  93
     6.1.  Spoofing  . . . . . . . . . . . . . . . . . . . . . . . .  93
     6.2.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  94
     6.3.  Denial of Service . . . . . . . . . . . . . . . . . . . .  95
   7.  Areas for Future Study  . . . . . . . . . . . . . . . . . . .  96
   8.  Changes from RFC 5036 . . . . . . . . . . . . . . . . . . . .  97
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  97
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  98
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  98
     10.2.  Informative References . . . . . . . . . . . . . . . . . 100
   Appendix A.  LDP Label Distribution Procedures  . . . . . . . . . 101
     A.1.  Handling Label Distribution Events  . . . . . . . . . . . 104



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       A.1.1.  Receive Label Request . . . . . . . . . . . . . . . . 104
       A.1.2.  Receive Label Mapping . . . . . . . . . . . . . . . . 108
       A.1.3.  Receive Label Abort Request . . . . . . . . . . . . . 114
       A.1.4.  Receive Label Release . . . . . . . . . . . . . . . . 115
       A.1.5.  Receive Label Withdraw  . . . . . . . . . . . . . . . 117
       A.1.6.  Recognize New FEC . . . . . . . . . . . . . . . . . . 119
       A.1.7.  Detect Change in FEC Next Hop . . . . . . . . . . . . 122
       A.1.8.  Receive Notification / Label Request Aborted  . . . . 124
       A.1.9.  Receive Notification / No Label Resources . . . . . . 125
       A.1.10. Receive Notification / No Route . . . . . . . . . . . 126
       A.1.11. Receive Notification / Loop Detected  . . . . . . . . 127
       A.1.12. Receive Notification / Label Resources Available  . . 127
       A.1.13. Detect Local Label Resources Have Become Available  . 128
       A.1.14. LSR Decides to No Longer Label Switch a FEC . . . . . 129
       A.1.15. Timeout of Deferred Label Request . . . . . . . . . . 130
     A.2.  Common Label Distribution Procedures  . . . . . . . . . . 130
       A.2.1.  Send_Label  . . . . . . . . . . . . . . . . . . . . . 130
       A.2.2.  Send_Label_Request  . . . . . . . . . . . . . . . . . 132
       A.2.3.  Send_Label_Withdraw . . . . . . . . . . . . . . . . . 133
       A.2.4.  Send_Notification . . . . . . . . . . . . . . . . . . 133
       A.2.5.  Send_Message  . . . . . . . . . . . . . . . . . . . . 134
       A.2.6.  Check_Received_Attributes . . . . . . . . . . . . . . 134
       A.2.7.  Prepare_Label_Request_Attributes  . . . . . . . . . . 136
       A.2.8.  Prepare_Label_Mapping_Attributes  . . . . . . . . . . 137
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 140
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 141

1.  Editors notes - this section will be removed before publication

   This entire section will be removed before publication.

1.1.  Scope of RFC5036bis work

   The goal of this document is to take the LDP specification to
   Internet Standard.

   Currently RFC 5036 - the LDP Specification - is a Draft Standard;
   this step on the Standards Track has been removed.  It is therefore
   the plan to move the document to Internet Standard.

   Thhis document includes updates to the LDP Specification defined
   since the document was published, including:

   1.  Updates all references and citations.

       RFC 5036 obsoleted RFC 3036, and will in turn be obsoleted by the
       RFC5036-bis-to-be.




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   2.  RFC 5036 is updated by RFC 6720, RFC 6790, RFC 7358, RFC 7552.

   3.  Incorporate all outstanding Errata.

       These include the following approved Erratum with IDs: 3156,
       2133, 3155, 3415, 3425.

       [Ed Note: Done in rev -01]

   4.  Close loops with Stephen on the security section.

   5.  Have IANA review the IANA section.

1.2.  ToDo

   1.  Evaluation of which of the RFCs that updated RFC 5036 need to be
       incorporated into the rfc5036bis document.  Specifically, these
       RFCs updated RFC 5036: RFC 6720, RFC 6790, RFC 7358, RFC 7552.
       RFCs that updated RFC 5036 and will be incorporated into this
       rfc5036bis, will be Obsoleted by rfc5036bis.

   2.  Review IANA Allocations.  Review the IANA sections (there are
       currently two) and merge them into one.

   3.  Evaluate if there are things, based on e.g. non-deployment, that
       should be removed from the rfc5036bis-to-be.

   4.  Evaluate what to do about RFC 7349, it is not listed as an update
       of LDP, but it is an extension of the LDP security mechanisms
       (Hello crypto).

   5.  RFC 2385 (The TCP Authentication Option) has been obsoleted by
       RFC 5925, the changes are such that the text we refer to is no
       longer there.  We probably need a minor re-write of section
       2.9.1.

   6.  This document now carries a pre RFC 5378 copyright statement,
       since there clearly are material included in the document prior
       to RFC 5378 (10 Nov 2008), Verify that this is the right
       approach.

   7.  Revisit section 4.5.3 "Initialization Message" if we decide to
       remove ATM and FR.

   8.  Check if how to forward messages relating path-vector and hop-
       count from multiple downstreams needs to be specified/clarified.





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2.  LDP Overview

   The MPLS architecture RFC 3031 [RFC3031] defines a label distribution
   protocol as a set of procedures by which one Label Switched Router
   (LSR) informs another of the meaning of labels used to forward
   traffic between and through them.

   The MPLS architecture does not assume a single label distribution
   protocol.  In fact, a number of different label distribution
   protocols are being standardized.  Existing protocols have been
   extended so that label distribution can be piggybacked on them.  New
   protocols have also been defined for the explicit purpose of
   distributing labels.  The MPLS architecture discusses some of the
   considerations when choosing a label distribution protocol for use in
   particular MPLS applications such as Traffic Engineering RFC 2702
   [RFC2702].

   The Label Distribution Protocol (LDP) is a protocol defined for
   distributing labels.  It was originally published as RFC 3036 in
   January 2001.  It was produced by the MPLS Working Group of the IETF
   and was jointly authored by Loa Andersson, Paul Doolan, Nancy
   Feldman, Andre Fredette, and Bob Thomas.

   LDP is a protocol defined for distributing labels.  It is the set of
   procedures and messages by which Label Switched Routers (LSRs)
   establish Label Switched Paths (LSPs) through a network by mapping
   network-layer routing information directly to data-link layer
   switched paths.  These LSPs may have an endpoint at a directly
   attached neighbor (comparable to IP hop-by-hop forwarding), or may
   have an endpoint at a network egress node, enabling switching via all
   intermediary nodes.

   LDP associates a Forwarding Equivalence Class (FEC) RFC 3031
   [RFC3031] with each LSP it creates.  The FEC associated with an LSP
   specifies which packets are "mapped" to that LSP.  LSPs are extended
   through a network as each LSR "splices" incoming labels for a FEC to
   the outgoing label assigned to the next hop for the given FEC.

   More information about the applicability of LDP can be found in RFC
   3037 [RFC3037].

   This document assumes (but does not require) familiarity with the
   MPLS architecture RFC 3031 [RFC3031].  Note that RFC 3031 [RFC3031]
   includes a glossary of MPLS terminology, such as ingress, label
   switched path, etc.






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2.1.  LDP Peers

   Two LSRs that use LDP to exchange label/FEC mapping information are
   known as "LDP Peers" with respect to that information, and we speak
   of there being an "LDP Session" between them.  A single LDP session
   allows each peer to learn the other's label mappings; i.e., the
   protocol is bidirectional.

2.2.  LDP Message Exchange

   There are four categories of LDP messages:

   1.  Discovery messages, used to announce and maintain the presence of
       an LSR in a network.

   2.  Session messages, used to establish, maintain, and terminate
       sessions between LDP peers.

   3.  Advertisement messages, used to create, change, and delete label
       mappings for FECs.

   4.  Notification messages, used to provide advisory information and
       to signal error information.

   Discovery messages provide a mechanism whereby LSRs indicate their
   presence in a network by sending a Hello message periodically.  This
   is transmitted as a UDP packet to the LDP port at the 'all routers on
   this subnet' group multicast address.  When an LSR chooses to
   establish a session with another LSR learned via the Hello message,
   it uses the LDP initialization procedure over TCP transport.  Upon
   successful completion of the initialization procedure, the two LSRs
   are LDP peers, and may exchange advertisement messages.

   When to request a label or advertise a label mapping to a peer is
   largely a local decision made by an LSR.  In general, the LSR
   requests a label mapping from a neighboring LSR when it needs one,
   and advertises a label mapping to a neighboring LSR when it wishes
   the neighbor to use a label.

   Correct operation of LDP requires reliable and in-order delivery of
   messages.  To satisfy these requirements, LDP uses the TCP transport
   for Session, Advertisement, and Notification messages, i.e., for
   everything but the UDP-based discovery mechanism.








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2.3.  LDP Message Structure

   All LDP messages have a common structure that uses a Type-Length-
   Value (TLV) encoding scheme; see Section "Type-Length-Value
   Encoding".  The Value part of a TLV-encoded object, or TLV for short,
   may itself contain one or more TLVs.

2.4.  LDP Error Handling

   LDP errors and other events of interest are signaled to an LDP peer
   by Notification messages.

   There are two kinds of LDP Notification messages:

   1.  Error Notifications, used to signal fatal errors.  If an LSR
       receives an Error Notification from a peer for an LDP session, it
       terminates the LDP session by closing the TCP transport
       connection for the session and discarding all label mappings
       learned via the session.

   2.  Advisory Notifications, used to pass on LSR information about the
       LDP session or the status of some previous message received from
       the peer.

2.5.  LDP Extensibility and Future Compatibility

   Functionality may be added to LDP in the future.  It is likely that
   future functionality will utilize new messages and object types
   (TLVs).  It may be desirable to employ such new messages and TLVs
   within a network using older implementations that do not recognize
   them.  While it is not possible to make every future enhancement
   backwards compatible, some prior planning can ease the introduction
   of new capabilities.  This specification defines rules for handling
   unknown message types and unknown TLVs for this purpose.

2.6.  Specification Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  LDP Operation

3.1.  FECs

   It is necessary to precisely specify which packets may be mapped to
   each LSP.  This is done by providing a FEC specification for each




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   LSP.  The FEC identifies the set of IP packets that may be mapped to
   that LSP.

   Each FEC is specified as a set of one or more FEC elements.  Each FEC
   element identifies a set of packets that may be mapped to the
   corresponding LSP.  When an LSP is shared by multiple FEC elements,
   that LSP is terminated at (or before) the node where the FEC elements
   can no longer share the same path.

   This specification defines a single type of FEC element, the "Address
   Prefix FEC element".  This element is an address prefix of any length
   from 0 to a full address, inclusive.

   Additional FEC elements may be defined, as needed, by other
   specifications.

   In the remainder of this section, we give the rules to be used for
   mapping packets to LSPs that have been set up using an Address Prefix
   FEC element.

   We say that a particular address "matches" a particular address
   prefix if and only if that address begins with that prefix.  We also
   say that a particular packet matches a particular LSP if and only if
   that LSP has an Address Prefix FEC element that matches the packet's
   destination address.  With respect to a particular packet and a
   particular LSP, we refer to any Address Prefix FEC element that
   matches the packet as the "matching prefix".

   The procedure for mapping a particular packet to a particular LSP
   uses the following rules.  Each rule is applied in turn until the
   packet can be mapped to an LSP.

   -  If a packet matches exactly one LSP, the packet is mapped to that
      LSP.

   -  If a packet matches multiple LSPs, it is mapped to the LSP whose
      matching prefix is the longest.  If there is no one LSP whose
      matching prefix is longest, the packet is mapped to one from the
      set of LSPs whose matching prefix is longer than the others.  The
      procedure for selecting one of those LSPs is beyond the scope of
      this document.

   -  If it is known that a packet must traverse a particular egress
      router, and there is an LSP that has an Address Prefix FEC element
      that is a /32 address of that router, then the packet is mapped to
      that LSP.  The procedure for obtaining this knowledge is beyond
      the scope of this document.




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   The procedure for determining that a packet must traverse a
   particular egress router is beyond the scope of this document.  (As
   an example, if one is running a link state routing algorithm, it may
   be possible to obtain this information from the link state data base.
   As another example, if one is running BGP, it may be possible to
   obtain this information from the BGP next hop attribute of the
   packet's route.)

3.2.  Label Spaces, Identifiers, Sessions, and Transport

3.2.1.  Label Spaces

   The notion of "label space" is useful for discussing the assignment
   and distribution of labels.  There are two types of label spaces:

   -  Per interface label space.  Interface-specific incoming labels are
      used for interfaces that use interface resources for labels.  An
      example of such an interface is a label-controlled ATM interface
      that uses VCIs (Virtual Channel Identifiers) as labels, or a Frame
      Relay interface that uses DLCIs (Data Link Connection Identifiers)
      as labels.

      Note that the use of a per interface label space only makes sense
      when the LDP peers are "directly connected" over an interface, and
      the label is only going to be used for traffic sent over that
      interface.

   -  Per platform label space.  Platform-wide incoming labels are used
      for interfaces that can share the same labels.

3.2.2.  LDP Identifiers

   An LDP Identifier is a six octet quantity used to identify an LSR
   label space.  The first four octets identify the LSR and must be a
   globally unique value, such as a 32-bit router Id assigned to the
   LSR.  The last two octets identify a specific label space within the
   LSR.  The last two octets of LDP Identifiers for platform-wide label
   spaces are always both zero.  This document uses the following print
   representation for LDP Identifiers:

      <LSR Id> : <label space id>

   e.g., lsr171:0, lsr19:2.

   Note that an LSR that manages and advertises multiple label spaces
   uses a different LDP Identifier for each such label space.





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   A situation where an LSR would need to advertise more than one label
   space to a peer and hence use more than one LDP Identifier occurs
   when the LSR has two links to the peer and both are ATM (and use per
   interface labels).  Another situation would be where the LSR had two
   links to the peer, one of which is ethernet (and uses per platform
   labels) and the other of which is ATM.

3.2.3.  LDP Sessions

   LDP sessions exist between LSRs to support label exchange between
   them.

      When an LSR uses LDP to advertise more than one label space to
      another LSR, it uses a separate LDP session for each label space.

3.2.4.  LDP Transport

   LDP uses TCP as a reliable transport for sessions.

      When multiple LDP sessions are required between two LSRs, there is
      one TCP session for each LDP session.

3.3.  LDP Sessions between Non-Directly Connected LSRs

   LDP sessions between LSRs that are not directly connected at the link
   level may be desirable in some situations.

   For example, consider a "traffic engineering" application where LSRa
   sends traffic matching some criteria via an LSP to non-directly
   connected LSRb rather than forwarding the traffic along its normally
   routed path.

   The path between LSRa and LSRb would include one or more intermediate
   LSRs (LSR1,...LSRn).  An LDP session between LSRa and LSRb would
   enable LSRb to label switch traffic arriving on the LSP from LSRa by
   providing LSRb means to advertise labels for this purpose to LSRa.

   In this situation, LSRa would apply two labels to traffic it forwards
   on the LSP to LSRb: a label learned from LSR1 to forward traffic
   along the LSP path from LSRa to LSRb; and a label learned from LSRb
   to enable LSRb to label switch traffic arriving on the LSP.

   LSRa first adds the label learned via its LDP session with LSRb to
   the packet label stack (either by replacing the label on top of the
   packet label stack with it if the packet arrives labeled or by
   pushing it if the packet arrives unlabeled).  Next, it pushes the
   label for the LSP learned from LSR1 onto the label stack.




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3.4.  LDP Discovery

   LDP discovery is a mechanism that enables an LSR to discover
   potential LDP peers.  Discovery makes it unnecessary to explicitly
   configure an LSR's label switching peers.

   There are two variants of the discovery mechanism:

   -  A Basic Discovery mechanism used to discover LSR neighbors that
      are directly connected at the link level.

   -  An Extended Discovery mechanism used to locate LSRs that are not
      directly connected at the link level.

3.4.1.  Basic Discovery Mechanism

   To engage in LDP Basic Discovery on an interface, an LSR periodically
   sends LDP Link Hellos out the interface.  LDP Link Hellos are sent as
   UDP packets addressed to the well-known LDP discovery port for the
   "all routers on this subnet" group multicast address.

   An LDP Link Hello sent by an LSR carries the LDP Identifier for the
   label space the LSR intends to use for the interface and possibly
   additional information.

   Receipt of an LDP Link Hello on an interface identifies a "Hello
   adjacency" with a potential LDP peer reachable at the link level on
   the interface as well as the label space the peer intends to use for
   the interface.

3.4.2.  Extended Discovery Mechanism

   LDP sessions between non-directly connected LSRs are supported by LDP
   Extended Discovery.

   To engage in LDP Extended Discovery, an LSR periodically sends LDP
   Targeted Hellos to a specific address.  LDP Targeted Hellos are sent
   as UDP packets addressed to the well-known LDP discovery port at the
   specific address.

   An LDP Targeted Hello sent by an LSR carries the LDP Identifier for
   the label space the LSR intends to use and possibly additional
   optional information.

   Extended Discovery differs from Basic Discovery in the following
   ways:





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   -  A Targeted Hello is sent to a specific address rather than to the
      "all routers" group multicast address for the outgoing interface.

   -  Unlike Basic Discovery, which is symmetric, Extended Discovery is
      asymmetric.

      One LSR initiates Extended Discovery with another targeted LSR,
      and the targeted LSR decides whether to respond to or ignore the
      Targeted Hello.  A targeted LSR that chooses to respond does so by
      periodically sending Targeted Hellos to the initiating LSR.

   Receipt of an LDP Targeted Hello identifies a "Hello adjacency" with
   a potential LDP peer reachable at the network level and the label
   space the peer intends to use.

3.5.  Establishing and Maintaining LDP Sessions

3.5.1.  LDP Session Establishment

   The exchange of LDP Discovery Hellos between two LSRs triggers LDP
   session establishment.  Session establishment is a two step process:

   -  Transport connection establishment

   -  Session initialization

   The following describes establishment of an LDP session between LSRs
   LSR1 and LSR2 from LSR1's point of view.  It assumes the exchange of
   Hellos specifying label space LSR1:a for LSR1 and label space LSR2:b
   for LSR2.

3.5.2.  Transport Connection Establishment

   The exchange of Hellos results in the creation of a Hello adjacency
   at LSR1 that serves to bind the link (L) and the label spaces LSR1:a
   and LSR2:b.

   1.  If LSR1 does not already have an LDP session for the exchange of
       label spaces LSR1:a and LSR2:b, it attempts to open a TCP
       connection for a new LDP session with LSR2.

       LSR1 determines the transport addresses to be used at its end
       (A1) and LSR2's end (A2) of the LDP TCP connection.  Address A1
       is determined as follows:

       a.  If LSR1 uses the Transport Address optional object (TLV) in
           Hellos it sends to LSR2 to advertise an address, A1 is the
           address LSR1 advertises via the optional object;



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       b.  If LSR1 does not use the Transport Address optional object,
           A1 is the source address used in Hellos it sends to LSR2.

       Similarly, address A2 is determined as follows:

       a.  If LSR2 uses the Transport Address optional object, A2 is the
           address LSR2 advertises via the optional object;

       b.  If LSR2 does not use the Transport Address optional object,
           A2 is the source address in Hellos received from LSR2.

   2.  LSR1 determines whether it will play the active or passive role
       in session establishment by comparing addresses A1 and A2 as
       unsigned integers.  If A1 > A2, LSR1 plays the active role;
       otherwise, it is passive.


       The procedure for comparing A1 and A2 as unsigned integers is:

       -  If A1 and A2 are not in the same address family, they are
          incomparable, and no session can be established.

       -  Let U1 be the abstract unsigned integer obtained by treating
          A1 as a sequence of bytes, where the byte that appears
          earliest in the message is the most significant byte of the
          integer and the byte that appears latest in the message is the
          least significant byte of the integer.

       -  Let U2 be the abstract unsigned integer obtained from A2 in a
          similar manner.

       -  Compare U1 with U2.  If U1 > U2, then A1 > A2; if U1 < U2,
          then A1 < A2.

   3.  If LSR1 is active, it attempts to establish the LDP TCP
       connection by connecting to the well-known LDP port at address
       A2.  If LSR1 is passive, it waits for LSR2 to establish the LDP
       TCP connection to its well-known LDP port.

   Note that when an LSR sends a Hello, it selects the transport address
   for its end of the session connection and uses the Hello to advertise
   the address, either explicitly by including it in an optional
   Transport Address TLV or implicitly by omitting the TLV and using it
   as the Hello source address.

   An LSR MUST advertise the same transport address in all Hellos that
   advertise the same label space.  This requirement ensures that two




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   LSRs linked by multiple Hello adjacencies using the same label spaces
   play the same connection establishment role for each adjacency.

3.5.3.  Session Initialization

   After LSR1 and LSR2 establish a transport connection, they negotiate
   session parameters by exchanging LDP Initialization messages.  The
   parameters negotiated include LDP protocol version, label
   distribution method, timer values, VPI/VCI (Virtual Path Identifier /
   Virtual Channel Identifier) ranges for label controlled ATM, DLCI
   (Data Link Connection Identifier) ranges for label controlled Frame
   Relay, etc.

   Successful negotiation completes establishment of an LDP session
   between LSR1 and LSR2 for the advertisement of label spaces LSR1:a
   and LSR2:b.

   The following describes the session initialization from LSR1's point
   of view.

   After the connection is established, if LSR1 is playing the active
   role, it initiates negotiation of session parameters by sending an
   Initialization message to LSR2.  If LSR1 is passive, it waits for
   LSR2 to initiate the parameter negotiation.

   In general when there are multiple links between LSR1 and LSR2 and
   multiple label spaces to be advertised by each, the passive LSR
   cannot know which label space to advertise over a newly established
   TCP connection until it receives the LDP Initialization message on
   the connection.  The Initialization message carries both the LDP
   Identifier for the sender's (active LSR's) label space and the LDP
   Identifier for the receiver's (passive LSR's) label space.

   By waiting for the Initialization message from its peer, the passive
   LSR can match the label space to be advertised by the peer (as
   determined from the LDP Identifier in the PDU header for the
   Initialization message) with a Hello adjacency previously created
   when Hellos were exchanged.

   1.  When LSR1 plays the passive role:

       a.  If LSR1 receives an Initialization message, it attempts to
           match the LDP Identifier carried by the message PDU with a
           Hello adjacency.

       b.  If there is a matching Hello adjacency, the adjacency
           specifies the local label space for the session.




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           Next LSR1 checks whether the session parameters proposed in
           the message are acceptable.  If they are, LSR1 replies with
           an Initialization message of its own to propose the
           parameters it wishes to use and a KeepAlive message to signal
           acceptance of LSR2's parameters.  If the parameters are not
           acceptable, LSR1 responds by sending a Session Rejected/
           Parameters Error Notification message and closing the TCP
           connection.

       c.  If LSR1 cannot find a matching Hello adjacency, it sends a
           Session Rejected/No Hello Error Notification message and
           closes the TCP connection.

       d.  If LSR1 receives a KeepAlive in response to its
           Initialization message, the session is operational from
           LSR1's point of view.

       e.  If LSR1 receives an Error Notification message, LSR2 has
           rejected its proposed session and LSR1 closes the TCP
           connection.

   2.  When LSR1 plays the active role:

       a.  If LSR1 receives an Error Notification message, LSR2 has
           rejected its proposed session and LSR1 closes the TCP
           connection.

       b.  If LSR1 receives an Initialization message, it checks whether
           the session parameters are acceptable.  If so, it replies
           with a KeepAlive message.  If the session parameters are
           unacceptable, LSR1 sends a Session Rejected/Parameters Error
           Notification message and closes the connection.

       c.  If LSR1 receives a KeepAlive message, LSR2 has accepted its
           proposed session parameters.

       d.  When LSR1 has received both an acceptable Initialization
           message and a KeepAlive message, the session is operational
           from LSR1's point of view.

           Until the LDP session is established, no other messages
           except those listed in the procedures above may be exchanged,
           and the rules for processing the U-bit in LDP messages are
           overridden.  If a message other than those listed in the
           procedures above is received, a Shutdown msg MUST be
           transmitted and the transport connection MUST be closed.





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   It is possible for a pair of incompatibly configured LSRs that
   disagree on session parameters to engage in an endless sequence of
   messages as each NAKs the other's Initialization messages with Error
   Notification messages.

   An LSR MUST throttle its session setup retry attempts with an
   exponential backoff in situations where Initialization messages are
   being NAK'd.  It is also recommended that an LSR detecting such a
   situation take action to notify an operator.

   The session establishment setup attempt following a NAK'd
   Initialization message MUST be delayed no less than 15 seconds, and
   subsequent delays MUST grow to a maximum delay of no less than 2
   minutes.  The specific session establishment action that must be
   delayed is the attempt to open the session transport connection by
   the LSR playing the active role.

   The throttled sequence of Initialization NAKs is unlikely to cease
   until operator intervention reconfigures one of the LSRs.  After such
   a configuration action, there is no further need to throttle
   subsequent session establishment attempts (until their Initialization
   messages are NAK'd).

   Due to the asymmetric nature of session establishment,
   reconfiguration of the passive LSR will go unnoticed by the active
   LSR without some further action.  Section "Hello Message" describes
   an optional mechanism an LSR can use to signal potential LDP peers
   that it has been reconfigured.

3.5.4.  Initialization State Machine

   It is convenient to describe LDP session negotiation behavior in
   terms of a state machine.  We define the LDP state machine to have
   five possible states and present the behavior as a state transition
   table and as a state transition diagram.  Note that a Shutdown
   message is implemented as a Notification message with a Status TLV
   indicating a fatal error.


