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

LDP Specification

Status of This Memo

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

Abstract

   The architecture for Multiprotocol Label Switching (MPLS) is
   described in RFC 3031.  A fundamental concept in MPLS is that two
   Label Switching Routers (LSRs) must agree on the meaning of the
   labels used to forward traffic between and through them.  This common
   understanding is achieved by using a set of procedures, called a
   label distribution protocol, by which one LSR informs another of
   label bindings it has made.  This document defines a set of such
   procedures called LDP (for Label Distribution Protocol) by which LSRs
   distribute labels to support MPLS forwarding along normally routed
   paths.

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

      2.3. LDP Sessions between Non-Directly Connected LSRs ..........10
      2.4. LDP Discovery .............................................11
           2.4.1. Basic Discovery Mechanism ..........................11
           2.4.2. Extended Discovery Mechanism .......................11
      2.5. Establishing and Maintaining LDP Sessions .................12
           2.5.1. LDP Session Establishment ..........................12
           2.5.2. Transport Connection Establishment .................12
           2.5.3. Session Initialization .............................14
           2.5.4. Initialization State Machine .......................16
           2.5.5. Maintaining Hello Adjacencies ......................19
           2.5.6. Maintaining LDP Sessions ...........................19
      2.6. Label Distribution and Management .........................20
           2.6.1. Label Distribution Control Mode ....................20
                  2.6.1.1. Independent Label Distribution Control ....20
                  2.6.1.2. Ordered Label Distribution Control ........20
           2.6.2. Label Retention Mode ...............................21
                  2.6.2.1. Conservative Label Retention Mode .........21
                  2.6.2.2. Liberal Label Retention Mode ..............21
           2.6.3. Label Advertisement Mode ...........................22
      2.7. LDP Identifiers and Next Hop Addresses ....................22
      2.8. Loop Detection ............................................23
           2.8.1. Label Request Message ..............................23
           2.8.2. Label Mapping Message ..............................25
           2.8.3. Discussion .........................................26
      2.9. Authenticity and Integrity of LDP Messages ................27
           2.9.1. TCP MD5 Signature Option ...........................27
           2.9.2. LDP Use of TCP MD5 Signature Option ................29
      2.10. Label Distribution for Explicitly Routed LSPs ............29
   3. Protocol Specification .........................................30
      3.1. LDP PDUs ..................................................30
      3.2. LDP Procedures ............................................31
      3.3. Type-Length-Value Encoding ................................31
      3.4. TLV Encodings for Commonly Used Parameters ................33
           3.4.1. FEC TLV ............................................33
                  3.4.1.1. FEC Procedures ............................35
           3.4.2. Label TLVs .........................................35
                  3.4.2.1. Generic Label TLV .........................36
                  3.4.2.2. ATM Label TLV .............................36
                  3.4.2.3. Frame Relay Label TLV .....................37
           3.4.3. Address List TLV ...................................38
           3.4.4. Hop Count TLV ......................................39
                  3.4.4.1. Hop Count Procedures ......................39
           3.4.5. Path Vector TLV ....................................41
                  3.4.5.1. Path Vector Procedures ....................41
                           3.4.5.1.1. Label Request Path Vector ......41
                           3.4.5.1.2. Label Mapping Path Vector ......42
           3.4.6. Status TLV .........................................43

