Network Working Group                                      Loa Andersson
Internet Draft                                      Nortel Networks Inc.
Expiration Date: July November 1999
                                                             Paul Doolan
                                                       Ennovate Networks

                                                           Nancy Feldman
                                                                IBM Corp

                                                          Andre Fredette
                                                    Nortel Networks Inc.

                                                              Bob Thomas
                                                     Cisco Systems, Inc.

                                                            January

                                                                May 1999

                           LDP Specification

                       draft-ietf-mpls-ldp-03.txt

                       draft-ietf-mpls-ldp-04.txt

Status of this Memo

   This document is an Internet-Draft. Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Abstract

   An overview of Multi Protocol Label Switching (MPLS) is provided in
   [FRAMEWORK] and a proposed architecture in [ARCH].  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 the
   Label Distribution Protocol (LDP) referenced in [ARCH]. a set
   of procedures, called a label distribution protocol, by which one LSR
   informs another of label bindins it has made.  This document defines the
   a set of such procedures called LDP protocol. (for Label Distribution
   Protocol).

Changes from Previous Draft

     - This draft includes by reference addresses issues raised during the CR-LDP-based and RSVP-based
       methods for establishing explicitly routed LSPs. LDP last call held
       February 8, 1999 through February 24, 1999.

Open Issues

   The following LDP issues are left unresolved with this version of the
   spec:

     - This draft modifies hop count procedures slightly for Label
       Mapping messages to correctly LDP support TTL maintenance for packets
       traversing LSPs which include multiple clouds of devices which do CoS is not perform 'TTL-decrement'.

     - This draft removes the E-bit (Null Encapsulation bit) from the
       ATM Session Parameters TLV used specified in the Initialization message
       because draft-ietf-mpls-atm-01.txt leaves no encapsulation
       parameters to negotiate at session setup time.

     - This draft adds the D-bit, (VC Directionality bit) to the ATM
       Session Paramameters TLV this version.  CoS
       support may be addressed in order to allow interoperability with
       ATM switches with 'paired' cross connects.  When such a switch
       establishes a VC in one direction, connectivity future version.

     - LDP support for multicast is established
       automatically not specified in the other direction.

     - This draft specifies the representation of the Implicit NULL
       label [see ARCH]. this version.
       Multicast support will be addressed in a future version.

     - This draft updates the procedure LDP support for the "Detect change multipath label switching is not specified in FEC
       next hop" event in order to explicitly address the case where
       there is no next hop.

     - This draft expands the PVLim field of the Common Session
       Parameters TLV to allow specification of loop detection path
       vector length limits of up to 255.

     - This draft corrects several errors of omission (e.g., failure to
       specify certain TLV type codes, failure to note that Frame Relay,
       like ATM, requires use of Hop Count TLV in Label Mapping and
       Request messages), corrects numerous typos, and includes minor
       re-wordings intended to clarify meaning.

Open Issues

   The following LDP issues are left unresolved with this version of the
   spec:

     - LDP support for CoS is not completely specified in this version.
       Cos support will be more fully addressed in a future version.

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

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

Table of Contents

    1          LDP Overview  .......................................   7   6
    1.1        LDP Peers  ..........................................   7   6
    1.2        LDP Message Exchange  ...............................   7
    1.3        LDP Message Structure  ..............................   8   7
    1.4        LDP Error Handling  .................................   8
    1.5        LDP Extensibility and Future Compatibility  .........   9   8
    2          LDP Operation  ......................................   9   8
    2.1        FECs  ...............................................   9   8
    2.2        Label Spaces, Identifiers, Sessions and Transport  ..  10
    2.2.1      Label Spaces  .......................................  10
    2.2.2      LDP Identifiers  ....................................  11
    2.2.3      LDP Sessions  .......................................  11
    2.2.4      LDP Transport  ......................................  11
    2.3        LDP Sessions between non-Directly Connected LSRs  ...  12
    2.4        LDP Discovery   .....................................  12
    2.4.1      Basic Discovery Mechanism  ..........................  12
    2.4.2      Extended Discovery Mechanism  .......................  13
    2.5        Establishing and Maintaining LDP Sessions  ..........  14
    2.5.1      LDP Session Establishment  ..........................  14
    2.5.2      Transport Connection Establishment  .................  14
    2.5.3      Session Initialization  .............................  15
    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  25
    2.8.2      Label Mapping Message  ..............................  26
    2.8.3      Discussion  .........................................  27  28
    2.9        Label Distribution for Explicitly Routed LSPs  ......  28
    3          Protocol Specification  .............................  28  29
    3.1        LDP PDUs  ...........................................  29
    3.2        LDP Procedures  .....................................  30
    3.3        Type-Length-Value Encoding  .........................  30
    3.4        TLV Encodings for Commonly Used Parameters  .........  32
    3.4.1      FEC TLV  ............................................  32
    3.4.1.1    FEC Procedures  .....................................  34  35
    3.4.2      Label TLVs  .........................................  34  35
    3.4.2.1    Generic Label TLV  ..................................  34  35
    3.4.2.2    ATM Label TLV  ......................................  35
    3.4.2.3    Frame Relay Label TLV  ..............................  36
    3.4.3      Address List TLV  ...................................  36
    3.4.4      COS TLV  ............................................  37
    3.4.5
    3.4.4      Hop Count TLV  ......................................  38
    3.4.5.1
    3.4.4.1    Hop Count Procedures  ...............................  38
    3.4.6
    3.4.5      Path Vector TLV  ....................................  39
    3.4.6.1
    3.4.5.1    Path Vector Procedures  .............................  40
    3.4.6.1.1
    3.4.5.1.1  Label Request Path Vector  ..........................  40
    3.4.6.1.2
    3.4.5.1.2  Label Mapping Path Vector  ..........................  41
    3.4.7
    3.4.6      Status TLV  .........................................  41  42
    3.5        LDP Messages  .......................................  43
    3.5.1      Notification Message  ...............................  45
    3.5.1.1    Notification Message Procedures  ....................  46  47
    3.5.1.2    Events Signaled by Notification Messages  ...........  46  47
    3.5.1.2.1  Malformed PDU or Message  ...........................  47
    3.5.1.2.2  Unknown or Malformed TLV  ...........................  47  48
    3.5.1.2.3  Session Hold KeepAlive Timer Expiration  ......................  48  .................  49
    3.5.1.2.4  Unilateral Session Shutdown  ........................  48  49
    3.5.1.2.5  Initialization Message Events  ......................  48  49
    3.5.1.2.6  Events Resulting From Other Messages  ...............  48  49
    3.5.1.2.7  Miscellaneous Events  ...............................  49
    3.5.2      Hello Message  ......................................  49  50
    3.5.2.1    Hello Message Procedures  ...........................  51  52
    3.5.3      Initialization Message  .............................  52  53
    3.5.3.1    Initialization Message Procedures  ..................  60  61
    3.5.4      KeepAlive Message  ..................................  60  61
    3.5.4.1    KeepAlive Message Procedures  .......................  60  62
    3.5.5      Address Message  ....................................  61  62
    3.5.5.1    Address Message Procedures  .........................  61  63
    3.5.6      Address Withdraw Message  ...........................  62  63
    3.5.6.1    Address Withdraw Message Procedures  ................  62  64
    3.5.7      Label Mapping Message  ..............................  63  64
    3.5.7.1    Label Mapping Message Procedures  ...................  64  65
    3.5.7.1.1  Independent Control Mapping  ........................  64  65
    3.5.7.1.2  Ordered Control Mapping  ............................  65  66
    3.5.7.1.3  Downstream on Demand Label Advertisement  ...........  65  66
    3.5.7.1.4  Downstream Unsolicited Label Advertisement  .........  66  67
    3.5.8      Label Request Message  ..............................  66  68
    3.5.8.1    Label Request Message Procedures  ...................  67  69
    3.5.9      Label Abort Request Message  ........................  70
    3.5.9.1    Label Abort Request Message Procedures  .............  71
    3.5.10     Label Withdraw Message  .............................  68
    3.5.9.1  72
    3.5.10.1   Label Withdraw Message Procedures  ..................  69
    3.5.10  73
    3.5.11     Label Release Message  ..............................  70
    3.5.10.1  74
    3.5.11.1   Label Release Message Procedures  ...................  71  74
    3.6        Messages and TLVs for Extensibility  ................  72  75
    3.6.1      LDP Vendor-private Extensions  ......................  72  76
    3.6.1.1    LDP Vendor-private TLVs  ............................  72  76
    3.6.1.2    LDP Vendor-private Messages  ........................  73  77
    3.6.2      LDP Experimental Extensions  ........................  75  78
    3.7        Message Summary  ....................................  75  79
    3.8        TLV Summary  ........................................  76  79
    3.9        Status Code Summary  ................................  77  80
    3.10       Well-known Numbers  .................................  78  81
    3.10.1     UDP and TCP Ports  ..................................  78  81
    3.10.2     Implicit NULL Label  ................................  78  81
    4          Security  ...........................................  78 Considerations  ............................  81
    4.1        The TCP MD5 Signature Option  .......................  78  81
    4.2        LDP Use of the TCP MD5 Signature Option  ............  80  83
    5          Intellectual Property Considerations  ...............  80  84
    6          Acknowledgments  ....................................  80  84
    7          References  .........................................  81  84
    8          Author Information  .................................  82  85

