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rfc:rfc3036

Network Working Group L. Andersson Request for Comments: 3036 Nortel Networks Inc. Category: Standards Track P. Doolan

                                                      Ennovate Networks
                                                             N. Feldman
                                                               IBM Corp
                                                            A. Fredette
                                                          PhotonEx Corp
                                                              B. Thomas
                                                    Cisco Systems, Inc.
                                                           January 2001
                         LDP Specification

Status of this Memo

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

Copyright Notice

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

Abstract

 The architecture for Multi Protocol 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.

Andersson, et al. Standards Track [Page 1] RFC 3036 LDP Specification January 2001

Table of Contents

 1          LDP Overview  .......................................   5
 1.1        LDP Peers  ..........................................   6
 1.2        LDP Message Exchange  ...............................   6
 1.3        LDP Message Structure  ..............................   7
 1.4        LDP Error Handling  .................................   7
 1.5        LDP Extensibility and Future Compatibility  .........   7
 1.6        Specification Language  .............................   7
 2          LDP Operation  ......................................   8
 2.1        FECs  ...............................................   8
 2.2        Label Spaces, Identifiers, Sessions and Transport  ..   9
 2.2.1      Label Spaces  .......................................   9
 2.2.2      LDP Identifiers  ....................................  10
 2.2.3      LDP Sessions  .......................................  10
 2.2.4      LDP Transport  ......................................  11
 2.3        LDP Sessions between non-Directly Connected LSRs  ...  11
 2.4        LDP Discovery   .....................................  11
 2.4.1      Basic Discovery Mechanism  ..........................  12
 2.4.2      Extended Discovery Mechanism  .......................  12
 2.5        Establishing and Maintaining LDP Sessions  ..........  13
 2.5.1      LDP Session Establishment  ..........................  13
 2.5.2      Transport Connection Establishment  .................  13
 2.5.3      Session Initialization  .............................  14
 2.5.4      Initialization State Machine  .......................  17
 2.5.5      Maintaining Hello Adjacencies  ......................  20
 2.5.6      Maintaining LDP Sessions  ...........................  20
 2.6        Label Distribution and Management  ..................  21
 2.6.1      Label Distribution Control Mode  ....................  21
 2.6.1.1    Independent Label Distribution Control  .............  21
 2.6.1.2    Ordered Label Distribution Control  .................  21
 2.6.2      Label Retention Mode  ...............................  22
 2.6.2.1    Conservative Label Retention Mode  ..................  22
 2.6.2.2    Liberal Label Retention Mode  .......................  22
 2.6.3      Label Advertisement Mode  ...........................  23
 2.7        LDP Identifiers and Next Hop Addresses  .............  23
 2.8        Loop Detection  .....................................  24
 2.8.1      Label Request Message  ..............................  24
 2.8.2      Label Mapping Message  ..............................  26
 2.8.3      Discussion  .........................................  27
 2.9        Authenticity and Integrity of LDP Messages  .........  28
 2.9.1      TCP MD5 Signature Option  ...........................  28
 2.9.2      LDP Use of TCP MD5 Signature Option  ................  30
 2.10       Label Distribution for Explicitly Routed LSPs  ......  30
 3          Protocol Specification  .............................  31
 3.1        LDP PDUs  ...........................................  31
 3.2        LDP Procedures  .....................................  32
 3.3        Type-Length-Value Encoding  .........................  32

Andersson, et al. Standards Track [Page 2] RFC 3036 LDP Specification January 2001

 3.4        TLV Encodings for Commonly Used Parameters  .........  34
 3.4.1      FEC TLV  ............................................  34
 3.4.1.1    FEC Procedures  .....................................  37
 3.4.2      Label TLVs  .........................................  37
 3.4.2.1    Generic Label TLV  ..................................  37
 3.4.2.2    ATM Label TLV  ......................................  38
 3.4.2.3    Frame Relay Label TLV  ..............................  38
 3.4.3      Address List TLV  ...................................  39
 3.4.4      Hop Count TLV  ......................................  40
 3.4.4.1    Hop Count Procedures  ...............................  40
 3.4.5      Path Vector TLV  ....................................  41
 3.4.5.1    Path Vector Procedures  .............................  42
 3.4.5.1.1  Label Request Path Vector  ..........................  42
 3.4.5.1.2  Label Mapping Path Vector  ..........................  43
 3.4.6      Status TLV  .........................................  43
 3.5        LDP Messages  .......................................  45
 3.5.1      Notification Message  ...............................  47
 3.5.1.1    Notification Message Procedures  ....................  48
 3.5.1.2    Events Signaled by Notification Messages  ...........  49
 3.5.1.2.1  Malformed PDU or Message  ...........................  49
 3.5.1.2.2  Unknown or Malformed TLV  ...........................  50
 3.5.1.2.3  Session KeepAlive Timer Expiration  .................  50
 3.5.1.2.4  Unilateral Session Shutdown  ........................  51
 3.5.1.2.5  Initialization Message Events  ......................  51
 3.5.1.2.6  Events Resulting From Other Messages  ...............  51
 3.5.1.2.7  Internal Errors  ....................................  51
 3.5.1.2.8  Miscellaneous Events  ...............................  51
 3.5.2      Hello Message  ......................................  51
 3.5.2.1    Hello Message Procedures  ...........................  54
 3.5.3      Initialization Message  .............................  55
 3.5.3.1    Initialization Message Procedures  ..................  63
 3.5.4      KeepAlive Message  ..................................  63
 3.5.4.1    KeepAlive Message Procedures  .......................  63
 3.5.5      Address Message  ....................................  64
 3.5.5.1    Address Message Procedures  .........................  64
 3.5.6      Address Withdraw Message  ...........................  65
 3.5.6.1    Address Withdraw Message Procedures  ................  66
 3.5.7      Label Mapping Message  ..............................  66
 3.5.7.1    Label Mapping Message Procedures  ...................  67
 3.5.7.1.1  Independent Control Mapping  ........................  67
 3.5.7.1.2  Ordered Control Mapping  ............................  68
 3.5.7.1.3  Downstream on Demand Label Advertisement  ...........  68
 3.5.7.1.4  Downstream Unsolicited Label Advertisement  .........  69
 3.5.8      Label Request Message  ..............................  69
 3.5.8.1    Label Request Message Procedures  ...................  70
 3.5.9      Label Abort Request Message  ........................  72
 3.5.9.1    Label Abort Request Message Procedures  .............  73
 3.5.10     Label Withdraw Message  .............................  74

Andersson, et al. Standards Track [Page 3] RFC 3036 LDP Specification January 2001

 3.5.10.1   Label Withdraw Message Procedures  ..................  75
 3.5.11     Label Release Message  ..............................  76
 3.5.11.1   Label Release Message Procedures  ...................  77
 3.6        Messages and TLVs for Extensibility  ................  78
 3.6.1      LDP Vendor-private Extensions  ......................  78
 3.6.1.1    LDP Vendor-private TLVs  ............................  78
 3.6.1.2    LDP Vendor-private Messages  ........................  80
 3.6.2      LDP Experimental Extensions  ........................  81
 3.7        Message Summary  ....................................  81
 3.8        TLV Summary  ........................................  82
 3.9        Status Code Summary  ................................  83
 3.10       Well-known Numbers  .................................  84
 3.10.1     UDP and TCP Ports  ..................................  84
 3.10.2     Implicit NULL Label  ................................  84
 4          IANA Considerations  ................................  84
 4.1        Message Type Name Space  ............................  84
 4.2        TLV Type Name Space  ................................  85
 4.3        FEC Type Name Space  ................................  85
 4.4        Status Code Name Space  .............................  86
 4.5        Experiment ID Name Space  ...........................  86
 5          Security Considerations  ............................  86
 5.1        Spoofing  ...........................................  86
 5.2        Privacy  ............................................  87
 5.3        Denial of Service  ..................................  87
 6          Areas for Future Study  .............................  89
 7          Intellectual Property Considerations  ...............  89
 8          Acknowledgments  ....................................  89
 9          References  .........................................  89
 10         Authors' Addresses  .................................  92
 Appendix A LDP Label Distribution Procedures  ..................  93
 A.1        Handling Label Distribution Events  .................  95
 A.1.1      Receive Label Request  ..............................  96
 A.1.2      Receive Label Mapping  ..............................  99
 A.1.3      Receive Label Abort Request  ........................ 105
 A.1.4      Receive Label Release  .............................. 107
 A.1.5      Receive Label Withdraw  ............................. 109
 A.1.6      Recognize New FEC  .................................. 110
 A.1.7      Detect Change in FEC Next Hop  ...................... 113
 A.1.8      Receive Notification / Label Request Aborted  ....... 116
 A.1.9      Receive Notification / No Label Resources  .......... 116
 A.1.10     Receive Notification / No Route  .................... 117
 A.1.11     Receive Notification / Loop Detected  ............... 118
 A.1.12     Receive Notification / Label Resources Available  ... 118
 A.1.13     Detect local label resources have become available  . 119
 A.1.14     LSR decides to no longer label switch a FEC  ........ 120
 A.1.15     Timeout of deferred label request  .................. 121
 A.2        Common Label Distribution Procedures  ............... 121
 A.2.1      Send_Label  ......................................... 121

Andersson, et al. Standards Track [Page 4] RFC 3036 LDP Specification January 2001

 A.2.2      Send_Label_Request  ................................. 123
 A.2.3      Send_Label_Withdraw  ................................ 124
 A.2.4      Send_Notification  .................................. 125
 A.2.5      Send_Message  ....................................... 125
 A.2.6      Check_Received_Attributes  .......................... 126
 A.2.7      Prepare_Label_Request_Attributes  ................... 127
 A.2.8      Prepare_Label_Mapping_Attributes  ................... 129
 Full Copyright Statement  ...................................... 132

1. LDP Overview

 The MPLS architecture [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 [RFC2702].
 The Label Distribution Protocol (LDP) defined in this document is a
 new 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) [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
 [RFC3037].
 This document assumes familiarity with the MPLS architecture
 [RFC3031].  Note that [RFC3031] includes a glossary of MPLS
 terminology, such as ingress, label switched path, etc.