                  Session Initialization State Transition Table

      STATE         EVENT                               NEW STATE

      NON EXISTENT  Session TCP connection established  INITIALIZED

      INITIALIZED   Transmit Initialization msg         OPENSENT
                          (Active Role)




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                    Receive acceptable                  OPENREC
                          Initialization msg
                          (Passive Role)
                      Action: Transmit Initialization
                              msg and KeepAlive msg

                    Receive Any other LDP msg           NON EXISTENT
                      Action: Transmit Error Notification msg
                              (NAK) and close transport connection

      OPENREC       Receive KeepAlive msg               OPERATIONAL

                    Receive Any other LDP msg           NON EXISTENT
                      Action: Transmit Error Notification msg
                              (NAK) and close transport connection

      OPENSENT      Receive acceptable                  OPENREC
                          Initialization msg
                      Action: Transmit KeepAlive msg

                    Receive Any other LDP msg           NON EXISTENT
                      Action: Transmit Error Notification msg
                              (NAK) and close transport connection

      OPERATIONAL   Receive Shutdown msg                NON EXISTENT
                      Action: Transmit Shutdown msg and
                              close transport connection

                    Receive other LDP msgs              OPERATIONAL
                    Timeout                             NON EXISTENT
                      Action: Transmit Shutdown msg and
                              close transport connection


                 Session Initialization State Transition Diagram

                                 +------------+
                                 |            |
                   +------------>|NON EXISTENT|<--------------------+
                   |             |            |                     |
                   |             +------------+                     |
                   | Session        |    ^                          |
                   |   connection   |    |                          |
                   |   established  |    | Rx any LDP msg except    |
                   |                V    |   Init msg or Timeout    |
                   |            +-----------+                       |
      Rx Any other |            |           |                       |
         msg or    |            |INITIALIZED|                       |



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         Timeout / |        +---|           |-+                     |
      Tx NAK msg   |        |   +-----------+ |                     |
                   |        | (Passive Role)  | (Active Role)       |
                   |        | Rx Acceptable   | Tx Init msg         |
                   |        |    Init msg /   |                     |
                   |        | Tx Init msg     |                     |
                   |        |    Tx KeepAlive |                     |
                   |        V    msg          V                     |
                   |   +-------+        +--------+                  |
                   |   |       |        |        |                  |
                   +---|OPENREC|        |OPENSENT|----------------->|
                   +---|       |        |        | Rx Any other msg |
                   |   +-------+        +--------+    or Timeout    |
      Rx KeepAlive |        ^                |     Tx NAK msg       |
         msg       |        |                |                      |
                   |        |                | Rx Acceptable        |
                   |        |                |    Init msg /        |
                   |        +----------------+ Tx KeepAlive msg     |
                   |                                                |
                   |      +-----------+                             |
                   +----->|           |                             |
                          |OPERATIONAL|                             |
                          |           |---------------------------->+
                          +-----------+     Rx Shutdown msg
                   All other  |   ^            or Timeout /
                     LDP msgs |   |         Tx Shutdown msg
                              |   |
                              +---+


                        Figure 1: LDP State Machine

3.5.5.  Maintaining Hello Adjacencies

   An LDP session with a peer has one or more Hello adjacencies.

   An LDP session has multiple Hello adjacencies when a pair of LSRs is
   connected by multiple links that share the same label space; for
   example, multiple PPP links between a pair of routers.  In this
   situation, the Hellos an LSR sends on each such link carry the same
   LDP Identifier.

   LDP includes mechanisms to monitor the necessity of an LDP session
   and its Hello adjacencies.

   LDP uses the regular receipt of LDP Discovery Hellos to indicate a
   peer's intent to use the label space identified by the Hello.  An LSR
   maintains a hold timer with each Hello adjacency that it restarts



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   when it receives a Hello that matches the adjacency.  If the timer
   expires without receipt of a matching Hello from the peer, LDP
   concludes that the peer no longer wishes to label switch using that
   label space for that link (or target, in the case of Targeted Hellos)
   or that the peer has failed.  The LSR then deletes the Hello
   adjacency.  When the last Hello adjacency for an LDP session is
   deleted, the LSR terminates the LDP session by sending a Notification
   message and closing the transport connection.

3.5.6.  Maintaining LDP Sessions

   LDP includes mechanisms to monitor the integrity of the LDP session.

   LDP uses the regular receipt of LDP PDUs on the session transport
   connection to monitor the integrity of the session.  An LSR maintains
   a KeepAlive Timer for each peer session that it resets whenever it
   receives an LDP PDU from the session peer.  If the KeepAlive Timer
   expires without receipt of an LDP PDU from the peer, the LSR
   concludes that the transport connection is bad or that the peer has
   failed, and it terminates the LDP session by closing the transport
   connection.

   After an LDP session has been established, an LSR must arrange that
   its peer receive an LDP PDU from it at least every KeepAlive time
   period to ensure the peer restarts the session KeepAlive Timer.  The
   LSR may send any protocol message to meet this requirement.  In
   circumstances where an LSR has no other information to communicate to
   its peer, it sends a KeepAlive message.

   An LSR may choose to terminate an LDP session with a peer at any
   time.  Should it choose to do so, it informs the peer with a Shutdown
   message.

3.6.  Label Distribution and Management

   The MPLS architecture RFC 3031 [RFC3031] allows an LSR to distribute
   a FEC label binding in response to an explicit request from another
   LSR.  This is known as Downstream On Demand label distribution.  It
   also allows an LSR to distribute label bindings to LSRs that have not
   explicitly requested them.  RFC 3031 [RFC3031] calls this method of
   label distribution Unsolicited Downstream; this document uses the
   term Downstream Unsolicited.

   Both of these label distribution techniques may be used in the same
   network at the same time.  However, for any given LDP session, each
   LSR must be aware of the label distribution method used by its peer
   in order to avoid situations where one peer using Downstream




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   Unsolicited label distribution assumes its peer is also.  See
   Section "Downstream on Demand Label Advertisement".

3.6.1.  Label Distribution Control Mode

   The behavior of the initial setup of LSPs is determined by whether
   the LSR is operating with independent or Ordered LSP Control.  An LSR
   may support both types of control as a configurable option.

3.6.1.1.  Independent Label Distribution Control

   When using independent LSP control, each LSR may advertise label
   mappings to its neighbors at any time it desires.  For example, when
   operating in independent Downstream on Demand mode, an LSR may answer
   requests for label mappings immediately, without waiting for a label
   mapping from the next hop.  When operating in independent Downstream
   Unsolicited mode, an LSR may advertise a label mapping for a FEC to
   its neighbors whenever it is prepared to label-switch that FEC.

   A consequence of using independent mode is that an upstream label can
   be advertised before a downstream label is received.

3.6.1.2.  Ordered Label Distribution Control

   When using LSP Ordered Control, an LSR may initiate the transmission
   of a label mapping only for a FEC for which it has a label mapping
   for the FEC next hop, or for which the LSR is the egress.  For each
   FEC for which the LSR is not the egress and no mapping exists, the
   LSR MUST wait until a label from a downstream LSR is received before
   mapping the FEC and passing corresponding labels to upstream LSRs.
   An LSR may be an egress for some FECs and a non-egress for others.

   An LSR may act as an egress LSR, with respect to a particular FEC,
   under any of the following conditions:

   1.  The FEC refers to the LSR itself (including one of its directly
       attached interfaces).

   2.  The next hop router for the FEC is outside of the Label Switching
       Network.

   3.  FEC elements are reachable by crossing a routing domain boundary,
       such as another area for OSPF summary networks, or another
       autonomous system for OSPF AS externals and BGP routes RFC 2328
       [RFC2328] and RFC 4271 [RFC4271].






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   Note that whether an LSR is an egress for a given FEC may change over
   time, depending on the state of the network and LSR configuration
   settings.

3.6.2.  Label Retention Mode

   The MPLS architecture RFC 3031 [RFC3031] introduces the notion of
   label retention mode which specifies whether an LSR maintains a label
   binding for a FEC learned from a neighbor that is not its next hop
   for the FEC.

3.6.2.1.  Conservative Label Retention Mode

   In Downstream Unsolicited advertisement mode, label mapping
   advertisements for all routes may be received from all peer LSRs.
   When using Conservative Label retention, advertised label mappings
   are retained only if they will be used to forward packets (i.e., if
   they are received from a valid next hop according to routing).  If
   operating in Downstream on Demand mode, an LSR will request label
   mappings only from the next hop LSR according to routing.  Since
   Downstream on Demand mode is primarily used when label conservation
   is desired (e.g., an ATM switch with limited cross connect space), it
   is typically used with the Conservative Label retention mode.

   The main advantage of the conservative mode is that only the labels
   that are required for the forwarding of data are allocated and
   maintained.  This is particularly important in LSRs where the label
   space is inherently limited, such as in an ATM switch.  A
   disadvantage of the conservative mode is that if routing changes the
   next hop for a given destination, a new label must be obtained from
   the new next hop before labeled packets can be forwarded.

3.6.2.2.  Liberal Label Retention Mode

   In Downstream Unsolicited advertisement mode, label mapping
   advertisements for all routes may be received from all LDP peers.
   When using Liberal Label retention, every label mappings received
   from a peer LSR is retained regardless of whether the LSR is the next
   hop for the advertised mapping.  When operating in Downstream on
   Demand mode with Liberal Label retention, an LSR might choose to
   request label mappings for all known prefixes from all peer LSRs.
   Note, however, that Downstream on Demand mode is typically used by
   devices such as ATM switch-based LSRs for which the conservative
   approach is recommended.

   The main advantage of the Liberal Label retention mode is that
   reaction to routing changes can be quick because labels already




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   exist.  The main disadvantage of the liberal mode is that unneeded
   label mappings are distributed and maintained.

3.6.3.  Label Advertisement Mode

   Each interface on an LSR is configured to operate in either
   Downstream Unsolicited or Downstream on Demand advertisement mode.
   LSRs exchange advertisement modes during initialization.  The major
   difference between Downstream Unsolicited and Downstream on Demand
   modes is in which LSR takes responsibility for initiating mapping
   requests and mapping advertisements.

3.7.  LDP Identifiers and Next Hop Addresses

   An LSR maintains learned labels in a Label Information Base (LIB).
   When operating in Downstream Unsolicited mode, the LIB entry for an
   address prefix associates a collection of (LDP Identifier, label)
   pairs with the prefix, one such pair for each peer advertising a
   label for the prefix.

   When the next hop for a prefix changes, the LSR must retrieve the
   label advertised by the new next hop from the LIB for use in
   forwarding.  To retrieve the label, the LSR must be able to map the
   next hop address for the prefix to an LDP Identifier.

   Similarly, when the LSR learns a label for a prefix from an LDP peer,
   it must be able to determine whether that peer is currently a next
   hop for the prefix to determine whether it needs to start using the
   newly learned label when forwarding packets that match the prefix.
   To make that decision, the LSR must be able to map an LDP Identifier
   to the peer's addresses to check whether any are a next hop for the
   prefix.

   To enable LSRs to map between a peer LDP Identifier and the peer's
   addresses, LSRs advertise their addresses using LDP Address and
   Withdraw Address messages.

   An LSR sends an Address message to advertise its addresses to a peer.
   An LSR sends a Withdraw Address message to withdraw previously
   advertised addresses from a peer.

3.8.  Loop Detection

   Loop Detection is a configurable option that provides a mechanism for
   finding looping LSPs and for preventing Label Request messages from
   looping in the presence of non-merge capable LSRs.





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   The mechanism makes use of Path Vector and Hop Count TLVs carried by
   Label Request and Label Mapping messages.  It builds on the following
   basic properties of these TLVs:

   -  A Path Vector TLV contains a list of the LSRs that its containing
      message has traversed.  An LSR is identified in a Path Vector list
      by its unique LSR Identifier (Id), which is the first four octets
      of its LDP Identifier.  When an LSR propagates a message
      containing a Path Vector TLV, it adds its LSR Id to the Path
      Vector list.  An LSR that receives a message with a Path Vector
      that contains its LSR Id detects that the message has traversed a
      loop.  LDP supports the notion of a maximum allowable Path Vector
      length; an LSR that detects a Path Vector has reached the maximum
      length behaves as if the containing message has traversed a loop.

   -  A Hop Count TLV contains a count of the LSRS that the containing
      message has traversed.  When an LSR propagates a message
      containing a Hop Count TLV, it increments the count.  An LSR that
      detects a Hop Count has reached a configured maximum value behaves
      as if the containing message has traversed a loop.  By convention,
      a count of 0 is interpreted to mean the hop count is unknown.
      Incrementing an unknown hop count value results in an unknown hop
      count value (0).

   The following paragraphs describe LDP Loop Detection procedures.  For
   these paragraphs, and only these paragraphs, "MUST" is redefined to
   mean "MUST if configured for Loop Detection".  The paragraphs specify
   messages that MUST carry Path Vector and Hop Count TLVs.  Note that
   the Hop Count TLV and its procedures are used without the Path Vector
   TLV in situations when Loop Detection is not configured (see RFC 3035
   [RFC3035] and RFC 3034 [RFC3034]).

3.8.1.  Label Request Message

   The use of the Path Vector TLV and Hop Count TLV prevent Label
   Request messages from looping in environments that include non-merge
   capable LSRs.

   The rules that govern use of the Hop Count TLV in Label Request
   messages by LSR R when Loop Detection is enabled are the following:

   -  The Label Request message MUST include a Hop Count TLV.

   -  If R is sending the Label Request because it is a FEC ingress, it
      MUST include a Hop Count TLV with hop count value 1.

   -  If R is sending the Label Request as a result of having received a
      Label Request from an upstream LSR, and if the received Label



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      Request contains a Hop Count TLV, R MUST increment the received
      hop count value by 1 and MUST pass the resulting value in a Hop
      Count TLV to its next hop along with the Label Request message.

   The rules that govern use of the Path Vector TLV in Label Request
   messages by LSR R when Loop Detection is enabled are the following:

   -  If R is sending the Label Request because it is a FEC ingress,
      then if R is non-merge capable, it MUST include a Path Vector TLV
      of length 1 containing its own LSR Id.

   -  If R is sending the Label Request as a result of having received a
      Label Request from an upstream LSR, then if the received Label
      Request contains a Path Vector TLV or if R is non-merge capable:

         R MUST add its own LSR Id to the Path Vector, and MUST pass the
         resulting Path Vector to its next hop along with the Label
         Request message.  If the Label Request contains no Path Vector
         TLV, R MUST include a Path Vector TLV of length 1 containing
         its own LSR Id.

   Note that if R receives a Label Request message for a particular FEC,
   and R has previously sent a Label Request message for that FEC to its
   next hop and has not yet received a reply, and if R intends to merge
   the newly received Label Request with the existing outstanding Label
   Request, then R does not propagate the Label Request to the next hop.

   If R receives a Label Request message from its next hop with a Hop
   Count TLV that exceeds the configured maximum value, or with a Path
   Vector TLV containing its own LSR Id or which exceeds the maximum
   allowable length, then R detects that the Label Request message has
   traveled in a loop.

   When R detects a loop, it MUST send a Loop Detected Notification
   message to the source of the Label Request message and drop the Label
   Request message.

3.8.2.  Label Mapping Message

   The use of the Path Vector TLV and Hop Count TLV in the Label Mapping
   message provide a mechanism to find and terminate looping LSPs.  When
   an LSR receives a Label Mapping message from a next hop, the message
   is propagated upstream as specified below until an ingress LSR is
   reached or a loop is found.

   The rules that govern the use of the Hop Count TLV in Label Mapping
   messages sent by an LSR R when Loop Detection is enabled are the
   following:



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   -  R MUST include a Hop Count TLV.

   -  If R is the egress, the hop count value MUST be 1.

   -  If the Label Mapping message is being sent to propagate a Label
      Mapping message received from the next hop to an upstream peer,
      the hop count value MUST be determined as follows:

      o  If R is a member of the edge set of an LSR domain whose LSRs do
         not perform 'TTL-decrement' (e.g., an ATM LSR domain or a Frame
         Relay LSR domain) and the upstream peer is within that domain,
         R MUST reset the hop count to 1 before propagating the message.

      o  Otherwise, R MUST increment the hop count received from the
         next hop before propagating the message.

   -  If the Label Mapping message is not being sent to propagate a
      Label Mapping message, the hop count value MUST be the result of
      incrementing R's current knowledge of the hop count learned from
      previous Label Mapping messages.  Note that this hop count value
      will be unknown if R has not received a Label Mapping message from
      the next hop.

   Any Label Mapping message MAY contain a Path Vector TLV.  The rules
   that govern the mandatory use of the Path Vector TLV in Label Mapping
   messages sent by LSR R when Loop Detection is enabled are the
   following:

   -  If R is the egress, the Label Mapping message need not include a
      Path Vector TLV.

   -  If R is sending the Label Mapping message to propagate a Label
      Mapping message received from the next hop to an upstream peer,
      then:

      o  If R is merge capable and if R has not previously sent a Label
         Mapping message to the upstream peer, then it MUST include a
         Path Vector TLV.

      o  If the received message contains an unknown hop count, then R
         MUST include a Path Vector TLV.

      o  If R has previously sent a Label Mapping message to the
         upstream peer, then it MUST include a Path Vector TLV if the
         received message reports an LSP hop count increase, a change in
         hop count from unknown to known, or a change from known to
         unknown.




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   If the above rules require R include a Path Vector TLV in the Label
   Mapping message, R computes it as follows:



      o  If the received Label Mapping message included a Path Vector,
         the Path Vector sent upstream MUST be the result of adding R's
         LSR Id to the received Path Vector.

      o  If the received message had no Path Vector, the Path Vector
         sent upstream MUST be a Path Vector of length 1 containing R's
         LSR Id.

   -  If the Label Mapping message is not being sent to propagate a
      received message upstream, the Label Mapping message MUST include
      a Path Vector of length 1 containing R's LSR Id.

      If R receives a Label Mapping message from its next hop with a Hop
      Count TLV that exceeds the configured maximum value, or with a
      Path Vector TLV containing its own LSR Id or that exceeds the
      maximum allowable length, then R detects that the corresponding
      LSP contains a loop.

      When R detects a loop, it MUST stop using the label for
      forwarding, drop the Label Mapping message, and signal Loop
      Detected status to the source of the Label Mapping message.

3.8.3.  Discussion

   If Loop Detection is desired in an MPLS domain, then it should be
   turned on in ALL LSRs within that MPLS domain, else Loop Detection
   will not operate properly and may result in undetected loops or in
   falsely detected loops.

   LSRs that are configured for Loop Detection are NOT expected to store
   the Path Vectors as part of the LSP state.

   Note that in a network where only non-merge capable LSRs are present,
   Path Vectors are passed downstream from ingress to egress, and are
   not passed upstream.  Even when merge is supported, Path Vectors need
   not be passed upstream along an LSP that is known to reach the
   egress.  When an LSR experiences a change of next hop, it need pass
   Path Vectors upstream only when it cannot tell from the hop count
   that the change of next hop does not result in a loop.

   In the case of ordered label distribution, Label Mapping messages are
   propagated from egress toward ingress, naturally creating the Path
   Vector along the way.  In the case of independent label distribution,



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   an LSR may originate a Label Mapping message for a FEC before
   receiving a Label Mapping message from its downstream peer for that
   FEC.  In this case, the subsequent Label Mapping message for the FEC
   received from the downstream peer is treated as an update to LSP
   attributes, and the Label Mapping message must be propagated
   upstream.  Thus, it is recommended that Loop Detection be configured
   in conjunction with ordered label distribution, to minimize the
   number of Label Mapping update messages.

3.9.  Authenticity and Integrity of LDP Messages

   This section specifies a mechanism to protect against the
   introduction of spoofed TCP segments into LDP session connection
   streams.  The use of this mechanism MUST be supported as a
   configurable option.

   The mechanism is based on use of the TCP MD5 Signature Option
   specified in RFC 2385 [RFC2385] for use by BGP [RFC4271].  See RFC
   1321 [RFC1321] for a specification of the MD5 hash function.  From a
   standards maturity point of view, the current document relates to RFC
   2385 the same way as RFC 4271 relates to RFC 2385.  This is explained
   in RFC 4278 [RFC4278].

3.9.1.  TCP MD5 Signature Option

   The following quotes from RFC 2385 [RFC2385] outline the security
   properties achieved by using the TCP MD5 Signature Option and
   summarize its operation:

   "IESG Note

      This document describes current existing practice for securing BGP
      against certain simple attacks.  It is understood to have security
      weaknesses against concerted attacks."

   "Abstract

      This memo describes a TCP extension to enhance security for BGP.
      It defines a new TCP option for carrying an MD5 RFC 1321 [RFC1321]
      digest in a TCP segment.  This digest acts like a signature for
      that segment, incorporating information known only to the
      connection end points.  Since BGP uses TCP as its transport, using
      this option in the way described in this paper significantly
      reduces the danger from certain security attacks on BGP."

   "Introduction





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      The primary motivation for this option is to allow BGP to protect
      itself against the introduction of spoofed TCP segments into the
      connection stream.  Of particular concern are TCP resets.

      To spoof a connection using the scheme described in this paper, an
      attacker would not only have to guess TCP sequence numbers, but
      would also have had to obtain the password included in the MD5
      digest.  This password never appears in the connection stream, and
      the actual form of the password is up to the application.  It
      could even change during the lifetime of a particular connection
      so long as this change was synchronized on both ends (although
      retransmission can become problematical in some TCP
      implementations with changing passwords).

      Finally, there is no negotiation for the use of this option in a
      connection, rather it is purely a matter of site policy whether or
      not its connections use the option."

   "MD5 as a Hashing Algorithm

      Since this memo was first issued (under a different title), the
      MD5 algorithm has been found to be vulnerable to collision search
      attacks [Dobb], and is considered by some to be insufficiently
      strong for this type of application.

      This memo still specifies the MD5 algorithm, however, since the
      option has already been deployed operationally, and there was no
      "algorithm type" field defined to allow an upgrade using the same
      option number.  The original document did not specify a type field
      since this would require at least one more byte, and it was felt
      at the time that taking 19 bytes for the complete option (which
      would probably be padded to 20 bytes in TCP implementations) would
      be too much of a waste of the already limited option space.

      This does not prevent the deployment of another similar option
      which uses another hashing algorithm (like SHA-1).  Also, if most
      implementations pad the 18 byte option as defined to 20 bytes
      anyway, it would be just as well to define a new option which
      contains an algorithm type field.

      This would need to be addressed in another document, however."

   End of quotes from RFC 2385 [RFC2385].








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3.9.2.  LDP Use of TCP MD5 Signature Option

   LDP uses the TCP MD5 Signature Option as follows:

   -  Use of the MD5 Signature Option for LDP TCP connections is a
      configurable LSR option.

   -  An LSR that uses the MD5 Signature Option is configured with a
      password (shared secret) for each potential LDP peer.

   -  The LSR applies the MD5 algorithm as specified in RFC 2385
      [RFC2385] to compute the MD5 digest for a TCP segment to be sent
      to a peer.  This computation makes use of the peer password as
      well as the TCP segment.

   -  When the LSR receives a TCP segment with an MD5 digest, it
      validates the segment by calculating the MD5 digest (using its own
      record of the password) and compares the computed digest with the
      received digest.  If the comparison fails, the segment is dropped
      without any response to the sender.

   -  The LSR ignores LDP Hellos from any LSR for which a password has
      not been configured.  This ensures that the LSR establishes LDP
      TCP connections only with LSRs for which a password has been
      configured.

4.  Protocol Specification

   Previous sections that describe LDP operation have discussed
   scenarios that involve the exchange of messages among LDP peers.
   This section specifies the message encodings and procedures for
   processing the messages.

   LDP message exchanges are accomplished by sending LDP protocol data
   units (PDUs) over LDP session TCP connections.

   Each LDP PDU can carry one or more LDP messages.  Note that the
   messages in an LDP PDU need not be related to one another.  For
   example, a single PDU could carry a message advertising FEC-label
   bindings for several FECs, another message requesting label bindings
   for several other FECs, and a third Notification message signaling
   some event.

4.1.  LDP PDUs

   Each LDP PDU is an LDP header followed by one or more LDP messages.
   The LDP header is:




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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Version                      |         PDU Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         LDP Identifier                        |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Figure 2: LDP PDU

   Version
      Two octet unsigned integer containing the version number of the
      protocol.  This version of the specification specifies LDP
      protocol version 1.

   PDU Length
      Two octet integer specifying the total length of this PDU in
      octets, excluding the Version and PDU Length fields.

      The maximum allowable PDU Length is negotiable when an LDP session
      is initialized.  Prior to completion of the negotiation, the
      maximum allowable length is 4096 octets.

   LDP Identifier
      Six octet field that uniquely identifies the label space of the
      sending LSR for which this PDU applies.  The first four octets
      identify the LSR and MUST be a globally unique value.  It SHOULD
      be a 32-bit router Id assigned to the LSR and also used to
      identify it in Loop Detection Path Vectors.  The last two octets
      identify a label space within the LSR.  For a platform-wide label
      space, these SHOULD both be zero.

   Note that there is no alignment requirement for the first octet of an
   LDP PDU.

4.2.  LDP Procedures

   LDP defines messages, TLVs, and procedures in the following areas:

   -  Peer discovery

   -  Session management

   -  Label distribution

   -  Notification of errors and advisory information



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   The sections that follow describe the message and TLV encodings for
   these areas and the procedures that apply to them.

   The label distribution procedures are complex and are difficult to
   describe fully, coherently, and unambiguously as a collection of
   separate message and TLV specifications.

   Appendix A, "LDP Label Distribution Procedures", describes the label
   distribution procedures in terms of label distribution events that
   may occur at an LSR and how the LSR must respond.  Appendix A is the
   specification of LDP label distribution procedures.  If a procedure
   described elsewhere in this document conflicts with Appendix A,
   Appendix A specifies LDP behavior.

4.3.  Type-Length-Value Encoding

   LDP uses a Type-Length-Value (TLV) encoding scheme to encode much of
   the information carried in LDP messages.