      3.5. LDP Messages ..............................................44
           3.5.1. Notification Message ...............................46
                  3.5.1.1. Notification Message Procedures ...........48
                  3.5.1.2. Events Signaled by Notification Messages ..48
                           3.5.1.2.1. Malformed PDU or Message .......48
                           3.5.1.2.2. Unknown or Malformed TLV .......49
                           3.5.1.2.3. Session KeepAlive Timer
                                      Expiration .....................50
                           3.5.1.2.4. Unilateral Session Shutdown ....50
                           3.5.1.2.5. Initialization Message Events ..50
                           3.5.1.2.6. Events Resulting from
                                      Other Messages .................50
                           3.5.1.2.7. Internal Errors ................51
                           3.5.1.2.8. Miscellaneous Events ...........51
           3.5.2. Hello Message ......................................51
                  3.5.2.1. Hello Message Procedures ..................53
           3.5.3. Initialization Message .............................54
                  3.5.3.1. Initialization Message Procedures .........63
           3.5.4. KeepAlive Message ..................................63
                  3.5.4.1. KeepAlive Message Procedures ..............63
           3.5.5. Address Message ....................................64
                  3.5.5.1. Address Message Procedures ................64
           3.5.6. Address Withdraw Message ...........................65
                  3.5.6.1. Address Withdraw Message Procedures .......66
           3.5.7. Label Mapping Message ..............................66
                  3.5.7.1. Label Mapping Message Procedures ..........67
                           3.5.7.1.1. Independent Control Mapping ....67
                           3.5.7.1.2. Ordered Control Mapping ........68
                           3.5.7.1.3. Downstream on Demand
                                      Label Advertisement ............68
                           3.5.7.1.4. Downstream Unsolicited
                                      Label Advertisement ............69
           3.5.8. Label Request Message ..............................70
                  3.5.8.1. Label Request Message Procedures ..........71
           3.5.9. Label Abort Request Message ........................72
                  3.5.9.1. Label Abort Request Message Procedures ....73
           3.5.10. Label Withdraw Message ............................74
                  3.5.10.1. Label Withdraw Message Procedures ........75
           3.5.11. Label Release Message .............................76
                  3.5.11.1. Label Release Message Procedures .........77
      3.6. Messages and TLVs for Extensibility .......................78
           3.6.1. LDP Vendor-Private Extensions ......................78
                  3.6.1.1. LDP Vendor-Private TLVs ...................78
                  3.6.1.2. LDP Vendor-Private Messages ...............80
           3.6.2. LDP Experimental Extensions ........................81
      3.7. Message Summary ...........................................81
      3.8. TLV Summary ...............................................82
      3.9. Status Code Summary .......................................83

      3.10. Well-Known Numbers .......................................84
           3.10.1. UDP and TCP Ports .................................84
           3.10.2. Implicit NULL Label ...............................84
   4. IANA Considerations ............................................84
      4.1. Message Type Name Space ...................................84
      4.2. TLV Type Name Space .......................................85
      4.3. FEC Type Name Space .......................................85
      4.4. Status Code Name Space ....................................86
      4.5. Experiment ID Name Space ..................................86
   5. Security Considerations ........................................86
      5.1. Spoofing ..................................................86
      5.2. Privacy ...................................................87
      5.3. Denial of Service .........................................88
   6. Areas for Future Study .........................................89
   7. Changes from RFC 3036 ..........................................90
   8. Acknowledgments ................................................93
   9. References .....................................................93
      9.1. Normative References ......................................93
      9.2. Informative References ....................................94
   Appendix A. LDP Label Distribution Procedures  ....................95
      A.1. Handling Label Distribution Events  .......................97
           A.1.1. Receive Label Request  .............................98
           A.1.2.  Receive Label Mapping  ...........................101
           A.1.3.  Receive Label Abort Request  .....................107
           A.1.4.  Receive Label Release  ...........................109
           A.1.5.  Receive Label Withdraw  ..........................111
           A.1.6.  Recognize New FEC  ...............................113
           A.1.7.  Detect Change in FEC Next Hop  ...................115
           A.1.8.  Receive Notification / Label Request Aborted  ....118
           A.1.9.  Receive Notification / No Label Resources  .......119
           A.1.10.  Receive Notification / No Route  ................119
           A.1.11.  Receive Notification / Loop Detected  ...........120
           A.1.12.  Receive Notification / Label Resources Available 121
           A.1.13.  Detect Local Label Resources Have Become
                    Available  ......................................122
           A.1.14.  LSR Decides to No Longer Label Switch a FEC  ....123
           A.1.15.  Timeout of Deferred Label Request  ..............123
      A.2. Common Label Distribution Procedures  ....................124
           A.2.1.  Send_Label  ......................................124
           A.2.2.  Send_Label_Request  ..............................125
           A.2.3.  Send_Label_Withdraw  .............................127
           A.2.4.  Send_Notification  ...............................127
           A.2.5.  Send_Message  ....................................128
           A.2.6.  Check_Received_Attributes  .......................128
           A.2.7.  Prepare_Label_Request_Attributes  ................129
           A.2.8.  Prepare_Label_Mapping_Attributes  ................131

1. LDP Overview

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

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

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

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

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

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

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

1.1. LDP Peers

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

1.2. LDP Message Exchange

   There are four categories of LDP messages:

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

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

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

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

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

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

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

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

1.5. LDP Extensibility and Future Compatibility

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

1.6. Specification Language

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

2. LDP Operation

2.1. FECs

   It is necessary to precisely specify which packets may be mapped to
   each LSP.  This is done by providing a FEC specification for each
   LSP.  The FEC identifies the set of IP packets that may be mapped to
   that LSP.

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

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

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

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

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

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

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

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

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

   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.)

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 (Virtual Channel Identifiers) as
         labels, or a Frame Relay interface that uses DLCIs (Data Link
         Connection Identifiers) as labels.