Andersson, et al. Standards Track [Page 5] RFC 3036 LDP Specification January 2001

1.1. LDP Peers

 Two LSRs which 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 bi-directional.

1.2. 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.

Andersson, et al. Standards Track [Page 6] RFC 3036 LDP Specification January 2001

1.3. 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.

1.4. 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 an LSR information about
       the LDP session or the status of some previous message received
       from the peer.

1.5. 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.

1.6. 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 [RFC2119].

Andersson, et al. Standards Track [Page 7] RFC 3036 LDP Specification January 2001

2. LDP Operation

2.1. 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
 LSP.  The FEC identifies the set of IP packets which 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 which 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.
 Following are the currently defined types of FEC elements.  New
 element types may be added as needed:
    1. Address Prefix.  This element is an address prefix of any
       length from 0 to a full address, inclusive.
    2. Host Address.  This element is a full host address.
 (We will see below that an Address Prefix FEC element which is a full
 address has a different effect than a Host Address FEC element which
 has the same address.)
 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 which matches the packet's
 destination address.  With respect to a particular packet and a
 particular LSP, we refer to any Address Prefix FEC element which
 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.
  1. If there is exactly one LSP which has a Host Address FEC

element that is identical to the packet's destination address,

       then the packet is mapped to that LSP.
  1. If there are multiple LSPs, each containing a Host Address FEC

element that is identical to the packet's destination address,

       then the packet is mapped to one of those LSPs.  The procedure
       for selecting one of those LSPs is beyond the scope of this
       document.

Andersson, et al. Standards Track [Page 8] RFC 3036 LDP Specification January 2001

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

that LSP.

  1. 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.
  1. If it is known that a packet must traverse a particular egress

router, and there is an LSP which has an Address Prefix FEC

       element which is an 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.
 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.)
 It is worth pointing out a few consequences of these rules:
  1. A packet may be sent on the LSP whose Address Prefix FEC

element is the address of the packet's egress router ONLY if

       there is no LSP matching the packet's destination address.
  1. A packet may match two LSPs, one with a Host Address FEC

element and one with an Address Prefix FEC element. In this

       case, the packet is always assigned to the former.
  1. A packet which does not match a particular Host Address FEC

element may not be sent on the corresponding LSP, even if the

       Host Address FEC element identifies the packet's egress router.

2.2. Label Spaces, Identifiers, Sessions and Transport

2.2.1. Label Spaces

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

Andersson, et al. Standards Track [Page 9] RFC 3036 LDP Specification January 2001

  1. 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 as labels, or a Frame Relay
       interface that uses DLCIs 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.
  1. Per platform label space. Platform-wide incoming labels are

used for interfaces that can share the same labels.

2.2.2. 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.
 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.

2.2.3. 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.

Andersson, et al. Standards Track [Page 10] RFC 3036 LDP Specification January 2001

2.2.4. 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.

2.3. 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.

2.4. 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:
  1. A basic discovery mechanism used to discover LSR neighbors that

are directly connected at the link level.

  1. An extended discovery mechanism used to locate LSRs that are

not directly connected at the link level.

Andersson, et al. Standards Track [Page 11] RFC 3036 LDP Specification January 2001

2.4.1. 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.

2.4.2. 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:
  1. A Targeted Hello is sent to a specific address rather than to

the "all routers" group multicast address for the outgoing

       interface.
  1. 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.

Andersson, et al. Standards Track [Page 12] RFC 3036 LDP Specification January 2001

 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.

2.5. Establishing and Maintaining LDP Sessions

2.5.1. LDP Session Establishment

 The exchange of LDP Discovery Hellos between two LSRs triggers LDP
 session establishment.  Session establishment is a two step process:
  1. Transport connection establishment.
  2. 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.

2.5.2. 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
          Hello's it sends to LSR2 to advertise an address, A1 is the
          address LSR1 advertises via the optional object;
       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.

Andersson, et al. Standards Track [Page 13] RFC 3036 LDP Specification January 2001

    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:
  1. If A1 and A2 are not in the same address family, they are

incomparable, and no session can be established.

  1. Let U1 be the abstract unsigned integer obtained by treating

A1 as a sequence of bytes, where the byte which appears

          earliest in the message is the most significant byte of the
          integer and the byte which 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.
  1. 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
 LSRs linked by multiple Hello adjacencies using the same label spaces
 play the same connection establishment role for each adjacency.

2.5.3. 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 ranges for label
 controlled ATM, DLCI ranges for label controlled Frame Relay, etc.

Andersson, et al. Standards Track [Page 14] RFC 3036 LDP Specification January 2001

 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.
          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.

Andersson, et al. Standards Track [Page 15] RFC 3036 LDP Specification January 2001

       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.
    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.

Andersson, et al. Standards Track [Page 16] RFC 3036 LDP Specification January 2001

    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.

2.5.4. 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.

Andersson, et al. Standards Track [Page 17] RFC 3036 LDP Specification January 2001

             Session Initialization State Transition Table
    STATE         EVENT                               NEW STATE
    NON EXISTENT  Session TCP connection established  INITIALIZED
                  established
    INITIALIZED   Transmit Initialization msg         OPENSENT
                        (Active Role)
                  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

Andersson, et al. Standards Track [Page 18] RFC 3036 LDP Specification January 2001

             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|                       |
       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
                            |   |
                            +---+

Andersson, et al. Standards Track [Page 19] RFC 3036 LDP Specification January 2001

2.5.5. 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 which it restarts
 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 a LDP session is
 deleted, the LSR terminates the LDP session by sending a Notification
 message and closing the transport connection.

2.5.6. 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 which 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.

Andersson, et al. Standards Track [Page 20] RFC 3036 LDP Specification January 2001

2.6. Label Distribution and Management

 The MPLS architecture [RF3031] 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.  [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
 Unsolicited label distribution assumes its peer is also.  See Section
 "Downstream on Demand label Advertisement".

2.6.1. 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.

2.6.1.1. 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.

2.6.1.2. 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:

Andersson, et al. Standards Track [Page 21] RFC 3036 LDP Specification January 2001

    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
       [RFC2328] [RFC1771].
 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.

2.6.2. Label Retention Mode

 The MPLS architecture [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.

2.6.2.1. 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.

2.6.2.2. 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

Andersson, et al. Standards Track [Page 22] RFC 3036 LDP Specification January 2001

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

2.6.3. 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.

2.7. 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.

Andersson, et al. Standards Track [Page 23] RFC 3036 LDP Specification January 2001

 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

2.8. Loop Detection

 Loop detection is a configurable option which provides a mechanism
 for finding looping LSPs and for preventing Label Request messages
 from looping in the presence of non-merge capable LSRs.
 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:
  1. 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.
  1. 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 describes 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 [RFC3035] and [RFC3034]).

2.8.1. 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.

Andersson, et al. Standards Track [Page 24] RFC 3036 LDP Specification January 2001

 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:
  1. The Label Request message MUST include a Hop Count TLV.
  1. 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.

  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

    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:
  1. 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.
  1. 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 which 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.

Andersson, et al. Standards Track [Page 25] RFC 3036 LDP Specification January 2001

2.8.2. 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:
  1. R MUST include a Hop Count TLV.
  1. If R is the egress, the hop count value MUST be 1.
  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.
  1. 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:
  1. If R is the egress, the Label Mapping message need not include a

Path Vector TLV.

  1. If R is sending the Label Mapping message to propagate a Label

Mapping message received from the next hop to an upstream peer,

    then:

Andersson, et al. Standards Track [Page 26] RFC 3036 LDP Specification January 2001

    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.
    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.
  1. 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 which 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 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.

2.8.3. 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 which are configured for loop detection are NOT expected to
 store the path vectors as part of the LSP state.

Andersson, et al. Standards Track [Page 27] RFC 3036 LDP Specification January 2001

 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 which 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,
 an LSR may originate a Label Mapping message for an 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.