   An LDP TLV is encoded as a 2 octet field that uses 14 bits to specify
   a Type and 2 bits to specify behavior when an LSR doesn't recognize
   the Type, followed by a 2 octet Length field, followed by a variable
   length Value field.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |U|F|        Type               |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                             Value                             |
      ~                                                               ~
      |                                                               |
      |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 3: TLV Encoding

   U-bit
      Unknown TLV bit.  Upon receipt of an unknown TLV, if U is clear
      (=0), a notification MUST be returned to the message originator
      and the entire message MUST be ignored; if U is set (=1), the
      unknown TLV MUST be silently ignored and the rest of the message
      processed as if the unknown TLV did not exist.  The sections
      following that define TLVs specify a value for the U-bit.

   F-bit



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      Forward unknown TLV bit.  This bit applies only when the U-bit is
      set and the LDP message containing the unknown TLV is to be
      forwarded.  If F is clear (=0), the unknown TLV is not forwarded
      with the containing message; if F is set (=1), the unknown TLV is
      forwarded with the containing message.  The sections following
      that define TLVs specify a value for the F-bit.  By setting both
      the U- and F-bits, a TLV can be propagated as opaque data through
      nodes that do not recognize the TLV.

   Type
      Encodes how the Value field is to be interpreted.

   Length
      Specifies the length of the Value field in octets.

   Value
      Octet string of Length octets that encodes information to be
      interpreted as specified by the Type field.

   Note that there is no alignment requirement for the first octet of a
   TLV.

   Note that the Value field itself may contain TLV encodings.  That is,
   TLVs may be nested.

   The TLV encoding scheme is very general.  In principle, everything
   appearing in an LDP PDU could be encoded as a TLV.  This
   specification does not use the TLV scheme to its full generality.  It
   is not used where its generality is unnecessary and its use would
   waste space unnecessarily.  These are usually places where the type
   of a value to be encoded is known, for example by its position in a
   message or an enclosing TLV, and the length of the value is fixed or
   readily derivable from the value encoding itself.

   Some of the TLVs defined for LDP are similar to one another.  For
   example, there is a Generic Label TLV, an ATM Label TLV, and a Frame
   Relay TLV; see Sections "Generic Label TLV", "ATM Label TLV", and
   "Frame Relay TLV".

   While it is possible to think about TLVs related in this way in terms
   of a TLV type that specifies a TLV class and a TLV subtype that
   specifies a particular kind of TLV within that class, this
   specification does not formalize the notion of a TLV subtype.

   The specification assigns type values for related TLVs, such as the
   label TLVs, from a contiguous block in the 16-bit TLV type number
   space.




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   Section 4.8 "TLV Summary" lists the TLVs defined in this version of
   the protocol and the section in this document that describes each.

4.4.  TLV Encodings for Commonly Used Parameters

   There are several parameters used by more than one LDP message.  The
   TLV encodings for these commonly used parameters are specified in
   this section.

4.4.1.  FEC TLV

   Labels are bound to Forwarding Equivalence Classes (FECs).  A FEC is
   a list of one or more FEC elements.  The FEC TLV encodes FEC items.

   Its encoding is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| FEC (0x0100)              |      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        FEC Element 1                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                                                               ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        FEC Element n                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 4: FEC TLV Encoding

    FEC Element 1 to FEC Element n
      There are several types of FEC elements; see Section "FECs".  The
      FEC element encoding depends on the type of FEC element.

      A FEC Element value is encoded as a 1 octet field that specifies
      the element type, and a variable length field that is the type-
      dependent element value.  Note that while the representation of
      the FEC element value is type-dependent, the FEC element encoding
      itself is one where standard LDP TLV encoding is not used.

      The FEC Element value encoding is:








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   +---------------------+------+--------------------------------------+
   | FEC Element type    | Type | Value                                |
   | name                |      |                                      |
   +---------------------+------+--------------------------------------+
   | Wildcard            | 0x01 | No value; i.e., 0 value octets; see  |
   |                     |      | below.                               |
   | Prefix              | 0x02 | See below.                           |
   +---------------------+------+--------------------------------------+

                        Table 1: FEC Element Types

   Note that this version of LDP supports the use of multiple FEC
   Elements per FEC for the Label Mapping message only.  The use of
   multiple FEC Elements in other messages is not permitted in this
   version, and is a subject for future study.

   Wildcard FEC Element

      To be used only in the Label Withdraw and Label Release messages.
      Indicates the withdraw/release is to be applied to all FECs
      associated with the label within the following label TLV.  Must be
      the only FEC Element in the FEC TLV.

   Prefix FEC Element value encoding:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Prefix (0x02) |     Address Family            |     PreLen    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Prefix                                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 5: Prefix FEC Element Value Encoding

   Address Family
      Two octet quantity containing a value from ADDRESS FAMILY NUMBERS
      in [ASSIGNED_AF] that encodes the address family for the address
      prefix in the Prefix field.

   Prelen
      One octet unsigned integer containing the length in bits of the
      address prefix that follows.  A length of zero indicates a prefix
      that matches all addresses (the default destination); in this
      case, the Prefix itself is zero octets).

   Prefix




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      An address prefix encoded according to the Address Family field,
      whose length, in bits, was specified in the PreLen field, padded
      to a byte boundary.

4.4.1.1.  FEC Procedures

   If in decoding a FEC TLV an LSR encounters a FEC Element with an
   Address Family it does not support, it SHOULD stop decoding the FEC
   TLV, abort processing the message containing the TLV, and send an
   "Unsupported Address Family" Notification message to its LDP peer
   signaling an error.

   If it encounters a FEC Element type it cannot decode, it SHOULD stop
   decoding the FEC TLV, abort processing the message containing the
   TLV, and send an "Unknown FEC" Notification message to its LDP peer
   signaling an error.

4.4.2.  Label TLVs

   Label TLVs encode labels.  Label TLVs are carried by the messages
   used to advertise, request, release, and withdraw label mappings.

   There are several different kinds of Label TLVs that can appear in
   situations that require a Label TLV.

4.4.2.1.  Generic Label TLV

   An LSR uses Generic Label TLVs to encode labels for use on links for
   which label values are independent of the underlying link technology.
   Examples of such links are PPP and Ethernet.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| Generic Label (0x0200)    |      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Label                                                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 6: Generic Label

   For further information, see RFC 3032 [RFC3032].

   Label

      This is a 20-bit label value represented as a 20-bit number in a 4
      octet field as follows:




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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Label                             |                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                              Figure 7: Label

   For further information, see RFC 3032 [RFC3032].

4.4.2.2.  ATM Label TLV

   An LSR uses ATM Label TLVs to encode labels for use on ATM links.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| ATM Label (0x0201)        |         Length                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Res| V |          VPI          |         VCI                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 8: ATM Label TLV

   Res

      This field is reserved.  It MUST be set to zero on transmission
      and MUST be ignored on receipt.

   V-bits

      Two-bit switching indicator.  If V-bits is 00, both the VPI and
      VCI are significant.  If V-bits is 01, only the VPI field is
      significant.  If V-bit is 10, only the VCI is significant.

   VPI

      Virtual Path Identifier.  If VPI is less than 12-bits it SHOULD be
      right justified in this field and preceding bits SHOULD be set to
      0.

   VCI

      Virtual Channel Identifier.  If the VCI is less than 16-bits, it
      SHOULD be right justified in the field and the preceding bits MUST
      be set to 0.  If Virtual Path switching is indicated in the V-bits
      field, then this field MUST be ignored by the receiver and set to
      0 by the sender.



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   Editors Note: We have a discussion on whether we need ATM in the
   Internet Standard version of the LDP specification.

4.4.2.3.  Frame Relay Label TLV

   An LSR uses Frame Relay Label TLVs to encode labels for use on Frame
   Relay links.


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| Frame Relay Label (0x0202)|       Length                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Reserved    |Len|                     DLCI                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 9: Frame Relay Label TLV

   Reserved

      This field is reserved.  It MUST be set to zero on transmission
      and MUST be ignored on receipt.

   Len

      This field specifies the number of bits of the DLCI.  The
      following values are supported:

                          +------+--------------+
                          | Code | bits of DLCI |
                          +------+--------------+
                          | 0    | 10           |
                          | 1    | reserved     |
                          | 2    | 32           |
                          | 3    | reserved     |
                          +------+--------------+

                  Table 2: Frame Relay Label Length codes

   DLCI

      The Data Link Connection Identifier

      For a 10-bit DLCI, the encoding is:






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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0|0| Frame Relay Label (0x0202)|       Length                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Reserved    |Len|            0            |    10-bit DLCI    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 10: Frame Relay Label TLV for a 10 bit DLCI

   For a 23-bit DLCI, the encoding is:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0|0| Frame Relay Label (0x0202)|       Length                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Reserved    |Len|              23-bit DLCI                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 11: Frame Relay Label TLV for a 10 bit DLCI

   For further information, see RFC 3034 [RFC3034].

4.4.3.  Address List TLV

   The Address List TLV appears in Address and Address Withdraw
   messages.

   Its encoding is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| Address List (0x0101)     |      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Address Family            |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
      |                                                               |
      |                        Addresses                              |
      ~                                                               ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 12: Address List TLV

   Address Family




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      Two octet quantity containing a value from ADDRESS FAMILY NUMBERS
      in [ASSIGNED_AF] that encodes the addresses contained in the
      Addresses field.

   Addresses

      A list of addresses from the specified Address Family.  The
      encoding of the individual addresses depends on the Address
      Family.
      The following address encodings are defined by this version of the
      protocol:

              +----------------+----------------------------+
              | Address Family | Address Encoding           |
              +----------------+----------------------------+
              | IPv4           | 4 octet full IPv4vaddress  |
              | IPv6           | 16 octet full IPv6 address |
              +----------------+----------------------------+

                         Table 3: Address Families

4.4.4.  Hop Count TLV

   The Hop Count TLV appears as an optional field in messages that set
   up LSPs.  It calculates the number of LSR hops along an LSP as the
   LSP is being set up.

   Note that setup procedures for LSPs that traverse ATM and Frame Relay
   links require use of the Hop Count TLV (see RFC 3035 [RFC3035]and RFC
   3034 [RFC3034]).

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| Hop Count (0x0103)        |      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     HC Value  |
      +-+-+-+-+-+-+-+-+

                         Figure 13: Hop Count TLV

   HC Value

      1 octet unsigned integer hop count value.







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4.4.4.1.  Hop Count Procedures

   During setup of an LSP, an LSR R may receive a Label Mapping or Label
   Request message for the LSP that contains the Hop Count TLV.  If it
   does, it SHOULD record the hop count value.

   If LSR R then propagates the Label Mapping message for the LSP to an
   upstream peer or the Label Request message to a downstream peer to
   continue the LSP setup, it must determine a hop count to include in
   the propagated message as follows:

   -  If the message is a Label Request message, R MUST increment the
      received hop count;

   -  If the message is a Label Mapping message, R determines the hop
      count as follows:

      o  If R is a member of the edge set of an LSR domain whose LSRs do
         not perform 'TTL-decrement' and the upstream peer is within
         that domain, R MUST reset the hop count to 1 before propagating
         the message.

      o  Otherwise, R MUST increment the received hop count.

   The first LSR in the LSP (ingress for a Label Request message, egress
   for a Label Mapping message) SHOULD set the hop count value to 1.

   By convention, a value of 0 indicates an unknown hop count.  The
   result of incrementing an unknown hop count is itself an unknown hop
   count (0).

   Use of the unknown hop count value greatly reduces the signaling
   overhead when independent control is used.  When a new LSP is
   established, each LSR starts with an unknown hop count.  Addition of
   a new LSR whose hop count is also unknown does not cause a hop count
   update to be propagated upstream since the hop count remains unknown.
   When the egress is finally added to the LSP, then the LSRs propagate
   hop count updates upstream via Label Mapping messages.

   Without use of the unknown hop count, each time a new LSR is added to
   the LSP a hop count update would need to be propagated upstream if
   the new LSR is closer to the egress than any of the other LSRs.
   These updates are useless overhead since they don't reflect the hop
   count to the egress.

   From the perspective of the ingress node, the fact that the hop count
   is unknown implies nothing about whether a packet sent on the LSP




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   will actually make it to the egress.  All it implies is that the hop
   count update from the egress has not yet reached the ingress.

   If an LSR receives a message containing a Hop Count TLV, it MUST
   check the hop count value to determine whether the hop count has
   exceeded its configured maximum allowable value.  If so, it MUST
   behave as if the containing message has traversed a loop by sending a
   Notification message signaling Loop Detected in reply to the sender
   of the message.

   If Loop Detection is configured, the LSR MUST follow the procedures
   specified in Section 3.8 "Loop Detection".

4.4.5.  Path Vector TLV

   The Path Vector TLV is used with the Hop Count TLV in Label Request
   and Label Mapping messages to implement the optional LDP Loop
   Detection mechanism.  See Section "Loop Detection".  Its use in the
   Label Request message records the path of LSRs the request has
   traversed.  Its use in the Label Mapping message records the path of
   LSRs a label advertisement has traversed to set up an LSP.  Its
   encoding is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| Path Vector (0x0104)      |        Length                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            LSR Id 1                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                                                               ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            LSR Id n                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 14: Path Vector TLV

   One or more LSR Ids

      A list of router-ids indicating the path of LSRs the message has
      traversed.  Each LSR Id is the first four octets (router-id) of
      the LDP Identifier for the corresponding LSR.  This ensures it is
      unique within the LSR network.






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4.4.5.1.  Path Vector Procedures

   The Path Vector TLV is carried in Label Mapping and Label Request
   messages when Loop Detection is configured.

4.4.5.1.1.  Label Request Path Vector

   Section 3.8 "Loop Detection" specifies situations when an LSR must
   include a Path Vector TLV in a Label Request message.

   An LSR that receives a Path Vector in a Label Request message MUST
   perform the procedures described in Section "Loop Detection".

   If the LSR detects a loop, it MUST reject the Label Request message.

   The LSR MUST:

   1.  Transmit a Notification message to the sending LSR signaling
       "Loop Detected".

   2.  Not propagate the Label Request message further.

   Note that a Label Request message with a Path Vector TLV is forwarded
   until:

   1.  A loop is found,

   2.  The LSP egress is reached, or

   3.  The maximum Path Vector limit or maximum Hop Count limit is
       reached.  This is treated as if a loop had been detected.

4.4.5.1.2.  Label Mapping Path Vector

   Section 3.8 "Loop Detection" specifies the situations when an LSR
   must include a Path Vector TLV in a Label Mapping message.

   An LSR that receives a Path Vector in a Label Mapping message MUST
   perform the procedures described in Section "Loop Detection".

   If the LSR detects a loop, it MUST reject the Label Mapping message
   in order to prevent a forwarding loop.  The LSR MUST:

   1.  Transmit a Label Release message carrying a Status TLV to the
       sending LSR to signal "Loop Detected".

   2.  Not propagate the message further.




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   3.  Check whether the Label Mapping message is for an existing LSP.
       If so, the LSR must unsplice any upstream labels that are spliced
       to the downstream label for the FEC.

   Note that a Label Mapping message with a Path Vector TLV is forwarded
   until:

   1.  A loop is found,

   2.  An LSP ingress is reached, or

   3.  The maximum Path Vector or maximum Hop Count limit is reached.
       This is treated as if a loop had been detected.

4.4.6.  Status TLV

   Notification messages carry Status TLVs to specify events being
   signaled.

   The encoding for the Status TLV is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |U|F| Status (0x0300)           |      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Status Code                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Message Type             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 15: Status TLV

   U-bit

      SHOULD be 0 when the Status TLV is sent in a Notification message.
      SHOULD be 1 when the Status TLV is sent in some other message.

   F-bit

      SHOULD be the same as the setting of the F-bit in the Status Code
      field.

   Status Code





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      32-bit unsigned integer encoding the event being signaled.  The
      structure of a Status Code is:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |E|F|                 Status Data                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 16: Path Vector TLV

   E-bit

      Fatal error bit.  If set (=1), this is a fatal Error Notification.
      If clear (=0), this is an Advisory Notification.

   F-bit

      Forward bit.  If set (=1), the notification SHOULD be forwarded to
      the LSR for the next-hop or previous-hop for the LSP, if any,
      associated with the event being signaled.  If clear (=0), the
      notification SHOULD NOT be forwarded.

   Status Data

      30-bit unsigned integer that specifies the status information.

      This specification defines Status Codes (32-bit unsigned integers
      with the above encoding).

      A Status Code of 0 signals success.

   Message ID

      If non-zero, 32-bit value that identifies the peer message to
      which the Status TLV refers.  If zero, no specific peer message is
      being identified.

   Message Type

      If non-zero, the type of the peer message to which the Status TLV
      refers.  If zero, the Status TLV does not refer to any specific
      message type.

   Note that use of the Status TLV is not limited to Notification
   messages.  A message other than a Notification message may carry a
   Status TLV as an Optional Parameter.  When a message other than a
   Notification carries a Status TLV, the U-bit of the Status TLV SHOULD



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   be set to 1 to indicate that the receiver SHOULD silently discard the
   TLV if unprepared to handle it.

4.5.  LDP Messages

   All LDP messages have the following format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |U|   Message Type              |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                     Mandatory Parameters                      |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                     Optional Parameters                       |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 17: LDP message format

   U-bit

      Unknown message bit.  Upon receipt of an unknown message, if U is
      clear (=0), a notification is returned to the message originator;
      if U is set (=1), the unknown message is silently ignored.  The
      sections following that define messages specify a value for the
      U-bit.

   Message Type

      Identifies the type of message.

   Message Length

      Specifies the cumulative length in octets of the Message ID,
      Mandatory Parameters, and Optional Parameters.

   Message ID




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      32-bit value used to identify this message.  Used by the sending
      LSR to facilitate identifying Notification messages that may apply
      to this message.  An LSR sending a Notification message in
      response to this message SHOULD include this Message ID in the
      Status TLV carried by the Notification message; see Section 4.5.1
      "Notification Message".

   Mandatory Parameters

      Variable length set of required message parameters.  Some messages
      have no required parameters.

      For messages that have required parameters, the required
      parameters MUST appear in the order specified by the individual
      message specifications in the sections that follow.

   Optional Parameters

      Variable length set of optional message parameters.  Many messages
      have no optional parameters.

      For messages that have optional parameters, the optional
      parameters may appear in any order.

   Note that there is no alignment requirement for the first octet of an
   LDP message and that there is no padding at the end of a message;
   that is, parameters can end at odd-byte boundaries.

   The following message types are defined in this version of LDP:

   +---------------------+---------------------------------------------+
   | Message Name        | Section Number and Title                    |
   +---------------------+---------------------------------------------+
   | Notification        | Section 4.5.1 "Notification Message"        |
   | Hello               | Section 4.5.2 "Hello Message"               |
   | Initialization      | Section 4.5.3 "Initialization Message"      |
   | KeepAlive           | Section 4.5.4 "KeepAlive Message"           |
   | Address             | Section 4.5.5 "Address Message"             |
   | Address Withdraw    | Section 4.5.6 "Address Withdraw Message"    |
   | Label Mapping       | Section 4.5.7 "Label Mapping Message"       |
   | Label Request       | Section 4.5.8 "Label Request Message"       |
   | Label Abort Request | Section 4.5.5 "Label Abort Request Message" |
   | Label Withdraw      | Section 4.5.10 "Label Withdraw Message"     |
   | Label Release       | Section 4.5.11 "Label Release Message"      |
   +---------------------+---------------------------------------------+

                           Table 4: LDP Messages




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   The sections that follow specify the encodings and procedures for
   these messages.

   Some of the above messages are related to one another, for example
   the Label Mapping, Label Request, Label Withdraw, and Label Release
   messages.

   While it is possible to think about messages related in this way in
   terms of a message type that specifies a message class and a message
   subtype that specifies a particular kind of message within that
   class, this specification does not formalize the notion of a message
   subtype.

   The specification assigns type values for related messages, such as
   the Label messages, from a contiguous block in the 16-bit message
   type number space.

4.5.1.  Notification Message

   An LSR sends a Notification message to inform an LDP peer of a
   significant event.  A Notification message signals a fatal error or
   provides advisory information such as the outcome of processing an
   LDP message or the state of the LDP session.

   The encoding for the Notification message is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Notification (0x0001)     |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Status (TLV)                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 18: Notification Message

   Message ID

      32-bit value used to identify this message.

   Status TLV

      Indicates the event being signaled.  The encoding for the Status
      TLV is specified in Section 4.4.6 "Status TLV".



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   Optional Parameters

      This variable length field contains 0 or more parameters, each
      encoded as a TLV.  The following Optional Parameters are generic
      and may appear in any Notification message:

           +--------------------+--------+--------+-----------+
           | Optional Parameter | Type   | Length | Value     |
           +--------------------+--------+--------+-----------+
           | Extended Status    | 0x0301 | 4      | See below |
           | Returned PDU       | 0x0302 | var    | See below |
           | Returned Message   | 0x0303 | var    | See below |
           +--------------------+--------+--------+-----------+

                           Table 5: LDP Messages

   Other Optional Parameters, specific to the particular event being
   signaled by the Notification messages, may appear.  These are
   described elsewhere.

   Extended Status

      The 4 octet value is an Extended Status Code that encodes
      additional information that supplements the status information
      contained in the Notification Status Code.

   Returned PDU

      An LSR uses this parameter to return part of an LDP PDU to the LSR
      that sent it.  The value of this TLV is the PDU header and as much
      PDU data following the header as appropriate for the condition
      being signaled by the Notification message.

   Returned Message

      An LSR uses this parameter to return part of an LDP message to the
      LSR that sent it.  The value of this TLV is the message type and
      length fields and as much message data following the type and
      length fields as appropriate for the condition being signaled by
      the Notification message.

4.5.1.1.  Notification Message Procedures

   If an LSR encounters a condition requiring it to notify its peer with
   advisory or error information, it sends the peer a Notification
   message containing a Status TLV that encodes the information and
   optionally additional TLVs that provide more information about the
   condition.



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   If the condition is one that is a fatal error, the Status Code
   carried in the Notification will indicate that.  In this case, after
   sending the Notification message the LSR SHOULD terminate the LDP
   session by closing the session TCP connection and discard all state
   associated with the session, including all label-FEC bindings learned
   via the session.

   When an LSR receives a Notification message that carries a Status
   Code that indicates a fatal error, it SHOULD terminate the LDP
   session immediately by closing the session TCP connection and discard
   all state associated with the session, including all label-FEC
   bindings learned via the session.

   The above statement does not apply to the processing of the Shutdown
   message in the session initialization procedure.  When an LSR
   receives a Shutdown message during session initialization, it SHOULD
   transmit a Shutdown message and then close the transport connection.

4.5.1.2.  Events Signaled by Notification Messages

   It is useful for descriptive purpose to classify events signaled by
   Notification messages into the following categories.

4.5.1.2.1.  Malformed PDU or Message

   Malformed LDP PDUs or messages that are part of the LDP Discovery
   mechanism are handled by silently discarding them.

   An LDP PDU received on a TCP connection for an LDP session is
   malformed if:

   -  The LDP Identifier in the PDU header is unknown to the receiver,
      or it is known but is not the LDP Identifier associated by the
      receiver with the LDP peer for this LDP session.  This is a fatal
      error signaled by the Bad LDP Identifier Status Code.

   -  The LDP protocol version is not supported by the receiver, or it
      is supported but is not the version negotiated for the session
      during session establishment.  This is a fatal error signaled by
      the Bad Protocol Version Status Code.

   -  The PDU Length field is too small (< 14) or too large (> maximum
      PDU length).  This is a fatal error signaled by the Bad PDU Length
      Status Code.  Section "Initialization Message" describes how the
      maximum PDU length for a session is determined.

   An LDP message is malformed if:




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   -  The Message Type is unknown.

      If the Message Type is < 0x8000 (high order bit = 0), it is an
      error signaled by the Unknown Message Type Status Code.

      If the Message Type is >= 0x8000 (high order bit = 1), it is
      silently discarded.

   -  The Message Length is too large, that is, indicates that the
      message extends beyond the end of the containing LDP PDU.  This is
      a fatal error signaled by the Bad Message Length Status Code.

   -  The Message Length is too small, that is, smaller than the
      smallest possible value component.  This is a fatal error signaled
      by the Bad Message Length Status Code.

   -  The message is missing one or more Mandatory Parameters.  This is
      a non-fatal error signaled by the Missing Message Parameters
      Status Code.

4.5.1.2.2.  Unknown or Malformed TLV

   Malformed TLVs contained in LDP messages that are part of the LDP
   Discovery mechanism are handled by silently discarding the containing
   message.

   A TLV contained in an LDP message received on a TCP connection of an
   LDP is malformed if:

   -  The TLV Length is too large, that is, indicates that the TLV
      extends beyond the end of the containing message.  This is a fatal
      error signaled by the Bad TLV Length Status Code.

   -  - The TLV type is unknown.

      If the TLV type is < 0x8000 (high order bit = 0), it is an error
      signaled by the Unknown TLV Status Code.

      If the TLV type is >= 0x8000 (high order bit = 1), the TLV is
      silently dropped.

   -  The TLV Value is malformed.  This occurs when the receiver handles
      the TLV but cannot decode the TLV Value.  This is interpreted as
      indicative of a bug in either the sending or receiving LSR.  It is
      a fatal error signaled by the Malformed TLV Value Status Code.






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4.5.1.2.3.  Session KeepAlive Timer Expiration

   This is a fatal error signaled by the KeepAlive Timer Expired Status
   Code.

4.5.1.2.4.  Unilateral Session Shutdown

   This is a fatal event signaled by the Shutdown Status Code.  The
   Notification message may optionally include an Extended Status TLV to
   provide a reason for the Shutdown.  The sending LSR terminates the
   session immediately after sending the Notification.

4.5.1.2.5.  Initialization Message Events

   The session initialization negotiation (see Section "Session
   Initialization") may fail if the session parameters received in the
   Initialization message are unacceptable.  This is a fatal error.  The
   specific Status Code depends on the parameter deemed unacceptable,
   and is defined in Section 4.5.3 "Initialization Message".