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

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

2.2.2. LDP Identifiers

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

         <LSR Id> : <label space id>

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

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

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

2.2.3. LDP Sessions

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

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

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 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 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
            Hellos it sends to LSR2 to advertise an address, A1 is the
            address LSR1 advertises via the optional object;

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

         Similarly, address A2 is determined as follows:

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

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

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

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

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

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

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

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

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

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

   An LSR MUST advertise the same transport address in all Hellos that
   advertise the same label space.  This requirement ensures that two
   LSRs linked by multiple Hello adjacencies using the same label spaces
   play the same connection establishment role for each adjacency.

2.5.3. Session Initialization

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

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

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

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

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

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

      1. When LSR1 plays the passive role:

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

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

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

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

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

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

      2. When LSR1 plays the active role:

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

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

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

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

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

   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.  Note that a Shutdown
   message is implemented as a Notification message with a Status TLV
   indicating a fatal error.

               Session Initialization State Transition Table

   STATE         EVENT                               NEW STATE

   NON EXISTENT  Session TCP connection established  INITIALIZED
                 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 that 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 an 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 that it resets whenever it
   receives an LDP PDU from the session peer.  If the KeepAlive Timer
   expires without receipt of an LDP PDU from the peer, the LSR
   concludes that the transport connection is bad or that the peer has
   failed, and it terminates the LDP session by closing the transport
   connection.

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

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

2.6. Label Distribution and Management

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

   Both of these label distribution techniques may be used in the same
   network at the same time.  However, for any given LDP session, each
   LSR must be aware of the label distribution method used by its peer
   in order to avoid situations where one peer using Downstream
   Unsolicited label distribution assumes its peer is also.  See Section
   "Downstream on Demand Label Advertisement".

2.6.1. Label Distribution Control Mode

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

2.6.1.1. Independent Label Distribution Control

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

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

2.6.1.2. Ordered Label Distribution Control

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

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

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

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

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

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

2.6.2. Label Retention Mode

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

2.6.2.1. Conservative Label Retention Mode

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

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

2.6.2.2. Liberal Label Retention Mode

   In Downstream Unsolicited advertisement mode, label mapping
   advertisements for all routes may be received from all LDP peers.
   When using Liberal Label retention, every label mappings received

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

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

2.6.3. Label Advertisement Mode

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

2.7. LDP Identifiers and Next Hop Addresses

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

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

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

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

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

2.8. Loop Detection

   Loop Detection is a configurable option that 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 first four octets of its LDP Identifier.  When an LSR
         propagates a message containing a Path Vector TLV, it adds its
         LSR Id to the Path Vector list.  An LSR that receives a message
         with a Path Vector that contains its LSR Id detects that the
         message has traversed a loop.  LDP supports the notion of a
         maximum allowable Path Vector length; an LSR that detects a
         Path Vector has reached the maximum length behaves as if the
         containing message has traversed a loop.

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

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

2.8.1. Label Request Message

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

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

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

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

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

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

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

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

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

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

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

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

2.8.2. Label Mapping Message

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

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

   -  R MUST include a Hop Count TLV.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

2.8.3. Discussion

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

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

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

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

2.9. Authenticity and Integrity of LDP Messages

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

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

2.9.1. TCP MD5 Signature Option

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

      "IESG Note

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

      "Abstract

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

      "Introduction

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

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

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

      "MD5 as a Hashing Algorithm

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

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

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

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

   End of quotes from [RFC2385].

2.9.2. LDP Use of TCP MD5 Signature Option

   LDP uses the TCP MD5 Signature Option as follows:

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

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

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

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

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

2.10. Label Distribution for Explicitly Routed LSPs

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

3.  Protocol Specification

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

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

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

3.1. LDP PDUs

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

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

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

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

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

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

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

3.2. LDP Procedures

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

      - 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 MUST be silently ignored and the rest of the message
      processed as if the unknown TLV did not exist.  The sections
      following that define TLVs specify a value for the U-bit.

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

   Type
      Encodes how the Value field is to be interpreted.

   Length
      Specifies the length of the Value field in octets.

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

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

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

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

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

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

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

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

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

      Wildcard FEC Element

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

      Prefix FEC Element value encoding:

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

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

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

         Prefix
            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.

3.4.1.1. FEC Procedures

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

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

3.4.2. Label TLVs

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

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

3.4.2.1. Generic Label TLV

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

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

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

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

      For further information, see [RFC3032].