2.9. 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 [RFC2385] for use by BGP.  See [RFC1321] for a
 specification of the MD5 hash function.

2.9.1. TCP MD5 Signature Option

 The following quotes from [RFC2385] outline the security properties
 achieved by using the TCP MD5 Signature Option and summarizes 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."

Andersson, et al. Standards Track [Page 28] RFC 3036 LDP Specification January 2001

    "Abstract
       This memo describes a TCP extension to enhance security for
       BGP.  It defines a new TCP option for carrying an MD5 [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
       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.

Andersson, et al. Standards Track [Page 29] RFC 3036 LDP Specification January 2001

       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 [RFC2385].

2.9.2. LDP Use of TCP MD5 Signature Option

 LDP uses the TCP MD5 Signature Option as follows:
  1. Use of the MD5 Signature Option for LDP TCP connections is a

configurable LSR option.

  1. An LSR that uses the MD5 Signature Option is configured with a

password (shared secret) for each potential LDP peer.

  1. The LSR applies the MD5 algorithm as specified in [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.
  1. 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.
  1. 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.

2.10. Label Distribution for Explicitly Routed LSPs

 Traffic Engineering [RFC2702] is expected to be an important MPLS
 application.  MPLS support for Traffic Engineering uses explicitly
 routed LSPs, which need not follow normally-routed (hop-by-hop) paths
 as determined by destination-based routing protocols.  CR-LDP [CRLDP]
 defines extensions to LDP to use LDP to set up explicitly routed
 LSPs.

Andersson, et al. Standards Track [Page 30] RFC 3036 LDP Specification January 2001

3. 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.

3.1. LDP PDUs

 Each LDP PDU is an LDP header followed by one or more LDP messages.
 The LDP header 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Version                      |         PDU Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         LDP Identifier                        |
 +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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 bytes.

Andersson, et al. Standards Track [Page 31] RFC 3036 LDP Specification January 2001

 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.

3.2. LDP Procedures

 LDP defines messages, TLVs and procedures in the following areas:
  1. Peer discovery;
  2. Session management;
  3. Label distribution;
  4. Notification of errors and advisory information.
 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.

3.3. 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.

Andersson, et al. Standards Track [Page 32] RFC 3036 LDP Specification January 2001

  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                             |
 ~                                                               ~
 |                                                               |
 |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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 sections
    following that define TLVs specify a value for the U-bit.
 F bit
    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.
 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

Andersson, et al. Standards Track [Page 33] RFC 3036 LDP Specification January 2001

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

3.4. 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.

3.4.1. 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.

Andersson, et al. Standards Track [Page 34] RFC 3036 LDP Specification January 2001

 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                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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:
       FEC Element       Type      Value
       type name
         Wildcard        0x01      No value; i.e., 0 value octets;
                                       see below.
         Prefix          0x02      See below.
         Host Address    0x03      Full host address; see below.
    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.

Andersson, et al. Standards Track [Page 35] RFC 3036 LDP Specification January 2001

    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 (2)   |     Address Family            |     PreLen    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Prefix                                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Address Family
       Two octet quantity containing a value from ADDRESS FAMILY
       NUMBERS in [RFC1700] 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
       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.
    Host Address FEC Element 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Host Addr (3) |     Address Family            | Host Addr Len |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                     Host Addr                                 |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Address Family
       Two octet quantity containing a value from ADDRESS FAMILY
       NUMBERS in [RFC1700] that encodes the address family for the
       address prefix in the Prefix field.
    Host Addr Len
       Length of the Host address in octets.
    Host Addr
       An address encoded according to the Address Family field.

Andersson, et al. Standards Track [Page 36] RFC 3036 LDP Specification January 2001

3.4.1.1. 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.

3.4.2. 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 which can appear in
 situations that require a Label TLV.

3.4.2.1. 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                                                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Label
    This is a 20-bit label value as specified in [RFC3032] represented
    as a 20-bit number in a 4 octet field.

Andersson, et al. Standards Track [Page 37] RFC 3036 LDP Specification January 2001

3.4.2.2. 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                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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.

3.4.2.3. 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                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Andersson, et al. Standards Track [Page 38] RFC 3036 LDP Specification January 2001

 Res
    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:
       0 = 10 bits DLCI
       2 = 23 bits DLCI
    Len values 1 and 3 are reserved.
 DLCI
    The Data Link Connection Identifier.  Refer to [RFC3034] for the
    label values and formats.

3.4.3. 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                              |
 ~                                                               ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Address Family
    Two octet quantity containing a value from ADDRESS FAMILY NUMBERS
    in [RFC1700] 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.

Andersson, et al. Standards Track [Page 39] RFC 3036 LDP Specification January 2001

    The following address encodings are defined by this version of the
    protocol:
       Address Family      Address Encoding
       IPv4                4 octet full IPv4 address
       IPv6                16 octet full IPv6 address

3.4.4. 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 setup.
 Note that setup procedures for LSPs that traverse ATM and Frame Relay
 links require use of the Hop Count TLV (see [RFC3035] and [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  |
 +-+-+-+-+-+-+-+-+
 HC Value
    1 octet unsigned integer hop count value.

3.4.4.1. 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 must determine a hop count to include
 in the propagated message as follows:
  1. If the message is a Label Request message, R must increment the

received hop count;

  1. If the message is a Label Mapping message, R determines the hop

count as follows:

Andersson, et al. Standards Track [Page 40] RFC 3036 LDP Specification January 2001

    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 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
 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 "Loop Detection".

3.4.5. 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

Andersson, et al. Standards Track [Page 41] RFC 3036 LDP Specification January 2001

 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 setup 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                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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.

3.4.5.1. Path Vector Procedures

 The Path Vector TLV is carried in Label Mapping and Label Request
 messages when loop detection is configured.

3.4.5.1.1. Label Request Path Vector

 Section "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".

Andersson, et al. Standards Track [Page 42] RFC 3036 LDP Specification January 2001

    2. Not propagate the Label Request message further.
 Note that a Label Request message with Path Vector TLV is forwarded
 until:
    1. A loop is found,
    2. The LSP egress is reached,
    3. The maximum Path Vector limit or maximum Hop Count limit is
       reached.  This is treated as if a loop had been detected.

3.4.5.1.2. Label Mapping Path Vector

 Section "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.
    3. Check whether the Label Mapping message is for an existing LSP.
       If so, the LSR must unsplice any upstream labels which 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.

3.4.6. Status TLV

 Notification messages carry Status TLVs to specify events being
 signaled.

Andersson, et al. Standards Track [Page 43] RFC 3036 LDP Specification January 2001

 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             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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
    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                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    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 which specifies the status information.
    This specification defines Status Codes (32-bit unsigned integers
    with the above encoding).

Andersson, et al. Standards Track [Page 44] RFC 3036 LDP Specification January 2001

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

3.5. 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                       |
 +                                                               +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Andersson, et al. Standards Track [Page 45] RFC 3036 LDP Specification January 2001

 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
    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
    "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.
 The following message types are defined in this version of LDP:
    Message Name            Section Title
    Notification            "Notification Message"
    Hello                   "Hello Message"
    Initialization          "Initialization Message"
    KeepAlive               "KeepAlive Message"

Andersson, et al. Standards Track [Page 46] RFC 3036 LDP Specification January 2001

    Address                 "Address Message"
    Address Withdraw        "Address Withdraw Message"
    Label Mapping           "Label Mapping Message"
    Label Request           "Label Request Message"
    Label Abort Request     "Label Abort Request Message"
    Label Withdraw          "Label Withdraw Message"
    Label Release           "Label Release Message"
 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 of a contiguous block in the 16-bit message
 type number space.

3.5.1. 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                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Message ID
    32-bit value used to identify this message.

Andersson, et al. Standards Track [Page 47] RFC 3036 LDP Specification January 2001

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

3.5.1.1. 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.
 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

Andersson, et al. Standards Track [Page 48] RFC 3036 LDP Specification January 2001

 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.

3.5.1.2. Events Signaled by Notification Messages

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

3.5.1.2.1. 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:
  1. 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.
  1. 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.
  1. 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:
  1. 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.

Andersson, et al. Standards Track [Page 49] RFC 3036 LDP Specification January 2001

       If the Message Type is >= 0x8000 (high order bit = 1) it is
       silently discarded.
  1. 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.
  1. The message is missing one or more Mandatory Parameters. This

is a non-fatal error signalled by the Missing Message

       Parameters Status Code.

3.5.1.2.2. 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:
  1. 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.
  1. 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.  Section "Unknown TLV in Known Message Type"
       elaborates on this behavior.
  1. 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.

3.5.1.2.3. Session KeepAlive Timer Expiration

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

Andersson, et al. Standards Track [Page 50] RFC 3036 LDP Specification January 2001

3.5.1.2.4. 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.

3.5.1.2.5. 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 Sections "Initialization Message".

3.5.1.2.6. 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.

3.5.1.2.7. 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.

3.5.1.2.8. 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.