4.5.1.2.6.  Events Resulting from Other Messages

   Messages other than the Initialization message may result in events
   that must be signaled to LDP peers via Notification messages.  These
   events and the Status Codes used in the Notification messages to
   signal them are described in the sections that describe these
   messages.

4.5.1.2.7.  Internal Errors

   An LDP implementation may be capable of detecting problem conditions
   specific to its implementation.  When such a condition prevents an
   implementation from interacting correctly with a peer, the
   implementation should, when capable of doing so, use the Internal
   Error Status Code to signal the peer.  This is a fatal error.

4.5.1.2.8.  Miscellaneous Events

   These are events that fall into none of the categories above.  There
   are no miscellaneous events defined in this version of the protocol.

4.5.2.  Hello Message

   LDP Hello messages are exchanged as part of the LDP Discovery
   Mechanism; see Section 3.4 "LDP Discovery".

   The encoding for the Hello message is:




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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Hello (0x0100)            |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Common Hello Parameters TLV               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 19: Hello Message

   Message ID

      32-bit value used to identify this message.

   Common Hello Parameters TLV

      Specifies parameters common to all Hello messages.  The encoding
      for the Common Hello Parameters TLV is:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0|0| Common Hello Parms(0x0400)|      Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Hold Time                |T|R| Reserved                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 20: Hello Message

   Hold Time

      Hello hold time in seconds.  An LSR maintains a record of Hellos
      received from potential peers (see Section 4.5.2.1 "Hello Message
      Procedures").  Hello Hold Time specifies the time the sending LSR
      will maintain its record of Hellos from the receiving LSR without
      receipt of another Hello.

      A pair of LSRs negotiates the hold times they use for Hellos from
      each other.  Each proposes a hold time.  The hold time used is the
      minimum of the hold times proposed in their Hellos.

      A value of 0 means use the default, which is 15 seconds for Link
      Hellos and 45 seconds for Targeted Hellos.  A value of 0xffff
      means infinite.



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   T, Targeted Hello

      A value of 1 specifies that this Hello is a Targeted Hello.  A
      value of 0 specifies that this Hello is a Link Hello.

   R, Request Send Targeted Hellos

      A value of 1 requests the receiver to send periodic Targeted
      Hellos to the source of this Hello.  A value of 0 makes no
      request.

      An LSR initiating Extended Discovery sets R to 1.  If R is 1, the
      receiving LSR checks whether it has been configured to send
      Targeted Hellos to the Hello source in response to Hellos with
      this request.  If not, it ignores the request.  If so, it
      initiates periodic transmission of Targeted Hellos to the Hello
      source.

   Reserved

      This field is reserved.  It MUST be set to zero on transmission
      and ignored on receipt.

   Optional Parameters

      This variable length field of the Hello message contains 0 or more
      parameters, each encoded as a TLV.  The optional parameters
      defined by this version of the protocol are:

      +-------------------------------+--------+--------+-----------+
      | Optional Parameters           | Type   | Length | Value     |
      +-------------------------------+--------+--------+-----------+
      | IPv4 Transport Address        | 0x0401 | 4      | See below |
      | Configuration Sequence Number | 0x0402 | 4      | See below |
      | IPv6 Transport Address        | 0x0403 | 16     | See below |
      +-------------------------------+--------+--------+-----------+

                Table 6: Optional Hello Message Parameters

   IPv4 Transport Address

      Specifies the IPv4 address to be used for the sending LSR when
      opening the LDP session TCP connection.  If this optional TLV is
      not present, the IPv4 source address for the UDP packet carrying
      the Hello SHOULD be used.

   Configuration Sequence Number




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      Specifies a 4 octet unsigned configuration sequence number that
      identifies the configuration state of the sending LSR.  Used by
      the receiving LSR to detect configuration changes on the sending
      LSR.

   IPv6 Transport Address

      Specifies the IPv6 address to be used for the sending LSR when
      opening the LDP session TCP connection.  If this optional TLV is
      not present the IPv6 source address for the UDP packet carrying
      the Hello SHOULD be used.

4.5.2.1.  Hello Message Procedures

   An LSR receiving Hellos from another LSR maintains a Hello adjacency
   corresponding to the Hellos.  The LSR maintains a hold timer with the
   Hello adjacency, which it restarts whenever it receives a Hello that
   matches the Hello adjacency.  If the hold timer for a Hello adjacency
   expires the LSR discards the Hello adjacency: see Section 3.5.5
   "Maintaining Hello Adjacencies" and Section 3.5.6 "Maintaining LDP
   Sessions".

   We recommend that the interval between Hello transmissions be at most
   one third of the Hello hold time.

   An LSR processes a received LDP Hello as follows:

   1.  The LSR checks whether the Hello is acceptable.  The criteria for
       determining whether a Hello is acceptable are implementation
       dependent (see below for example criteria).

   2.  If the Hello is not acceptable, the LSR ignores it.

   3.  If the Hello is acceptable, the LSR checks whether it has a Hello
       adjacency for the Hello source.  If so, it restarts the hold
       timer for the Hello adjacency.  If not, it creates a Hello
       adjacency for the Hello source and starts its hold timer.

   4.  If the Hello carries any optional TLVs, the LSR processes them
       (see below).

   5.  Finally, if the LSR has no LDP session for the label space
       specified by the LDP Identifier in the PDU header for the Hello,
       it follows the procedures of Section "LDP Session Establishment".

   The following are examples of acceptability criteria for Link and
   Targeted Hellos:




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      A Link Hello is acceptable if the interface on which it was
      received has been configured for label switching.

      A Targeted Hello from source address A is acceptable if either:





      -  The LSR has been configured to accept Targeted Hellos, or

      -  The LSR has been configured to send Targeted Hellos to A.

      The following describes how an LSR processes Hello optional TLVs:

      Transport Address

         The LSR associates the specified transport address with the
         Hello adjacency.

      Configuration Sequence Number

         The Configuration Sequence Number optional parameter is used by
         the sending LSR to signal configuration changes to the
         receiving LSR.  When a receiving LSR playing the active role in
         LDP session establishment detects a change in the sending LSR
         configuration, it may clear the session setup backoff delay, if
         any, associated with the sending LSR (see Section 3.5.3
         "Session Initialization").

         A sending LSR using this optional parameter is responsible for
         maintaining the configuration sequence number it transmits in
         Hello messages.  Whenever there is a configuration change on
         the sending LSR, it increments the configuration sequence
         number.

4.5.3.  Initialization Message

   Note: We have an open discussion on whether we can remove ATM and FR
   from this document, if we decide to do that this section needs to be
   revisited.

   The LDP Initialization message is exchanged as part of the LDP
   session establishment procedure; see Section 3.5.1 "LDP Session
   Establishment".

   The encoding for the Initialization message is:




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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Initialization (0x0200)   |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Common Session Parameters TLV             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 21: Initialization Message

   Message ID

      32-bit value used to identify this message.

   Common Session Parameters TLV

      Specifies values proposed by the sending LSR for parameters that
      must be negotiated for every LDP session.

      The encoding for the Common Session Parameters TLV is:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0|0| Common Sess Parms (0x0500)|      Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Protocol Version              |      KeepAlive Time           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |A|D|  Reserved |     PVLim     |      Max PDU Length           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Receiver LDP Identifier                       |
       +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |
       -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++

                 Figure 22: Common Session Parameters TLV

   Protocol Version

      Two octet unsigned integer containing the version number of the
      protocol.  This version of the specification specifies LDP
      protocol version 1.

   KeepAlive Time



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      Two octet unsigned non zero integer that indicates the number of
      seconds that the sending LSR proposes for the value of the
      KeepAlive Time.  The receiving LSR MUST calculate the value of the
      KeepAlive Timer by using the smaller of its proposed KeepAlive
      Time and the KeepAlive Time received in the PDU.  The value chosen
      for KeepAlive Time indicates the maximum number of seconds that
      may elapse between the receipt of successive PDUs from the LDP
      peer on the session TCP connection.  The KeepAlive Timer is reset
      each time a PDU arrives.

   A, Label Advertisement Discipline

      Indicates the type of Label advertisement.  A value of 0 means
      Downstream Unsolicited advertisement; a value of 1 means
      Downstream On Demand.

      If one LSR proposes Downstream Unsolicited and the other proposes
      Downstream on Demand, the rules for resolving this difference is:


      -  If the session is for a label-controlled ATM link or a label-
         controlled Frame Relay link, then Downstream on Demand MUST be
         used.

      -  Otherwise, Downstream Unsolicited MUST be used.

         If the label advertisement discipline determined in this way is
         unacceptable to an LSR, it MUST send a Session Rejected/
         Parameters Advertisement Mode Notification message in response
         to the Initialization message and not establish the session.

   D, Loop Detection

      Indicates whether Loop Detection based on Path Vectors is enabled.
      A value of 0 means that Loop Detection is disabled; a value of 1
      means that Loop Detection is enabled.

   PVLim, Path Vector Limit

      The configured maximum Path Vector length.  MUST be 0 if Loop
      Detection is disabled (D = 0).  If the Loop Detection procedures
      would require the LSR to send a Path Vector that exceeds this
      limit, the LSR will behave as if a loop had been detected for the
      FEC in question.

      When Loop Detection is enabled in a portion of a network, it is
      recommended that all LSRs in that portion of the network be
      configured with the same Path Vector limit.  Although knowledge of



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      a peer's Path Vector limit will not change an LSR's behavior, it
      does enable the LSR to alert an operator to a possible
      misconfiguration.

   Reserved

      This field is reserved.  It MUST be set to zero on transmission
      and ignored on receipt.

   Max PDU Length

      Two octet unsigned integer that proposes the maximum allowable
      length for LDP PDUs for the session.  A value of 255 or less
      specifies the default maximum length of 4096 octets.

      The receiving LSR MUST calculate the maximum PDU length for the
      session by using the smaller of its and its peer's proposals for
      Max PDU Length.  The default maximum PDU length applies before
      session initialization completes.  If the maximum PDU length
      determined this way is unacceptable to an LSR, it MUST send a
      Session Rejected/Parameters Max PDU Length Notification message in
      response to the Initialization message and not establish the
      session.

   Receiver LDP Identifier

      Identifies the receiver's label space.  This LDP Identifier,
      together with the sender's LDP Identifier in the PDU header,
      enables the receiver to match the Initialization message with one
      of its Hello adjacencies; see Section 4.5.2.1 "Hello Message
      Procedures".

      If there is no matching Hello adjacency, the LSR MUST send a
      Session Rejected/No Hello Notification message in response to the
      Initialization message and not establish the session.

   Optional Parameters

      This variable length field contains 0 or more parameters, each
      encoded as a TLV.  The optional parameters are:











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     +--------------------------------+--------+--------+-----------+
     | Optional Parameters            | Type   | Length | Value     |
     +--------------------------------+--------+--------+-----------+
     | ATM Session Parameters         | 0x0501 | var    | See below |
     | Frame Relay Session Parameters | 0x0502 | var    | See below |
     +--------------------------------+--------+--------+-----------+

           Table 7: Initialization Message; Optional Parameters

   ATM Session Parameters

      Used when an LDP session manages label exchange for an ATM link to
      specify ATM-specific session parameters.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0|0|   ATM Sess Parms (0x0501) |      Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | M |   N   |D|                        Reserved                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 ATM Label Range Component 1                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                                                               ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 ATM Label Range Component N                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 23: ATM Session Parameters (optional)

   M, ATM Merge Capabilities

      Specifies the merge capabilities of an ATM switch.  The following
      values are supported in this version of the specification:

                    +-------+-------------------------+
                    | Value | Meaning                 |
                    +-------+-------------------------+
                    | 0     | Merge not supported     |
                    | 1     | VP Merge supported      |
                    | 2     | VC Merge supported      |
                    | 3     | VP & VC Merge supported |
                    +-------+-------------------------+

                      Table 8: ATM Merge Capabilities




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   If the merge capabilities of the LSRs differ, then:

   -  Non-merge and VC-merge LSRs may freely interoperate.

   -  The interoperability of VP-merge-capable switches with non- VP-
      merge-capable switches is a subject for future study.  When the
      LSRs differ on the use of VP merge, the session is established,
      but VP merge is not used.

      Note that if VP merge is used, it is the responsibility of the
      ingress node to ensure that the chosen VCI is unique within the
      LSR domain.

   N, Number of label range components

      Specifies the number of ATM Label Range Components included in the
      TLV.

   D, VC Directionality

      A value of 0 specifies bidirectional VC capability, meaning the
      LSR can (within a given VPI) support the use of a given VCI as a
      label for both link directions independently.  A value of 1
      specifies unidirectional VC capability, meaning (within a given
      VPI) a given VCI may appear in a label mapping for one direction
      on the link only.  When either or both of the peers specifies
      unidirectional VC capability, both LSRs use unidirectional VC
      label assignment for the link as follows.  The LSRs compare their
      LDP Identifiers as unsigned integers.  The LSR with the larger LDP
      Identifier may assign only odd- numbered VCIs in the VPI/VCI range
      as labels.  The system with the smaller LDP Identifier may assign
      only even-numbered VCIs in the VPI/VCI range as labels.

   Reserved

      This field is reserved.  It MUST be set to zero on transmission
      and ignored on receipt.

   One or more ATM Label Range Components

      A list of ATM Label Range Components that together specify the
      Label range supported by the transmitting LSR.

      A receiving LSR MUST calculate the intersection between the
      received range and its own supported label range.  The
      intersection is the range in which the LSR may allocate and accept
      labels.  LSRs MUST NOT establish a session with neighbors for
      which the intersection of ranges is NULL.  In this case, the LSR



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      MUST send a Session Rejected/Parameters Label Range Notification
      message in response to the Initialization message and not
      establish the session.

      The encoding for an ATM Label Range Component is:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Res  |    Minimum VPI        |      Minimum VCI              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Res  |    Maximum VPI        |      Maximum VCI              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 24: ATM Label Range Component

   Res

      This field is reserved.  It MUST be set to zero on transmission
      and ignored on receipt.

   Minimum VPI (12 bits)

      This 12-bit field specifies the lower bound of a block of Virtual
      Path Identifiers that is supported on the originating switch.  If
      the VPI is less than 12 bits, it SHOULD be right justified in this
      field and preceding bits SHOULD be set to 0.

   Minimum VCI (16 bits)

      This 16-bit field specifies the lower bound of a block of Virtual
      Channel Identifiers that is supported on the originating switch.
      If the VCI is less than 16 bits, it SHOULD be right justified in
      this field and preceding bits SHOULD be set to 0.

   Maximum VPI (12 bits)

      This 12-bit field specifies the upper bound of a block of Virtual
      Path Identifiers that is supported on the originating switch.  If
      the VPI is less than 12 bits, it SHOULD be right justified in this
      field and preceding bits SHOULD be set to 0.

   Maximum VCI (16 bits)

      This 16-bit field specifies the upper bound of a block of Virtual
      Connection Identifiers that is supported on the originating
      switch.  If the VCI is less than 16 bits, it SHOULD be right
      justified in this field and preceding bits SHOULD be set to 0.



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   When peer LSRs are connected indirectly by means of an ATM VP, the
   sending LSR SHOULD set the Minimum and Maximum VPI fields to 0, and
   the receiving LSR MUST ignore the Minimum and Maximum VPI fields.

   Frame Relay Session Parameters

      Used when an LDP session manages label exchange for a Frame Relay
      link to specify Frame Relay-specific session parameters.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0|0|   FR Sess Parms (0x0502)  |      Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | M |   N   |D|                        Reserved                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Frame Relay Label Range Component 1               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                                                               ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Frame Relay Label Range Component N               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 25: Frame Relay Session Parameters

   M, Frame Relay Merge Capabilities

      Specifies the merge capabilities of a Frame Relay switch.  The
      following values are supported in this version of the
      specification:

                      +-------+---------------------+
                      | Value | Meaning             |
                      +-------+---------------------+
                      | 0     | Merge not supported |
                      | 1     | VP Merge supported  |
                      | 2     | Merge supported     |
                      +-------+---------------------+

                  Table 9: Frame Relay Merge Capabilities

      Non-merge and merge Frame Relay LSRs may freely interoperate.

   N, Number of label range components





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      Specifies the number of Frame Relay Label Range Components
      included in the TLV.

   D, VC Directionality

      A value of 0 specifies bidirectional VC capability, meaning the
      LSR can support the use of a given DLCI as a label for both link
      directions independently.  A value of 1 specifies unidirectional
      VC capability, meaning a given DLCI may appear in a label mapping
      for one direction on the link only.  When either or both of the
      peers specifies unidirectional VC capability, both LSRs use
      unidirectional VC label assignment for the link as follows.  The
      LSRs compare their LDP Identifiers as unsigned integers.  The LSR
      with the larger LDP Identifier may assign only odd-numbered DLCIs
      in the range as labels.  The system with the smaller LDP
      Identifier may assign only even-numbered DLCIs in the range as
      labels.

   Reserved

      This field is reserved.  It MUST be set to zero on transmission
      and ignored on receipt.

   One or more Frame Relay Label Range Components

      A list of Frame Relay Label Range Components that together specify
      the Label range supported by the transmitting LSR.

      A receiving LSR MUST calculate the intersection between the
      received range and its own supported label range.  The
      intersection is the range in which the LSR may allocate and accept
      labels.  LSRs MUST NOT establish a session with neighbors for
      which the intersection of ranges is NULL.  In this case, the LSR
      MUST send a Session Rejected/Parameters Label Range Notification
      message in response to the Initialization message and not
      establish the session.

      The encoding for a Frame Relay Label Range Component is:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Reserved    |Len|                     Minimum DLCI            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Reserved        |                     Maximum DLCI            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 26: Frame Relay Label Range Component



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   Reserved

      This field is reserved.  It MUST be set to zero on transmission
      and ignored on receipt.

   Len

      This field specifies the number of bits of the DLCI.  The
      following values are supported:

                            +-----+-----------+
                            | Len | DLCI bits |
                            +-----+-----------+
                            | 0   | 10        |
                            | 1   | reserved  |
                            | 2   | 23        |
                            | 3   | reserved  |
                            +-----+-----------+

                       Table 10: Number of DLCI bits

   Minimum DLCI

      This 23-bit field specifies the lower bound of a block of Data
      Link Connection Identifiers (DLCIs) that is supported on the
      originating switch.  The DLCI SHOULD be right justified in this
      field and unused bits SHOULD be set to 0.

   Maximum DLCI

      This 23-bit field specifies the upper bound of a block of Data
      Link Connection Identifiers (DLCIs) that is supported on the
      originating switch.  The DLCI SHOULD be right justified in this
      field and unused bits SHOULD be set to 0.

   Note that there is no Generic Session Parameters TLV for sessions
   that advertise Generic Labels.

4.5.3.1.  Initialization Message Procedures

   See Section 3.5.1 "LDP Session Establishment" and particularly
   Section 3.5.3 "Session Initialization" for general procedures for
   handling the Initialization message.








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4.5.4.  KeepAlive Message

   An LSR sends KeepAlive messages as part of a mechanism that monitors
   the integrity of the LDP session transport connection.

   The encoding for the KeepAlive message is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   KeepAlive (0x0201)        |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 27: KeepAlive Message

   Message ID

      32-bit value used to identify this message.

   Optional Parameters

      No optional parameters are defined for the KeepAlive message.

4.5.4.1.  KeepAlive Message Procedures

   The KeepAlive Timer mechanism described in Section 3.5.6 "Maintaining
   LDP Sessions" resets a session KeepAlive Timer every time an LDP PDU
   is received on the session TCP connection.  The KeepAlive message is
   provided to allow reset of the KeepAlive Timer in circumstances where
   an LSR has no other information to communicate to an LDP peer.

   An LSR MUST arrange that its peer receive an LDP message from it at
   least every KeepAlive Time period.  Any LDP protocol message will do
   but, in circumstances where no other LDP protocol messages have been
   sent within the period, a KeepAlive message MUST be sent.

4.5.5.  Address Message

   An LSR sends the Address message to an LDP peer to advertise its
   interface addresses.

   The encoding for the Address message is:





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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Address (0x0300)          |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                     Address List TLV                          |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 28: Address Message

   Message ID

      32-bit value used to identify this message.

   Address List TLV

      The list of interface addresses being advertised by the sending
      LSR.  The encoding for the Address List TLV is specified in
      Section 4.4.3 "Address List TLV".

   Optional Parameters

      No optional parameters are defined for the Address message.

4.5.5.1.  Address Message Procedures

   An LSR that receives an Address message uses the addresses it learns
   to maintain a database for mapping between peer LDP Identifiers and
   next hop addresses; see Section 3.7 LDP Identifiers and Next Hop
   Addresses".

   When a new LDP session is initialized and before sending Label
   Mapping or Label Request messages, an LSR SHOULD advertise its
   interface addresses with one or more Address messages.

   Whenever an LSR "activates" a new interface address, it SHOULD
   advertise the new address with an Address message.

   Whenever an LSR "de-activates" a previously advertised address, it
   SHOULD withdraw the address with an Address Withdraw message; see
   Section "Address Withdraw Message".




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   If an LSR does not support the Address Family specified in the
   Address List TLV, it SHOULD send an Unsupported Address Family
   Notification to its LDP signaling an error and abort processing the
   message.

   An LSR may re-advertise an address (A) that it has previously
   advertised without explicitly withdrawing the address.  If the
   receiver already has address binding (LSR, A), it SHOULD take no
   further action.

   An LSR may withdraw an address (A) without having previously
   advertised it.  If the receiver has no address binding (LSR, A), it
   SHOULD take no further action.

4.5.6.  Address Withdraw Message

   An LSR sends the Address Withdraw message to an LDP peer to withdraw
   previously advertised interface addresses.

   The encoding for the Address Withdraw message is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Address Withdraw (0x0301) |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                     Address List TLV                          |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 29: Address Withdraw Message

   Message ID

      32-bit value used to identify this message.

   Address List TLV

      The list of interface addresses being withdrawn by the sending
      LSR.  The encoding for the Address List TLV is specified in
      Section "Address List TLV".

   Optional Parameters



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      No optional parameters are defined for the Address Withdraw
      message.

4.5.6.1.  Address Withdraw Message Procedures

   See Section 4.5.5.1 "Address Message Procedures".

4.5.7.  Label Mapping Message

   An LSR sends a Label Mapping message to an LDP peer to advertise FEC-
   label bindings to the peer.

   The encoding for the Label Mapping message is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Label Mapping (0x0400)    |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     FEC TLV                                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Label TLV                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 30: Label Mapping Message

   Message ID

      32-bit value used to identify this message.

   FEC TLV

      Specifies the FEC component of the FEC-Label mapping being
      advertised.  See Section 4.4.1 "FEC TLVs" for encoding.

   Label TLV

      Specifies the Label component of the FEC-Label mapping.  See
      Section 4.4.2 "Label TLVs" for encoding.

   Optional Parameters

      This variable length field contains 0 or more parameters, each
      encoded as a TLV.  The optional parameters are:



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          +------------------------------+----------+-----------+
          | Optional Parameter           | Length   | Value     |
          +------------------------------+----------+-----------+
          | Label Request Message ID TLV | 4        | See below |
          | Hop Count TLV                | 1        | See below |
          | Path Vector TLV              | variable | See below |
          +------------------------------+----------+-----------+

           Table 11: Label Mapping Message: Optional Parameters

   The encodings for the Hop Count and Path Vector TLVs can be found in
   Section 4.4 "TLV Encodings for Commonly Used Parameters".

   Label Request Message ID

      If this Label Mapping message is a response to a Label Request
      message, it MUST include the Label Request Message ID optional
      parameter.  The value of this optional parameter is the Message ID
      of the corresponding Label Request message.

   Hop Count

      Specifies the running total of the number of LSR hops along the
      LSP being set up by the Label message.  Section "Hop Count
      Procedures" describes how to handle this TLV.

   Path Vector

      Specifies the LSRs along the LSP being set up by the Label
      message.  Section 4.4.5.1 "Path Vector Procedures" describes how
      to handle this TLV.

4.5.7.1.  Label Mapping Message Procedures

   The Mapping message is used by an LSR to distribute a label mapping
   for a FEC to an LDP peer.  If an LSR distributes a mapping for a FEC
   to multiple LDP peers, it is a local matter whether it maps a single
   label to the FEC, and distributes that mapping to all its peers, or
   whether it uses a different mapping for each of its peers.

   An LSR is responsible for the consistency of the label mappings it
   has distributed and that its peers have these mappings.

   An LSR receiving a Label Mapping message from a downstream LSR for a
   Prefix SHOULD NOT use the label for forwarding unless its routing
   table contains an entry that exactly matches the FEC Element.





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   See Appendix A, "LDP Label Distribution Procedures", for more
   details.

4.5.7.1.1.  Independent Control Mapping

   If an LSR is configured for independent control, a mapping message is
   transmitted by the LSR upon any of the following conditions:

   1.  The LSR recognizes a new FEC via the forwarding table, and the
       label advertisement mode is Downstream Unsolicited advertisement.

   2.  The LSR receives a Request message from an upstream peer for a
       FEC present in the LSR's forwarding table.

   3.  The next hop for a FEC changes to another LDP peer, and Loop
       detection is configured.

   4.  The attributes of a mapping change.

   5.  The receipt of a mapping from the downstream next hop AND

       a.)    no upstream mapping has been created OR

       b.)    loop detection is configured OR

       c.)    the attributes of the mapping have changed.

4.5.7.1.2.  Ordered Control Mapping

   If an LSR is doing Ordered Control, a Mapping message is transmitted
   by downstream LSRs upon any of the following conditions:

   1.  The LSR recognizes a new FEC via the forwarding table and is the
       egress for that FEC.

   2.  The LSR receives a Request message from an upstream peer for a
       FEC present in the LSR's forwarding table, and the LSR is the
       egress for that FEC OR has a downstream mapping for that FEC.