3.4.2.2. ATM Label TLV

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

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

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

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

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

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

3.4.2.3. Frame Relay Label TLV

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

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

   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 of DLCI
2 = 23 bits of DLCI

      Len values 1 and 3 are reserved.

   DLCI
      The Data Link Connection Identifier

      For a 10-bit DLCI, the encoding is:

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

      For a 23-bit DLCI, the encoding is:

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

      For further information, see [RFC3034].

3.4.3. Address List TLV

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

   Its encoding is:

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

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

   Addresses
      A list of addresses from the specified Address Family.  The
      encoding of the individual addresses depends on the Address
      Family.

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

         Address Family      Address Encoding

         IPv4                4 octet full IPv4 address
         IPv6                16 octet full IPv6 address

3.4.4. Hop Count TLV

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

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

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

   HC Value
      1 octet unsigned integer hop count value.

3.4.4.1. Hop Count Procedures

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

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

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

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

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

      o  Otherwise, R MUST increment the received hop count.

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

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

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

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

   From the perspective of the ingress node, the fact that the hop count
   is unknown implies nothing about whether a packet sent on the LSP
   will actually make it to the egress.  All it implies is that the hop
   count update from the egress has not yet reached the ingress.

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

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

3.4.5. Path Vector TLV

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

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

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

3.4.5.1. Path Vector Procedures

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

3.4.5.1.1. Label Request Path Vector

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

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

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

   The LSR MUST:

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

      2. Not propagate the Label Request message further.

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

      1. A loop is found,

      2. The LSP egress is reached, or

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

3.4.5.1.2. Label Mapping Path Vector

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

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

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

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

      2. Not propagate the message further.

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

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

      1. A loop is found,

      2. An LSP ingress is reached, or

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

3.4.6. Status TLV

   Notification messages carry Status TLVs to specify events being
   signaled.

   The encoding for the Status TLV is:

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

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

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

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

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

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

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

      Status Data
         30-bit unsigned integer that specifies the status information.

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

         A Status Code of 0 signals success.

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

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

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

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

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

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

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

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

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

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

      Message Name            Section 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
   significant event.  A Notification message signals a fatal error or
   provides advisory information such as the outcome of processing an
   LDP message or the state of the LDP session.

   The encoding for the Notification message is:

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

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

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

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

         Optional Parameter     Type     Length  Value

         Extended Status        0x0301    4      See below
         Returned PDU           0x0302    var    See below
         Returned Message       0x0303    var    See below

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

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

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

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

3.5.1.1. Notification Message Procedures

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

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

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

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

3.5.1.2. Events Signaled by Notification Messages

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

3.5.1.2.1. Malformed PDU or Message

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

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

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

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

   An LDP message is malformed if:

      -  The Message Type is unknown.

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

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

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

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

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

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 an
         error signaled by the Unknown TLV Status Code.

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

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

3.5.1.2.3. Session KeepAlive Timer Expiration

This is a fatal error signaled by the 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
   Initialization") may fail if the session parameters received in the
   Initialization message are unacceptable.  This is a fatal error.  The
   specific Status Code depends on the parameter deemed unacceptable,
   and is defined in Sections "Initialization Message".

3.5.1.2.6. Events Resulting from Other Messages

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

3.5.1.2.7. Internal Errors

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

3.5.1.2.8. Miscellaneous Events

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

3.5.2. Hello Message

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

   The encoding for the Hello message is:

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

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

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

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

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

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

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

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

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

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

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

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

      Optional Parameter         Type     Length  Value

      IPv4 Transport Address     0x0401     4     See below
      Configuration              0x0402     4     See below
         Sequence Number
      IPv6 Transport Address     0x0403    16     See below

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

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

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

3.5.2.1. Hello Message Procedures

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

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

   An LSR processes a received LDP Hello as follows:

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

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

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

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

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

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

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

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

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

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

      The following describes how an LSR processes Hello optional TLVs:

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

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

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

3.5.3. Initialization Message

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

   The encoding for the Initialization message is:

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

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

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

      The encoding for the Common Session Parameters TLV is:

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

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

      KeepAlive Time
         Two octet unsigned non zero integer that indicates the number
         of seconds that the sending LSR proposes for the value of the
         KeepAlive Time.  The receiving LSR MUST calculate the value of
         the KeepAlive Timer by using the smaller of its proposed
         KeepAlive Time and the KeepAlive Time received in the PDU.  The

         value chosen for KeepAlive Time indicates the maximum number of
         seconds that may elapse between the receipt of successive PDUs
         from the LDP peer on the session TCP connection.  The KeepAlive
         Timer is reset each time a PDU arrives.