3.5.2. Hello Message

 LDP Hello Messages are exchanged as part of the LDP Discovery
 Mechanism; see Section "LDP Discovery".
 The encoding for the Hello Message is:

Andersson, et al. Standards Track [Page 51] RFC 3036 LDP Specification January 2001

  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                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Hold Time,
       Hello hold time in seconds.  An LSR maintains a record of
       Hellos received from potential peers (see Section "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.
    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.

Andersson, et al. Standards Track [Page 52] RFC 3036 LDP Specification January 2001

    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 contains 0 or more parameters, each
       encoded as a TLV.  The optional parameters defined by this
       version of the protocol are
       Optional Parameter         Type     Length  Value
       IPv4 Transport Address     0x0401     4      See below
       Configuration              0x0402     4      See below
          Sequence Number
       IPv6 Transport Address     0x0403    16      See below
    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
       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.

Andersson, et al. Standards Track [Page 53] RFC 3036 LDP Specification January 2001

3.5.2.1. 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 sections
 "Maintaining Hello Adjacencies" and "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:
    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:
  1. The LSR has been configured to accept Targeted Hellos, or
  1. The LSR has been configured to send Targeted Hellos to A.
    The following describes how an LSR processes Hello optional TLVs:

Andersson, et al. Standards Track [Page 54] RFC 3036 LDP Specification January 2001

    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 "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.

3.5.3. Initialization Message

 The LDP Initialization Message is exchanged as part of the LDP
 session establishment procedure; see Section "LDP Session
 Establishment".
 The encoding for the Initialization 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|   Initialization (0x0200)   |      Message Length           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Message ID                                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Common Session Parameters TLV             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Optional Parameters                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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:

Andersson, et al. Standards Track [Page 55] RFC 3036 LDP Specification January 2001

     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                       |
    +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               |
    -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++
    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
       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:
  1. If the session is for a label-controlled ATM link or a

label-controlled Frame Relay link, then Downstream on Demand

          must be used.
  1. 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

Andersson, et al. Standards Track [Page 56] RFC 3036 LDP Specification January 2001

       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 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 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 "Hello Message
       Procedures".

Andersson, et al. Standards Track [Page 57] RFC 3036 LDP Specification January 2001

       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:
       Optional Parameter       Type     Length  Value
       ATM Session Parameters   0x0501   var     See below
       Frame Relay Session      0x0502   var     See below
         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                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    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
       If the merge capabilities of the LSRs differ, then:

Andersson, et al. Standards Track [Page 58] RFC 3036 LDP Specification January 2001

  1. Non-merge and VC-merge LSRs may freely interoperate.
  1. 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 (see [ATM-VP]).
    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 which 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 an ATM Label Range Component is:

Andersson, et al. Standards Track [Page 59] RFC 3036 LDP Specification January 2001

     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              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Res
          This field is reserved. It must be set to zero on
          transmission and must be 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 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.
       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.
    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.
    See [ATM-VP] for specification of the fields for ATM Label Range
    Components to be used with VP merge LSRs.

Andersson, et al. Standards Track [Page 60] RFC 3036 LDP Specification January 2001

    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               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    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            Merge supported
       Non-merge and merge Frame Relay LSRs may freely interoperate.
    N, Number of label range components
       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

Andersson, et al. Standards Track [Page 61] RFC 3036 LDP Specification January 2001

       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 which 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            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       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
               2       23
          Len values 1 and 3 are reserved.

Andersson, et al. Standards Track [Page 62] RFC 3036 LDP Specification January 2001

       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
 which advertise Generic Labels.

3.5.3.1. Initialization Message Procedures

 See Section "LDP Session Establishment" and particularly Section
 "Session Initialization" for general procedures for handling the
 Initialization Message.

3.5.4. 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                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Message ID
    32-bit value used to identify this message.
 Optional Parameters
    No optional parameters are defined for the KeepAlive message.

3.5.4.1. KeepAlive Message Procedures

 The KeepAlive Timer mechanism described in Section "Maintaining LDP
 Sessions" resets a session KeepAlive timer every time an LDP PDU is

Andersson, et al. Standards Track [Page 63] RFC 3036 LDP Specification January 2001

 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.

3.5.5. Address Message

 An LSR sends the Address Message to an LDP peer to advertise its
 interface addresses.
 The encoding for the Address Message is:
  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                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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
    "Address List TLV".
 Optional Parameters
    No optional parameters are defined for the Address message.

3.5.5.1. Address Message Procedures

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

Andersson, et al. Standards Track [Page 64] RFC 3036 LDP Specification January 2001

 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".
 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 signalling an error and abort processing the
 message.

3.5.6. 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                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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
    No optional parameters are defined for the Address Withdraw
    message.

Andersson, et al. Standards Track [Page 65] RFC 3036 LDP Specification January 2001

3.5.6.1. Address Withdraw Message Procedures

 See Section "Address Message Procedures"

3.5.7. 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                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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 "FEC TLV" for encoding.
 Label TLV
    Specifies the Label component of the FEC-Label mapping.  See
    Section "Label TLV" for encoding.
 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 Request         4            See below
           Message ID TLV
       Hop Count TLV         1            See below
       Path Vector TLV       variable     See below

Andersson, et al. Standards Track [Page 66] RFC 3036 LDP Specification January 2001

    The encodings for the Hop Count, and Path Vector TLVs can be found
    in Section "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 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 setup by the Label Message.  Section "Hop Count
       Procedures" describes how to handle this TLV.
    Path Vector
       Specifies the LSRs along the LSP being setup by the Label
       Message.  Section "Path Vector Procedures" describes how to
       handle this TLV.

3.5.7.1. 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 or Host Address FEC Element should not use the label for
 forwarding unless its routing table contains an entry that exactly
 matches the FEC Element.
 See Appendix A "LDP Label Distribution Procedures" for more details.

3.5.7.1.1. 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.

Andersson, et al. Standards Track [Page 67] RFC 3036 LDP Specification January 2001

    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.

3.5.7.1.2. 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
          b) loop detection is configured   OR
          c) the attributes of the mapping have changed.

3.5.7.1.3. 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.

Andersson, et al. Standards Track [Page 68] RFC 3036 LDP Specification January 2001

 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.

3.5.7.1.4. 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 readvertise 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.
 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 readvertising 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.

3.5.8. Label Request Message

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

Andersson, et al. Standards Track [Page 69] RFC 3036 LDP Specification January 2001

 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                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Message ID
    32-bit value used to identify this message.
 FEC TLV
    The FEC for which a label is being requested.  See Section "FEC
    TLV" for encoding.
 Optional Parameters
    This variable length field contains 0 or more parameters, each
    encoded as a TLV.  The optional parameters are:
       Optional Parameter     Length       Value
       Hop Count TLV          1            See below
       Path Vector TLV        variable     See below
    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 setup by the Label Request Message.  Section "Hop
       Count Procedures" describes how to handle this TLV.
    Path Vector
       Specifies the LSRs along the LSR being setup by the Label
       Request Message.  Section "Path Vector Procedures" describes
       how to handle this TLV.

3.5.8.1. 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.

Andersson, et al. Standards Track [Page 70] RFC 3036 LDP Specification January 2001

 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 setup 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 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
 or a Host Address 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 or Host Address,
 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 which
 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:

Andersson, et al. Standards Track [Page 71] RFC 3036 LDP Specification January 2001

    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.

3.5.9. 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                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Message ID
    32-bit value used to identify this message.
 FEC TLV
    Identifies the FEC for which the Label Request is being aborted.

Andersson, et al. Standards Track [Page 72] RFC 3036 LDP Specification January 2001

 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.

3.5.9.1. Label Abort Request Message Procedures

 An LSR Ru may send a Label Abort Request message to abort an
 outstanding Label Request message for FEC sent to LSR Rd in the
 following circumstances:
    1. Ru's next hop for 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 FEC from an upstream peer Y.
    3. Ru is a merge, non-ingress LSR and has received a Label Abort
       Request for FEC from an upstream peer Y and Y is the only
       (last) upstream LSR requesting a label for 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.

Andersson, et al. Standards Track [Page 73] RFC 3036 LDP Specification January 2001

 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:
  1. A Label Request Aborted Notification message acknowledging the

abort;

  1. A Label Mapping message in response to the Label Request

message being aborted;

  1. 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 time out
 period should be relatively long (several minutes).  If the time out
 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.
 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.

3.5.10. 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.

Andersson, et al. Standards Track [Page 74] RFC 3036 LDP Specification January 2001

 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                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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:
       Optional Parameter    Length       Value
       Label TLV             variable     See below
    The encoding for Label TLVs are found in Section "Label TLVs".
    Label
       If present, specifies the label being withdrawn (see procedures
       below).

3.5.10.1. 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.

Andersson, et al. Standards Track [Page 75] RFC 3036 LDP Specification January 2001

 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.

3.5.11. 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                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Message ID
    32-bit value used to identify this message.
 FEC TLV
    Identifies the FEC for which the FEC-label mapping is being
    released.