   3.  The next hop for a FEC changes to another LDP peer, and Loop
       Detection is configured.

   4.  The attributes of a mapping change.

   5.  The receipt of a mapping from the downstream next hop AND

       a.)    no upstream mapping has been created OR




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       b.)    loop detection is configured OR

       c.)    the attributes of the mapping have changed.

4.5.7.1.3.  Downstream on Demand Label Advertisement

   In general, the upstream LSR is responsible for requesting label
   mappings when operating in Downstream on Demand mode.  However,
   unless some rules are followed, it is possible for neighboring LSRs
   with different advertisement modes to get into a livelock situation
   where everything is functioning properly, but no labels are
   distributed.  For example, consider two LSRs Ru and Rd where Ru is
   the upstream LSR and Rd is the downstream LSR for a particular FEC.
   In this example, Ru is using Downstream Unsolicited advertisement
   mode and Rd is using Downstream on Demand mode.  In this case, Rd may
   assume that Ru will request a label mapping when it wants one and Ru
   may assume that Rd will advertise a label if it wants Ru to use one.
   If Rd and Ru operate as suggested, no labels will be distributed from
   Rd to Ru.

   This livelock situation can be avoided if the following rule is
   observed: an LSR operating in Downstream on Demand mode SHOULD NOT be
   expected to send unsolicited mapping advertisements.  Therefore, if
   the downstream LSR is operating in Downstream on Demand mode, the
   upstream LSR is responsible for requesting label mappings as needed.

4.5.7.1.4.  Downstream Unsolicited Label Advertisement

   In general, the downstream LSR is responsible for advertising a label
   mapping when it wants an upstream LSR to use the label.  An upstream
   LSR may issue a mapping request if it so desires.

   The combination of Downstream Unsolicited mode and Conservative Label
   retention can lead to a situation where an LSR releases the label for
   a FEC that it later needs.  For example, if LSR Rd advertises to LSR
   Ru the label for a FEC for which it is not Ru's next hop, Ru will
   release the label.  If Ru's next hop for the FEC later changes to Rd,
   it needs the previously released label.

   To deal with this situation, either Ru can explicitly request the
   label when it needs it, or Rd can periodically re-advertise it to Ru.
   In many situations Ru will know when it needs the label from Rd.  For
   example, when its next hop for the FEC changes to Rd.  However, there
   could be situations when Ru does not.  For example, Rd may be
   attempting to establish an LSP with non-standard properties.  Forcing
   Ru to explicitly request the label in this situation would require it
   to maintain state about a potential LSP with non-standard properties.




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   In situations where Ru knows it needs the label, it is responsible
   for explicitly requesting the label by means of a Label Request
   message.  In situations where Ru may not know that it needs the
   label, Rd is responsible for periodically re-advertising the label to
   Ru.

   For this version of LDP, the only situation where Ru knows it needs a
   label for a FEC from Rd is when Rd is its next hop for the FEC, Ru
   does not have a label from Rd, and the LSP for the FEC is one that
   can be established with TLVs defined in this document.

4.5.8.  Label Request Message

   An LSR sends the Label Request message to an LDP peer to request a
   binding (mapping) for a FEC.

   The encoding for the Label Request message is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Label Request (0x0401)    |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     FEC TLV                                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 31: Label Request Message

   Message ID

      32-bit value used to identify this message.

   FEC TLV

      The FEC for which a label is being requested.  See Section 4.4.1
      "FEC TLV" for encoding.

   Optional Parameters
      This variable length field contains 0 or more parameters, each
      encoded as a TLV.  The optional parameters are:







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               +--------------------+----------+-----------+
               | Optional Parameter | Length   | Value     |
               +--------------------+----------+-----------+
               | Hop Count TLV      | 1        | See below |
               | Path Vector TLV    | variable | See below |
               +--------------------+----------+-----------+

           Table 12: Label Request Message: Optional Parameters

   The encodings for the Hop Count and Path Vector TLVs can be found in
   Section "TLV Encodings for Commonly Used Parameters".

   Hop Count
      Specifies the running total of the number of LSR hops along the
      LSP being set up by the Label Request message.  Section "Hop Count
      Procedures" describes how to handle this TLV.

   Path Vector
      Specifies the LSRs along the LSP being set up by the Label Request
      message.  Section "Path Vector Procedures" describes how to handle
      this TLV.

4.5.8.1.  Label Request Message Procedures

   The Request message is used by an upstream LSR to explicitly request
   that the downstream LSR assign and advertise a label for a FEC.

   An LSR may transmit a Request message under any of the following
   conditions:

   1.  The LSR recognizes a new FEC via the forwarding table, and the
       next hop is an LDP peer, and the LSR doesn't already have a
       mapping from the next hop for the given FEC.

   2.  The next hop to the FEC changes, and the LSR doesn't already have
       a mapping from that next hop for the given FEC.

       Note that if the LSR already has a pending Label Request message
       for the new next hop, it SHOULD NOT issue an additional Label
       Request in response to the next hop change.

   3.  The LSR receives a Label Request for a FEC from an upstream LDP
       peer, the FEC next hop is an LDP peer, and the LSR doesn't
       already have a mapping from the next hop.
       Note that since a non-merge LSR must set up a separate LSP for
       each upstream peer requesting a label, it must send a separate
       Label Request for each such peer.  A consequence of this is that




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       a non-merge LSR may have multiple Label Request messages for a
       given FEC outstanding at the same time.

   The receiving LSR SHOULD respond to a Label Request message with a
   Label Mapping for the requested label or with a Notification message
   indicating why it cannot satisfy the request.

   When the FEC for which a label is requested is a Prefix FEC Element,
   the receiving LSR uses its routing table to determine its response.
   Unless its routing table includes an entry that exactly matches the
   requested Prefix, the LSR MUST respond with a No Route Notification
   message.

   The message ID of the Label Request message serves as an identifier
   for the Label Request transaction.  When the receiving LSR responds
   with a Label Mapping message, the mapping message MUST include a
   Label Request/Returned Message ID TLV optional parameter that
   includes the message ID of the Label Request message.  Note that
   since LSRs use Label Request message IDs as transaction identifiers,
   an LSR SHOULD NOT reuse the message ID of a Label Request message
   until the corresponding transaction completes.

   This version of the protocol defines the following Status Codes for
   the Notification message that signals a request cannot be satisfied:

   No Route
      The FEC for which a label was requested includes a FEC Element for
      which the LSR does not have a route.

   No Label Resources
      The LSR cannot provide a label because of resource limitations.
      When resources become available, the LSR MUST notify the
      requesting LSR by sending a Notification message with the Label
      Resources Available Status Code.

      An LSR that receives a No Label Resources response to a Label
      Request message MUST NOT issue further Label Request messages
      until it receives a Notification message with the Label Resources
      Available Status Code.

   Loop Detected
      The LSR has detected a looping Label Request message.

   See Appendix A, "LDP Label Distribution Procedures", for more
   details.






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4.5.9.  Label Abort Request Message

   The Label Abort Request message may be used to abort an outstanding
   Label Request message.

   The encoding for the Label Abort Request message is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Label Abort Req (0x0404)  |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     FEC TLV                                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Label Request Message ID TLV              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 32: Label Abort Request Message

   Message ID
      32-bit value used to identify this message.

   FEC TLV
      Identifies the FEC for which the Label Request is being aborted.

   Label Request Message ID TLV
      Specifies the message ID of the Label Request message to be
      aborted.

   Optional Parameters
      No optional parameters are defined for the Label Abort Req
      message.

4.5.9.1.  Label Abort Request Message Procedures

   An LSR Ru may send a Label Abort Request message to abort an
   outstanding Label Request message for a FEC sent to an LSR Rd in the
   following circumstances:

   1.  Ru's next hop for the FEC has changed from LSR Rd to LSR X; or

   2.  Ru is a non-merge, non-ingress LSR and has received a Label Abort
       Request for the FEC from an upstream peer Y.




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   3.  Ru is a merge, non-ingress LSR and has received a Label Abort
       Request for the FEC from an upstream peer Y and Y is the only
       (last) upstream LSR requesting a label for the FEC.

   There may be other situations where an LSR may choose to abort an
   outstanding Label Request message in order to reclaim resource
   associated with the pending LSP.  However, specification of general
   strategies for using the abort mechanism is beyond the scope of LDP.

   When an LSR receives a Label Abort Request message, if it has not
   previously responded to the Label Request being aborted with a Label
   Mapping message or some other Notification message, it MUST
   acknowledge the abort by responding with a Label Request Aborted
   Notification message.  The Notification MUST include a Label Request
   Message ID TLV that carries the message ID of the aborted Label
   Request message.

   If an LSR receives a Label Abort Request Message after it has
   responded to the Label Request in question with a Label Mapping
   message or a Notification message, it ignores the abort request.

   If an LSR receives a Label Mapping message in response to a Label
   Request message after it has sent a Label Abort Request message to
   abort the Label Request, the label in the Label Mapping message is
   valid.  The LSR may choose to use the label or to release it with a
   Label Release message.

   An LSR aborting a Label Request message may not reuse the Message ID
   for the Label Request message until it receives one of the following
   from its peer:

   -  A Label Request Aborted Notification message acknowledging the
      abort;

   -  A Label Mapping message in response to the Label Request message
      being aborted;

   -  A Notification message in response to the Label Request message
      being aborted (e.g., Loop Detected, No Label Resources, etc.).

   To protect itself against tardy peers or faulty peer implementations
   an LSR may choose to time out receipt of the above.  The timeout
   period should be relatively long (several minutes).  If the timeout
   period elapses with no reply from the peer, the LSR may reuse the
   Message ID of the Label Request message; if it does so, it should
   also discard any record of the outstanding Label Request and Label
   Abort messages.




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   Note that the response to a Label Abort Request message is never
   "ordered".  That is, the response does not depend on the downstream
   state of the LSP setup being aborted.  An LSR receiving a Label Abort
   Request message MUST process it immediately, regardless of the
   downstream state of the LSP, responding with a Label Request Aborted
   Notification or ignoring it, as appropriate.

4.5.10.  Label Withdraw Message

   An LSR sends a Label Withdraw Message to an LDP peer to signal the
   peer that the peer may not continue to use specific FEC-label
   mappings the LSR had previously advertised.  This breaks the mapping
   between the FECs and the labels.

   The encoding for the Label Withdraw Message is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Label Withdraw (0x0402)   |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     FEC TLV                                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Label TLV (optional)                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 33: Label Withdraw Message

   Message ID
      32-bit value used to identify this message.

   FEC TLV
      Identifies the FEC for which the FEC-label mapping is being
      withdrawn.

   Optional Parameters
      This variable length field contains 0 or more parameters, each
      encoded as a TLV.  The optional parameters are:









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               +--------------------+----------+-----------+
               | Optional Parameter | Length   | Value     |
               +--------------------+----------+-----------+
               | Label TLV          | variable | See below |
               +--------------------+----------+-----------+

           Table 13: Label Withdraw Message: Optional Parameters

   The encoding for Label TLVs are found in Section 4.4.2 "Label TLVs"

   Label
      If present, specifies the label being withdrawn (see procedures
      below).

4.5.10.1.  Label Withdraw Message Procedures

   An LSR transmits a Label Withdraw message under the following
   conditions:

   1.  The LSR no longer recognizes a previously known FEC for which it
       has advertised a label.

   2.  The LSR has decided unilaterally (e.g., via configuration) to no
       longer label switch a FEC (or FECs) with the label mapping being
       withdrawn.

   The FEC TLV specifies the FEC for which labels are to be withdrawn.
   If no Label TLV follows the FEC, all labels associated with the FEC
   are to be withdrawn; otherwise, only the label specified in the
   optional Label TLV is to be withdrawn.

   The FEC TLV may contain the Wildcard FEC Element; if so, it may
   contain no other FEC Elements.  In this case, if the Label Withdraw
   message contains an optional Label TLV, then the label is to be
   withdrawn from all FECs to which it is bound.  If there is not an
   optional Label TLV in the Label Withdraw message, then the sending
   LSR is withdrawing all label mappings previously advertised to the
   receiving LSR.

   An LSR that receives a Label Withdraw message MUST respond with a
   Label Release message.

   See Appendix A, "LDP Label Distribution Procedures", for more
   details.







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4.5.11.  Label Release Message

   An LSR sends a Label Release message to an LDP peer to signal the
   peer that the LSR no longer needs specific FEC-label mappings
   previously requested of and/or advertised by the peer.

   The encoding for the Label Release Message is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   Label Release (0x0403)   |      Message Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     FEC TLV                                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Label TLV (optional)                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Optional Parameters                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 34: Label Release Message

   Message ID
      32-bit value used to identify this message.

   FEC TLV
      Identifies the FEC for which the FEC-label mapping is being
      released.

   Optional Parameters
      This variable length field contains 0 or more parameters, each
      encoded as a TLV.  The optional parameters are:

               +--------------------+----------+-----------+
               | Optional Parameter | Length   | Value     |
               +--------------------+----------+-----------+
               | Label TLV          | variable | See below |
               +--------------------+----------+-----------+

           Table 14: Label Release Message: Optional Parameters

   The encodings for Label TLVs are found in Section 4.4.2.

    Label
      If present, the label being released (see procedures below).




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4.5.11.1.  Label Release Message Procedures

   An LSR transmits a Label Release message to a peer when it no longer
   needs a label previously received from or requested of that peer.

   An LSR MUST transmit a Label Release message under any of the
   following conditions:

   1.  The LSR that sent the label mapping is no longer the next hop for
       the mapped FEC, and the LSR is configured for conservative
       operation.

   2.  The LSR receives a label mapping from an LSR that is not the next
       hop for the FEC, and the LSR is configured for conservative
       operation.

   3.  The LSR receives a Label Withdraw message.

   Note that if an LSR is configured for "liberal mode", a release
   message will never be transmitted in the case of conditions (1) and
   (2) as specified above.  In this case, the upstream LSR keeps each
   unused label, so that it can immediately be used later if the
   downstream peer becomes the next hop for the FEC.

   The FEC TLV specifies the FEC for which labels are to be released.
   If no Label TLV follows the FEC, all labels associated with the FEC
   are to be released; otherwise, only the label specified in the
   optional Label TLV is to be released.

   The FEC TLV may contain the Wildcard FEC Element; if so, it may
   contain no other FEC Elements.  In this case, if the Label Release
   message contains an optional Label TLV, then the label is to be
   released for all FECs to which it is bound.  If there is not an
   optional Label TLV in the Label Release message, then the sending LSR
   is releasing all label mappings previously learned from the receiving
   LSR.

   See Appendix A, "LDP Label Distribution Procedures", for more
   details.

4.6.  Messages and TLVs for Extensibility

   Support for LDP extensibility includes the rules for the U- and F-
   bits that specify how an LSR handles unknown TLVs and messages.

   This section specifies TLVs and messages for vendor-private and
   experimental use.




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4.6.1.  LDP Vendor-Private Extensions

   Vendor-private TLVs and messages are used to convey vendor-private
   information between LSRs.

4.6.1.1.  LDP Vendor-Private TLVs

   The Type range 0x3E00 through 0x3EFF is reserved for vendor-private
   TLVs.

   The encoding for a vendor-private TLV is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |U|F|    Type (0x3E00-0x3EFF)   |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Vendor ID                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                           Data....                            |
      ~                                                               ~
      |                                                               |
      |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 35: LDP Vendor-Private TLVs

   U-bit
      Unknown TLV bit.  Upon receipt of an unknown TLV, if U is clear
      (=0), a notification MUST be returned to the message originator
      and the entire message MUST be ignored; if U is set (=1), the
      unknown TLV is silently ignored and the rest of the message is
      processed as if the unknown TLV did not exist.

      The determination as to whether a vendor-private message is
      understood is based on the Type and the mandatory Vendor ID field.

      Implementations that support vendor-private TLVs MUST support a
      user-accessible configuration interface that causes the U-bit to
      be set on all transmitted vendor-private TLVs; this requirement
      MAY be satisfied by a user-accessible configuration interface that
      prevents transmission of all vendor-private TLVs for which the U-
      bit is clear.

   F-bit




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      Forward unknown TLV bit.  This bit only applies when the U-bit is
      set and the LDP message containing the unknown TLV is to be
      forwarded.  If F is clear (=0), the unknown TLV is not forwarded
      with the containing message; if F is set (=1), the unknown TLV is
      forwarded with the containing message.

   Type
      Type value in the range 0x3E00 through 0x3EFF.  Together, the Type
      and Vendor ID field specify how the Data field is to be
      interpreted.

   Length
      Specifies the cumulative length in octets of the Vendor ID and
      Data fields.

   Vendor ID
      802 Vendor ID as assigned by the IEEE.

   Data
      The remaining octets after the Vendor ID in the Value field are
      optional vendor-dependent data.

4.6.1.2.  LDP Vendor-Private Messages

   The Message Type range 0x3E00 through 0x3EFF is reserved for Vendor-
   Private messages.

























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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |U|    Msg Type (0x3E00-0x3EFF) |      Message Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Message ID                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Vendor ID                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +                                                               +
      |                     Remaining Mandatory Parameters            |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                     Optional Parameters                       |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 36: LDP Vendor-Private Messages

   U-bit
      Unknown message bit.  Upon receipt of an unknown message, if U is
      clear (=0), a notification is returned to the message originator;
      if U is set (=1), the unknown message is silently ignored.

      The determination as to whether a Vendor-Private message is
      understood is based on the Msg Type and the Vendor ID parameter.

      Implementations that support Vendor-Private messages MUST support
      a user-accessible configuration interface that causes the U-bit to
      be set on all transmitted Vendor-Private messages; this
      requirement MAY be satisfied by a user-accessible configuration
      interface that prevents transmission of all Vendor-Private
      messages for which the U-bit is clear.

   Msg Type
      Message Type value in the range 0x3E00 through 0x3EFF.  Together,
      the Msg Type and the Vendor ID specify how the message is to be
      interpreted.

   Message Length
      Specifies the cumulative length in octets of the Message ID,
      Vendor ID, Remaining Mandatory Parameters, and Optional
      Parameters.




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   Message ID
      32-bit integer used to identify this message.  Used by the sending
      LSR to facilitate identifying Notification messages that may apply
      to this message.  An LSR sending a Notification message in
      response to this message will include this Message ID in the
      notification message; see Section "Notification Message".

   Vendor ID
      802 Vendor ID as assigned by the IEEE.

   Remaining Mandatory Parameters
      Variable length set of remaining required message parameters.

   Optional Parameters
      Variable length set of optional message parameters.

4.6.2.  LDP Experimental Extensions

   LDP support for experimentation is similar to support for vendor-
   private extensions with the following differences:

   -  The Type range 0x3F00 through 0x3FFF is reserved for experimental
      TLVs.

   -  The Message Type range 0x3F00 through 0x3FFF is reserved for
      experimental messages.

   -  The encodings for experimental TLVs and messages are similar to
      the vendor-private encodings with the following difference.

      Experimental TLVs and messages use an Experiment ID field in place
      of a Vendor ID field.  The Experiment ID field is used with the
      Type or Message Type field to specify the interpretation of the
      experimental TLV or Message.

      Administration of Experiment IDs is the responsibility of the
      experimenters.

4.7.  Message Summary

   The following are the LDP messages defined in this version of the
   protocol.









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   +---------------+---------+---------+-------------------------------+
   | Message Name  | Type    | Section | Section Title                 |
   |               |         | Number  |                               |
   +---------------+---------+---------+-------------------------------+
   | Notification  | 0x0001  | 4.5.1   | Notification Message          |
   | Hello         | 0x0100  | 4.5.2   | Hello Message                 |
   | Initializatio | 0x0200  | 4.5.3   | Initialization Message        |
   | n             |         |         |                               |
   | KeepAlive     | 0x0201  | 4.5.4   | KeepAlive Message             |
   | Address       | 0x0300  | 4.5.5   | Address Message               |
   | Address       | 0x0301  | 4.5.6   | Address Withdraw Message      |
   | Withdraw      |         |         |                               |
   | Label Mapping | 0x0400  | 4.5.7   | Label Mapping Message         |
   | Label Request | 0x0401  | 4.5.8   | Label Request Message         |
   | Label         | 0x0402  | 4.5.10  | Label Withdraw Message        |
   | Withdraw      |         |         |                               |
   | Label Release | 0x0403  | 4.5.11  | Label Release Message         |
   | Label Abort   | 0x0404  | 4.5.9   | Label Abort Request Message   |
   | Request       |         |         |                               |
   | Vendor-       | 0x3E00- | 4.6.1   | LDP Vendor-Private Extensions |
   | Private       | 0x3EFF  |         |                               |
   | Experimental  | 0x3F00- | 4.6.2   | LDP Experimental Extensions   |
   |               | 0x3FFF  |         |                               |
   +---------------+---------+---------+-------------------------------+

                         Table 15: Message Summary

4.8.  TLV Summary

   The following are the TLVs defined in this version of the protocol.





















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   +---------------+---------+---------+-------------------------------+
   | Message Name  | Type    | Section | Section Title                 |
   |               |         | Number  |                               |
   +---------------+---------+---------+-------------------------------+
   | FEC           | 0x0100  | 4.4.1   | FEC TLV                       |
   | Address List  | 0x0101  | 4.4.3   | Address List TLV              |
   | Hop Count     | 0x0103  | 4.4.4   | Hop Count TLV                 |
   | Path Vector   | 0x0104  | 4.4.5   | Path Vector TLV               |
   | Generic Label | 0x0200  | 4.4.2.1 | Generic Label TLV             |
   | ATM Label     | 0x0201  | 4.4.2.2 | ATM Label TLV                 |
   | Frame Relay   | 0x0202  | 4.4.2.3 | Frame Relay Label TLV         |
   | Status        | 0x0300  | 4.4.6   | Status TLV                    |
   | Extended      | 0x0301  | 4.5.1   | Notification Message          |
   | Status        |         |         |                               |
   | Returned PDU  | 0x0302  | 4.5.1   | Notification Message          |
   | Returned      | 0x0303  | 4.5.1   | Notification Message          |
   | Message       |         |         |                               |
   | Common Hello  | 0x0400  | 4.5.2   | Hello Message                 |
   | Parameters    |         |         |                               |
   | IPv4          | 0x0401  | 4.5.2   | Hello Message                 |
   | Transport     |         |         |                               |
   | Address       |         |         |                               |
   | Configuration | 0x0402  | 4.5.2   | Hello Message                 |
   | Sequence      |         |         |                               |
   | Number        |         |         |                               |
   | IPv6          | 0x0403  | 4.5.2   | Hello Message                 |
   | Transport     |         |         |                               |
   | Address       |         |         |                               |
   | Common        | 0x0500  | 4.5.3   | Initialization Message        |
   | Session       |         |         |                               |
   | Parameters    |         |         |                               |
   | ATM Session   | 0x0501  | 4.5.3   | Initialization Message        |
   | Parameters    |         |         |                               |
   | Frame Relay   | 0x0502  | 4.5.3   | Initialization Message        |
   | Session       |         |         |                               |
   | Parameters    |         |         |                               |
   | Label Request | 0x0600  | 4.5.7   | Label Mapping Message         |
   | Message ID    |         |         |                               |
   | Vendor-       | 0x3E00- | 4.6.1   | LDP Vendor-Private Extensions |
   | Private       | 0x3EFF  |         |                               |
   | Experimental  | 0x3F00- | 4.6.2   | LDP Experimental Extensions   |
   |               | 0x3FFF  |         |                               |
   +---------------+---------+---------+-------------------------------+

                           Table 16: TLV Summary






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4.9.  Status Code Summary

   The following are the Status Codes defined in this version of the
   protocol.

   The "E" column is the required setting of the Status Code E-bit; the
   "Status Data" column is the value of the 30-bit Status Data field in
   the Status Code TLV.  Note that the setting of the Status Code F-bit
   is at the discretion of the LSR originating the Status TLV.

   +--------------------------------+---+------------+-----------------+
   | Status Code                    | E | Status     | Section Number  |
   |                                |   | Data       |                 |
   +--------------------------------+---+------------+-----------------+
   | Success                        | 0 | 0x00000000 | 4.4.6           |
   | Bad LDP Identifier             | 1 | 0x00000001 | 4.5.1.2         |
   | Bad Protocol Version           | 1 | 0x00000002 | 4.5.1.2         |
   | Bad PDU Length                 | 1 | 0x00000003 | 4.5.1.2         |
   | Unknown Message Type           | 0 | 0x00000004 | 4.5.1.2         |
   | Bad Message Length             | 1 | 0x00000005 | 4.5.1.2         |
   | Unknown TLV                    | 0 | 0x00000006 | 4.5.1.2         |
   | Bad TLV Length                 | 1 | 0x00000007 | Section 4.5.1.2 |
   | Malformed TLV Value            | 1 | 0x00000008 | 4.5.1.2         |
   | Hold Timer Expired             | 1 | 0x00000009 | 4.5.1.2         |
   | Shutdown                       | 1 | 0x0000000A | 4.5.1.2         |
   | Loop Detected                  | 0 | 0x0000000B | 3.8             |
   | Unknown FEC                    | 0 | 0x0000000C | 4.4.1.1         |
   | No Route                       | 0 | 0x0000000D | 4.5.8           |
   | No Label Resources             | 0 | 0x0000000E | 4.5.8           |
   | Label Resources / Available    | 0 | 0x0000000F | 4.5.8           |
   | Session Rejected / No Hello    | 1 | 0x00000010 | 3.5.3           |
   | Session Rejected / Parameters  | 1 | 0x00000011 | 3.5.3           |
   | Advertisement Mode             |   |            |                 |
   | Session Rejected / Parameters  | 1 | 0x00000012 | 3.5.3           |
   | Max PDU Length                 |   |            |                 |
   | Session Rejected / Parameters  | 1 | 0x00000013 | Section 3.5.3   |
   | Label Range                    |   |            |                 |
   | KeepAlive Timer Expired        | 1 | 0x00000014 | 3.5.3           |
   | Label Request Aborted          | 0 | 0x00000015 | 4.5.9           |
   | Missing Message Parameters     | 0 | 0x00000016 | 4.5.1.2         |
   | Unsupported Address Family     | 0 | 0x00000017 | 4.4.1.1         |
   | Session Rejected / Bad Keep    | 0 | 0x00000018 | 3.5.3           |
   | Alive Time                     |   |            |                 |
   | Internal Error                 | 0 | 0x00000019 | 4.5.1.2         |
   +--------------------------------+---+------------+-----------------+

                       Table 17: Status Code Summary




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4.10.  Well-Known Numbers

   prov text

4.10.1.  UDP and TCP Ports

   The UDP port for LDP Hello messages is 646.