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

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

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

         -  Otherwise, Downstream Unsolicited MUST be used.

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

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

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

         When Loop Detection is enabled in a portion of a network, it is
         recommended that all LSRs in that portion of the network be
         configured with the same Path Vector limit.  Although knowledge
         of a peer's Path Vector limit will not change an LSR's
         behavior, it does enable the LSR to alert an operator to a
         possible misconfiguration.

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

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

         The receiving LSR MUST calculate the maximum PDU length for the
         session by using the smaller of its and its peer's proposals
         for Max PDU Length.  The default maximum PDU length applies
         before session initialization completes.

         If the maximum PDU length determined this way is unacceptable
         to an LSR, it MUST send a Session Rejected/Parameters Max PDU
         Length Notification message in response to the Initialization
         message and not establish the session.

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

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

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

      Optional Parameter       Type     Length  Value

      ATM Session Parameters   0x0501   var     See below
      Frame Relay Session      0x0502   var     See below
        Parameters

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

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

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

               Value          Meaning

                 0            Merge not supported
                 1            VP Merge supported
                 2            VC Merge supported
                 3            VP & VC Merge supported

         If the merge capabilities of the LSRs differ, then:

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

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

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

      N, Number of label range components
         Specifies the number of ATM Label Range Components included in
         the TLV.

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

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

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

         A receiving LSR MUST calculate the intersection between the
         received range and its own supported label range.  The
         intersection is the range in which the LSR may allocate and
         accept labels.  LSRs MUST NOT establish a session with
         neighbors for which the intersection of ranges is NULL.  In
         this case, the LSR 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
            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 Channel Identifiers that is supported on the
            originating switch.  If the VCI is less than 16 bits, it
            SHOULD be right justified in this field and preceding bits
            SHOULD be set to 0.

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

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

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

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

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

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

               Value          Meaning

                 0            Merge not supported
                 1            Merge supported

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

      N, Number of label range components
         Specifies the number of Frame Relay Label Range Components
         included in the TLV.

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

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

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

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

         The encoding for a Frame Relay Label Range Component is:

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

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

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

               Len    DLCI Bits

               0       10
               2       23

         Len values 1 and 3 are reserved.

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

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

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

3.5.3.1. Initialization Message Procedures

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

3.5.4. KeepAlive Message

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

   The encoding for the KeepAlive message is:

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

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

   Optional Parameters
      No optional parameters are defined for the KeepAlive message.

3.5.4.1. KeepAlive Message Procedures

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

   An LSR MUST arrange that its peer receive an LDP message from it at
   least every KeepAlive Time period.  Any LDP protocol message will do

but, in circumstances where no other LDP protocol messages have been
sent within the period, a KeepAlive message MUST be sent.

3.5.5. Address Message

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

   The encoding for the Address message is:

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

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

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

   Optional Parameters
      No optional parameters are defined for the Address message.

3.5.5.1. Address Message Procedures

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

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

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

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

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

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

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

3.5.6. Address Withdraw Message

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

   The encoding for the Address Withdraw message is:

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

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

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

   Optional Parameters
      No optional parameters are defined for the Address Withdraw
      message.

3.5.6.1. Address Withdraw Message Procedures

See Section "Address Message Procedures".

3.5.7. Label Mapping Message

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

   The encoding for the Label Mapping message is:

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

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

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

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

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

         Optional Parameter    Length       Value

         Label Request         4            See below
             Message ID TLV
         Hop Count TLV         1            See below
         Path Vector TLV       variable     See below

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

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

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

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

3.5.7.1. Label Mapping Message Procedures

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

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

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

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

3.5.7.1.1. Independent Control Mapping

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

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

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

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

      4. The attributes of a mapping change.

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

            a) no upstream mapping has been created  OR
            b) loop detection is configured  OR
            c) the attributes of the mapping have changed.

3.5.7.1.2. Ordered Control Mapping

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

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

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

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

      4. The attributes of a mapping change.

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

            a) no upstream mapping has been created   OR
            b) Loop Detection is configured   OR
            c) the attributes of the mapping have changed.

3.5.7.1.3. Downstream on Demand Label Advertisement

   In general, the upstream LSR is responsible for requesting label
   mappings when operating in Downstream on Demand mode.  However,
   unless some rules are followed, it is possible for neighboring LSRs
   with different advertisement modes to get into a livelock situation
   where everything is functioning properly, but no labels are

   distributed.  For example, consider two LSRs Ru and Rd where Ru is
   the upstream LSR and Rd is the downstream LSR for a particular FEC.
   In this example, Ru is using Downstream Unsolicited advertisement
   mode and Rd is using Downstream on Demand mode.  In this case, Rd may
   assume that Ru will request a label mapping when it wants one and Ru
   may assume that Rd will advertise a label if it wants Ru to use one.
   If Rd and Ru operate as suggested, no labels will be distributed from
   Rd to Ru.