Andersson, et al. Standards Track [Page 76] RFC 3036 LDP Specification January 2001

 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
    The encodings for Label TLVs are found in Section "Label TLVs".
    Label
       If present, the label being released (see procedures below).

3.5.11.1. Label Release Message Procedures

 An LSR transmits a Label Release message to a peer when it is 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 which 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 which 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

Andersson, et al. Standards Track [Page 77] RFC 3036 LDP Specification January 2001

 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.

3.6. Messages and TLVs for Extensibility

 Support for LDP extensibility includes the rules for the U and F bits
 that specify how an LSR should handle unknown TLVs and messages.
 This section specifies TLVs and messages for vendor-private and
 experimental use.

3.6.1. LDP Vendor-private Extensions

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

3.6.1.1. 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....                            |
 ~                                                               ~
 |                                                               |
 |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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.

Andersson, et al. Standards Track [Page 78] RFC 3036 LDP Specification January 2001

    The determination as to whether a vendor-private message is
    understood is based on the Type and the mandatory Vendor ID field.
 F bit
    Forward unknown TLV bit.  This bit only applies when the U bit is
    set and the LDP message containing the unknown TLV is 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.

Andersson, et al. Standards Track [Page 79] RFC 3036 LDP Specification January 2001

3.6.1.2. LDP Vendor-private Messages

 The Message Type range 0x3E00 through 0x3EFF is reserved for vendor-
 private Messages.
  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                       |
 +                                                               +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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.
 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.

Andersson, et al. Standards Track [Page 80] RFC 3036 LDP Specification January 2001

 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.

3.6.2. LDP Experimental Extensions

 LDP support for experimentation is similar to support for vendor-
 private extensions with the following differences:
  1. The Type range 0x3F00 through 0x3FFF is reserved for

experimental TLVs.

  1. The Message Type range 0x3F00 through 0x3FFF is reserved for

experimental messages.

  1. 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.

3.7. Message Summary

 The following are the LDP messages defined in this version of the
 protocol.
    Message Name            Type     Section Title
    Notification            0x0001   "Notification Message"
    Hello                   0x0100   "Hello Message"
    Initialization          0x0200   "Initialization Message"

Andersson, et al. Standards Track [Page 81] RFC 3036 LDP Specification January 2001

    KeepAlive               0x0201   "KeepAlive Message"
    Address                 0x0300   "Address Message"
    Address Withdraw        0x0301   "Address Withdraw Message"
    Label Mapping           0x0400   "Label Mapping Message"
    Label Request           0x0401   "Label Request Message"
    Label Withdraw          0x0402   "Label Withdraw Message"
    Label Release           0x0403   "Label Release Message"
    Label Abort Request     0x0404   "Label Abort Request Message"
    Vendor-Private          0x3E00-  "LDP Vendor-private Extensions"
                            0x3EFF
    Experimental            0x3F00-  "LDP Experimental Extensions"
                            0x3FFF

3.8. TLV Summary

 The following are the TLVs defined in this version of the protocol.
    TLV                      Type      Section Title
    FEC                      0x0100    "FEC TLV"
    Address List             0x0101    "Address List TLV"
    Hop Count                0x0103    "Hop Count TLV"
    Path Vector              0x0104    "Path Vector TLV"
    Generic Label            0x0200    "Generic Label TLV"
    ATM Label                0x0201    "ATM Label TLV"
    Frame Relay Label        0x0202    "Frame Relay Label TLV"
    Status                   0x0300    "Status TLV"
    Extended Status          0x0301    "Notification Message"
    Returned PDU             0x0302    "Notification Message"
    Returned Message         0x0303    "Notification Message"
    Common Hello             0x0400    "Hello Message"
       Parameters
    IPv4 Transport Address   0x0401    "Hello Message"
    Configuration            0x0402    "Hello Message"
       Sequence Number
    IPv6 Transport Address   0x0403    "Hello Message"
    Common Session           0x0500    "Initialization Message"
       Parameters
    ATM Session Parameters   0x0501    "Initialization Message"
    Frame Relay Session      0x0502    "Initialization Message"
       Parameters
    Label Request            0x0600    "Label Mapping Message"
        Message ID
    Vendor-Private           0x3E00-   "LDP Vendor-private Extensions"
                             0x3EFF
    Experimental             0x3F00-   "LDP Experimental Extensions"
                             0x3FFF

Andersson, et al. Standards Track [Page 82] RFC 3036 LDP Specification January 2001

3.9. 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 Data   Section Title
    Success               0   0x00000000    "Status TLV"
    Bad LDP Identifier    1   0x00000001    "Events Signaled by ..."
    Bad Protocol Version  1   0x00000002    "Events Signaled by ..."
    Bad PDU Length        1   0x00000003    "Events Signaled by ..."
    Unknown Message Type  0   0x00000004    "Events Signaled by ..."
    Bad Message Length    1   0x00000005    "Events Signaled by ..."
    Unknown TLV           0   0x00000006    "Events Signaled by ..."
    Bad TLV length        1   0x00000007    "Events Signaled by ..."
    Malformed TLV Value   1   0x00000008    "Events Signaled by ..."
    Hold Timer Expired    1   0x00000009    "Events Signaled by ..."
    Shutdown              1   0x0000000A    "Events Signaled by ..."
    Loop Detected         0   0x0000000B    "Loop Detection"
    Unknown FEC           0   0x0000000C    "FEC Procedures"
    No Route              0   0x0000000D    "Label Request Mess ..."
    No Label Resources    0   0x0000000E    "Label Request Mess ..."
    Label Resources /     0   0x0000000F    "Label Request Mess ..."
        Available
    Session Rejected/     1   0x00000010    "Session Initialization"
       No Hello
    Session Rejected/     1   0x00000011    "Session Initialization"
       Parameters Advertisement Mode
    Session Rejected/     1   0x00000012    "Session Initialization"
       Parameters Max PDU Length
    Session Rejected/     1   0x00000013    "Session Initialization"
       Parameters Label Range
    KeepAlive Timer       1   0x00000014    "Events Signaled by ..."
        Expired
    Label Request Aborted 0   0x00000015    "Label Request Abort ..."
    Missing Message       0   0x00000016    "Events Signaled by ..."
        Parameters
    Unsupported Address   0   0x00000017    "FEC Procedures"
        Family                              "Address Message Proc ..."

Andersson, et al. Standards Track [Page 83] RFC 3036 LDP Specification January 2001

    Session Rejected/     1   0x00000018    "Session Initialization"
       Bad KeepAlive Time
    Internal Error        1   0x00000019    "Events Signaled by ..."

3.10. Well-known Numbers

3.10.1. UDP and TCP Ports

 The UDP port for LDP Hello messages is 646.
 The TCP port for establishing LDP session connections is 646.

3.10.2. Implicit NULL Label

 The Implicit NULL label (see [RFC3031]) is represented as a Generic
 Label TLV with a Label field value as specified by [RFC3032].

4. IANA Considerations

 LDP defines the following name spaces which require management:
  1. Message Type Name Space.
  2. TLV Type Name Space.
  3. FEC Type Name Space.
  4. Status Code Name Space.
  5. Experiment ID Name Space.
 The following sections provide guidelines for managing these name
 spaces.

4.1. Message Type Name Space

 LDP divides the name space for message types into three ranges.  The
 following are the guidelines for managing these ranges:
  1. Message Types 0x0000 - 0x3DFF. Message types in this range are

part of the LDP base protocol. Following the policies outlined

       in [IANA], Message types in this range are allocated through an
       IETF Consensus action.
  1. 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.

Andersson, et al. Standards Track [Page 84] RFC 3036 LDP Specification January 2001

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

4.2. TLV Type Name Space

 LDP divides the name space for TLV types into three ranges.  The
 following are the guidelines for managing these ranges:
  1. TLV Types 0x0000 - 0x3DFF. TLV types in this range are part of

the LDP base protocol. Following the policies outlined in

       [IANA], TLV types in this range are allocated through an IETF
       Consensus action.
  1. 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.
  1. 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).

4.3. FEC Type Name Space

 The range for FEC types is 0 - 255.
 Following the policies outlined in [IANA], 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.

Andersson, et al. Standards Track [Page 85] RFC 3036 LDP Specification January 2001

4.4. Status Code Name Space

 The range for Status Codes is 0x00000000 - 0x3FFFFFFF.
 Following the policies outlined in [IANA], 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.

4.5. Experiment ID Name Space

 The range for Experiment Ids is 0x00000000 - 0xffffffff.
 Following the policies outlined in [IANA], 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.

5. Security Considerations

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

5.1. Spoofing

 There are two types of LDP communication that could be the target of
 a spoofing attack.
 1. Discovery exchanges carried by UDP.
    LSRs 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
    by filtering them and accepting only those originating at sources
    permitted by an access list.

Andersson, et al. Standards Track [Page 86] RFC 3036 LDP Specification January 2001

 2. Session communication carried by TCP.
    LDP specifies use of the TCP MD5 Signature Option to provide for
    the authenticity and integrity of session messages.
    [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.