   The TCP port for establishing LDP session connections is 646.

4.10.2.  Implicit NULL Label

   The Implicit NULL label is defined in RFC 3031 [RFC3031] as follows:

   "The Implicit NULL label is a label with special semantics which an
   LSR can bind to an address prefix.  If LSR Ru, by consulting its ILM
   (Incoming Label Map) sees that labeled packet P must be forwarded
   next to Rd, but that Rd has distributed a binding of Implicit NULL to
   the corresponding address prefix, then instead of replacing the value
   of the label on top of the label stack, Ru pops the label stack, and
   then forwards the resulting packet to Rd."

   The implicit NULL label is represented in LDP as a Generic Label TLV
   with a Label field value of 3, as defined in RFC 3032 [RFC3032].

5.  RFC 5036 IANA Considerations

   Note: In version -00 of this document does only minimal changes to
   the RFC 5036 IANA considerationns.  The author believe that some
   further minor changes will be made eventually.  The "IANA
   consideration" section (see Section 9) is included to capture
   anything new that relates to IANA, Before publication the two section
   will be merged.

   The LDP specification defines the following name spaces that are
   managed by IANA and found at [LDP_NAME_SPACE]:

   -  Message Type Name Space, found at [MSG_TYPE_NAME_SPACE]
   -  TLV Type Name Space, found at [TLV_TYPE_NAME_SPACE]
   -  FEC Type Name Space, found at [FEC_TYPE_NAME_SPACE]
   -  Status Code Name Space, found at [STATUS_CODE_NAME_SPACE]
   -  Experiment ID Name Space, found at [EXP_ID_NAME_SPACE]

   Section 5.1 "Message Type Name Space" to Section 5.5
   "Experiment ID Name Space" provide guidelines for managing these name
   spaces.





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   LDP Name Spaces have also been defined by other RFCs since the LDP
   Specification was first published and are managed by IANA as addition
   to the original LDP Name Space at [LDP_NAME_SPACE].

      RFC 6388 [RFC6388] defined:



         LDP MP Opaque Value Element basic type, found at
         [MP_BASIC_OPAQUE]

         LDP MP Opaque Value Element extended type, found at
         [EXT_BASIC_OPAQUE]

         LDP MP Status Value Element type, found at [MP_STATUS_VALUE]

      RFC 7361 [RFC7361]defined



         MAC Flush Flags, found at [MAC_FLUSH]

   The guidelines for how to manage the name spaces defined in other
   RFCs than RFC 5036 (or this document when it gets approved) is found
   in the RFCs that defined the name spaces.

5.1.  Message Type Name Space

   LDP divides the name space for message types into three ranges.  The
   following are the guidelines for managing these ranges:

   -  Message Types 0x0000 - 0x3DFF.  Message types in this range are
      part of the LDP base protocol.  Following the policies outlined in
      RFC 5226 [RFC5226] and RFC 2434 [RFC2434], Message types in this
      range are allocated through an IETF Consensus action.

   -  Message Types 0x3E00 - 0x3EFF.  Message types in this range are
      reserved for Vendor-Private extensions and are the responsibility
      of the individual vendors (see Section "LDP Vendor-Private
      Messages").  IANA management of this range of the Message Type
      Name Space is unnecessary.

   -  Message Types 0x3F00 - 0x3FFF.  Message types in this range are
      reserved for Experimental extensions and are the responsibility of
      the individual experimenters (see Sections "LDP Experimental
      Extensions" and "Experiment ID Name Space").  IANA management of
      this range of the Message Type Name Space is unnecessary; however,




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      IANA is responsible for managing part of the Experiment ID Name
      Space (see below).

5.2.  TLV Type Name Space

   LDP divides the name space for TLV types into three ranges.  The
   following are the guidelines for managing these ranges:

   -  TLV Types 0x0000 - 0x3DFF.  TLV types in this range are part of
      the LDP base protocol.  Following the policies outlined in RFC
      5226 [RFC5226] and RFC 2434 [RFC2434], TLV types in this range are
      allocated through an IETF Consensus action.

   -  TLV Types 0x3E00 - 0x3EFF.  TLV types in this range are reserved
      for Vendor-Private extensions and are the responsibility of the
      individual vendors (see Section "LDP Vendor-Private TLVs").  IANA
      management of this range of the TLV Type Name Space is
      unnecessary.

   -  TLV Types 0x3F00 - 0x3FFF.  TLV types in this range are reserved
      for Experimental extensions and are the responsibility of the
      individual experimenters (see Sections "LDP Experimental
      Extensions" and "Experiment ID Name Space").  IANA management of
      this range of the TLV Name Space is unnecessary; however, IANA is
      responsible for managing part of the Experiment ID Name Space (see
      below).

5.3.  FEC Type Name Space

   The range for FEC types is 0 - 255.

   Following the policies outlined in RFC 5226 [RFC5226] and RFC 2434
   [RFC2434], FEC types in the range 0 - 127 are allocated through an
   IETF Consensus action, types in the range 128 - 191 are allocated as
   First Come First Served, and types in the range 192 - 255 are
   reserved for Private Use.

5.4.  Status Code Name Space

   The range for Status Codes is 0x00000000 - 0x3FFFFFFF.

   Following the policies outlined in RFC 5226 [RFC5226] and RFC 2434
   [RFC2434], Status Codes in the range 0x00000000 - 0x1FFFFFFF are
   allocated through an IETF Consensus action, codes in the range
   0x20000000 - 0x3EFFFFFF are allocated as First Come First Served, and
   codes in the range 0x3F000000 - 0x3FFFFFFF are reserved for Private
   Use.




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5.5.  Experiment ID Name Space

   The range for Experiment IDs is 0x00000000 - 0xffffffff.

   Following the policies outlined in RFC 5226 [RFC5226] and RFC 2434
   [RFC2434], Experiment IDs in the range 0x00000000 - 0xefffffff are
   allocated as First Come First Served and Experiment IDs in the range
   0xf0000000 - 0xffffffff are reserved for Private Use.

6.  Security Considerations

   Editors Note: This is still working in progress.

   This section identifies threats to which LDP may be vulnerable and
   discusses means by which those threats might be mitigated.

6.1.  Spoofing

   There are two types of LDP communication that could be the target of
   a spoofing attack.

   1.  Discovery exchanges carried by UDP

       LSRs indicate their willingness to establish and maintain LDP
       sessions by periodically sending Hello messages.  Receipt of a
       Hello serves to create a new "Hello adjacency", if one does not
       already exist, or to refresh an existing one.  Spoofing a Hello
       packet for an existing adjacency can cause the adjacency to time
       out and that can result in termination of the associated session.
       This can occur when the spoofed Hello specifies a small Hold
       Time, causing the receiver to expect Hellos within this interval,
       while the true neighbor continues sending Hellos at the lower,
       previously agreed to, frequency.

       LSRs directly connected at the link level exchange Basic Hello
       messages over the link.  The threat of spoofed Basic Hellos can
       be reduced by:

       o  Accepting Basic Hellos only on interfaces to which LSRs that
          can be trusted are directly connected.

       o  Ignoring Basic Hellos not addressed to the All Routers on this
          Subnet multicast group.

       LSRs not directly connected at the link level may use Extended
       Hello messages to indicate willingness to establish an LDP
       session.  An LSR can reduce the threat of spoofed Extended Hellos




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       by filtering them and accepting only those originating at sources
       permitted by an access list.

   2.  LSRs not directly connected at the link level may use Extended
       Hello messages to indicate willingness to establish an LDP
       session.  An LSR can reduce the threat of spoofed Extended Hellos
       by filtering them and accepting only those originating at sources
       permitted by an access list.

   3.  Session communication carried by TCP

       LDP specifies use of the TCP MD5 Signature Option to provide for
       the authenticity and integrity of session messages.

       RFC 2385 [RFC2385] asserts that MD5 authentication is now
       considered by some to be too weak for this application.  It also
       points out that a similar TCP option with a stronger hashing
       algorithm (it cites SHA-1 as an example) could be deployed.  To
       our knowledge, no such TCP option has been defined and deployed.
       However, we note that LDP can use whatever TCP message digest
       techniques are available, and when one stronger than MD5 is
       specified and implemented, upgrading LDP to use it would be
       relatively straightforward.

6.2.  Privacy

   LDP provides no mechanism for protecting the privacy of label
   distribution.

   The security requirements of label distribution protocols are
   essentially identical to those of the protocols that distribute
   routing information.  By providing a mechanism to ensure the
   authenticity and integrity of its messages, LDP provides a level of
   security that is at least as good as, though no better than, that
   which can be provided by the routing protocols themselves.  The more
   general issue of whether privacy should be required for routing
   protocols is beyond the scope of this document.

   One might argue that label distribution requires privacy to address
   the threat of label spoofing.  However, that privacy would not
   protect against label spoofing attacks since data packets carry
   labels in the clear.  Furthermore, label spoofing attacks can be made
   without knowledge of the FEC bound to a label.

   To avoid label spoofing attacks, it is necessary to ensure that
   labeled data packets are labeled by trusted LSRs and that the labels
   placed on the packets are properly learned by the labeling LSRs.




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6.3.  Denial of Service

   LDP provides two potential targets for Denial of Service (DoS)
   attacks:

   1.  Well-known UDP Port for LDP Discovery

       An LSR administrator can address the threat of DoS attacks via
       Basic Hellos by ensuring that the LSR is directly connected only
       to peers that can be trusted to not initiate such an attack.
       Interfaces to peers interior to the administrator's domain should
       not represent a threat since interior peers are under the
       administrator's control.  Interfaces to peers exterior to the
       domain represent a potential threat since exterior peers are not.
       An administrator can reduce that threat by connecting the LSR
       only to exterior peers that can be trusted to not initiate a
       Basic Hello attack.

       DoS attacks via Extended Hellos are potentially a more serious
       threat.  This threat can be addressed by filtering Extended
       Hellos using access lists that define addresses with which
       Extended Discovery is permitted.  However, performing the
       filtering requires LSR resource.

       In an environment where a trusted MPLS cloud can be identified,
       LSRs at the edge of the cloud can be used to protect interior
       LSRs against DoS attacks via Extended Hellos by filtering out
       Extended Hellos originating outside of the trusted MPLS cloud,
       accepting only those originating at addresses permitted by access
       lists.  This filtering protects LSRs in the interior of the cloud
       but consumes resources at the edges.

   2.  Well-known TCP port for LDP Session Establishment

       Like other control plane protocols that use TCP, LDP may be the
       target of DoS attacks, such as SYN attacks.  LDP is no more or
       less vulnerable to such attacks than other control plane
       protocols that use TCP.

       The threat of such attacks can be mitigated somewhat by the
       following:

       o  An LSR SHOULD avoid promiscuous TCP listens for LDP session
          establishment.  It SHOULD use only listens that are specific
          to discovered peers.  This enables it to drop attack packets
          early in their processing since they are less likely to match
          existing or in-progress connections.




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       o  The use of the MD5 option helps somewhat since it prevents a
          SYN from being accepted unless the MD5 segment checksum is
          valid.  However, the receiver must compute the checksum before
          it can decide to discard an otherwise acceptable SYN segment.

       o  The use of access list mechanisms applied at the boundary of
          the MPLS cloud in a manner similar to that suggested above for
          Extended Hellos can protect the interior against attacks
          originating from outside the cloud.

7.  Areas for Future Study

   The following topics not addressed in this version of LDP are
   possible areas for future study:

   Note: in the -00 version of this document this section has not been
   changed from RFC 5036.  It will need to be reviewed and uodated.

   -  Section 2.16 of the MPLS architecture RFC 3031 [RFC3031] requires
      that the initial label distribution protocol negotiation between
      peer LSRs enable each LSR to determine whether its peer is capable
      of popping the label stack.  This version of LDP assumes that LSRs
      support label popping for all link types except ATM and Frame
      Relay.  A future version may specify means to make this
      determination part of the session initiation negotiation.

   -  LDP support for CoS (Class of Service) is not specified in this
      version.  CoS support may be addressed in a future version.

   -  LDP support for multicast is not specified in this version.
      Multicast support may be addressed in a future version.

   -  LDP support for multipath label switching is not specified in this
      version.  Multipath support may be addressed in a future version.

   -  LDP support for signaling the maximum transmission unit is not
      specified in this version.  It is discussed in the experimental
      document RFC 3988 [RFC3988].

   -  The current specification does not address basic peer discovery on
      Non-Broadcast Multi-Access (NBMA) media.  The solution available
      in the current specification is to use extended peer discovery in
      such setups.  The issue of defining a mechanism semantically
      similar to Basic Discovery (1 hop limit, bind the hello adjacency
      to an interface) that uses preconfigured neighbor addresses is
      left for further study.





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   -  The current specification does not support shutting down an
      adjacency.  The motivation for doing it and the mechanisms for
      achieving it are left for further study.

   -  The current specification does not include a method for securing
      Hello messages, to detect spoofing of Hellos.  The scenarios where
      this is necessary, as well as the mechanism for achieving it are
      left for future study.

   -  The current specification does not have the ability to detect a
      stateless fast control plane restart.  The method for achieving
      this, possibly through an "incarnation/instance" number carried in
      the Hello message, is left for future study.

   -  The current specification does not support an "end of LIB"
      message, analogous to BGP's "end of RIB" message that an LDP LSR
      (operating in DU mode) would use following session establishment.
      The discussion on the need for such a mechanism and its
      implementation is left for future study.

   -  The current specification does not deal with situations where
      different LSRs advertise the same address.  Such situations
      typically occur as the result of configuration errors, and the
      goal in this case is to provide the LSRs advertising the same
      address with enough information to enable operators to take
      corrective action.  The specification of this mechanism is left
      for a separate document.

8.  Changes from RFC 5036

   Here is a list of changes from RFC 5036

   1.  Some editorial changes has been made, e.g. internal references is
       more frequently used, some implicit lists has been replaced by
       tables, e.g. for Optional Parameters carried in LDP messages.

   2.  The refrence to CR-LDP has been removed.

   3.  References to the LDP registries create outside the LDP
       Specification has been added.

9.  IANA Considerations

   There are no requests for IANA actions in this document.

   Note to the RFC Editor - this section can be removed before
   publication.




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10.  References

10.1.  Normative References

   [ASSIGNED_AF]
              "IANA Assigned Address Families",
              <http://www.iana.org/assignments/address-family-numbers>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <http://www.rfc-editor.org/info/rfc1321>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <http://www.rfc-editor.org/info/rfc2328>.

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, DOI 10.17487/RFC2385, August
              1998, <http://www.rfc-editor.org/info/rfc2385>.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 2434,
              DOI 10.17487/RFC2434, October 1998,
              <http://www.rfc-editor.org/info/rfc2434>.

   [RFC2702]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
              McManus, "Requirements for Traffic Engineering Over MPLS",
              RFC 2702, DOI 10.17487/RFC2702, September 1999,
              <http://www.rfc-editor.org/info/rfc2702>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <http://www.rfc-editor.org/info/rfc3031>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <http://www.rfc-editor.org/info/rfc3032>.







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   [RFC3034]  Conta, A., Doolan, P., and A. Malis, "Use of Label
              Switching on Frame Relay Networks Specification",
              RFC 3034, DOI 10.17487/RFC3034, January 2001,
              <http://www.rfc-editor.org/info/rfc3034>.

   [RFC3035]  Davie, B., Lawrence, J., McCloghrie, K., Rosen, E.,
              Swallow, G., Rekhter, Y., and P. Doolan, "MPLS using LDP
              and ATM VC Switching", RFC 3035, DOI 10.17487/RFC3035,
              January 2001, <http://www.rfc-editor.org/info/rfc3035>.

   [RFC3037]  Thomas, B. and E. Gray, "LDP Applicability", RFC 3037,
              DOI 10.17487/RFC3037, January 2001,
              <http://www.rfc-editor.org/info/rfc3037>.

   [RFC3988]  Black, B. and K. Kompella, "Maximum Transmission Unit
              Signalling Extensions for the Label Distribution
              Protocol", RFC 3988, DOI 10.17487/RFC3988, January 2005,
              <http://www.rfc-editor.org/info/rfc3988>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5860]  Vigoureux, M., Ed., Ward, D., Ed., and M. Betts, Ed.,
              "Requirements for Operations, Administration, and
              Maintenance (OAM) in MPLS Transport Networks", RFC 5860,
              DOI 10.17487/RFC5860, May 2010,
              <http://www.rfc-editor.org/info/rfc5860>.

   [RFC5921]  Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed., Levrau,
              L., and L. Berger, "A Framework for MPLS in Transport
              Networks", RFC 5921, DOI 10.17487/RFC5921, July 2010,
              <http://www.rfc-editor.org/info/rfc5921>.

   [RFC6371]  Busi, I., Ed. and D. Allan, Ed., "Operations,
              Administration, and Maintenance Framework for MPLS-Based
              Transport Networks", RFC 6371, DOI 10.17487/RFC6371,
              September 2011, <http://www.rfc-editor.org/info/rfc6371>.

   [RFC6372]  Sprecher, N., Ed. and A. Farrel, Ed., "MPLS Transport
              Profile (MPLS-TP) Survivability Framework", RFC 6372,
              DOI 10.17487/RFC6372, September 2011,
              <http://www.rfc-editor.org/info/rfc6372>.







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10.2.  Informative References

   [EXP_ID_NAME_SPACE]
              "Experiment ID Name Space",
              <http://www.iana.org/assignments/ldp-namespaces/
              ldp-namespaces.xhtml#ldp-namespaces-10>.

   [EXT_BASIC_OPAQUE]
              "LDP MP Opaque Value Element extended type",
              <http://www.iana.org/assignments/ldp-namespaces/
              ldp-namespaces.xhtml#ldp-namespaces-13>.

   [FEC_TYPE_NAME_SPACE]
              "Forwarding Equivalence Class (FEC) Type Name Space",
              <http://www.iana.org/assignments/ldp-namespaces/
              ldp-namespaces.xhtml#fec-type>.

   [LDP_NAME_SPACE]
              "Label Distribution Protocol (LDP) Parameters",
              <http://www.iana.org/assignments/ldp-namespaces/
              ldp-namespaces.xhtml>.

   [MAC_FLUSH]
              "MAC Flush Flags", <http://www.iana.org/assignments/ldp-
              namespaces/ldp-namespaces.xhtml#mac-flush-flags>.

   [MP_BASIC_OPAQUE]
              "LDP MP Opaque Value Element basic type",
              <http://www.iana.org/assignments/ldp-namespaces/
              ldp-namespaces.xhtml#ldp-namespaces-11>.

   [MP_STATUS_VALUE]
              "LDP MP Opaque Value Element extended type",
              <http://www.iana.org/assignments/ldp-namespaces/
              ldp-namespaces.xhtml#ldp-namespaces-14>.

   [MSG_TYPE_NAME_SPACE]
              "Message Type Name Space",
              <http://www.iana.org/assignments/ldp-namespaces/
              ldp-namespaces.xhtml#ldp-namespaces-2>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <http://www.rfc-editor.org/info/rfc4271>.






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   [RFC4278]  Bellovin, S. and A. Zinin, "Standards Maturity Variance
              Regarding the TCP MD5 Signature Option (RFC 2385) and the
              BGP-4 Specification", RFC 4278, DOI 10.17487/RFC4278,
              January 2006, <http://www.rfc-editor.org/info/rfc4278>.

   [RFC6388]  Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
              Thomas, "Label Distribution Protocol Extensions for Point-
              to-Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011,
              <http://www.rfc-editor.org/info/rfc6388>.

   [RFC7361]  Dutta, P., Balus, F., Stokes, O., Calvignac, G., and D.
              Fedyk, "LDP Extensions for Optimized MAC Address
              Withdrawal in a Hierarchical Virtual Private LAN Service
              (H-VPLS)", RFC 7361, DOI 10.17487/RFC7361, September 2014,
              <http://www.rfc-editor.org/info/rfc7361>.

   [STATUS_CODE_NAME_SPACE]
              "Status Code Name Space",
              <http://www.iana.org/assignments/ldp-namespaces/
              ldp-namespaces.xhtml#status-codes>.

   [TLV_TYPE_NAME_SPACE]
              "TLV Type Name Space", <http://www.iana.org/assignments/
              ldp-namespaces/ldp-namespaces.xhtml#ldp-namespaces-4>.

Appendix A.  LDP Label Distribution Procedures

   This section specifies label distribution behavior in terms of LSR
   response to the following events:

   -  Receive Label Request Message;
   -  Receive Label Mapping Message;
   -  Receive Label Abort Request Message;
   -  Receive Label Release Message;
   -  Receive Label Withdraw Message;
   -  Recognize new FEC;
   -  Detect change in FEC next hop;
   -  Receive Notification Message / Label Request Aborted;
   -  Receive Notification Message / No Label Resources;
   -  Receive Notification Message / No Route;
   -  Receive Notification Message / Loop Detected;
   -  Receive Notification Message / Label Resources Available;
   -  Detect local label resources have become available;
   -  LSR decides to no longer label switch a FEC;
   -  Timeout of deferred label request.





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   The specification of LSR behavior in response to an event has three
   parts:

   1.  Summary.  Prose that describes LSR response to the event in
       overview.

   2.  Context.  A list of elements referred to by the Algorithm part of
       the specification.  (See 3.)

   3.  Algorithm.  An algorithm for LSR response to the event.

   The summary may omit details of the LSR response, such as bookkeeping
   action or behavior dependent on the LSR label advertisement mode,
   control mode, or label retention mode in use.  The intent is that the
   Algorithm fully and unambiguously specify the LSR response.

   The algorithms in this section use procedures defined in the MPLS
   architecture specification RFC 3031 [RFC3031] for hop-by-hop routed
   traffic.  These procedures are:

   -  Label Distribution procedure, which is performed by a downstream
      LSR to determine when to distribute a label for a FEC to LDP
      peers.  The architecture defines four Label Distribution
      procedures:

      o  Downstream Unsolicited Independent Control, called
         PushUnconditional in RFC 3031 [RFC3031].

      o  Downstream Unsolicited Ordered Control, called PushConditional
         in RFC 3031 [RFC3031].

      o  Downstream On Demand Independent Control, called
         PulledUnconditional in RFC 3031 [RFC3031].

      o  Downstream On Demand Ordered Control, called PulledConditional
         in RFC 3031 [RFC3031].

   -  Label Withdrawal procedure, which is performed by a downstream LSR
      to determine when to withdraw a FEC label mapping previously
      distributed to LDP peers.  The architecture defines a single Label
      Withdrawal procedure.  Whenever an LSR breaks the binding between
      a label and a FEC, it MUST withdraw the FEC label mapping from all
      LDP peers to which it has previously sent the mapping.

   -  Label Request procedure, which is performed by an upstream LSR to
      determine when to explicitly request that a downstream LSR bind a
      label to a FEC and send it the corresponding label mapping.  The
      architecture defines three Label Request procedures:



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      o  Request Never.  The LSR never requests a label.

      o  Request When Needed.  The LSR requests a label whenever it
         needs one.

      o  Request On Request.  This procedure is used by non-label
         merging LSRs.  The LSR requests a label when it receives a
         request for one, in addition to whenever it needs one.

   -  Label Release procedure, which is performed by an upstream LSR to
      determine when to release a previously received label mapping for
      a FEC.  The architecture defines two Label Release procedures:

      o  Conservative Label retention, called ReleaseOnChange in RFC
         3031 [RFC3031].

      o  Liberal Label retention, called NoReleaseOnChange in RFC 3031
         [RFC3031].

   -  Label Use procedure, which is performed by an LSR to determine
      when to start using a FEC label for forwarding/switching.  The
      architecture defines three Label Use procedures:

      o  Use Immediate.  The LSR immediately uses a label received from
         a FEC next hop for forwarding/switching.

      o  Use If Loop Free.  The LSR uses a FEC label received from a FEC
         next hop for forwarding/switching only if it has determined
         that by doing so it will not cause a forwarding loop.

      o  Use If Loop Not Detected.  This procedure is the same as Use
         Immediate unless the LSR has detected a loop in the FEC LSP.
         Use of the FEC label for forwarding/switching will continue
         until the next hop for the FEC changes or the loop is no longer
         detected.

   This version of LDP does not include a loop prevention mechanism;
   therefore, the procedures below do not make use of the Use If Loop
   Free procedure.

   -  Label No Route procedure (called the NotAvailable procedure in RFC
      3031 [RFC3031]), which is performed by an upstream LSR to
      determine how to respond to a No Route notification from a
      downstream LSR in response to a request for a FEC label mapping.
      The architecture specification defines two Label No Route
      procedures:





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      o  Request Retry.  The LSR should issue the label request at a
         later time.

      o  No Request Retry.  The LSR should assume that the downstream
         LSR will provide a label mapping when the downstream LSR has a
         next hop, and it should not reissue the request.

A.1.  Handling Label Distribution Events

   This section defines LDP label distribution procedures by specifying
   an algorithm for each label distribution event.  The requirement on
   an LDP implementation is that its event handling must have the effect
   specified by the algorithms.  That is, an implementation need not
   follow exactly the steps specified by the algorithms as long as the
   effect is identical.

   The algorithms for handling label distribution events share common
   actions.  The specifications below package these common actions into
   procedure units.  Specifications for these common procedures are in
   their own Section, "Common Label Distribution Procedures", which
   follows this.

   An implementation would use data structures to store information
   about protocol activity.  This appendix specifies the information to
   be stored in sufficient detail to describe the algorithms, and
   assumes the ability to retrieve the information as needed.  It does
   not specify the details of the data structures.