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

3.5.7.1.4. Downstream Unsolicited Label Advertisement

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

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

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

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

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

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 encoding for the Label Request message is:

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

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

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

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

         Optional Parameter     Length       Value

         Hop Count TLV          1            See below
         Path Vector TLV        variable     See below

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

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

   Path Vector
      Specifies the LSRs along the LSR being set up 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 next hop, it SHOULD NOT issue an additional
         Label Request in response to the next hop change.

      3. The LSR receives a Label Request for a FEC from an upstream LDP
         peer, the FEC next hop is an LDP peer, and the LSR doesn't
         already have a mapping from the next hop.

         Note that since a non-merge LSR must set up a separate LSP for
         each upstream peer requesting a label, it must send a separate
         Label Request for each such peer.  A consequence of this is
         that a non-merge LSR may have multiple Label Request messages
         for a given FEC outstanding at the same time.

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

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

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

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

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

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

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

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

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

3.5.9. Label Abort Request Message

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

   The encoding for the Label Abort Request message is:

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

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

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

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

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

3.5.9.1. Label Abort Request Message Procedures

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

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

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

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

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

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

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

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

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

      -  A Label Request Aborted Notification message acknowledging the
         abort;

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

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

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

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

3.5.10. Label Withdraw Message

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

   The encoding for the Label Withdraw Message is:

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

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

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

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

         Optional Parameter    Length       Value

         Label TLV             variable     See below

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

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

3.5.10.1. Label Withdraw Message Procedures

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

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

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

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

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

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

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

3.5.11. Label Release Message

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

   The encoding for the Label Release Message is:

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

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

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

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

         Optional Parameter    Length       Value

         Label TLV             variable     See below

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

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

3.5.11.1. Label Release Message Procedures

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

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

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

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

      3. The LSR receives a Label Withdraw message.

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

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

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

   is releasing all label mappings previously learned from the receiving
   LSR.

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

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 handles unknown TLVs and messages.

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

3.6.1. LDP Vendor-Private Extensions

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

3.6.1.1. LDP Vendor-Private TLVs

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

   The encoding for a vendor-private TLV is:

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

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

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

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

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

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

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

   Vendor ID
      802 Vendor ID as assigned by the IEEE.

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

3.6.1.2. LDP Vendor-Private Messages

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

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

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

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

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

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

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

   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 responsibility of the
         experimenters.

3.7. Message Summary

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

      Message Name            Type     Section Title

      Notification            0x0001   "Notification Message"
      Hello                   0x0100   "Hello Message"
      Initialization          0x0200   "Initialization Message"
      KeepAlive               0x0201   "KeepAlive Message"
      Address                 0x0300   "Address Message"
      Address Withdraw        0x0301   "Address Withdraw Message"
      Label Mapping           0x0400   "Label Mapping Message"
      Label Request           0x0401   "Label Request Message"
      Label Withdraw          0x0402   "Label Withdraw Message"
      Label Release           0x0403   "Label Release Message"
      Label Abort Request     0x0404   "Label Abort Request Message"
      Vendor-Private          0x3E00-  "LDP Vendor-Private Extensions"
                              0x3EFF
      Experimental            0x3F00-  "LDP Experimental Extensions"
                              0x3FFF

3.8. TLV Summary

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

      TLV                      Type      Section Title

      FEC                      0x0100    "FEC TLV"
      Address List             0x0101    "Address List TLV"
      Hop Count                0x0103    "Hop Count TLV"
      Path Vector              0x0104    "Path Vector TLV"
      Generic Label            0x0200    "Generic Label TLV"
      ATM Label                0x0201    "ATM Label TLV"
      Frame Relay Label        0x0202    "Frame Relay Label TLV"
      Status                   0x0300    "Status TLV"
      Extended Status          0x0301    "Notification Message"
      Returned PDU             0x0302    "Notification Message"
      Returned Message         0x0303    "Notification Message"
      Common Hello             0x0400    "Hello Message"
         Parameters
      IPv4 Transport Address   0x0401    "Hello Message"
      Configuration            0x0402    "Hello Message"
         Sequence Number
      IPv6 Transport Address   0x0403    "Hello Message"
      Common Session           0x0500    "Initialization Message"
         Parameters
      ATM Session Parameters   0x0501    "Initialization Message"
      Frame Relay Session      0x0502    "Initialization Message"
         Parameters
      Label Request            0x0600    "Label Mapping Message"
          Message ID