5.2. 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 which distribute
 routing information.  By providing a mechanism to ensure the
 authenticity and integrity of its messages LDP provides a level of
 security which 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.

5.3. 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 which can be trusted to not initiate such an attack.

Andersson, et al. Standards Track [Page 87] RFC 3036 LDP Specification January 2001

    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 a 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.
       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.

Andersson, et al. Standards Track [Page 88] RFC 3036 LDP Specification January 2001

6. Areas for Future Study

 The following topics not addressed in this version of LDP are
 possible areas for future study:
  1. Section 2.16 of the MPLS architecture [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.
  1. LDP support for CoS is not specified in this version. CoS

support may be addressed in a future version.

  1. LDP support for multicast is not specified in this version.

Multicast support may be addressed in a future version.

  1. LDP support for multipath label switching is not specified in

this version. Multipath support may be addressed in a future

       version.

7. Intellectual Property Considerations

 The IETF has been notified of intellectual property rights claimed in
 regard to some or all of the specification contained in this
 document.  For more information consult the online list of claimed
 rights.

8. Acknowledgments

 The ideas and text in this document have been 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.

9. References

 [ATM-VP]    N. Feldman, B. Jamoussi, S. Komandur, A, Viswanathan, T
             Worster, "MPLS using ATM VP Switching", Work in Progress.
 [CRLDP]     L. Andersson, A. Fredette, B. Jamoussi, R. Callon, P.
             Doolan, N. Feldman, E. Gray, J. Halpern, J. Heinanen T.
             E. Kilty, A. G.  Malis, M. Girish, K. Sundell, P.
             Vaananen, T. Worster, L. Wu, R.  Dantu, "Constraint-Based
             LSP Setup using LDP", Work in Progress.

Andersson, et al. Standards Track [Page 89] RFC 3036 LDP Specification January 2001

 [DIFFSERV]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
             and W. Weiss, "An Architecture for Differentiated
             Services", RFC 2475, December 1998.
 [IANA]      Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 2434,
             October 1998.
 [RFC1321]   Rivest, R., "The MD5 Message-Digest Algorithm," RFC 1321,
             April 1992.
 [RFC1483]   Heinanen, J., "Multiprotocol Encapsulation over ATM
             Adaptation Layer 5", RFC 1483, July 1993.
 [RFC2328]   Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
 [RFC1700]   Reynolds, J. and J. Postel, "ASSIGNED NUMBERS", STD 2,
             RFC 1700, October 1994.
 [RFC1771]   Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
             (BGP-4)", RFC 1771, March 1995.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2205]   Braden, R., Zhang, L., Berson, S., Herzog, S. and S.
             Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
             Functional Specification", RFC 2205, September 1997.
 [RFC2385]   Heffernan, A., "Protection of BGP Sessions via the TCP
             MD5 Signature Option", RFC 2385, August 1998.
 [RFC2702]   Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.
             McManus, "Requirements for Traffic Engineering over
             MPLS", RFC 2702, September 1999.
 [RFC3031]   Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol
             Label Switching Architecture", RFC 3031, January 2001.
 [RFC3032]   Rosen, E., Rekhter, Y., Tappan, D., Farinacci, D.,
             Fedorkow, G.,  Li, T. and A. Conta, "MPLS Label Stack
             Encoding", RFC 3032, January 2001.
 [RFC3034]   Conta, A., Doolan, P. and A. Malis, "Use of Label
             Switching on Frame Relay Networks Specification", RFC
             3034, January 2001.

Andersson, et al. Standards Track [Page 90] RFC 3036 LDP Specification January 2001

 [RFC3035]   Davie, B., Lawrence, J., McCloghrie, K., Rekhter, Y.,
             Rosen, E., Swallow, G. and P. Doolan, "MPLS using LDP and
             ATM VC Switching", RFC 3035, January 2001.
 [RFC3037]   Thomas, B. and E. Gray, "LDP Applicability", RFC 3037,
             January 2001.

Andersson, et al. Standards Track [Page 91] RFC 3036 LDP Specification January 2001

10. Authors' Addresses

 Loa Andersson
 Nortel Networks Inc
 St Eriksgatan 115, PO Box 6701
 113 85 Stockholm
 Sweden
 Phone: +46 8 5088 36 34
 Mobile: +46 70 522 78 34
 EMail: loa.andersson@nortelnetworks.com
 Paul Doolan
 Ennovate Networks
 60 Codman Hill Rd
 Marlborough MA 01719
 Phone: 978-263-2002
 EMail: pdoolan@ennovatenetworks.com
 Nancy Feldman
 IBM Research
 30 Saw Mill River Road
 Hawthorne, NY 10532
 Phone:  914-784-3254
 EMail: nkf@us.ibm.com
 Andre Fredette
 PhotonEx Corporation
 8C Preston Court
 Bedford, MA 01730
 Phone: 781-301-4655
 EMail: fredette@photonex.com
 Bob Thomas
 Cisco Systems, Inc.
 250 Apollo Dr.
 Chelmsford, MA 01824
 Phone:  978-244-8078
 EMail: rhthomas@cisco.com

Andersson, et al. Standards Track [Page 92] RFC 3036 LDP Specification January 2001

Appendix A. LDP Label Distribution Procedures

 This section specifies label distribution behavior in terms of LSR
 response to the following events:
  1. Receive Label Request Message;
  2. Receive Label Mapping Message;
  3. Receive Label Abort Request Message;
  4. Receive Label Release Message;
  5. Receive Label Withdraw Message;
  6. Recognize new FEC;
  7. Detect change in FEC next hop;
  8. Receive Notification Message / Label Request Aborted;
  9. Receive Notification Message / No Label Resources;
  10. Receive Notification Message / No Route;
  11. Receive Notification Message / Loop Detected;
  12. Receive Notification Message / Label Resources Available;
  13. Detect local label resources have become available;
  14. LSR decides to no longer label switch a FEC;
  15. Timeout of deferred label request.
 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 [RFC3031] for hop-by-hop routed traffic.
 These procedures are:
  1. 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:

Andersson, et al. Standards Track [Page 93] RFC 3036 LDP Specification January 2001

       .  Downstream Unsolicited Independent Control, called
          PushUnconditional in [RFC3031].
       .  Downstream Unsolicited Ordered Control, called
          PushConditional in [RFC3031].
       .  Downstream On Demand Independent Control, called
          PulledUnconditional in [RFC3031].
       .  Downstream On Demand Ordered Control, called
          PulledConditional in [RFC3031].
  1. 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.
  1. 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:
       .  Request Never.  The LSR never requests a label.
       .  Request When Needed.  The LSR requests a label whenever
          it needs one.
       .  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.
  1. 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:
       .  Conservative label retention, called Release On Change in
          [RFC3031].
       .  Liberal label retention, called No Release On Change in
          [RFC3031].

Andersson, et al. Standards Track [Page 94] RFC 3036 LDP Specification January 2001

  1. 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:
       .  Use Immediate.  The LSR immediately uses a label received
          from a FEC next hop for forwarding/switching.
       .  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.
       .  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.
  1. Label No Route procedure (called Label Not Available procedure

in [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:
       .  Request Retry.  The LSR should issue the label request at a
          later time.
       .  No Request Retry.  The LSR should assume 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.

Andersson, et al. Standards Track [Page 95] RFC 3036 LDP Specification January 2001

 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:
  1. Transmission of a notification message to the requesting LSR

indicating why a label mapping for the FEC cannot be provided;

  1. Transmission of a FEC label mapping to the requesting LSR;
  1. Transmission of a FEC label request to the FEC next hop;
  1. Installation of labels for forwarding/switching use by the LSR.
 Context:
  1. LSR. The LSR handling the event.
  1. MsgSource. The LDP peer that sent the message.
  1. FEC. The FEC specified in the message.
  1. RAttributes. Attributes received with the message. E.g., Hop

Count, Path Vector.

  1. SAttributes. Attributes to be included in Label Request

message, if any, propagated to FEC Next Hop.

  1. StoredHopCount. The hop count, if any, previously recorded for

the FEC.

Andersson, et al. Standards Track [Page 96] RFC 3036 LDP Specification January 2001

 Algorithm:
    LRq.1   Execute procedure Check_Received_Attributes (MsgSource,
            LabelRequest, RAttributes).
            If Loop Detected, goto LRq.13.
    LRq.2   Is there a Next Hop for FEC?
            If not, goto LRq.5.
    LRq.3   Is MsgSource the Next Hop?
            Ifnot, 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:
          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
                Prepare_Label_Mapping_Attributes(MsgSource, FEC,
                RAttributes, SAttributes, Propagating,
                StoredHopCount).
             3. Execute procedure Send_Label (MsgSource, FEC,
                SAttributes).

Andersson, et al. Standards Track [Page 97] RFC 3036 LDP Specification January 2001

             4. Is LSR egress for FEC? OR
                Has LSR previously received and retained a label
                mapping for FEC from Next Hop?
                If 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
             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

Andersson, et al. Standards Track [Page 98] RFC 3036 LDP Specification January 2001

             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 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.