A.1.1.  Receive Label Request

   Summary:

      The response by an LSR to receipt of a FEC label request from an
      LDP peer may involve one or more of the following actions:



      -  Transmission of a notification message to the requesting LSR
         indicating why a label mapping for the FEC cannot be provided;

      -  Transmission of a FEC label mapping to the requesting LSR;

      -  Transmission of a FEC label request to the FEC next hop;

      -  Installation of labels for forwarding/switching use by the LSR.

   Context:




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   -  LSR.  The LSR handling the event.

   -  MsgSource.  The LDP peer that sent the message.

   -  FEC.  The FEC specified in the message.

   -  RAttributes.  Attributes received with the message, e.g., Hop
      Count, Path Vector.

   -  SAttributes.  Attributes to be included in the Label Request
      message, if any, propagated to FEC Next Hop.

   -  StoredHopCount.  The hop count, if any, previously recorded for
      the FEC.

   Algorithm:

   LRq.1:   Execute procedure Check_Received_Attributes (MsgSource,
            LabelRequest, RAttributes).
            If Loop Detected, goto LRq.4.

   LRq.2:   Is there a Next Hop for FEC?
            If not, goto LRq.5.

   LRq.3:   Is MsgSource the Next Hop?
            If not, goto LRq.6.

   LRq.4:   Execute procedure Send_Notification (MsgSource, Loop
            Detected).
            Goto LRq.13

   LRq.5:   Execute procedure Send_Notification (MsgSource, No Route).
            Goto LRq.13.

   LRq.6:   Has LSR previously received a label request for FEC from
            MsgSource?
            If not, goto LRq.8.  (See Note 1.)

   LRq.7:   Is the label request a duplicate request?
            If so, goto LRq.13.  (See Note 2.)

   LRq.8:   Record label request for FEC received from MsgSource and
            mark it pending.


   LRq.9:   Perform LSR Label Distribution procedure:





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               For Downstream Unsolicited Independent Control OR For
               Downstream On Demand Independent Control

               1.  Has LSR previously received and retained a label
                   mapping for FEC from Next Hop?
                   Is so, set Propagating to IsPropagating.
                   If not, set Propagating to NotPropagating.

               2.  Execute procedure
                   Is so, set Propagating to IsPropagating.
                   If not, set Propagating to NotPropagating.

               3.  Execute procedure Send_Label (MsgSource, FEC,
                   SAttributes).

               4.  Is LSR egress for FEC?  OR Has LSR previously
                   received and retained a label mapping for FEC from
                   Next Hop?
                   Is so, goto LRq.11.
                   If not, goto LRq.10.

               For Downstream Unsolicited Ordered Control OR For
               Downstream On Demand Ordered Control

               1.  Is LSR egress for FEC?  OR Has LSR previously
                   received and retained a label mapping for FEC from
                   Next Hop?
                   (See Note 3.)
                   If not, goto LRq.10.

               2.  Execute procedure
                   Prepare_Label_Mapping_Attributes(MsgSource, FEC,
                   RAttributes, SAttributes, IsPropagating,
                   StoredHopCount)

               3.  Execute procedure Send_Label (MsgSource, FEC,
                   SAttributes).
                   Goto LRq.11.

   LRq.10:  Perform LSR Label Request procedure:

               For Request Never

               1.  Goto LRq.13.

               For Request When Needed OR
               For Request On Request




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               1.  Execute procedure Prepare_Label_Request_Attributes
                   (Next Hop, FEC, RAttributes, SAttributes);

               2.  Execute procedure Send_Label_Request (Next Hop, FEC,
                   SAttributes).
                   Goto LRq.13.

   LRq.11:  Has LSR successfully sent a label for FEC to MsgSource?
            If not, goto LRq.13.  (See Note 4.)

   LRq.12:  Perform LSR Label Use procedure.

            For Use Immediate OR For Use If Loop Not Detected

            1.  Install label sent to MsgSource and label from Next
                Hop (if LSR is not egress) for forwarding/switching use.

   LRq.13:  DONE.

   Notes:

   1.  In the case where MsgSource is a non-label merging LSR, it will
       send a label request for each upstream LDP peer that has
       requested a label for FEC from it.  The LSR must be able to
       distinguish such requests from a non-label merging MsgSource from
       duplicate label requests.

       The LSR uses the message ID of received Label Request messages to
       detect duplicate requests.  This means that an LSR (the upstream
       peer) may not reuse the message ID used for a Label Request until
       the Label Request transaction has completed.

   2.  When an LSR sends a label request to a peer, it records that the
       request has been sent and marks it as outstanding.  As long as
       the request is marked outstanding, the LSR SHOULD NOT send
       another request for the same label to the peer.  Such a second
       request would be a duplicate.  The Send_Label_Request procedure
       described below obeys this rule.

       A duplicate label request is considered a protocol error and
       SHOULD be dropped by the receiving LSR (perhaps with a suitable
       notification returned to MsgSource).

   3.  If the LSR is not merge-capable, this test will fail.

   4.  The Send_Label procedure may fail due to lack of label resources,
       in which case the LSR SHOULD NOT perform the Label Use procedure.




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A.1.2.  Receive Label Mapping

   Summary:

   The response by an LSR to receipt of a FEC label mapping from an LDP
   peer may involve one or more of the following actions:

   -  Transmission of a Label Release message for the FEC label to the
      LDP peer;

   -  Transmission of Label Mapping messages for the FEC to one or more
      LDP peers;

   -  Installation of the newly learned label for forwarding/switching
      use by the LSR.

   Context:

   -  LSR.  The LSR handling the event.

   -  MsgSource.  The LDP peer that sent the message.

   -  FEC.  The FEC specified in the message.

   -  Label.  The label specified in the message.

   -  PrevAdvLabel.  The label for the FEC, if any, previously
      advertised to an upstream peer.  Assuming no label was previously
      advertised, this is the same label as the one in the Label Mapping
      message being processed.

   -  StoredHopCount.  The hop count previously recorded for the FEC.

   -  RAttributes.  Attributes received with the message, e.g., Hop
      Count, Path Vector.

   -  SAttributes to be included in the Label Mapping message, if any,
      propagated to upstream peers.

   Algorithm:

   LMp.1:   Does the received label mapping match an outstanding label
            request for FEC previously sent to MsgSource?

            If not,goto LMp.3.

   LMp.2:   Delete record of outstanding FEC label request.




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   LMp.3:   Execute procedure Check_Received_Attributes (MsgSource,
            LabelMapping, RAttributes).

            If No Loop Detected, goto LMp.9.

   LMp.4:   Does the LSR have a previously received label mapping for
            FEC from MsgSource?  (See Note 1.)

            If not, goto LMp.8.  (See Note 2.)

   LMp.5:   Does the label previously received from MsgSource match
            Label (i.e., the label received in the message)?  (See Note
            3.)

            If not, goto LMp.8.  (See Note 4.)

   LMp.6:   Delete matching label mapping for FEC previously received
            from MsgSource.

   LMp.7:   Remove Label from forwarding/switching use.  (See Note 5.)

   LMp.8:   Execute procedure Send_Message (MsgSource, Label Release,
            FEC, Label, Loop Detected Status code).

            Goto LMp.33.

   LMp.9:   Does LSR have a previously received label mapping for FEC
            from MsgSource for the LSP in question?  (See Note 6.)

            If not, goto LMp.11.

   LMp.10:  Does the label previously received from MsgSource match
            Label (i.e., the label received in the message)?  (See Note
            3.)
            OR
            Is the received label mapping in response to a previously
            outstanding label request sent to MsgSource?  (See Note12.)

            If so, goto LMp.11.

   LMp.10a: Is LSR operating in Downstream Unsolicited mode?  If so,
            delete the label mapping for the label previously received
            from MsgSource and remove it from forwarding/switching use.
            Execute procedure Send_Message (MsgSource, Label Release,
            FEC, label previously received from MsgSource).

   LMp.11:  Determine the Next Hop for FEC.




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   LMp.12:  Is MsgSource the Next Hop for FEC?

            If so, goto LMp.14.

   LMp.13:  Perform LSR Label Release procedure:

               For Conservative Label retention:

               1.  Goto LMp.32.

               For Liberal Label retention:

               1.  Record label mapping for FEC with Label and
                   RAttributes has been received from MsgSource.
                   Goto LMp.33.

   LMp.14:  Is LSR an ingress for FEC?
            If not, goto LMp.16.

   LMp.15:  Install Label for forwarding/switching use.

   LMp.16:  Record label mapping for FEC with Label and RAttributes has
            been received from MsgSource.

   LMp.17:  Iterate through LMp.31 for each Peer.  (See Note 7).

   LMp.18:  Has LSR previously sent a label mapping for FEC to Peer for
            the LSP in question?  (See Note 8.)

            If so, goto LMp.22.

   LMp.19:  Is the Downstream Unsolicited Ordered Control Label
            Distribution procedure being used by LSR?

            If not, goto LMp.28.

   LMp.20:  Execute procedure Prepare_Label_Mapping_Attributes (Peer,
            FEC, RAttributes, SAttributes, IsPropagating,
            StoredHopCount).

   LMp.21:  Execute procedure Send_Message (Peer, Label Mapping, FEC,
            PrevAdvLabel, SAttributes).  (See Note 13.)

            Goto LMp.28.

   LMp.22:  Iterate through LMp.27 for each label mapping for FEC
            previously sent to Peer.




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   LMp.23:  Are RAttributes in the received label mapping consistent
            with those previously sent to Peer?  If so, continue
            iteration from LMp.22 for next label mapping.  (See Note 9.)

   LMp.24:  Execute procedure Prepare_Label_Mapping_Attributes (Peer,
            FEC, RAttributes, SAttributes, IsPropagating,
            StoredHopCount).

   LMp.25:  Execute procedure Send_Message (Peer, Label Mapping, FEC,
            PrevAdvLabel, SAttributes).  (See Note 10.)

   LMp.26:  Update record of label mapping for FEC previously sent to
            Peer to include the new attributes sent.

   LMp.27:  End iteration from LMp.22.

   LMp.28:  Does LSR have any label requests for FEC from Peer marked as
            pending?

            If not, goto LMp.30.

   LMp.29:  Perform LSR Label Distribution procedure:

               For Downstream Unsolicited Independent Control OR For
               Downstream Unsolicited Ordered Control

               1.  Execute procedure Prepare_Label_Mapping_Attributes
                   (Peer, FEC, RAttributes, SAttributes, IsPropagating,
                   UnknownHopCount).

               2.  Execute procedure Send_Label (Peer, FEC,
                   SAttributes).

                   If the procedure fails, continue iteration for next
                   Peer at LMp.17.

               3.  If no pending requests exist for Peer, goto LMp.30.
                   (See Note 11.)

               For Downstream On Demand Independent Control
               OR
               For Downstream On Demand Ordered Control

               1.  Iterate through Step 5 for each pending label request
                   for FEC from Peer marked as pending.






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               2.  Execute procedure Prepare_Label_Mapping_Attributes
                   (Peer, FEC, RAttributes, SAttributes, IsPropagating,
                   UnknownHopCount)

               3.  Execute procedure Send_Label (Peer, FEC,
                   SAttributes).  If the procedure fails, continue
                   iteration for next Peer at LMp.17.

               4.  Delete record of pending request.

               5.  End iteration from Step 1.

               6.  Goto LMp.30.

   LMp.30:  Perform LSR Label Use procedure:

               For Use Immediate OR For Use If Loop Not Detected

               1.  Iterate through Step 3 for each label mapping for FEC
                   previously sent to Peer.

               2.  Install label received and label sent to Peer for
                   forwarding/switching use.

               3.  End iteration from Step 1.

               4.  Goto LMp.31.

   LMp.31:  End iteration from LMp.17.
            Go to LMp.33.

   LMp.32:  Execute procedure Send_Message (MsgSource, Label Release,
            FEC, Label).

   LMp.33:  DONE.

   Notes:

   1.   If the LSR is merging, there should be at most 1 received
        mapping for the FEC for the LSP in question.  In the non-
        merging case, there could be multiple received mappings for the
        FEC for the LSP in question.

   2.   If the LSR has detected a loop and it has not previously
        received a label mapping from MsgSource for the FEC, it simply
        releases the label.





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   3.   Does the Label received in the message match any of the 1 or
        more label mappings identified in the previous step (LMp.4 or
        LMp.9)?

   4.   An unsolicited mapping with a different label from the same peer
        would be an attempt to establish multipath label switching,
        which is not supported in this version of LDP.

   5.   If the Label is not in forwarding/switching use, LMp.7 has no
        effect.

   6.   If the received label mapping message matched an outstanding
        label request in LMp.1, then (by definition) the LSR has not
        previously received a label mapping for FEC for the LSP in
        question.  If the LSR is merging upstream labels for the LSP in
        question, there should be at most 1 received mapping.  In the
        non-merging case, there could be multiple received label
        mappings for the same FEC, one for each resulting LSP.

   7.   The LMp.17 iteration includes MsgSource in order to handle the
        case where the LSR is operating in Downstream Unsolicited
        Ordered Control mode.  Ordered Control prevents the LSR from
        advertising a label for the FEC until it has received a label
        mapping from its next hop (MsgSource) for the FEC.

   8.   If the LSR is merging the LSP, it may have previously sent label
        mappings for the FEC LSP to one or more peers.  If the LSR is
        not merging, it may have sent a label mapping for the LSP in
        question to at most one LSR.

   9.   he Loop Detection Path Vector attribute is considered in this
        check.  If the received RAttributes include a Path Vector and no
        Path Vector had been previously sent to the Peer, or if the
        received Path Vector is inconsistent with the Path Vector
        previously sent to the Peer, then the attributes are considered
        to be inconsistent.  Note that an LSR is not required to store a
        received Path Vector after it propagates the Path Vector in a
        mapping message.  If an LSR does not store the Path Vector, it
        has no way to check the consistency of a newly received Path
        Vector.  This means that whenever such an LSR receives a mapping
        message carrying a Path Vector it must always propagate the Path
        Vector.

   10.  LMp.22 through LMp.27 deal with a situation that can arise when
        the LSR is using independent control and it receives a mapping
        from the downstream peer after it has sent a mapping to an
        upstream peer.  In this situation, the LSR needs to propagate
        any changed attributes, such as Hop Count, upstream.  If Loop



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        Detection is configured on, the propagated attributes must
        include the Path Vector.

   11.  An LSR operating in Downstream Unsolicited mode MUST process any
        Label Request messages it receives.  If there are pending label
        requests, fall through into the Downstream on Demand procedures
        in order to satisfy the pending requests.

   12.  As determined by step LMp.1.

   13.  An LSR operating in Ordered Control mode may choose to skip at
        this stage the peer from which it received the advertisement
        that caused it to generate the label-map message.  Doing so will
        in effect provide a form of split-horizon.

A.1.3.  Receive Label Abort Request

   Summary:

      When an LSR receives a Label Abort Request message from a peer, it
      checks whether it has already responded to the label request in
      question.  If it has, it silently ignores the message.  If it has
      not, it sends the peer a Label Request Aborted Notification.  In
      addition, if it has a label request outstanding for the LSP in
      question to a downstream peer, it sends a Label Abort Request to
      the downstream peer to abort the LSP.

   Context:

   -  LSR.  The LSR handling the event.

   -  MsgSource.  The LDP peer that sent the message.

   -  FEC.  The FEC specified in the message.

   -  RequestMessageID.  The message ID of the label request message to
      be aborted.

   -  Next Hop.  The next hop for the FEC.

   Algorithm:

   LAbR.1   Does the message match a previously received Label Request
            message from MsgSource?  (See Note 1.)
            If not, goto LAbR.12.

   LAbR.2   Has LSR responded to the previously received label request?
            If so, goto LAbR.12.



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   LAbR.3   Execute procedure Send_Message (MsgSource, Notification,
            Label Request Aborted, TLV), where TLV is the Label Request
            Message ID TLV received in the Label Abort Request message.

   LAbR.4   Does LSR have a Label Request message outstanding for FEC?
            If so, goto LAbR.7.

   LAbR.5   Does LSR have a label mapping for FEC?
            If not, goto LAbR.11.

   LAbR.6   Generate Event: Received Label Release message for FEC from
            MsgSource.  (See Note 2.)
            Goto LAbR.11.

   LAbR.7   Is LSR merging the LSP for FEC?
            If not, goto LAbR.9.

   LAbR.8   Are there outstanding label requests for this FEC?
            If so, goto LAbR.11.

   LAbR.9   Execute procedure Send_Message (Next Hop, Label Abort
            Request, FEC, TLV), where TLV is a Label Request message ID
            TLV containing the Message ID used by the LSR in the
            outstanding Label Request message.

   LAbR.10  Record that a label abort request for FEC is pending.

   LAbR.11  Delete record of label request for FEC from MsgSource.

   LAbR.12  DONE.

   Notes:

   1.  LSR uses FEC and the Label Request message ID TLV carried by the
       label abort request to locate its record (if any) for the
       previously received label request from MsgSource.

   2.  If LSR has received a label mapping from NextHop, it should
       behave as if it had advertised a label mapping to MsgSource and
       MsgSource has released it.

A.1.4.  Receive Label Release

   Summary:

      When an LSR receives a Label Release message for a FEC from a
      peer, it checks whether other peers hold the released label.  If
      none do, the LSR removes the label from forwarding/switching use,



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      if it has not already done so, and if the LSR holds a label
      mapping from the FEC next hop, it releases the label mapping.

   Context:

   -  LSR.  The LSR handling the event.

   -  MsgSource.  The LDP peer that sent the message.

   -  Label.  The label specified in the message.

   -  FEC.  The FEC specified in the message.

   Algorithm:

   LRl.1   Does FEC match a known FEC?  If not, goto LRl.14.

   LRl.2   Remove MsgSource from record of peers that hold Label for
           FEC.  (See Note 1.)

   LRl.3   Does message match an outstanding label withdraw for FEC
           previously sent to MsgSource?
           If not, goto LRl.5

   LRl.4   Delete record of outstanding label withdraw for FEC
           previously sent to MsgSource.

   LRl.5   Is LSR merging labels for this FEC?  If not, goto LRl.7.
           (See Note 2.)

   LRl.6   Does LSR have outstanding label advertisements for this FEC?
           If so, goto LRl.11.

   LRl.7   Is LSR egress for the FEC?
           If so, goto LRl.11.

   LRl.8   Is there a Next Hop for FEC?  AND Does LSR have a previously
           received label mapping for FEC from Next Hop?
           If not, goto LRl.11.

   LRl.9   Is LSR configured to propagate releases?
           If not, goto LRl.11.  (See Note 3.)

   LRl.10  Execute procedure Send_Message (Next Hop, Label Release, FEC,
           Label from Next Hop).

   LRl.11  Remove Label from forwarding/switching use for traffic from
           MsgSource.



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   LRl.12  Do any peers still hold Label for FEC?
           If so, goto LRl.14.

   LRl.13  Free the Label.

   LRl.14  DONE.

   Notes:

   1.  If LSR is using Downstream Unsolicited label distribution, it
       SHOULD NOT re-advertise a label mapping for FEC to MsgSource
       until MsgSource requests it.

   2.  LRl.5 through LRl.9 deal with determining whether where the LSR
       should propagate the Label Release to a downstream peer (LRl.9).

   3.  If LRl.9 is reached, no upstream LSR holds a label for the FEC,
       and the LSR holds a label for the FEC from the FEC Next Hop.  The
       LSR could propagate the Label Release to the Next Hop.  By
       propagating the Label Release, the LSR releases a potentially
       scarce label resource.  In doing so, it also increases the
       latency for re-establishing the LSP should MsgSource or some
       other upstream LSR send it a new Label Request for FEC.
       Whether or not to propagate the release is not a protocol issue.
       Label distribution will operate properly whether or not the
       release is propagated.  The decision to propagate or not should
       take into consideration factors such as, whether labels are a
       scarce resource in the operating environment, the importance of
       keeping LSP setup latency low by keeping the amount of signaling
       required small, and whether LSP setup is ingress-controlled or
       egress-controlled in the operating environment.

A.1.5.  Receive Label Withdraw

   Summary:

      When an LSR receives a Label Withdraw message for a FEC from an
      LDP peer, it responds with a Label Release message and it removes
      the label from any forwarding/switching use.  If Ordered Control
      is in use, the LSR sends a Label Withdraw message to each LDP peer
      to which it had previously sent a label mapping for the FEC.  If
      the LSR is using Downstream on Demand label advertisement with
      independent control, it then acts as if it had just recognized the
      FEC.

   Context:

   -  LSR.  The LSR handling the event.



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   -  MsgSource.  The LDP peer that sent the message.

   -  Label.  The label specified in the message.

   -  FEC.  The FEC specified in the message.

   Algorithm:

   LWd.a   Remove Label from forwarding/switching use.  (See Note 1.)

   LWd.b   Execute procedure Send_Message (MsgSource, Label Release,
           FEC, Label).

   LWd.c   Has LSR previously received and retained a matching label
           mapping for FEC from MsgSource?
           If not, goto LWd.13.

   LWd.d   Delete matching label mapping for FEC previously received
           from MsgSource.

   LWd.e   Is LSR using Ordered Control?
           If so, goto LWd.8.

   LWd.f   Is MsgSource using Downstream On Demand label advertisement?
           If not, goto LWd.13.

   LWd.g   Generate Event: Recognize New FEC for FEC.
           Goto LWd.13.  (See Note 2.)

   LWd.h   Iterate through LWd.12 for each Peer, other than MsgSource.

   LWd.i   Has LSR previously sent a label mapping for FEC to Peer?
           If not, continue iteration for next Peer at LWd.8.

   LWd.j   Does the label previously sent to Peer "map" to the withdrawn
           Label?
           If not, continue iteration for next Peer at LWd.8.  (See Note
           3.)

   LWd.k   Execute procedure Send_Label_Withdraw (Peer, FEC, Label
           previously sent to Peer).

   LWd.l   End iteration from LWd.8.

   LWd.m   DONE.

   Notes:




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   1.  If the Label is not in forwarding/switching use, LWd.1 has no
       effect.

   2.  LWd.7 handles the case where the LSR is using Downstream On
       Demand label distribution with independent control.  In this
       situation, the LSR should send a label request to the FEC next
       hop as if it had just recognized the FEC.

   3.  LWd.10 handles both label merging (one or more incoming labels
       map to the same outgoing label) and no label merging (one label
       maps to the outgoing label) cases.

A.1.6.  Recognize New FEC

   Summary:

      The response by an LSR to learning a new FEC via the routing table
      may involve one or more of the following actions:



      -  Transmission of label mappings for the FEC to one or more LDP
         peers;

      -  Transmission of a label request for the FEC to the FEC next
         hop;

      -  Any of the actions that can occur when the LSR receives a label
         mapping for the FEC from the FEC next hop.

   Context:

      LSR.  The LSR handling the event.

      FEC.  The newly recognized FEC.

      Next Hop.  The next hop for the FEC.

      InitAttributes.  Attributes to be associated with the new FEC.
      (See Note 1.)

      SAttributes.  Attributes to be included in Label Mapping or Label
      Request messages, if any, sent to peers.

      StoredHopCount.  Hop count associated with FEC label mapping, if
      any, previously received from Next Hop.

   Algorithm:



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   FEC.1  Perform LSR Label Distribution procedure:

             For Downstream Unsolicited Independent Control

             1.  Iterate through 5 for each Peer.

             2.  Has LSR previously received and retained a label
                 mapping for FEC from Next Hop?
                 If so, set Propagating to IsPropagating.
                 If not, set Propagating to NotPropagating.

             3.  Execute procedure Prepare_Label_Mapping_Attributes
                 (Peer, FEC, InitAttributes, SAttributes, Propagating,
                 Unknown hop count(0)).

             4.  Execute procedure Send_Label (Peer, FEC, SAttributes).

             5.  End iteration from 1.
                 Goto FEC.2.

             For Downstream Unsolicited Ordered Control



             1.  Iterate through 5 for each Peer.

             2.  Is LSR egress for the FEC?  OR Has LSR previously
                 received and retained a label mapping for FEC from Next
                 Hop?
                 If not, continue iteration for next Peer.

             3.  Execute procedure Prepare_Label_Mapping_Attributes
                 (Peer, FEC, InitAttributes, SAttributes, Propagating,
                 StoredHopCount).

             4.  Execute procedure Send_Label (Peer, FEC, SAttributes).

             5.  End iteration from 1.
                 Goto FEC.2.

             For Downstream On Demand Independent Control
             OR
             For Downstream On Demand Ordered Control



             1.  Goto FEC.2.  (See Note 2.)




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   FEC.2  Has LSR previously received and retained a label mapping for
          FEC from Next Hop?
          If so, goto FEC.5

   FEC.3  Is Next Hop an LDP peer?
          If not, Goto FEC.6

   FEC.4  Perform LSR Label Request procedure:

             For Request Never

             1.  Goto FEC.6

             For Request When Needed
             OR
             For Request On Request



             1.  Execute procedure Prepare_Label_Request_Attributes
                 (Next Hop, FEC, InitAttributes, SAttributes);

             2.  Execute procedure Send_Label_Request (Next Hop, FEC,
                 Goto FEC.6.

   FEC.5  Generate Event: Received Label Mapping from Next Hop.  (See
          Note 3.)

   FEC.6  DONE.

   Notes:

   1.  An example of an attribute that might be part of InitAttributes
       is one that specifies desired LSP characteristics, such as Class
       of Service (CoS).  (Note that while the current version of LDP
       does not specify a CoS attribute, LDP extensions may.)

       The means by which FEC InitAttributes, if any, are specified is
       beyond the scope of LDP.  Note that the InitAttributes will not
       include a known Hop Count or a Path Vector.