      Vendor-Private           0x3E00-   "LDP Vendor-Private Extensions"
                               0x3EFF
      Experimental             0x3F00-   "LDP Experimental Extensions"
                               0x3FFF

3.9. Status Code Summary

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

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

      Status Code           E   Status Data   Section Title

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

      Unsupported Address   0   0x00000017    "FEC Procedures"
          Family                              "Address Message Proc ..."
      Session Rejected/     1   0x00000018    "Session Initialization"
         Bad KeepAlive Time
      Internal Error        1   0x00000019    "Events Signaled by ..."

3.10. Well-Known Numbers

3.10.1. UDP and TCP Ports

The UDP port for LDP Hello messages is 646.

The TCP port for establishing LDP session connections is 646.

3.10.2. Implicit NULL Label

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

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

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

4.  IANA Considerations

   LDP defines the following name spaces that require management:

      -  Message Type Name Space
      -  TLV Type Name Space
      -  FEC Type Name Space
      -  Status Code Name Space
      -  Experiment ID Name Space

   The following sections provide guidelines for managing these name
   spaces.

4.1. Message Type Name Space

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

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

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

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

4.2. TLV Type Name Space

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

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

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

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

4.3. FEC Type Name Space

The range for FEC types is 0 - 255.

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

4.4. Status Code Name Space

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

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

4.5. Experiment ID Name Space

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

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

5.  Security Considerations

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

5.1. Spoofing

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

   1. Discovery exchanges carried by UDP

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

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

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

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

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

   2. Session communication carried by TCP

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

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

5.2. Privacy

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

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

   One might argue that label distribution requires privacy to address
   the threat of label spoofing.  However, that privacy would not
   protect against label spoofing attacks since data packets carry

   labels in the clear.  Furthermore, label spoofing attacks can be made
   without knowledge of the FEC bound to a label.

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

5.3. Denial of Service

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

   1. Well-known UDP Port for LDP Discovery

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

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

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

   2. Well-known TCP port for LDP Session Establishment

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

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

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

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

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

6. Areas for Future Study

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

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

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

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

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

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

      -  The current specification does not address basic peer discovery
         on Non-Broadcast Multi-Access (NBMA) media.  The solution
         available in the current specification is to use extended peer

         discovery in such setups.  The issue of defining a mechanism
         semantically similar to Basic Discovery (1 hop limit, bind the
         hello adjacency to an interface) that uses preconfigured
         neighbor addresses is left for further study.

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

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

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

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

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

7. Changes from RFC 3036

   Here is a list of changes from RFC 3036

       1. Removed the Host Address FEC and references to it, since it is
          not used by any implementation.

       2. Split the reference list into normative and informative
          references

       3. Removed "MPLS using ATM VP Switching" from the list of
          normative references, and references to it.

        4. Removed reference to RFC 1700 and replaced it with a link to
          http://www.iana.org/assignments/address-family-numbers.

       5. Removed reference to RFC 1771 and replaced it with a reference
          to RFC 4271.

       6. Clarified the use of the F-bit.

       7. Added option to allow split horizon when doing Ordered
          Control.

8. Clarified the processing of messages with the U-bit set during
the session initialization procedures

9. Clarified the processing of the E-bit during session
initialization procedures.

      10. Added text explaining that the Shutdown message in the state
          transition diagram is implemented as a notification message
          with a Status TLV indicating a fatal error.

11. Added case for TLV length too short in the specification for
handling malformed TLVs.

12. Explained the security threat posed by hello spoofing.

13. Added reference to 4271 and 4278 and text for standards
maturity variance with regards to the MD5 option.

14. Added text from 3031 explaining the handling of implicit NULL
label.

15. Included the encoding of DLCIs to remove normative reference
to 3034.

16. Moved references to 3031, 3032, and 3034 to informative.

17. In the section describing handling of unknown TLV, removed
reference to inexistent section (errata in original document).

18. Added text clarifying how to achieve interoperability when
sending vendor-private TLVs and messages.

      19. In the "receive label request" procedures, if a loop is
          detected, changed the procedure to send a notification before
          aborting the rest of the processing.

20. In "receive label release" procedures, clarified the behavior
for merge-capable LSRs.

21. In "receive label release" procedures, clarified the behavior
for receipt of an unknown FEC.

      22. In note 4 of "Detect Change in FEC Next Hop", modified the
          text to reference the correct set of conditions for sending a
          label request procedure (typo in the original document).