    Appendix.A LDP Label Distribution Procedures  ..................  83  87
    A.1        Handling Label Distribution Events  .................  85  89
    A.1.1      Receive Label Request  ..............................  86  90
    A.1.2      Receive Label Mapping  ..............................  89  93
    A.1.3      Receive Label Abort Request  ........................  97
    A.1.4      Receive Label Release  ..............................  93
    A.1.4  99
    A.1.5      Receive Label Withdraw  .............................  95
    A.1.5 101
    A.1.6      Recognize New FEC  ..................................  96
    A.1.6 102
    A.1.7      Detect change Change in FEC next hop Next Hop  ......................  99
    A.1.7 105
    A.1.8      Receive Notification / Label Request Aborted  ....... 108
    A.1.9      Receive Notification / No Label Resources  .......... 101
    A.1.8 109
    A.1.10     Receive Notification / No Route  .................... 102
    A.1.9 109
    A.1.11     Receive Notification / Loop Detected  ............... 103
    A.1.10 110
    A.1.12     Receive Notification / Label Resources Available  ... 103
    A.1.11 111
    A.1.13     Detect local label resources have become available  . 104
    A.1.12 111
    A.1.14     LSR decides to no longer label switch a FEC  ........ 105
    A.1.13 112
    A.1.15     Timeout of deferred label request  .................. 105 113
    A.2        Common Label Distribution Procedures  ............... 106 114
    A.2.1      Send_Label  ......................................... 106 114
    A.2.2      Send_Label_Request  ................................. 108 115
    A.2.3      Send_Label_Withdraw  ................................ 109 117
    A.2.4      Send_Notification  .................................. 109 117
    A.2.5      Send_Message  ....................................... 110 118
    A.2.6      Check_Received_Attributes  .......................... 110 118
    A.2.7      Prepare_Label_Request_Attributes  ................... 111 119
    A.2.8      Prepare_Label_Mapping_Attributes  ................... 113 121

1. LDP Overview

   LDP

   Section 2.6 of the MPLS architecture [ARCH] 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.  Section 2.29 of the architecture [ARCH]
   discusses some of the considerations when chosing a label
   distribution protocol for use in particular MPLS applications such as
   Traffic Engineering [TE].

   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) [ARCH] 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.

   Note that this

   This document is written assumes familiarity with respect to unicast routing
   only. Multicast will be addressed in the MPLS architecture [ARCH].
   Note that [ARCH] includes a future revision. glossary of MPLS terminology, such as
   ingress, label switched path, etc.

1.1. LDP Peers

   Two LSRs which use LDP to exchange label/stream 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 the 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.

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.

2. LDP Operation

2.1. FECs

   It is necessary to precisely specify which IP 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 IP 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. IP Address Prefix.  This element is an IP address prefix of any
         length from 0 to 32 bits, a full address, inclusive.

      2. Host Address.  This element is a 32-bit IP full host address.

   We say

   (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 IP address "matches" a particular IP 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 IP Address Prefix FEC element which matches the packet's IP
   destination address.  With respect to a particular packet and a
   particular LSP, we refer to any IP 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.

     - If there is exactly one LSP which has a Host Address FEC element
       that is identical to the packet's IP destination address, then the
       packet is mapped to that LSP.

     - If there multiple LSPs, each containing a Host Address FEC
       element that is identical to the packet's IP 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.

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

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

     - If it is known that a packet must traverse a particular egress
       router, and there is an LSP which has an IP Address Prefix FEC
       element (of length 32 bits) 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:

     - 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.

     - 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.

     - 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:

     - 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.

     - 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 encode an IP address assigned to
   the LSR, and 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:

              <IP address> : <label space id>

   e.g., 171.32.27.28:0, 192.0.3.5:2.

   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.

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:

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

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

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 IP 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:

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

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

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

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

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:

           - Transport connection establishment.
           - Session initialization

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

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 IP 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 IP address in Hellos received from LSR2.

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

     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.

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.

   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 first LDP PDU on the connection.

   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.

         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.

       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.

               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
              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
                                 |   |
                                 +---+

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.

2.6. Label Distribution and Management

   The MPLS architecture [ARCH] 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.  This is known as Downstream Unsolicited label
   distribution.

   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
   Unsolicted 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:

      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

      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
         [rfc1583] [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

2.6.2.1. Conservative Label Retention Mode

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

   The main advantage of the liberal label retention mode is that reac-
   tion
   reaction to routing changes can be quick because labels already
   exist.  The main disadvantage of the liberal mode is that unneeded
   label map-
   pings mappings are distributed and maintained.

2.6.3. Label Advertisement Mode

   Each interface on an LSR is configured to operate in either Down-
   stream
   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 forward-
   ing.
   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 With-
   draw
   Withdraw Address messages.

   An LSR sends an Address message to advertise its addresses to a peer.
   An LSR sends a Withdraw Address message to withdraw previously adver-
   tised
   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:

     - 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 IP address
       component of its LDP Identifier.  When an LSR propagates a mes-
       sage
       message containing a Path Vector TLV it adds its LSR Id to the
       Path Vector list.  An LSR that receives a message with a Path
       Vector that contains its LSR Id detects that the message has
       traversed a loop.  LDP supports the notion of a maximum allowable
       Path Vector length; an LSR that detects a Path Vector has reached
       the maximum length behaves as if the containing message has
       traversed a loop.

     - A Hop Count TLV contains a count of the LSRS that the containing
       message has traversed.  When an LSR propagates a message contain-
       ing
       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.  In
   these paragraphs, "MUST" means "MUST if configured for loop detec-
   tion".
   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 detec-
   tion detection is not configured (see [ATM] and [FR]).

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.

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

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

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

   - If R is sending the Label Request as a result of having received a
     Label Request from an upstream LSR, and if the received Label
     Request contains a Hop Count TLV, R MUST increment the received hop
     count value by 1 and MUST pass the resulting value in a Hop Count
     TLV to its next hop along with the Label Request message;

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

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

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

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

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

   If R receives a Label Request message from its next hop with a Hop
   Count TLV 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 mes-
   sage
   message to the source of the Label Request message and drop the Label
   Request message.

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 fol-
   lowing:
   following:

   - R MUST include a Hop Count TLV.

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

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

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

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

   - If the Label Mapping message is not being sent to propagate a Label
     Mapping message, the hop count value MUST be the result of incre-
     menting
     incrementing R's current knowledge of the hop count learned from previ-
     ous
     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 follow-
   ing:
   following:

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

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

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

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

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

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

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

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

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

   If R receives a Label Mapping message from its next hop with a Hop
   Count TLV 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 send a Loop Detected Notification
   message to the source of the Label Mapping message.

2.8.3. Discussion

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

   Note that in a network where only non-merge capable LSRs are present,
   Path Vectors are passed downstream from ingress to egress, and are
   not passed upstream.  Even when merge is supported, Path Vectors need
   not be passed upstream along an LSP 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.

   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.

2.9. Label Distribution for Explicitly Routed LSPs

   Traffic Engineering [TE] is expected to be an important MPLS applica-
   tion.  It
   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.

   Two approaches for establishing explictily routed LSPs are under
   development within the MPLS WG. One approach  CR-LDP [CRLDP] uses
   defines extensions to LDP to accomplish label distribution; the other [LSPTUN] uses
   extensions use LDP to RSVP [rfc2205]. set up explicitly routed
   LSPs.

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 pro-
   cessing
   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 mes-
   sages
   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.

   LDP Identifier
     Six octet field that uniquely identifies the label space of the
     sending LSR for which this PDU applies.  The first four octets
     encode an IP address assigned to the LSR.  This address should be
     the router-id, also used to identify the LSR in loop detection Path
     Vectors.  The last two octets identify a label space within the
     LSR.  For a platform-
     wide 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:

     - Peer discovery;
     - Session management;
     - Label distribution;
     - 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.

    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 for-
     warded.
     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 for-
     warded
     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 inter-
     preted
     interpreted as specified by the Type field.

   Note that there is no alignment requirement for the first octect 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 specifica-
   tion
   specification does not use the TLV scheme to its full generality.  It
   is not used where its generality is unnecessary and its use would
   waste space unnecessarily.  These are usually places where the type
   of a value to be encoded is known, for example by its position in a mes-
   sage
   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 specifica-
   tion
   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.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|F|
   |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      4 octet full IP      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 asso-
       ciated
       associated with the label within the following label TLV.  Must
       be the only FEC Element in the FEC TLV.

     Prefix FEC Element value encoding:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Prefix (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.