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:
  1. Transmission of a label release message for the FEC label to

the LDP peer;

  1. Transmission of label mapping messages for the FEC to one or

more LDP peers,

Andersson, et al. Standards Track [Page 99] RFC 3036 LDP Specification January 2001

  1. Installation of the newly learned label for

forwarding/switching use by the LSR.

 Context:
  1. LSR. The LSR handling the event.
  1. MsgSource. The LDP peer that sent the message.
  1. FEC. The FEC specified in the message.
  1. Label. The label specified in the message.
  1. PrevAdvLabel. The label for FEC, if any, previously advertised

to an upstream peer.

  1. StoredHopCount. The hop count previously recorded for the FEC.
  1. RAttributes. Attributes received with the message. E.g., Hop

Count, Path Vector.

  1. SAttributes to be included in 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.
    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.

Andersson, et al. Standards Track [Page 100] RFC 3036 LDP Specification January 2001

    LMp.7   Remove Label from forwarding/switching use.  (See Note 5.)
            Goto LMp.33.
    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.)
            If not, goto LMp.32.  (See Note 4.)
    LMp.11  Determine the Next Hop for FEC.
    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.

Andersson, et al. Standards Track [Page 101] RFC 3036 LDP Specification January 2001

    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).
            Goto LMp.28
    LMp.22  Iterate through LMp.27 for each label mapping for FEC
            previously sent to Peer.
    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).

Andersson, et al. Standards Track [Page 102] RFC 3036 LDP Specification January 2001

            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.
            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.

Andersson, et al. Standards Track [Page 103] RFC 3036 LDP Specification January 2001

    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 LSR has detected a loop and it has not previously received
        a label mapping from MsgSource for the FEC, it simply releases
        the label.
    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 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) 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 LSR is operating in Downstream Unsolicited ordered
        control mode.  Ordered control prevents LSR from advertising a
        label for FEC until it has received a label mapping from its
        next hop (MsgSource) for FEC.
    8.  If LSR is merging the LSP it may have previously sent label
        mappings for the FEC LSP to one or more peers.  If LSR is not
        merging, it may have sent a label mapping for the LSP in
        question to at most one LSR.

Andersson, et al. Standards Track [Page 104] RFC 3036 LDP Specification January 2001

    9.  The 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 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.

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:
  1. LSR. The LSR handling the event.
  1. MsgSource. The LDP peer that sent the message.
  1. FEC. The FEC specified in the message.
  1. RequestMessageID. The message ID of the label request message

to be aborted.

Andersson, et al. Standards Track [Page 105] RFC 3036 LDP Specification January 2001

  1. 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.
    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 upstream peers other than MsgSource that have
            requested a label for 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

Andersson, et al. Standards Track [Page 106] RFC 3036 LDP Specification January 2001

 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,
    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:
  1. LSR. The LSR handling the event.
  1. MsgSource. The LDP peer that sent the message.
  1. Label. The label specified in the message.
  1. FEC. The FEC specified in the message.
 Algorithm:
    LRl.1   Remove MsgSource from record of peers that hold Label for
            FEC.  (See Note 1.)
    LRl.2   Does message match an outstanding label withdraw for FEC
            previously sent to MsgSource?
            If not, goto LRl.4
    LRl.3   Delete record of outstanding label withdraw for FEC
            previously sent to MsgSource.
    LRl.4   Is LSR merging labels for this FEC?
            If not, goto LRl.6.  (See Note 2.)
    LRl.5   Has LSR previously advertised a label for this FEC to
            other peers?
            If so, goto LRl.10.

Andersson, et al. Standards Track [Page 107] RFC 3036 LDP Specification January 2001

    LRl.6   Is LSR egress for the FEC?
            If so, goto LRl.10
    LRl.7   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.10.
    LRl.8   Is LSR configured to propagate releases?
            If not, goto LRl.10.  (See Note 3.)
    LRl.9   Execute procedure Send_Message (Next Hop, Label Release,
            FEC, Label from Next Hop).
    LRl.10  Remove Label from forwarding/switching use for traffic
            from MsgSource.
    LRl.11  Do any peers still hold Label for FEC?
            If so, goto LRl.13.
    LRl.12  Free the Label.
    LRl.13  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.4 through LRl.8 deal with determining whether where the LSR
       should propagate the label release to a downstream peer
       (LRl.9).
    3. If LRl.8 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

Andersson, et al. Standards Track [Page 108] RFC 3036 LDP Specification January 2001

       amount of signaling required small; 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:
  1. LSR. The LSR handling the event.
  1. MsgSource. The LDP peer that sent the message.
  1. Label. The label specified in the message.
  1. FEC. The FEC specified in the message.
 Algorithm:
    LWd.1   Remove Label from forwarding/switching use.  (See Note 1.)
    LWd.2   Execute procedure Send_Message (MsgSource, Label Release,
            FEC, Label)
    LWd.3   Has LSR previously received and retained a matching label
            mapping for FEC from MsgSource?
            If not, goto LWd.13.
    LWd.4   Delete matching label mapping for FEC previously received
            from MsgSource.
    LWd.5   Is LSR using ordered control?
            If so, goto LWd.8.
    LWd.6   Is MsgSource using Downstream On Demand label
            advertisement?
            If not, goto LWd.13.

Andersson, et al. Standards Track [Page 109] RFC 3036 LDP Specification January 2001

    LWd.7   Generate Event: Recognize New FEC for FEC.
            Goto LWd.13.  (See Note 2.)
    LWd.8   Iterate through LWd.12 for each Peer, other than
            MsgSource.
    LWd.9   Has LSR previously sent a label mapping for FEC to Peer?
            If not, continue iteration for next Peer at LWd.8.
    LWd.10  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.11  Execute procedure Send_Label_Withdraw (Peer, FEC, Label
            previously sent to Peer).
    LWd.12  End iteration from LWd.8.
    LWd.13  DONE
 Notes:
    1. If 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:
  1. Transmission of label mappings for the FEC to one or more LDP

peers;

  1. Transmission of a label request for the FEC to the FEC next

hop;

Andersson, et al. Standards Track [Page 110] RFC 3036 LDP Specification January 2001

  1. Any of the actions that can occur when the LSR receives a label

mapping for the FEC from the FEC next hop.

 Context:
  1. LSR. The LSR handling the event.
  1. FEC. The newly recognized FEC.
  1. Next Hop. The next hop for the FEC.
  1. InitAttributes. Attributes to be associated with the new FEC.

(See Note 1.)

  1. SAttributes. Attributes to be included in Label Mapping or

Label Request messages, if any, sent to peers.

  1. StoredHopCount. Hop count associated with FEC label mapping,

if any, previously received from Next Hop.

 Algorithm:
    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.

Andersson, et al. Standards Track [Page 111] RFC 3036 LDP Specification January 2001

             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.)
    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, SAttributes).
               Goto FEC.6.
    FEC.5   Generate Event: Received Label Mapping from Next Hop.
            (See Note 3.)
    FEC.6   DONE.

Andersson, et al. Standards Track [Page 112] RFC 3036 LDP Specification January 2001

 Notes:
    1. An example of an attribute that might be part of InitAttributes
       is one which 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
    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:
  1. Removal of the label from the FEC's old next hop from

forwarding/switching use;

  1. Transmission of label mapping messages for the FEC to one or

more LDP peers;

  1. Transmission of a label request to the FEC's new next hop;
  1. Any of the actions that can occur when the LSR receives a label

mapping from the FEC's new next hop.

 Context:
  1. LSR. The LSR handling the event.
  1. FEC. The FEC whose next hop changed.
  1. New Next Hop. The current next hop for the FEC.

Andersson, et al. Standards Track [Page 113] RFC 3036 LDP Specification January 2001

  1. Old Next Hop. The previous next hop for the FEC.
  1. OldLabel. Label, if any, previously received from Old Next

Hop.

  1. CurAttributes. The attributes, if any, currently associated

with the FEC.

  1. SAttributes. Attributes to be included in Label 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.
    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 a label abort request is pending for FEC with Old
           Next Hop.
    NH.10  Is there a New Next Hop for the 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.

Andersson, et al. Standards Track [Page 114] RFC 3036 LDP Specification January 2001

    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.
    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:
    1. If Label is not in forwarding/switching use, NH.2 has no
       effect.
    2. 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.
    3. 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 the FEC next hop had
       changed.
    4. Regardless of the Label Request procedure in use by the LSR, it
       must send a label request if the conditions in NH.8 hold.
       Therefore it executes the Send_Label_Request procedure directly
       rather than perform LSR Label Request procedure.

Andersson, et al. Standards Track [Page 115] RFC 3036 LDP Specification January 2001

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.
 Context:
  1. LSR. The LSR handling the event.
  1. FEC. The FEC for which a label was requested.
  1. RequestMessageID. The message ID of the label request message

to be aborted.