   2.  An LSR using Downstream On Demand label distribution would send a
       label only if it had a previously received label request marked
       as pending.  The LSR would have no such pending requests because
       it responds to any label request for an unknown FEC by sending
       the requesting LSR a No Route notification and discarding the
       label request; see LRq.3




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   3.  If the LSR has a label for the FEC from the Next Hop, it should
       behave as if it had just received the label from the Next Hop.
       This occurs in the case of Liberal Label retention mode.

A.1.7.  Detect Change in FEC Next Hop

   Summary:

      The response by an LSR to a change in the next hop for a FEC may
      involve one or more of the following actions:

      -  Removal of the label from the FEC's old next hop from
         forwarding/switching use;

      -  Transmission of label mapping messages for the FEC to one or
         more LDP peers;

      -  Transmission of a label request to the FEC's new next hop;

      -  Any of the actions that can occur when the LSR receives a label
         mapping from the FEC's new next hop.

      Context:



      -  LSR.  The LSR handling the event.

      -  FEC.  The FEC whose next hop changed.

      -  New Next Hop.  The current next hop for the FEC.

      -  Old Next Hop.  The previous next hop for the FEC.

      -  OldLabel.  Label, if any, previously received from Old Next
         Hop.

      -  CurAttributes.  The attributes, if any, currently associated
         with the FEC.

      -  SAttributes.  Attributes to be included in the Label Request
         message, if any, sent to New Next Hop.

   Algorithm:

   NH.1   Has LSR previously received and retained a label mapping for
          FEC from Old Next Hop?
          If not, goto NH.6.



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   NH.2   Remove label from forwarding/switching use.  (See Note 1.)

   NH.3   Is LSR using Liberal Label retention?

          If so, goto NH.6.

   NH.4   Execute procedure Send_Message (Old Next Hop, Label Release,
          OldLabel).

   NH.5   Delete label mapping for FEC previously received from Old Next
          Hop.

   NH.6   Does LSR have a label request pending with Old Next Hop?

          If not, goto NH.10.

   NH.7   Is LSR using Conservative Label retention?

          If not, goto NH.10.

   NH.8   Execute procedure Send_Message (Old Next Hop, Label Abort
          Request, FEC, TLV), where TLV is a Label Request Message ID
          TLV that carries the message ID of the pending label request.

   NH.9   Record that a label abort request is pending for FEC with Old
          Next Hop.

   NH.10  Is there a New Next Hop for FEC?

          If not, goto NH.16.

   NH.11  Has LSR previously received and retained a label mapping for
          FEC from New Next Hop?

          If not, goto NH.13.

   NH.12  Generate Event: Received Label Mapping from New Next Hop. Goto
          NH.20.  (See Note 2.)

   NH.13  Is LSR using Downstream on Demand advertisement?  OR Is Next
          Hop using Downstream on Demand advertisement?  OR Is LSR using
          Conservative Label retention?  (See Note 3.)

          If so, goto NH.14.

          If not, goto NH.20.





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   NH.14  Execute procedure Prepare_Label_Request_Attributes (Next Hop,
          FEC, CurAttributes, SAttributes).

   NH.15  Execute procedure Send_Label_Request (New Next Hop, FEC,
          SAttributes).  (See Note 4.)

          Goto NH.20.

   NH.16  Iterate through NH.19 for each Peer.

   NH.17  Has LSR previously sent a label mapping for FEC to Peer?  If
          not, continue iteration for next Peer at NH.16

   NH.18  Execute procedure Send_Label_Withdraw (Peer, FEC, Label
          previously sent to Peer).

   NH.19  End iteration from NH.16.

   NH.20  DONE.

   Notes:

   -  If the Label is not in forwarding/switching use, NH.2 has no
      effect.

   -  If the LSR has a label for the FEC from the New Next Hop, it
      should behave as if it had just received the label from the New
      Next Hop.

   -  The purpose of the check on label retention mode is to avoid a
      race with steps LMp.12-LMp.13 of the procedure for handling a
      Label Mapping message where the LSR operating in Conservative
      Label retention mode may have released a label mapping received
      from the New Next Hop before it detected that the FEC next hop had
      changed.

   -  Regardless of the Label Request procedure in use by the LSR, it
      MUST send a label request if the conditions in NH.13 hold.
      Therefore, it executes the Send_Label_Request procedure directly
      rather than perform the LSR Label Request procedure.

A.1.8.  Receive Notification / Label Request Aborted

   Summary:

   When an LSR receives a Label Request Aborted notification from an LDP
   peer, it records that the corresponding label request transaction, if
   any, has completed.



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   Context:

   -  LSR.  The LSR handling the event.

   -  FEC.  The FEC for which a label was requested.

   -  RequestMessageID.  The message ID of the label request message to
      be aborted.

   -  MsgSource.  The LDP peer that sent the Notification message.

   Algorithm:

   LRqA.a  Does the notification correspond to an outstanding label
           request abort for FEC?  (See Note 1.)

           If not, goto LRqA.3.

   LRqA.b  Record that the label request for FEC has been aborted.

   LRqA.c  DONE.

   Note:

   1.  The LSR uses the FEC and RequestMessageID to locate its record,
       if any, of the outstanding label request abort.

A.1.9.  Receive Notification / No Label Resources

   Summary:

      When an LSR receives a No Label Resources notification from an LDP
      peer, it stops sending label request messages to the peer until it
      receives a Label Resources Available Notification from the peer.

   Context:

      LSR.  The LSR handling the event.

      FEC.  The FEC for which a label was requested.

      MsgSource.  The LDP peer that sent the Notification message.

   Algorithm:

   NoRes.1  Delete record of outstanding label request for FEC sent to
            MsgSource.




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   NoRes.2  Record that label mapping for FEC from MsgSource is needed
            but that no label resources are available.

   NoRes.3  Set status record indicating it is not OK to send label
            requests to MsgSource.

   NoRes.4  DONE.

A.1.10.  Receive Notification / No Route

   Summary:

      When an LSR receives a No Route notification from an LDP peer in
      response to a Label Request message, the Label No Route procedure
      in use dictates its response.  The LSR either will take no further
      action, or it will defer the label request by starting a timer and
      send another Label Request message to the peer when the timer
      later expires.

   Context:

   -  LSR.  The LSR handling the event.

   -  FEC.  The FEC for which a label was requested.

   -  Attributes.  The attributes associated with the label request.

   -  MsgSource.  The LDP peer that sent the Notification message.

   Algorithm:

   NoNH.1  Delete record of outstanding label request for FEC sent to
           MsgSource.

   NoNH.2  Perform LSR Label No Route procedure.

              For Request No Retry

              1.  Goto NoNH.3.

              For Request Retry

              1.  Record deferred label request for FEC and Attributes
                  to be sent to MsgSource.

              2.  Start timeout.  Goto NoNH.3.

   NoNH.3  DONE.



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A.1.11.  Receive Notification / Loop Detected

   Summary:

      When an LSR receives a Loop Detected Status Code from an LDP peer
      in response to a Label Request message or a Label Mapping message,
      it behaves as if it had received a No Route notification.

   Context:

      See "Receive Notification / No Route".

   Algorithm:

      See "Receive Notification / No Route".

   Note:

   1.  When the Loop Detected notification is in response to a Label
       Request message, it arrives in a Status Code TLV in a
       Notification message.  When it is in response to a Label Mapping
       message, it arrives in a Status Code TLV in a Label Release
       message.

A.1.12.  Receive Notification / Label Resources Available

   Summary:

      When an LSR receives a Label Resources Available notification from
      an LDP peer, it resumes sending label requests to the peer.

   Context:

   -  LSR.  The LSR handling the event.

   -  MsgSource.  The LDP peer that sent the Notification message.

   -  SAttributes.  Attributes stored with the postponed Label Request
      message.

   Algorithm:

   Res.1  Set status record indicating it is OK to send label requests
          to MsgSource.

   Res.2  Iterate through Res.6 for each record of a FEC label mapping
          needed from MsgSource for which no label resources are
          available.



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   Res.3  Is MsgSource the next hop for FEC?

          If not, goto Res.5.

   Res.4  Execute procedure Send_Label_Request (MsgSource, FEC,
          SAttributes).  If the procedure fails, terminate iteration.

   Res.5  Delete record that no resources are available for a label
          mapping for FEC needed from MsgSource.

   Res.6  Res.6 End iteration from Res.2.

   Res.7  DONE.

A.1.13.  Detect Local Label Resources Have Become Available

   Summary:

      After an LSR has sent a No Label Resources notification to an LDP
      peer, when label resources later become available it sends a Label
      Resources Available notification to each such peer.

   Context:

   -  LSR.  The LSR handling the event.

   -  Attributes.  Attributes stored with the postponed Label Mapping
      message.

   Algorithm:

   ResA.1  Iterate through ResA.4 for each Peer to which LSR has
           previously sent a No Label Resources notification.

   ResA.2  Execute procedure Send_Notification (Peer, Label Resources
           Available).

   ResA.3  Delete record that No Label Resources notification was
           previously sent to Peer.

   ResA.4  End iteration from ResA.1.

   ResA.5  Iterate through ResA.8 for each record of a label mapping
           needed for FEC for Peer but no-label-resources.  (See Note
           1.)

   ResA.6  Execute procedure Send_Label (Peer, FEC, Attributes).  If the
           procedure fails, terminate iteration.



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   ResA.7  Clear record of FEC label mapping needed for peer but no-
           label-resources.

   ResA.8  End iteration from ResA.5

   ResA.9  DONE.

   Note:

   1.  Iteration ResA.5 through ResA.8 handles the situation where the
       LSR is using Downstream Unsolicited label distribution and was
       previously unable to allocate a label for a FEC.

A.1.14.  LSR Decides to No Longer Label Switch a FEC

   Summary:

      An LSR may unilaterally decide to no longer label switch a FEC for
      an LDP peer.  An LSR that does so MUST send a Label Withdraw
      message for the FEC to the peer.

   Context:

   -  Peer.  The peer.

   -  FEC.  The FEC.

   -  PrevAdvLabel.  The label for the FEC previously advertised to the
      Peer.

   Algorithm:

   NoLS.1  Execute procedure Send_Label_Withdraw (Peer, FEC,
           PrevAdvLabel).  (See Note 1.)

   DONE.

   Note:

   1.  The LSR may remove the label from forwarding/switching use as
       part of this event or as part of processing the Label Release
       from the peer in response to the label withdraw.  If the LSR
       doesn't wait for the Label Release message from the peer, it
       SHOULD NOT reuse the label until it receives the Label Release.







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A.1.15.  Timeout of Deferred Label Request

   Summary:

      Label requests are deferred in response to No Route and Loop
      Detected notifications.  When a deferred FEC label request for a
      peer times out, the LSR sends the label request.

   Context:

   -  LSR.  The LSR handling the event.

   -  FEC.  The FEC associated with the timeout event.

   -  Peer.  The LDP peer associated with the timeout event.

   -  Attributes.  Attributes stored with the deferred Label Request
      message.

   Algorithm:

   TO.1  Retrieve the record of the deferred label request.

   TO.2  Is Peer the next hop for FEC?

         If not, goto TO.4.

   TO.3  Execute procedure Send_Label_Request (Peer, FEC).

   TO.4  DONE.

A.2.  Common Label Distribution Procedures

   This section specifies utility procedures used by the algorithms that
   handle label distribution events.

A.2.1.  Send_Label

   Summary:

      The Send_Label procedure allocates a label for a FEC for an LDP
      peer, if possible, and sends a label mapping for the FEC to the
      peer.  If the LSR is unable to allocate the label and if it has a
      pending label request from the peer, it sends the LDP peer a No
      Label Resources notification.

   Parameters:




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   -  Peer.  The LDP peer to which the label mapping is to be sent.

   -  FEC.  The FEC for which a label mapping is to be sent.

   -  Attributes.  Attributes to be included with the label mapping.

   Additional Context:

   -  LSR.  The LSR executing the procedure.

   -  Label.  The label allocated and sent to Peer.

   Algorithm:

   SL.1   Does LSR have a label to allocate?
          If not, goto SL.9.

   SL.2   Allocate Label and bind it to the FEC.

   SL.3   Install Label for forwarding/switching use.

   SL.4   Execute procedure Send_Message (Peer, Label Mapping, FEC,
          Label, Attributes).

   SL.5   Record label mapping for FEC with Label and Attributes has
          been sent to Peer.

   SL.6   Does LSR have a record of a FEC label request from Peer marked
          as pending?
          If not, goto SL.8.

   SL.7   Delete record of pending label request for FEC from Peer.

   SL.8   Return success.

   SL.9   Does LSR have a label request for FEC from Peer marked as
          pending?
          If not, goto SL.13.

   SL.10  Execute procedure Send_Notification (Peer, No Label
          Resources).

   SL.11  Delete record of pending label request for FEC from Peer.

   SL.12  Record No Label Resources notification has been sent to Peer.
          Goto SL.14.





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   SL.13  Record label mapping needed for FEC and Attributes for Peer,
          but no-label-resources.  (See Note 1.)

   SL.14  Return failure.

   Note:

   1.  SL.13 handles the case of Downstream Unsolicited label
       distribution when the LSR is unable to allocate a label for a FEC
       to send to a Peer.

A.2.2.  Send_Label_Request

   Summary:

      An LSR uses the Send_Label_Request procedure to send a request for
      a label for a FEC to an LDP peer if currently permitted to do so.

   Parameters:

   -  Peer.  The LDP peer to which the label request is to be sent.

   -  FEC.  The FEC for which a label request is to be sent.

   -  Attributes.  Attributes to be included in the label request, e.g.,
      Hop Count, Path Vector.

   Additional Context:

   -  LSR.  The LSR executing the procedure.

   Algorithm:

   SLRq.1  Has a label request for FEC previously been sent to Peer and
           is it marked as outstanding?
           If so, Return success.  (See Note 1.)

   SLRq.2  Is status record indicating it is OK to send label requests
           to Peer set?
           If not, goto SLRq.6

   SLRq.3  Execute procedure Send_Message (Peer, Label Request, FEC,
           Attributes).

   SLRq.4  Record that label request for FEC has been sent to Peer and
           mark it as outstanding.

   SLRq.5  Return success.



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   SLRq.6  Postpone the label request by recording that label mapping
           for FEC and Attributes from Peer is needed but that no label
           resources are available.

   SLRq.7  Return failure.

   Note:

   1.  If the LSR is a non-merging LSR, it must distinguish between
       attempts to send label requests for a FEC triggered by different
       upstream LDP peers from duplicate requests.  This procedure will
       not send a duplicate label request.

A.2.3.  Send_Label_Withdraw

   Summary:

      An LSR uses the Send_Label_Withdraw procedure to withdraw a label
      for a FEC from an LDP peer.  To do this, the LSR sends a Label
      Withdraw message to the peer.

   Parameters:

   -  Peer.  The LDP peer to which the label withdraw is to be sent.

   -  FEC.  The FEC for which a label is being withdrawn.

   -  Label.  The label being withdrawn.

   Additional Context:

   -  LSR.  The LSR executing the procedure.

   Algorithm:

   SWd.1  Execute procedure Send_Message (Peer, Label Withdraw, FEC,
          Label).

   SWd.2  Record that label withdraw for FEC has been sent to Peer and
          mark it as outstanding.

A.2.4.  Send_Notification

   Summary:

      An LSR uses the Send_Notification procedure to send an LDP peer a
      Notification message.




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   Parameters

   -  Peer.  The LDP peer to which the Notification message is to be
      sent.

   -  Status.  Status code to be included in the Notification message.

   Additional Context:

      None.

   Algorithm:

   SNt.1  Execute procedure Send_Message (Peer, Notification, Status)

A.2.5.  Send_Message

   Summary:

      An LSR uses the Send_Message procedure to send an LDP peer an LDP
      message.

   Parameters:

      Peer.  The LDP peer to which the message is to be sent.

      Message Type.  The type of message to be sent.

      Additional message contents . . .  .

   Additional Context:

      None.

   Algorithm:

      This procedure is the means by which an LSR sends an LDP message
      of the specified type to the specified LDP peer.

A.2.6.  Check_Received_Attributes

   Summary:

      Check the attributes received in a Label Mapping or Label Request
      message.  If the attributes include a Hop Count or Path Vector,
      perform a Loop Detection check.  If a loop is detected, cause a
      Loop Detected Notification message to be sent to MsgSource.




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   Parameters:

   -  MsgSource.  The LDP peer that sent the message.

   -  MsgType.  The type of message received.

   -  RAttributes.  The attributes in the message.

   Additional Context:

      LSR Id.  The unique LSR Id of this LSR.

      Hop Count.  The Hop Count, if any, in the received attributes.

      Path Vector.  The Path Vector, if any, in the received attributes.

   Algorithm:

   CRa.1  Do RAttributes include Hop Count?
          If not, goto CRa.5.

   CRa.2  Does Hop Count exceed Max allowable hop count?
          If so, goto CRa.6.

   CRa.3  Do RAttributes include Path Vector?
          If not, goto CRa.5.

   CRa.4  Does Path Vector include LSR Id?  OR Does length of Path
          Vector exceed Max allowable length?
          If so, goto CRa.6

   CRa.5  Return No Loop Detected.

   CRa.6  Is MsgType LabelMapping?
          If so, goto CRa.8.  (See Note 1.)

   CRa.7  Execute procedure Send_Notification (MsgSource, Loop
          Detected).

   CRa.8  Return Loop Detected.

   CRa.9  DONE.

   Note:

   1.  When the attributes being checked were received in a Label
       Mapping message, the LSR sends the Loop Detected notification in




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       a Status Code TLV in a Label Release message.  (See
       Section "Receive Label Mapping".)

A.2.7.  Prepare_Label_Request_Attributes

   Summary:

      This procedure is used whenever a Label Request is to be sent to a
      Peer to compute the Hop Count and Path Vector, if any, to include
      in the message.

   Parameters:

      Peer.  The LDP peer to which the message is to be sent.

      FEC.  The FEC for which a label request is to be sent.

      RAttributes.  The attributes this LSR associates with the LSP for
      FEC.

      SAttributes.  The attributes to be included in the Label Request
      message.

   Additional Context:

      LSR Id.  The unique LSR Id of this LSR.

   Algorithm:

   PRqA.1   Is Hop Count required for this Peer?  (See Note 1.)  OR Do
            RAttributes include a Hop Count?  OR Is Loop Detection
            configured on LSR?
            If not, goto PRqA.14.

   PRqA.2   Is LSR ingress for FEC?
            If not, goto PRqA.6.

   PRqA.3   Include Hop Count of 1 in SAttributes.

   PRqA.4   Is Loop Detection configured on LSR?
            If not, goto PRqA.14.

   PRqA.5   Is LSR merge-capable?
            If so, goto PRqA.14.
            If not, goto PRqA.13.

   PRqA.6   Do RAttributes include a Hop Count?
            If not, goto PRqA.8.



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   PRqA.7   Increment RAttributes Hop Count and copy the resulting Hop
            Count to SAttributes.  (See Note 2.)
            Goto PRqA.9.

   PRqA.8   Include Hop Count of unknown (0) in SAttributes.

   PRqA.9   Is Loop Detection configured on LSR?
            If not, goto PRqA.14.

   PRqA.10  Do RAttributes have a Path Vector?
            If so, goto PRqA.12.

   PRqA.11  Is LSR merge-capable?
            If so, goto PRqA.14.
            If not, goto PRqA.13.

   PRqA.12  Add LSR Id to beginning of Path Vector from RAttributes and
            copy the resulting Path Vector into SAttributes.
            Goto PRqA.14.

   PRqA.13  Include Path Vector of length 1 containing LSR Id in
            SAttributes.

   PRqA.14  DONE.

   Notes:

   1.  The link with Peer may require that Hop Count be included in
       Label Request messages; for example, see RFC 3035 [RFC3034] and
       RFC 3034 [RFC3034].

   2.  For hop count arithmetic, unknown + 1 = unknown.

A.2.8.  Prepare_Label_Mapping_Attributes

   Summary:

      This procedure is used whenever a Label Mapping is to be sent to a
      Peer to compute the Hop Count and Path Vector, if any, to include
      in the message.

   Parameters:

   -  Peer.  The LDP peer to which the message is to be sent.

   -  FEC.  The FEC for which a label request is to be sent.





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   -  RAttributes.  The attributes this LSR associates with the LSP for
      FEC.

   -  SAttributes.  The attributes to be included in the Label Mapping
      message.

   -  IsPropagating.  The LSR is sending the Label Mapping message to
      propagate one received from the FEC next hop.

   -  PrevHopCount.  The Hop Count, if any, this LSR associates with the
      LSP for the FEC.

   Additional Context:

      LSR Id.  The unique LSR Id of this LSR.

   Algorithm:

   PMpA.1   Do the RAttributes include any unknown TLVs?
            If not, goto PMpA.4.

   PMpA.2   Do the settings of the U- and F-bits require forwarding of
            these TLVs?
            If not, goto PMpA.4.

   PMpA.3   Copy the unknown TLVs in SAttributes.

   PMpA.4   Is Hop Count required for this Peer?  (see Note 1.)  OR Do
            RAttributes include a Hop Count?  OR Is Loop Detection
            configured on LSR?
            If not, goto PMpA.24.

   PMpA.5   Is LSR egress for FEC?
            If not, goto PMpA.7.

   PMpA.6   Include Hop Count of 1 in SAttributes.
            Goto PMpA.24.

   PMpA.7   Do RAttributes have a Hop Count?
            If not, goto PMpA.11.

   PMpA.8   Is LSR a member of the edge set for an LSR domain whose LSRs
            do not perform TTL decrement AND Is Peer in that domain?
            (See Note 2.)
            If not, goto PMpA.10.

   PMpA.9   Include Hop Count of 1 in SAttributes.
            Goto PMpA.12.



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   PMpA.10  Increment RAttributes Hop Count and copy the resulting Hop
            Count to SAttributes.  (See Note 2.)
            Goto PMpA.12.

   PMpA.11  Include Hop Count of unknown (0) in SAttributes.

   PMpA.12  Is Loop Detection configured on LSR?
            If not, goto PMpA.24.

   PMpA.13  Do RAttributes have a Path Vector?
            If so, goto PMpA.22.

   PMpA.14  Is LSR propagating a received Label Mapping?
            If not, goto PMpA.23.

   PMpA.15  Does LSR support merging?
            If not, goto PMpA.17.

   PMpA.16  Has LSR previously sent a Label Mapping for FEC to Peer?
            If not, goto PMpA.23.

   PMpA.17  Do RAttributes include a Hop Count?
            If not, goto PMpA.24.

   PMpA.18  Is Hop Count in RAttributes unknown(0)?
            If so, goto PMpA.23.

   PMpA.19  Has LSR previously sent a Label Mapping for FEC to Peer?
            If not, goto PMpA.24.

   PMpA.20  Is Hop Count in RAttributes different from PrevHopCount?
            If not, goto PMpA.24.

   PMpA.21  Is the Hop Count in RAttributes > PrevHopCount?  OR Is
            PrevHopCount unknown(0)?
            If not, goto PMpA.24.

   PMpA.22  Add LSR Id to beginning of Path Vector from RAttributes and
            copy the resulting Path Vector into SAttributes.
            Goto PMpA.24.

   PMpA.23  Include Path Vector of length 1 containing LSR Id in
            SAttributes.

   PMpA.24  DONE.

   Notes:




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   1.  The link with Peer may require that Hop Count be included in
       Label Mapping messages; for example, see RFC 3035 [RFC3034] and
       RFC 3034 [RFC3034].

   2.  If the LSR is at the edge of a cloud of LSRs that do not perform
       TTL-decrement and it is propagating the Label Mapping message
       upstream into the cloud, it sets the Hop Count to 1 so that Hop
       Count across the cloud is calculated properly.  This ensures
       proper TTL management for packets forwarded across the part of
       the LSP that passes through the cloud.

   3.  For hop count arithmetic, unknown + 1 = unknown.

Acknowledgments

   The editors of this document relies heavily on, and would like to
   thank, everyone that contributed to the develoment and improvement of
   the LDP Specification.

   This document is produced as part of advancing the LDP specification
   to Internet Standard status.  The predessor (RFC 5036) was published
   as Draft Standard October 2007.  It was produced by the MPLS Working
   Group of the IETF and was jointly authored by Loa Andersson, Bob
   Thomas and Ina Minei.

   Since the Draft Standard version was published IETF has abandoned the
   3 steps standards ladder.  Now there is only proposed standard (PS)
   and Internet Standard (IS).  This is part of the motivation to make
   the effort to bring the LDP specification to Internet Standard.

   The LDP specification was originally published as RFC 3036 in January
   2001.  It was produced by the MPLS Working Group of the IETF and was
   jointly authored by Loa Andersson, Paul Doolan, Nancy Feldman, Andre
   Fredette, and Bob Thomas.

   The ideas and text in RFC 3036 were collected from a number of
   sources.  We would like to thank Rick Boivie, Ross Callon, Alex
   Conta, Eric Gray, Yoshihiro Ohba, Eric Rosen, Bernard Suter, Yakov
   Rekhter, and Arun Viswanathan for their input for RFC 3036.

   The authors would like to thank Eric Gray, David Black, and Sam
   Hartman for their input to and review of RFC 5036.  That input has
   been of great help also for the current document.

   In addition, the authors would like to thank the members of the MPLS
   Working Group for their ideas and the support they have given to this
   document, and in particular, to Eric Rosen, Luca Martini, Markus




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   Jork, Mark Duffy, Vach Kompella, Kishore Tiruveedhula, Rama
   Ramakrishnan, Nick Weeds, Adrian Farrel, and Andy Malis.

   Editor note - this section is still work in progress.

Authors' Addresses

   Xia Chen
   Huawei Technologies

   Email: jescia.chenxia@huawei.com


   Loa Andersson
   Huawei Technologies

   Email: loa@pi.nu


   Nic Leymann
   Deutsche Telekom

   Email: N.Leymann@telekom.de


   Ina Minei
   Google

   Email: inaminei@google.com


   Kamran Raza
   Cisco Systems, Inc.

   Email: skraza@cisco.com
















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