3.4.1.1. FEC Procedures

   If in decoding a FEC TLV an LSR encounters a FEC Element type it can-
   not
   cannot decode, it should stop decoding the FEC TLV, abort processing
   the message containing the TLV, and send an 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|F|
   |0|0| Generic Label (0x0200)    |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Label                                                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|F|
   |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 signifi-
     cant.
     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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|F|
   |0|0| Frame Relay Label (0x0202)|       Length                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Reserved    |Len|                     DLCI                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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
        1 = 17 bits DLCI
        2 = 23 bits DLCI

   DLCI
     The Data Link Connection Identifier.  Refer to [FR] for the label
     values and formats.

3.4.3. Address List TLV

   The Address List TLV appears in Address and Address Withdraw mes-
   sages.
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|F|
   |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 encod-
     ing
     encoding of the individual addresses depends on the Address Family.

     The following address encodings are defined by this version of the
     protocol:

         Address Family      Address Encoding

         IPv4                4 octet full IPv4 address

3.4.4. COS Hop Count TLV

   The COS (Class of Service) Hop Count TLV may appear appears as an optional field in messages that request and carry label mappings. set
   up LSPs.  It calculates the number of LSR hops along an LSP as the
   LSP is used to
   request being setup.

   Note that setup procedures for LSPs that traverse ATM and advertise (Label, FEC, class Frame Relay
   links require use of service) bindings.  Its
   encoding is: the Hop Count TLV (see [ATM] and [FR]).

    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| COS (0x0102)
   |0|0| Hop Count (0x0103)        |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |     COS     HC Value  |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   COS
   +-+-+-+-+-+-+-+-+

   HC Value
     The value field for this TLV is
     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 subject for further study.

     One possibility is to define a set of CoS values that map to Dif-
     ferentiated Services [DIFFSERV] code points.  Other CoS values
     could be supported in addition to or in place of the Differentiated
     Services code points.

3.4.5. 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 [ATM] and [FR]).

    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| Hop Count (0x0103)        |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     HC Value  |
   +-+-+-+-+-+-+-+-+

   HC Value
     1 octet unsigned integer hop count value.

3.4.5.1. Hop Count Procedures

   During setup of an LSP an LSR R may receive a Label Mapping or Label
   Request message 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:

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

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

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

     o Otherwise, R must increment the received hop count.

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

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

   If an

   Use of the unknown hop count value greatly reduces the signalling
   overhead when independent control is used.  When a new LSP is
   established, each LSR receives starts with unknown hop count.  Addition of a message containing
   new LSR whose hop count is also unknown does not cause a Hop Count TLV, it must
   check the hop count value
   update to determine whether be propagated upstream since the hop count has
   exceeded its configured maximum allowable value.  If so, it must
   behave as if remains unknown.
   When the containing message has traversed a loop by 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".

3.4.6.

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 detec-
   tion
   detection mechanism.  See Section "Loop Detection".  Its use in the
   Label Request message records the path of LSRs the request has
   traversed.  Its use in the Label Mapping message records the path of
   LSRs a label advertisement has traversed to 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|F|
   |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 IP address (router-id) component of
     the LDP identifier for the corresponding LSR.  This ensures it is
     unique within the LSR network.

3.4.6.1.

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.

3.4.6.1.1.

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".

      2. Not propagate the Label Reqeust 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.

3.4.6.1.2.

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 Notification message to the sending LSR signaling
         "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.

3.4.7.

3.4.6. Status TLV

   Notification messages carry Status TLVs to specify events being sig-
   naled.
   signaled.

   The encoding for the Status TLV is:

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

   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 notifica-
       tion.
       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).

     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                       |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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, Manda-
     tory
     Mandatory Parameters, and Optional Parameters.

   Message Id 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 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"
       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 signi-
   ficant
   significant event.  A Notification message signals a fatal error or pro-
   vides
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|
   |0|   Notification (0x0001)     |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Status (TLV)                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   Status TLV
     Indicates the event being signaled.  The encoding for the Status
     TLV is specified in Section "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 addi-
         tional
         additional information that supplements the status information con-
         tained
         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 signalled 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
         signalled 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 mes-
   sage
   message containing a Status TLV that encodes the information and option-
   ally
   optionally additional TLVs that provide more information about the
   event.

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

   When an LSR receives a Notification message that carries a Status
   Code that indicates a fatal error, it should terminate the LDP ses-
   sion
   session immediately by closing the session TCP connection and discard
   all state associated with the session, including all label-FEC bind-
   ings
   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 mal-
   formed
   malformed if:

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

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

     - The PDU Length field is too short small (< 18) 14) or too long 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:

     - The Message Type is unknown.

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

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

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

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

     - The TLV type is unknown.

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

       If the TLV type is >= 08000 (high order bit 1) the TLV is
       silently dropped.  Section "Unknown TLV in Known Message Type"
       elaborates on this behavior.

     - The TLV Value is malformed.  This occurs when the receiver han-
       dles
       handles the TLV but cannot decode the TLV Value.  This is inter-
       preted
       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 Hold KeepAlive Timer Expiration

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

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 Initial-
   ization")
   Initialization") may fail if the session parameters received in the Initial-
   ization
   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 sig-
   nal
   signal them are described in the sections that describe these
   messages.

3.5.1.2.7. 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 Mechan-
   ism;
   Mechanism; see Section "LDP Discovery".

   The encoding for the Hello 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|
   |0|   Hello (0x0100)            |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Common Hello Parameters TLV               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Message Id 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |U|F|
     |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 Pro-
       cedures").
       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.  There are interface type
       specific defaults default, which is 15 seconds for Link
       Hellos as well as a default and 45 seconds for Tar-
       geted Targeted Hellos.  A value of 0xfffff 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.

     R, Request Send Targeted Hellos
       A value of 1 requests the receiver to send periodic Targeted Hel-
       los
       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 Tar-
       geted
       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 ver-
       sion
       version of the protocol are

           Optional Parameter    Type     Length  Value

           Transport Address     0x0401     4      See below
           Configuration         0x0402     4      See below
              Sequence Number

       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 car-
         rying
         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.

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 "Maintain-
   ing
   "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 implementa-
         tion
         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 Estab-
         lishment".
         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 IP source address a.b.c.d is acceptable if
       either:

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

           - The LSR has been configured to send Targeted Hellos to
             a.b.c.d.

       The following describes how an LSR processes Hello optional TLVs:

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

       Configuration Sequence Number
         The Configuration Sequence Number optional parameter is used by
         the sending LSR to signal configuration changes to the receiv-
         ing
         receiving LSR.  When a receiving LSR playing the active role in
         LDP session establishment detects a change in the sending LSR con-
         figuration,
         configuration, it may clear the session setup backoff delay, if
         any, associated with the sending LSR (see Section "Session Ini-
         tialization").
         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 ses-
   sion
   session establishment procedure; see Section "LDP Session Establish-
   ment".
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|
   |0|   Initialization (0x0200)   |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Common Session Parameters TLV             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   Common Session Parameters TLV
     Specifies values proposed by the sending LSR for parameters common
     to all that
     must be negotiated for every LDP sessions. session.

     The encoding for the Common Session Parameters TLV is:

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

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

       Hold

       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 Interval. Time.  The receiving LSR MUST calculate the value of
         the KeepAlive Timer by using the smaller of its proposed
         Hold
         KeepAlive Time and the Hold KeepAlive Time received in the PDU.  The
         value chosen for Hold KeepAlive Time indicates the maximum number of
         seconds that may elapse between the receipt of successive PDUs
         from the LDP peer.  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 Down-
         stream
         Downstream On Demand.

         If one LSR proposes Downstream Unsolicted and the other pro-
         poses
         proposes Downstream on Demand, the rules for resolving this
         difference is:

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

           - Otherwise, Downstream Unsolicted must be used.

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

       D, Loop Detection
         Indicates whether loop detection based on path vectors is
         enabled.  A value of 0 means 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 pro-
         cedures
         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
         knowledege 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 Identifer
         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 Pro-
         cedures".
         Procedures".

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

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

         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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |U|F|
       |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 follow-
         ing
         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:

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

           - The interoperability of VP-merge-capable switches with
             non-VP-merge-capable switches is a subject for future
             study.

         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. 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 direc-
         tion
         direction on the link only.  When either or both of the peers speci-
         fies
         specifies unidirectional VC capability, both LSRs use
         unidirectional VC label assignement 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 odd-
         numbered VCIs in the VPI/VCI range as labels.  The system with
         the smaller LDP Iden-
         tifier 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 neigh-
         bors
         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:

          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 transmis-
           sion
           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 ori-
           ginating
           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 ori-
           ginating
           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.

       See [ATM-VP] for specification of the fields for ATM Label Range
       Components to be used with VP merge LSRs.

     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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |U|F|
       |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 specifi-
         cation:
         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 assignement
         for the link as follows.  The LSRs compare their LDP
         Identifiers as unsigned integers.  The LSR with the larger LDP
         Identifier may assign only odd-numbered DLCIs in the range as
         labels.  The system with the smaller LDP Identifier may assign
         only even-numbered DLCIs in the range as labels.

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

       One or more Frame Relay Label Range Components
         A list of Frame Relay Label Range Components 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 inter-
         section
         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 Notifi-
         cation 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 transmis-
           sion
           transmission and ignored on receipt.