  1. MsgSource. The LDP peer that sent the Notification message.
 Algorithm:
    LRqA.1  Does the notification correspond to an outstanding label
            request abort for FEC? (See Note 1).
            If not, goto LRqA.3.
    LRqA.2  Record that the label request for FEC has been aborted.
    LRqA.3  DONE
 Notes:
    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:
  1. LSR. The LSR handling the event.
  1. FEC. The FEC for which a label was requested.

Andersson, et al. Standards Track [Page 116] RFC 3036 LDP Specification January 2001

  1. MsgSource. The LDP peer that sent the Notification message.
 Algorithm:
    NoRes.1 Delete record of outstanding label request for FEC sent
            to MsgSource.
    NoRes.2 Record 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:
  1. LSR. The LSR handling the event.
  1. FEC. The FEC for which a label was requested.
  1. Attributes. The attributes associated with the label request.
  1. 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.

Andersson, et al. Standards Track [Page 117] RFC 3036 LDP Specification January 2001

          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.

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"
 Notes:
    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:
  1. LSR. The LSR handling the event.
  1. MsgSource. The LDP peer that sent the Notification message.
  1. SAttributes. Attributes stored with postponed Label Request

message.

Andersson, et al. Standards Track [Page 118] RFC 3036 LDP Specification January 2001

 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.
    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   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:
  1. LSR. The LSR handling the event.
  1. Attributes. Attributes stored with 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.

Andersson, et al. Standards Track [Page 119] RFC 3036 LDP Specification January 2001

    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.
    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.
 Notes:
    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:
  1. Peer. The peer.
  1. FEC. The FEC.
  1. PrevAdvLabel. The label for FEC previously advertised to Peer.
 Algorithm:
    NoLS.1  Execute procedure Send_Label_Withdraw (Peer, FEC,
            PrevAdvLabel).  (See Note 1.)
    NoLS.2  DONE.

Andersson, et al. Standards Track [Page 120] RFC 3036 LDP Specification January 2001

 Notes:
    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.

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:
  1. LSR. The LSR handling the event.
  1. FEC. The FEC associated with the timeout event.
  1. Peer. The LDP peer associated with the timeout event.
  1. Attributes. Attributes stored with 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

Andersson, et al. Standards Track [Page 121] RFC 3036 LDP Specification January 2001

    pending label request from the peer, it sends the LDP peer a No
    Label Resources notification.
 Parameters:
  1. Peer. The LDP peer to which the label mapping is to be sent.
  1. FEC. The FEC for which a label mapping is to be sent.
  1. Attributes. The attributes to be included with the label

mapping.

 Additional Context:
  1. LSR. The LSR executing the procedure.
  1. 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).

Andersson, et al. Standards Track [Page 122] RFC 3036 LDP Specification January 2001

    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.
    SL.13  Record label mapping needed for FEC and Attributes for
           Peer, but no-label-resources.  (See Note 1.)
    SL.14  Return failure.
 Notes:
    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:
  1. Peer. The LDP peer to which the label request is to be sent.
  1. FEC. The FEC for which a label request is to be sent.
  1. Attributes. Attributes to be included in the label request.

E.g., Hop Count, Path Vector.

 Additional Context:
  1. 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

Andersson, et al. Standards Track [Page 123] RFC 3036 LDP Specification January 2001

    SLRq.3  Execute procedure Send_Message (Peer, Label Request, FEC,
            Attributes).
    SLRq.4  Record label request for FEC has been sent to Peer and
            mark it as outstanding.
    SLRq.5  Return success.
    SLRq.6  Postpone the label request by recording label mapping for
            FEC and Attributes from Peer is needed but that no label
            resources are available.
    SLRq.7  Return failure.
 Notes:
    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:
  1. Peer. The LDP peer to which the label withdraw is to be sent.
  1. FEC. The FEC for which a label is being withdrawn.
  1. Label. The label being withdrawn
 Additional Context:
  1. LSR. The LSR executing the procedure.
 Algorithm:
    SWd.1  Execute procedure Send_Message (Peer, Label Withdraw, FEC,
           Label)
    SWd.2  Record label withdraw for FEC has been sent to Peer and
           mark it as outstanding.

Andersson, et al. Standards Track [Page 124] RFC 3036 LDP Specification January 2001

A.2.4. Send_Notification

 Summary:
    An LSR uses the Send_Notification procedure to send an LDP peer a
    notification message.
 Parameters:
  1. Peer. The LDP peer to which the Notification message is to be

sent.

  1. 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:
  1. Peer. The LDP peer to which the message is to be sent.
  1. Message Type. The type of message to be sent.
  1. 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.

Andersson, et al. Standards Track [Page 125] RFC 3036 LDP Specification January 2001

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.
 Parameters:
  1. MsgSource. The LDP peer that sent the message.
  1. MsgType. The type of message received.
  1. RAttributes. The attributes in the message.
 Additional Context:
  1. LSR Id. The unique LSR Id of this LSR.
  1. Hop Count. The Hop Count, if any, in the received attributes.
  1. 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)

Andersson, et al. Standards Track [Page 126] RFC 3036 LDP Specification January 2001

    CRa.8   Return Loop Detected.
    CRa.9   DONE
 Notes:
    1. When the attributes being checked were received in a Label
       Mapping message, the LSR sends the Loop Detected notification
       in 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:
  1. Peer. The LDP peer to which the message is to be sent.
  1. FEC. The FEC for which a label request is to be sent.
  1. RAttributes. The attributes this LSR associates with the LSP

for FEC.

  1. SAttributes. The attributes to be included in the Label

Request message.

 Additional Context:
  1. 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.

Andersson, et al. Standards Track [Page 127] RFC 3036 LDP Specification January 2001

    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.
    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 [RFC3035] and
       [RFC3034].
    2. For hop count arithmetic, unknown + 1 = unknown.

Andersson, et al. Standards Track [Page 128] RFC 3036 LDP Specification January 2001

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:
  1. Peer. The LDP peer to which the message is to be sent.
  1. FEC. The FEC for which a label request is to be sent.
  1. RAttributes. The attributes this LSR associates with the LSP

for FEC.

  1. SAttributes. The attributes to be included in the Label

Mapping message.

  1. IsPropagating. The LSR is sending the Label Mapping message to

propagate one received from the FEC next hop.

  1. PrevHopCount. The Hop Count, if any, this LSR associates with

the LSP for the FEC.

 Additional Context:
  1. LSR Id. The unique LSR Id of this LSR.
 Algorithm:
    PMpA.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 PMpA.21.
    PMpA.2  Is LSR egress for FEC?
            If not, goto PMpA.4.
    PMpA.3  Include Hop Count of 1 in SAttributes.  Goto PMpA.21.
    PMpA.4  Do RAttributes have a Hop Count?
            If not, goto PMpA.8.

Andersson, et al. Standards Track [Page 129] RFC 3036 LDP Specification January 2001

    PMpA.5  Is LSR member of 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.7.
    PMpA.6  Include Hop Count of 1 in SAttributes.  Goto PMpA.9.
    PMpA.7  Increment RAttributes Hop Count and copy the resulting
            Hop Count to SAttributes.  See Note 2.  Goto PMpA.9.
    PMpA.8  Include Hop Count of unknown (0) in SAttributes.
    PMpA.9  Is Loop Detection configured on LSR?
            If not, goto PMpA.21.
    PMpA.10 Do RAttributes have a Path Vector?
            If so, goto PMpA.19.
    PMpA.11 Is LSR propagating a received Label Mapping?
            If not, goto PMpA.20.
    PMpA.12 Does LSR support merging?
            If not, goto PMpA.14.
    PMpA.13 Has LSR previously sent a Label Mapping for FEC to Peer?
            If not, goto PMpA.20.
    PMpA.14 Do RAttributes include a Hop Count?
            If not, goto PMpA.21.
    PMpA.15 Is Hop Count in Rattributes unknown(0)?
            If so, goto PMpA.20.
    PMpA.16 Has LSR previously sent a Label Mapping for FEC to Peer?
            If not goto PMpA.21.
    PMpA.17 Is Hop Count in RAttributes different from PrevHopCount ?
            If not goto PMpA.21.
    PMpA.18 Is the Hop Count in RAttributes > PrevHopCount? OR
            Is PrevHopCount unknown(0)
            If not, goto PMpA.21.
    PMpA.19 Add LSR Id to beginning of Path Vector from RAttributes
            and copy the resulting Path Vector into SAttributes.
            Goto PMpA.21.

Andersson, et al. Standards Track [Page 130] RFC 3036 LDP Specification January 2001

    PMpA.20 Include Path Vector of length 1 containing LSR Id in
            SAttributes.
    PMpA.21 DONE.
 Notes:
    1. The link with Peer may require that Hop Count be included in
       Label Mapping messages; for example, see [RFC3035] and
       [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.

Andersson, et al. Standards Track [Page 131] RFC 3036 LDP Specification January 2001

Full Copyright Statement

 Copyright (C) The Internet Society (2001).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Andersson, et al. Standards Track [Page 132]

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