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

                Len    DLCI bits

                0       10
                1       17
                2       23
         Minimum DLCI
           This 23-bit vield 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 vield 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 Ini-
   tialization
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|
   |0|   KeepAlive (0x0201)        |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Message Id 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 Hold KeepAlive Timer mechanism described in Section "Maintaining LDP Ses-
   sions"
   Sessions" resets a session hold KeepAlive timer every time an LDP PDU is
   received. The KeepAlive Message is provided to allow reset of the Hold
   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 Hold 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|
   |0|   Address (0x0300)          |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Address List TLV                          |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Message Id 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 Identif-
   iers
   Identifiers and next hop addresses; see Section "LDP Identifiers and
   Next Hop Addresses".

   When a new LDP session is initialized and before sending Label Map-
   ping
   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 adver-
   tise
   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".

3.5.6. Address Withdraw Message

   An LSR sends the Address Withdraw Message to an LDP peer to withdraw previ-
   ously
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U
   |0|   Address Withdraw (0x0301) |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Address List TLV                          |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Message Id 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 mes-
     sage.
     message.

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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|
   |0|   Label Mapping (0x0400)    |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC TLV                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Label TLV                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   FEC TLV
     Specifies the FEC component of the FEC-Label mapping being adver-
     tised.
     advertised.  See Section "FEC TLV" for encoding.

   Label TLV
     Specifies the Label component of the FEC-Label mapping.  See Sec-
     tion
     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
         COS ID TLV               1            See below
         Hop Count TLV         1            See below
         Path Vector TLV       variable     See below

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

       Label Request Message Id ID
         If this Label Mapping message is a response to a Label Request
         message that carried the Return Message Id optional parameter
         (see Section "Label Request Message") the Label Mapping 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.

       COS
         Specifies the Class of Service (COS) to be associated with the
         FEC-Label mapping.  If not present, the LSR should use its
         default COS for IP packets as the COS.

       Hop Count
         Specifies the running total of the number of LSR hops along the
         LSP being setup by the Label Message.  Section "Hop Count Pro-
         cedures"
         Procedures" describes how to handle this TLV.

       Path Vector
         Specifies the LSRs along the LSP being setup by the Label Mes-
         sage.
         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 map- pings 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 Appendx 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 advertise-
         ment.
         advertisement.

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

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

      4. The attributes of a mapping change.

      5. The receipt of a mapping from the downstream next hop  AND
            a) no upstream mapping has been created  OR
            b) loop detection is configured  OR
            c) the attributes of the mapping have changed.

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

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

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.

3.5.8. Label Request Message

   An LSR sends the Label Request Message to an LDP peer

   The combination of Downstream Unsolicited mode and conservative label
   retention can lead to request a
   binding (mapping) situation where an LSR releases the label for
   a FEC.

   The encoding for the Label Request Message is:

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

   Message Id
     32-bit value used that it later needs.  For example, if LSR Rd advertises to identify this message.

   FEC TLV
     The LSR
   Ru the label for a FEC for which a label it is being requested.  See Section "FEC
     TLV" not Ru's next hop, Ru will
   release the label.  If Ru's next hop for encoding.

   Optional Parameters
     This variable length field contains 0 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 more parameters, each
     encoded as 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 TLV. 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.

   The optional parameters are:

         Optional Parameter     Length       Value

         Return encoding for the Label Request Message Id TLV is:

    0            See below
         COS TLV                   1            See below
         Hop Count TLV                   2                   3
    0 1            See below
         Path Vector TLV        variable     See below 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.

   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 hext 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 encodings for message ID of the COS, Hop Count, and Path Vector TLVs can be
     found in Section "TLV Encodings Label Request message serves as an identifier
   for Commonly Used Parameters".

       Return Message Id
         Requests the LDP peer include Label Request transaction.  When the receiving LSR responds
   with a Label Mapping message, the mapping message must include a
   Label Request/Returned Message Id ID TLV optional parameter which
   includes the message ID of this the Label Request message in its message.  Note that
   since LSRs use Label Mapping Request message response.  If IDs as transaction identifiers
   an
         LDP peer receives LSR should not reuse the message ID of a Label Request message with
   until the Return Mes-
         sage Id optional parameter, its corresponding transaction completes.

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

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

     No Label Mapping 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 contain 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 Requst message.

   See Appendx 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 Id optional parameter with
         the 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 (0x0402)  |      Message Id of the Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC TLV                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Label Request message.  See Section
         "Label Mapping Message".

       COS
         Specifies the Class of Service (COS) Message ID TLV              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Message ID
     32-bit value used to be associated with the
         requested FEC-Label mapping.  If not present, identify this message.

   FEC TLV
     Identifies the LSR should
         use its default COS FEC for IP packets as the COS.

       Hop Count
         Specifies the running total of the number of LSR hops along which the
         LSP FEC-label mapping is being setup by the
     withdrawn.

   Label Request Message.  Section "Hop
         Count Procedures" describes how to handle this TLV.

       Path Vector Message ID TLV
     Specifies the LSRs along the LSR being setup by message ID of the Label Request Message.  Section "Path Vector Procedures" describes
         how message to handle this TLV.

3.5.8.1. be
     aborted.

3.5.9.1. Label Abort Request Message Procedures

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

   An LSR Ru may transmit send a Label Abort Request message under any of the following con-
   ditions:

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

      2. The next hop to the FEC changes, and sent to LSR Rd in the LSR doesn't already
         have a mapping from that
   following circumstances:

      1. Ru's next hop for the given FEC.

      3. The FEC has changed from LSR receives Rd to LSR X; or

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

      3. Ru is a merge, non-ingress LSR and has received a Label Abort
         Request for FEC from an LDP peer, upstream peer Y and Y is the only
         (last) upstream LSR doesn't
         already have requesting a mapping from the next hop.

   The receiving label for FEC.

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

   When an LSR receives a Label Mapping for Abort Request message, if it has not
   previously responded to the requested label or Label Request being aborted with a Notification Label
   Mapping message
   indicating why or some other Notification message, it cannot satisfy the request.

   This version of the protocol defines the following Status Codes for must
   acknowledge the abort by responding with a Label Request Aborted
   Notification message message.  The Notification must include a Label Request
   Message ID TLV that signals carries the message ID of the aborted Label
   Request message.

   If an LSR receives a request cannot be satisfied:

     No Route
       The FEC for which Label Abort Request Message after it has
   responded to the Label Request in question with a label was requested includes Label Mapping
   message or a FEC Element
       for which Notification message, it ignores the abort request.

   If an LSR does not have receives a route.

     No Label Resources
       The LSR cannot provide 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 because of resource limitations.
       When resources become available in the Label Mapping message is
   valid.  The LSR must notify may choose to use the request-
       ing LSR by sending a Notification message label or to release it with the a
   Label
       Resources Available Status Code. Release mesage.

   An LSR that receives a No Label Resources response to aborting a Label Request message must may not issue further reuse the Message ID
   for the Label Request messages message until it receives a one of the following
   from its peer:

     - A Label Request Aborted Notfication message acknowledging the
       abort;

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

     - A Notification message with in response to the Label Resources
       Available Status code. Request message
       being aborted (e.g., Loop Detected 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).

   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 has detected receiving a looping Label Requst message.

   See Appendx A "LDP Abort
   Request message must process it immediately, regardless of the
   downstream state of the LSP, responding with a Label Distribution Procedures" for more details.

3.5.9. 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 map-
   pings
   mappings the LSR had previously advertised.  This breaks the mapping
   between the FECs and the labels.

   The encoding for the Label Withdraw Message is:

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

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

   FEC TLV
     Identifies the FEC for which the FEC-label mapping is being with-
     drawn.
     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).

3.5.9.1.

3.5.10.1. Label Withdraw Message Procedures

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

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

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

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

   The FEC TLV may contain the Wildcard FEC Element; if so, it may con-
   tain
   contain no other FEC Elements.  In this case, if the Label Withdraw mes-
   sage
   message contains an optional Label TLV, then the label is to be with-
   drawn
   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 Appendx A "LDP Label Distribution Procedures" for more details.

3.5.10.

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 previ-
   ously
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|
   |0|   Label Release (0x0403)   |      Message Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC TLV                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Label TLV (optional)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

         Optional Parameter    Length       Value

         Label TLV             variable     See below

     The encodings for Label TLVs are found in Section "Label TLVs".

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

3.5.10.1.

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 follow-
   ing
   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 conserva-
         tive
         conservative operation.

      3. The LSR has received receives a Label Withdraw message for a previously
         received label. message.

   Note that if an LSR is configured for "liberal mode", a release mes-
   sage
   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 con-
   tain
   contain no other FEC Elements.  In this case, if the Label Release mes-
   sage
   message contains an optional Label TLV, then the label is to be
   released for all FECs to which it is bound.  If there is not an
   optional Label TLV in the Label Release message, then the sending LSR
   is releasing all label mappings previously learned from the receiving
   LSR.

   See Appendx 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 0x2F00 0x3E00 through 0x2FFF 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 (0x2F00-0x2FFF) (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.

     The determination as to whether a vendor-private message is under-
     stood
     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 TLF is is to be for-
     warded.
     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 for-
     warded
     forwarded with the containing message.

   Type
     Type value in the range 0x2F00 0x3E00 through 0x2FFF. 0x3EFF.  Together, the Type
     and Vendor Id field specify how the Data field is to be inter-
     preted.
     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.

3.6.1.2. LDP Vendor-private Messages

   The Message Type range 0x2F00 0x3E00 through 0x2FFF 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 (0x2F00-0x2FFF) (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 under-
     stood
     understood is based on the Msg Type and the Vendor ID parameter.

   Msg Type
     Message type value in the range 0x2F00 0x3E00 through 0x2FFF. 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.

   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:

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

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

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

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

       Administration of Experiment IDs is the responsiblity 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"
       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          0x2F00-          0x3E00-  "LDP Vendor-private Extensions"
                               0x2FFF
                               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"
       COS                      0x0102    "COS 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
       Transport Address        0x0401    "Hello Message"
       Configuration            0x0402    "Hello Message"
          Sequence Number
       Common Session           0x0500    "Initialization Message"
          Parameters
       ATM Session Parameters   0x0501    "Initialization Message"
       Frame Relay Session      0x0502    "Initialization Message"
          Parameters
       Label Request            0x0600    "Label Request Message"
           Message Id
       Return Message Id        0x0601    "Label Mapping Message"
           Message ID
       Vendor-Private           0x2F00-           0x3E00-   "LDP Vendor-private Extensions"
                                0x2FFF
                                0x3EFF
       Experimental             0x3F00-   "LDP Experimental Extensions"
                                0x3FFF

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               Type 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 Identifer         0x80000001     1   0x00000001    "Events Signaled by ..."
       Bad Protocol Version      0x80000002  1   0x00000002    "Events Signaled by ..."
       Bad PDU Length            0x80000003        1   0x00000003    "Events Signaled by ..."
       Unknown Message Type      0x80000004  0   0x00000004    "Events Signaled by ..."
       Bad Message Length        0x80000005    1   0x00000005    "Events Signaled by ..."
       Unknown TLV               0x80000006           0   0x00000006    "Events Signaled by ..."
       Bad TLV length            0x80000007        1   0x00000007    "Events Signaled by ..."
       Malformed TLV Value       0x80000008   1   0x00000008    "Events Signaled by ..."
       Hold Timer Expired        0x80000009    1   0x00000009    "Events Signaled by ..."
       KeepAlive Timer       1   0x00000009    "Events Signaled by ..."
           Expired
       Shutdown                  0x8000000A              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 Available /     0   0x0000000F    "Label Request Mess ..."
           Available
       Session Rejected/         0x80000010     1   0x00000010    "Session Initialization"
          No Hello
       Session Rejected/         0x80000011     1   0x00000011    "Session Initialization"
          Parameters Advertisement Mode
       Session Rejected/         0x80000012     1   0x00000012    "Session Initialization"
          Parameters Max PDU Length
       Session Rejected/         0x80000013     1   0x00000013    "Session Initialization"
          Parameters Label Range

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 [ARCH]) is represented as a Generic
   Label TLV with a Label field of 0.

4. Security Considerations

   This section specifies an optional mechanism to protect against the
   introduction of spoofed TCP segments into LDP session connection
   streams.

   It 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.

4.1. The 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 currrent existing practice for securing
         BGP against certain simple attacks.  It is understood to have
         security weaknesses against concerted attacks."

      "Abstract

         This memo describes a TCP extension to enhance security for
         BGP.  It defines a new TCP option for carrying an MD5 [RFC1321]
         digest in a TCP segment.  This digest acts like a signature for
         that segment, incorporating information known only to the con-
         nection
         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 pro-
         tect
         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 appli-
         cation.
         application.  It could even change during the lifetime of a particu-
         lar
         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 insuffi-
         ciently
         insufficiently strong for this type of application.

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

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

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

   End of quotes from [rfc2385].

4.2. LDP Use of the TCP MD5 Signature Option

   LDP uses the TCP MD5 Signature Option as follows:

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

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

     - 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.

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

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

5. 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 docu-
   ment.
   document.  For more information consult the online list of claimed
   rights.

6. 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.

7. References

   [ARCH] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol Label
   Switching Architecture", Work in Progress, July 1998.

   [ATM] B. Davie, J. Lawrence, K. McCloghrie, Y. Rekhter, E. Rosen, G.
   Swallow, P. Doolan, "Use of Label Switching With ATM", Work in Pro-
   gress,
   Progress, September, 1998.

   [ATM-VP] N. Feldman, B. Jamoussi, S. Komandur, A, Viswanathan, T
   Worster, "MPLS using ATM VP Switching", Work in Progress, February,
   1999.

   [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,
   January, 1999.

   [DIFFSERV] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang, W.
   Weiss, "An Architecture for Differentiated Services", Work in Pro-
   gress,
   Progress, October, 1998.

   [ENCAP] E. Rosen, Y. Rekhter, D. Tappan, D. Farinacci, G. Fedorkow,
   T. Li, A. Conta, "MPLS Label Stack Encoding", Work in Progress, July,
   1998.

   [FR] A. Conta, P. Doolan, A. Malis, "Use of Label Switching on Frame
   Relay Networks", Work in Progress, October, 1998.

   [FRAMEWORK] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swal-
   low,
   Swallow, A. Viswanathan, "A Framework for Multiprotocol Label
   Switching", Work in Progress, November 1997.

   [LSPTUN] D. Awduche, L. Berger, D. Gan, T. Li, G. Swallow, Vijay
   Srinivasan, "Extensions to RSVP for LSP Tunnels", Work in Progress,
   November 1998.

   [rfc1321] Rivest, R., "The MD5 Message-Digest Algorithm," RFC 1321,
   April 1992.

   [rfc1483] J. Heinanen, "Multiprotocol Encapsulation over ATM Adapta-
   tion
   Adaptation Layer 5", RFC 1483, Telecom Finland, July 1993.

   [rfc1583] J. Moy, "OSPF Version 2", RFC 1583, Proteon Inc, March
   1994.

   [rfc1700] J. Reynolds, J.Postel, "ASSIGNED NUMBERS", October 1994.

   [rfc1771] Y. Rekhter, T. Li, "A Border Gateway Protocol 4 (BGP-4)",
   RFC 1771, IBM Corp, Cisco Systems, March 1995.

   [rfc2205] R. Braden, L. Zhang, S. Berson, S. Herzog, S. Jamin,
   "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specif-
   ication",
   Specification", RFC 2205, September 1997.

   [rfc2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
   Signature Option", RFC 2385, August 1998.

   [TE] D. Awduche, J. Malcolm, J Agogbua, M. O'Dell, J. McManus, "
   Requirements for Traffic Engineering over MPLS", Work in Progress,
   October 1998.

8. Author Information

   Loa Andersson                            Andre Fredette
   Nortel Networks Inc                      Nortel Networks Inc
   Kungsgatan 34, PO Box 1788               3 Federal Street
   111 97 Stockholm                         Billerica, MA  01821
   Sweden                                   Phone:  978-916-8524
   Phone: +46 8 441 78 34                   email: fredette@baynetworks.com
   Mobile: +46 70 522 78 34
   email: loa_andersson@baynetworks.com
   Paul Doolan                              Bob Thomas
   Ennovate Networks                        Cisco Systems, Inc.
   330 Codman Hill Rd                       250 Apollo Dr.
   Marlborough MA 01719                     Chelmsford, MA 01824
   Phone: 978-263-2002                      Phone:  978-244-8078
   email: pdoolan@ennovatenetworks.com      email: rhthomas@cisco.com

   Nancy Feldman
   IBM Corp.
   17 Skyline Drive
   Hawthorne NY 10532
   Phone:  914-784-3254
   email: nkf@us.ibm.com

Appendix A. LDP Label Distribution Procedures

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

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

   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 [ARCH] for hop-by-hop routed traffic.
   These procedures are:

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

         . Downstream Unsolicited Independent Control, called PushUncon-
           ditional
           PushUnconditional in [ARCH].

         . Downstream Unsolicited Ordered Control, called PushCondi-
           tional
           PushConditional in [ARCH].

         . Downstream On Demand Independent Control, called PulledUncon-
           ditional
           PulledUnconditional in [ARCH].

         . Downstream On Demand Ordered Control, called PulledCondi-
           tional
           PulledConditional in [ARCH].

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

     - Label Request procedure, which is performed by an upstream LSR to
       determine when to explicitly request that a downstrem 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 merg-
           ing
           merging LSRs. The LSR requests a label when it receives a
           request for one, in addition to whenever it needs one.

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

         . Conservative label retention, called Release On Change in
           [ARCH].

         . Liberal label retention, called No Release On Change in
           [ARCH].

     - 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 deter-
           mined
           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.

     - Label No Route procedure (called Label Not Available procedure in
       [ARCH]), 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
   specifid by the algorithms.  That is, an implementation need not
   follow exactly the steps specified by the algorithms as long as the
   effect is identical.

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

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

A.1.1. Receive Label Request

 Summary:

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

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

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

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

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

 Context:

     - LSR. The LSR handling the event.

     - MsgSource. The LDP peer that sent the message.

     - FEC. The FEC specified in the message.

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

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

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

 Algorithm:

   LRq.1   Execute procedure Check_Received_Attributes (MsgSource, RAt-
           tributes).
           RAttributes).
           If Loop Detected, goto LRq.11. LRq.13.

   LRq.2   Is there a Next Hop for FEC?
           If so, goto LRq.4. LRq.5.

   LRq.3   Is MsgSource the Next Hop?
           Ifnot, goto LRq.5.

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

   LRq.5   Execute procedure Send_Notification (MsgSource, No Route).
           Goto LRq.11.

   LRq.4 LRq.13.

   LRq.6   Has LSR previously received a label request for FEC from
           MsgSource?
           If not, goto LRq.6. LRq.8.  (See Note 1.)
   LRq.5

   LRq.7   Is the label request a duplicate request?
           If so, Goto LRq.11. LRq.13.  (See Note 2.)

   LRq.6

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

   LRq.7

   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 map-
                   ping
                   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, RAt-
                   tributes,
                   RAttributes, SAttributes, Propagating,
                   StoredHopCount).

               3.  Execute procedure Send_Label (MsgSource, FEC, SAttri-
                   butes).
                   SAttributes).

               4.  Is LSR egress for FEC? OR
                   Has LSR previously received and retained a label map-
                   ping
                   mapping for FEC from Next Hop?
                   If so, goto LRq.9. LRq.11.  If not, goto LRq.8. 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 map-
                   ping
                   mapping for FEC from Next Hop?
                   If not, goto LRq.8. LRq.10.

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

               3.  Execute procedure Send_Label (MsgSource, FEC, SAttri-
                   butes).
                   SAttributes).
                   Goto LRq.9.

   LRq.8 LRq.11.

   LRq.10   Perform LSR Label Request procedure:

             For Request Never

               1.  Goto LRq.11. 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.11.

   LRq.9 LRq.13.

   LRq.11   Has LSR successfully sent a label for FEC to MsgSource?
           If not, goto LRq.11. LRq.13.  (See Note 3.)

   LRq.10

   LRq.12  Perform LSR Label Use procedure.

             For Use Immediate OR
             For Use If Loop Not Detected

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

   LRq.11

   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 dis-
         tinguish such requests from a non-label merging MsgSource from
         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 label 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. 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:

     - Transmission of a label release message for the FEC label to the
       LDP peer;

     - Transmission of label mapping messages for the FEC to one or more
       LDP peers,

     - Installation of the newly learned label for forwarding/switching
       use by the LSR.

 Context:

     - LSR. The LSR handling the event.

     - MsgSource. The LDP peer that sent the message.

     - FEC. The FEC specified in the message.

     - Label. The label specified in the message.

     - PrevAdvLabel. The label for FEC, if any, previously advertised to
       an upstream peer.

     - StoredHopCount. The hop count previously recorded for the FEC.

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

     - SAttributes to be included in Label Mapping message, if any, pro-
       pagated
       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.9.

   LMp.2   Delete record of outstanding FEC label request.

   LMp.3   Execute procedure Check_Received_Attributes (MsgSource, RAt-
           tributes).
           RAttributes).
           If No Loop Detected, goto LMp.9.

   LMp.4   Does the LSR have a previously received label mapping for FEC
           from MsgSource?
           If not, goto LMp.8. (See Note 1.).

   LMp.5   Does the label previously received from MsgSource match Label
           (i.e., the label received in the message)?
           If not, goto LMp.8. (See Note 2.)

   LMp.6   Delete matching label mapping for FEC previously received
           from MsgSource.

   LMp.7   Remove Label from forwarding/switching use. (See Note 3.).
           Goto LMp.26.

   LMp.8   Execute procedure Send_Message (MsgSource, Label Release,
           FEC, Label).  Goto LMp.26.

   LMp.9   Determine the Next Hop for FEC.

   LMp.10  Is MsgSource the Next Hop for FEC?
           If so, goto LMp.12.

   LMp.11  Perform LSR Label Release procedure:

             For Conservative Label retention:

               1.  Execute procedure Send_Message (MsgSource, Label
                   Release, FEC, Label).
                   Goto LMp.26.

             For Liberal Label retention:

               1.  Record label mapping for FEC with Label and RAttri-
                   butes
                   RAttributes has been received from MsgSource.
                   Goto LMp.26.

   LMp.12  Does LSR have a previously received label mapping for FEC
           from MsgSource?
           If not, goto LMp.14

   LMp.13  Does the label previously received from MsgSource match Label
           (i.e., the label received in the message)?
           If not, goto LMp.8.  (See Note 2.)

   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 for LMp.25 for each Peer, other than
           MsgSource.

   LMp.18  Has LSR previously sent a label mapping for FEC to Peer?
           If not, goto LMp.23.

   LMp.19  Are RAttributes in the received label mapping consistent with
           those previously sent to Peer?
           If so, goto LMp.24.  (See Note 4.)

   LMp.20  Execute procedure Prepare_Label_Mapping_Attributes(Peer, FEC,
           RAttributes, SAttributes, IsPropagating, StoredHopCount).

   LMp.21  Execute procedure Send_Message (Peer, Label Mapping, FEC,
           PrevAdvLabel, SAttributes).  (See Note 5.)

   LMp.22  Update record of label mapping for FEC previously sent to
           Peer to include the new attributes sent.

           Goto LMp.24.

   LMp.23  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, RAttri-
                   butes,
                   RAttributes, SAttributes, IsPropagating,
                   UnknownHopCount).

               2.  Execute procedure Send_Label (Peer, FEC, SAttri-
                   butes).
                   SAttributes).
                   If the procedure fails, continue iteration for next
                   Peer at LMp.17.

               3.  Goto LMp.24.

             For Downstream On Demand Independent Control OR
             For Downstream On Demand Ordered Control

               1.  Does LSR have a label request for FEC from Peer
                   marked as pending?
                   If not, continue iteration for next Peer at LMp.17.

               2.  Execute procedure
                   Prepare_Label_Mapping_Attributes(Peer, FEC, RAttri-
                   butes,
                   RAttributes, SAttributes, IsPropagating,
                   UnknownHopCount)

               3.  Execute procedure Send_Label (Peer, FEC, SAttri-
                   butes).
                   SAttributes).
                   If the procedure fails, continue iteration for next
                   Peer at LMp.17.

               4.  Goto LMp.24.

   LMp.24  Perform LSR Label Use procedure:

             For Use Immediate OR
             For Use If Loop Not Detected

               1.  Install label received and label sent to Peer for
                   forwarding/switching use.
                   Goto LMp.25.

   LMp.25  End iteration from LMp.17.

   LMp.26  DONE.

 Notes:

      1. 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.

      2. A 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.

      3. If Label is not in forwarding/switching use, LMp.7 has no
         effect.

      4. The loop detection Path Vector attribute is considered in this
         check.  If the received RAttributes include 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.

      5. LMp.19 through LMp.21 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

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 Path Vector and
         no Path Vector had been previously sent 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 Peer, or if downstream peer to abort the
         received Path Vector is inconsistent with LSP.

 Context:

     - LSR. The LSR handling the Path Vector pre-
         viously event.

     - MsgSource. The LDP peer that sent to the Peer, then message.

     - FEC. The FEC specified in the attributes are considered message.

     - RequestMessageID. The message ID of the label request message to
       be inconsistent. aborted.

     - Next Hop. The next hop for the FEC.

 Algorithm:

   LAbR.1  Does the message match a previously received label request
           message from MsgSource? (See Note that an 1.)
           If not, goto LAbR.12.

   LAbR.2  Has LSR is not required responded to store a the previously received Path Vector after it propagates label request?
           If so, goto LAbR.12.

   LAbR.3  Execute procedure Send_Message(MsgSource, Notification, Label
           Request Aborted, TLV), where TLV is the Path Vector 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 message. for FEC?
           If an 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 does not store the Path Vector, it
         has no way to check merging the consistency of LSP for FEC?
           If not, goto LAbR.9.

   LAbR.8  Are there upstream peers other than MsgSource that have
           requested a newly label for FEC?
           If so, goto LAbR.11.

   LAbR.9  Execute procedure Send_Message (Next Hop, Label Abort
           Request, FEC, TLV), where TLV is the Label Request Message ID
           TLV received Path
         Vector.  This means that whenever such an LSR receives a map-
         ping in the label abort request message carrying from
           MsgSource.

   LAbR.10  Record that a Path Vector it must always propagate label abort request for FEC is pending.

   LAbR.11  Delete record of label request for FEC from MsgSource.

   LAbR.12  DONE

 Notes:

      1. LSR uses FEC and the Path Vector.

      5. LMp.19 through LMp.21 deal with a situation that can arise when 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 is using independent control and it receives has received a label mapping from the downstream peer after NextHop, it has sent should
         behave as if it had advertised a label mapping to an
         upstream peer. In this situation the LSR needs to propagate any
         changed attributes, such as Hop Count, upstream. If Loop Detec-
         tion is configured on, the propagated attributes must include
         the Path Vector

A.1.3. 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:

     - LSR. The LSR handling the event.

     - MsgSource. The LDP peer that sent the message.

     - Label. The label specified in the message.

     - FEC. The FEC specified in the message.

 Algorithm:

   LRl.1   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 pre-
           viously
           previously sent to MsgSource?
           If not, goto LRl.4

   LRl.3   Delete record of outstanding label withdraw for FEC previ-
           ously
           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.

   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 so, 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 Unsolicted 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 impor-
         tance
         importance of keeping LSP setup latency low by keeping the
         amount of signalling required small; whether LSP setup is ingress-
         controlled
         ingress-controlled or egress-controlled in the operating
         environment.

A.1.4.

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 indepen-
     dent
     independent control, it then acts as if it had just recognized the
     FEC.

 Context:

     - LSR. The LSR handling the event.

     - MsgSource. The LDP peer that sent the message.

     - Label. The label specified in the message.

     - FEC. The FEC specified in the message.

 Algorithm:

   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.

   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 interation 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 pre-
           viously
           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.1.5.

A.1.6. Recognize New FEC
 Summary:

     The response by an LSR to learning a new FEC via the routing table
     may involve one or more of the following actions:

     - Transmission of label mappings for the FEC to one or more LDP
       peers;

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

     - Any of the actions that can occur when the LSR receives a label
       mapping for the FEC from the FEC next hop.

 Context:

     - LSR. The LSR handling the event.

     - FEC. The newly recognized FEC.

     - Next Hop. The next hop for the FEC.

     - InitAttributes. Attributes to be associated with the new FEC.
       (See Note 1.)

     - SAttributes. Attributes to be included in Label Mapping or Label
       Request messages, if any, sent to peers.

     - StoredHopCount. Hop count associated with FEC label mapping , 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 map-
                   ping
                   mapping for FEC from Next Hop?
                   If so, set Propagating to IsPropagating.
                   If not, set Propagating to NotPropagating.

               3.  Execute procedure Prepare_Label_Mapping_Attributes
                   (Peer, FEC, InitAttributes, SAttributes, Propagating,
                   Unknown hop count(0)).

               4.  Execute procedure Send_Label (Peer, FEC, SAttributes)
               5.  End iteration from 1.
                   Goto FEC.2.

             For Downstream Unsolicited Ordered Control

               1.  Iterate through 5 for each Peer.

               2.  Is LSR egress for the FEC? OR
                   Has LSR previously received and retained a label map-
                   ping
                   mapping for FEC from Next Hop?
                   If not, continue iteration for next Peer.

               3.  xecute 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.

 Notes:

      1. An example of an attribute that might be part of InitAttributes
         is CoS. 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 InitAttri-
         butes 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 discard-
         ing
         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.1.6.

A.1.7. Detect change Change in FEC next hop Next Hop

 Summary:

     The response by an LSR to a change in the next hop for a FEC may
     involve one or more of the following actions:

     - Removal of the label from the FEC's old next hop from
       forwarding/switching use;

     - Transmission of label mappping messages for the FEC to one or
       more LDP peers;

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

     - Any of the actions that can occur when the LSR receives a label
       mapping from the FEC's new next hop.

 Context:

     - LSR. The LSR handling the event.

     - FEC. The FEC whose next hop changed.

     - New Next Hop. The current next hop for the FEC.

     - Old Next Hop. The previous next hop for the FEC.

     - OldLabel. Label, if any, previously received from Old Next Hop.

     - CurAttributes. The attributes, if any, currently associated with
       the FEC.

     - SAttributes. Attributes to be included in 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 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 Liberal Conservative label retention?
           If so, not, goto NH.6.

    NH.4 NH.10.

    NH.8   Execute procedure Send_Message (Old Next Hop, Label Release,
           OldLlabel).

    NH.5   Delete Abort
           Request, FEC, TLV), where TLV is a Label Request Message ID
           TLV that carries the message ID of the pending label mapping request.

    NH.9   Record a label abort request is pending for FEC previously received from with Old Next
           Hop.

    NH.6

    NH.10  Is there a New Next Hop for the FEC?
           If not, goto NH.12.

    NH.7 NH.16.

    NH.11  Has LSR previously received and retained a label mapping for
           FEC from New Next Hop?
           If not, goto NH.9.

    NH.8 NH.13.

    NH.12  Generate Event: Received Label Mapping from New Next Hop.
           Goto NH.12. NH.20. (See Note 2.)

    NH.9

    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.10. NH.14.
           If not, goto NH.12.

    NH.10 NH.20.

    NH.14  Execute procedure Prepare_Label_Request_Attributes (Next Hop,
           FEC, CurAttributes, SAttributes)

    NH.11

    NH.15  Execute procedure Send_Label_Request (New Next Hop, FEC, SAt-
           tributes).
           SAttributes).  (See Note 4.)
    NH.11
           Goto NH.20.

    NH.16  Iterate through NH.19 for each Peer.

    NH.17  Has LSR previously sent a label maping 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.10-LMp.11 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.

A.1.7.

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:

     - LSR. The LSR handling the event.

     - FEC. The FEC for which a label was requested.

     - RequestMessageID. The message ID of the label request message to
       be aborted.

     - MsgSource. The LDP peer that sent the Notification message.

 Algorithm:

   LRqA.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:

     - LSR. The LSR handling the event.

     - FEC. The FEC for which a label was requested.

     - MsgSource. The LDP peer that sent the Notification message.

 Algorithm:

   NoRes.1 Delete record of outstanding label request for FEC sent to
           MsgSource.

   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.1.8.

A.1.10. Receive Notification / No Route

 Summary:

     When an LSR receives a No Route notification from an LDP peer in
     response to a Label Request message, the Label No Route procedure
     in use dictates its response. The LSR either will take no further
     action, or it will defer the label request by starting a timer and
     send another Label Request message to the peer when the timer later
     expires.

 Context:

     - LSR. The LSR handling the event.

     - FEC. The FEC for which a label was requested.

     - Attributes. The attibutes associated with the label request.

     - MsgSource. The LDP peer that sent the Notification message.

 Algorithm:

   NoNH.1  Delete record of outstanding label request for FEC sent to
           MsgSource.

   NoNH.2  Perform LSR Label No Route procedure.

             For Request No Retry

               1.  Goto NoNH.3.

             For Request Retry

               1.  Record deferred label request for FEC and Attributes
                   to be sent to MsgSource.

               2.  Start timeout. Goto NoNH.3.

   NoNH.3  DONE.

A.1.9.

A.1.11. Receive Notification / Loop Detected

 Summary:

     When an LSR receives a Loop Detected notification from an LDP peer
     in response to a Label Request message, it behaves as if it had
     received a No Route notification.

     Context:

         See "Receive Notification / No Route".

     Algorithm:

         See "Receive Notification / No Route"

A.1.10.

A.1.12. Receive Notification / Label Resources Available

 Summary:

     When an LSR receives a Label Resources Available notification from
     an LDP peer, it resumes sending label requests to the peer.

 Context:

     - LSR. The LSR handling the event.

     - MsgSource. The LDP peer that sent the Notification message.

     - SAttributes. Attributes stored with postponed Label Request mes-
       sage.
       message.

 Algorithm:

   Res.1   Set status record indicating it is OK to send label requests
           to MsgSource.

   Res.2   Iterate through Res.6 for each record of a FEC label mapping
           needed from MsgSource for which no label resources are avail-
           able.
           available.

   Res.3   Is MsgSource the next hop for FEC?
           If not, goto Res.5.

   Res.4   Execute procedure Send_Label_Request (MsgSource, FEC, SAttri-
           butes).
           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.1.11.

A.1.13. Detect local label resources have become available

 Summary:

     After an LSR has sent a No Label Resources notification to an LDP
     peer, when label resources later become available it sends a Label
     Resources Available notification to each such peer.

 Context:

     - LSR. The LSR handling the event.

     - Attributes. Attributes stored with postponed Label Mapping mes-
       sage.
       message.

 Algorithm:

   ResA.1  Iterate through ResA.4 for each Peer to which LSR has previ-
           ously
           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 previ-
           ously
           previously sent to Peer.

   ResA.4  End iteration from ResA.1

   ResA.5  Iterate through ResA.8 for each record of a label mapping
           needed for FEC for Peer but no-label-resources.  (See Note
           1.)

   ResA.6  Execute procedure Send_Label (Peer, FEC, Attributes). If the
           procedure fails, terminate iteration.

   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.1.12.

A.1.14. LSR decides to no longer label switch a FEC
 Summary:

     An LSR may unilaterally decide to no longer label switch a FEC for
     an LDP peer. An LSR that does so must send a label withdraw message
     for the FEC to the peer.

 Context:

     - Peer. The peer.

     - FEC. The FEC.

     - PrevAdvLabel. The label for FEC previously advertised to Peer.

 Algorithm:

   NoLS.1  Execute procedure Send_Label_Withdraw (Peer, FEC, PrevAdvLa-
           bel).
           PrevAdvLabel).  (See Note 1.)

   NoLS.2  DONE.

 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.1.13.

A.1.15. Timeout of deferred label request

 Summary:

     Label requests are deferred in response to No Route and Loop
     Detected notifications.  When a deferred FEC label request for a
     peer times out, the LSR sends the label request.

 Context:

     - LSR. The LSR handling the event.

     - FEC. The FEC associated with the timeout event.

     - Peer. The LDP peer associated with the timeout event.

     - Attributes. Attributes stored with deferred Label Request mes-
       sage.
       message.

 Algorithm:

   TO.1    Retrieve the record of the deferred label request.

   TO.2    Is Peer the next hop for FEC?
           If not, goto TO.4.

   TO.3    Execute procedure Send_Label_Request (Peer, FEC).

   TO.4    DONE.

A.2. Common Label Distribution Procedures

   This section specifies utility procedures used by the algorithms that
   handle label distribution events.

A.2.1. Send_Label

 Summary:

     The Send_Label procedure allocates a label for a FEC for an LDP
     peer, if possible, and sends a label mapping for the FEC to the
     peer. If the LSR is unable to allocate the label and if it has a
     pending label request from the peer, it sends the LDP peer a No
     Label Resources notification.

 Parameters:

     - Peer. The LDP peer to which the label mapping is to be sent.

     - FEC. The FEC for which a label mapping is to be sent.

     - Attributes. The attributes to be included with the label mapping.

 Additional Context:

     - LSR. The LSR executing the procedure.

     - Label. The label allocated and sent to Peer.

 Algorithm:

    SL.1   Does LSR have a label to allocate?
           If not, goto SL.9.

    SL.2   Allocate Label and bind it to the FEC.

    SL.3   Install Label for forwarding/switchng use.

    SL.4   Execute procedure Send_Message (Peer, Label Mapping, FEC,
           Label, Attributes).

    SL.5   Record label mapping for FEC with Label and Attributes has
           been sent to Peer.

    SL.6   Does LSR have a record of a FEC label request from Peer
           marked as pending?
           If not, goto SL.8.

    SL.7   Delete record of pending label request for FEC from Peer.

    SL.8   Return success.

    SL.9   Does LSR have a label request for FEC from Peer marked as
           pending?
           If not, goto SL.13.

    SL.10  Execute procedure Send_Notification (Peer, No Label
           Resources).

    SL.11  Delete record of pending label request for FEC from Peer.

    SL.12  Record No Label Resources notification has been sent to Peer.
           Goto SL.14.

    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 distri-
         bution
         distribution when the LSR is unable to allocate a label for a
         FEC to send to a Peer.

A.2.2. Send_Label_Request

 Summary:

     An LSR uses the Send_Label_Request procedure to send a request for
     a label for a FEC to an LDP peer if currently permitted to do so.

 Parameters:

     - Peer. The LDP peer to which the label request is to be sent.

     - FEC. The FEC for which a label request is to be sent.

     - Attributes. Attributes to be included in the label request. E.g.,
       Hop Count, Path Vector, CoS. Vector.

 Additional Context:

     - LSR. The LSR executing the procedure.

 Algorithm:

   SLRq.1  Has a label request for FEC previously been sent to Peer and
           is it marked as outstanding?
           If so, Return success.  (See Note 1.)

   SLRq.2  Is status record indicating it is OK to send label requests
           to Peer set?
           If not, goto SLRq.6

   SLRq.3  Execute procedure Send_Message (Peer, Label Request, FEC,
           Attributes).

   SLRq.4  Record 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 dif-
         ferent
         different upstream LDP peers from duplicate requests. This pro-
         cedure
         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 With-
     draw
     Withdraw message to the peer.

 Parameters:

     - Peer. The LDP peer to which the label withdraw is to be sent.

     - FEC. The FEC for which a label is being withdrawn.

     - Label. The label being withdrawn

 Additional Context:

     - LSR. The LSR executing the procedure.

 Algorithm:

    SWd.1  Execute procedure Send_Message (Peer, Label Withdraw, FEC,
           Label)

    SWd.2  Record label withdraw for FEC has been sent to Peer and mark
           it as outstanding.

A.2.4. Send_Notification

 Summary:

     An LSR uses the Send_Notification procedure to send an LDP peer a
     notificaction message.

 Parameters:

     - Peer. The LDP peer to which the label withdraw is to be sent.

     - Status. Status code to be included in the Notification message.

 Additional Context:

     None.

 Algorithm:

   SNt.1  Execute procedure Send_Message (Peer, Notification, Status)

A.2.5. Send_Message

 Summary:

     An LSR uses the Send_Message procedure to send an LDP peer an LDP
     message.

 Parameters:

     - Peer. The LDP peer to which the message is to be sent.

     - Message Type. The type of message to be sent.

     - Additional message contents . . .  .

 Additional Context:

     None.

 Algorithm:

     This procedure is the means by which an LSR sends an LDP message of
     the specified type to the specified LDP peer.

A.2.6. Check_Received_Attributes

 Summary:

     Check the attributes received in a Label Mapping or Label Request
     message. If the attributes include a Hop Count or Path Vector, per-
     form
     perform a loop detection check. If a loop is detected, send a Loop
     Detected Notification message to MsgSource.

 Parameters:

     - MsgSource. The LDP peer that sent the message.

     - RAttributes. The attributes in the message.

 Additional Context:

     - LSR Id. The unique LSR Id of this LSR.

     - Hop Count. The Hop Count, if any, in the received attributes.

     - Path Vector. The Path Vector, if any in the received attributes.

 Algorithm:

   CRa.1   Do RAttributes include Hop Count?
           If not, goto CRa.5.

   CRa.2   Does Hop Count exceed Max allowable hop count?
           If so, goto CRa.6.

   CRa.3   Do RAttributes include Path Vector?
           If not, goto CRa.5.

   CRa.4   Does Path Vector Include LSR Id? OR
           Does length of Path Vector exceed Max allowable length?
           If so, goto CRa.6

   CRa.5   Return No Loop Detected.

   CRa.6   Execute procedure Send_Notification (MsgSource, Loop
           Detected)

   CRa.7   Return Loop Detected.

   CRa.8   DONE

A.2.7. Prepare_Label_Request_Attributes

 Summary:

     This procedure is used whenever a Label Request is to be sent to a
     Peer to compute the Hop Count and Path Vector, if any, to include
     in the message.

 Parameters:

     - Peer. The LDP peer to which the message is to be sent.

     - FEC. The FEC for which a label request is to be sent.

     - RAttributes. The attributes this LSR associates with the LSP for
       FEC.

     - SAttributes. The attributes to be included in the Label Request
       message.

 Additional Context:

     - LSR Id. The unique LSR Id of this LSR.

 Algorithm:

   PRqA.1  Is Hop Count required for this Peer (see Note 1.) ? OR
           Do RAttributes include a Hop Count? OR
           Is Loop Detection configured on LSR?
           If not, goto PRqA.14.

   PRqA.2  Is LSR ingress for FEC?
           If not, goto PRqA.6.

   PRqA.3  Include Hop Count of 1 in SAttributes.

   PRqA.4  Is Loop Detection configured on LSR?
           If not, goto PRqA.14.

   PRqA.5  Is LSR merge-capable?
           If so, goto PRqA.14.
           If not, goto PRqA.13.

   PRqA.6  Do RAttributes include a Hop Count?
           If not, goto PRqA.8.

   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 SAttri-
           butes.
           SAttributes.

   PRqA.14 DONE.

 Notes:

      1. The link with Peer may require that Hop Count be included in
         Label Request messages; for example, see [ATM] and [FR].

      2. For hop count arithmetic, unknown + 1 = unknown.

A.2.8. Prepare_Label_Mapping_Attributes

 Summary:

     This procedure is used whenever a Label Mapping is to be sent to a
     Peer to compute the Hop Count and Path Vector, if any, to include
     in the message.

 Parameters:

     - Peer. The LDP peer to which the message is to be sent.

     - FEC. The FEC for which a label request is to be sent.

     - RAttributes. The attributes this LSR associates with the LSP for
       FEC.

     - SAttributes. The attributes to be included in the Label Request
       message.

     - IsPropagating. The LSR is sending the Label Mapping message to
       propagate one received from the FEC next hop.

     - PrevHopCount. The Hop Count, if any, this LSR associates with the
       LSP for the FEC.

 Additional Context:

     - LSR Id. The unique LSR Id of this LSR.

 Algorithm:

   PMpA.1  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.

   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.

   Res.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.

   PMpA.20 Include Path Vector of length 1 containing LSR Id in SAttri-
           butes.
           SAttributes.

   PMpA.21 DONE.

 Notes:

      1. The link with Peer may require that Hop Count be included in
         Label Mapping messages; for example, see [ATM] and [FR].

      2. If the LSR is at the edge of a cloud of LSRs that do not per-
         form
         perform TTL-decrement and it is propagating the Label Mapping mes-
         sage
         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 mamagement management for packets forwarded across the
         part of the LSP that passes through the cloud.

      3. For hop count arithmetic, unknown + 1 = unknown.