      23. In the procedures for "LSR decides to no longer label switch a
          FEC", clarified the fact that the label must not be reused
          until a label release is received.

      24. In the routine "Prepare_Label_Mapping_Attributes", added a
          note regarding the treatment of unknown TLVs according to
          their U and F-bits.

      25. In the Address message processing procedures, clarified the
          behavior for the case where an LSR receives re-advertisement
          of an address previously advertised it, or withdrawal of an
          address from an LSR that has not previously advertised that
          address.

      26. In the routine "Receive Label Mapping", clarified the meaning
          of PrevAdvLabel when no label advertisement message has been
          sent previously.

      27. In the "Receive Label Mapping" procedures, if a loop is
          detected, modified the procedure to send a notification before
          aborting the rest of the processing.

      28. In the "Receive Label Mapping" procedures, corrected step
          LMp.10 to handle label mapping messages for additional (non-
          merged) LSPs for the FEC.

29. In the "Receive Label Mapping" procedures, clarified behavior
when receiving a duplicate label for the same FEC.

30. In the routine "Receive Label Abort Request", clarified the
behavior for non-merging LSRs.

31. Added the following items to the section discussing areas for
future study:

         o  extensions for communicating the maximum transmission unit
         o  basic peer discovery on NBMA media
         o  option of shutting down an adjacency
         o  mechanisms for securing Hello messages
         o  detection of a stateless fast control plane restart
         o  support of "end of LIB" message

         o  mechanisms for dealing with the case where different LSRs
            advertise the same address

8.  Acknowledgments

   This document is produced as part of advancing the LDP specification
   to draft standard status.  This document was originally published as
   RFC 3036 in January 2001.  It was produced by the MPLS Working Group
   of the IETF and was jointly authored by Loa Andersson, Paul Doolan,
   Nancy Feldman, Andre Fredette, and Bob Thomas.

   The ideas and text in RFC 3036 were collected from a number of
   sources.  We would like to thank Rick Boivie, Ross Callon, Alex
   Conta, Eric Gray, Yoshihiro Ohba, Eric Rosen, Bernard Suter, Yakov
   Rekhter, and Arun Viswanathan for their input for RFC 3036.

   The editors would like to thank Eric Gray, David Black, and Sam
   Hartman for their input to and review of the current document.

   In addition, the editors would like to thank the members of the MPLS
   Working Group for their ideas and the support they have given to this
   document, and in particular, to Eric Rosen, Luca Martini, Markus
   Jork, Mark Duffy, Vach Kompella, Kishore Tiruveedhula, Rama
   Ramakrishnan, Nick Weeds, Adrian Farrel, and Andy Malis.

9.  References

9.1. Normative References

   [IANA]        Narten, T. and H. Alvestrand, "Guidelines for Writing
                 an IANA Considerations Section in RFCs", BCP 26, RFC
                 2434, October 1998.

   [RFC1321]     Rivest, R., "The MD5 Message-Digest Algorithm", RFC
                 1321, April 1992.

   [ASSIGNED_AF] http://www.iana.org/assignments/address-family-numbers

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2385]     Heffernan, A., "Protection of BGP Sessions via the TCP
                 MD5 Signature Option", RFC 2385, August 1998.

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

[RFC3037] Thomas, B. and E. Gray, "LDP Applicability", RFC 3037,
January 2001.

9.2. Informative References

   [CRLDP]       Jamoussi, B., Ed., Andersson, L., Callon, R., Dantu,
                 R., Wu, L., Doolan, P., Worster, T., Feldman, N.,
                 Fredette, A., Girish, M., Gray, E., Heinanen, J.,
                 Kilty, T., and A. Malis, "Constraint-Based LSP Setup
                 using LDP", RFC 3212, January 2002.

   [LDP-MTU]     Black, B. and K. Kompella, "Maximum Transmission Unit
                 Signalling Extensions for the Label Distribution
                 Protocol", RFC 3988, January 2005.

   [RFC2328]     Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
                 1998.

   [RFC2702]     Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and
                 J. McManus, "Requirements for Traffic Engineering Over
                 MPLS", RFC 2702, September 1999.

   [RFC3031]     Rosen, E., Viswanathan, A., and R. Callon,
                 "Multiprotocol Label Switching Architecture", RFC 3031,
                 January 2001.

   [RFC3032]     Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
                 Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
                 Encoding", RFC 3032, January 2001.

   [RFC3034]     Conta, A., Doolan, P., and A. Malis, "Use of Label
                 Switching on Frame Relay Networks Specification", RFC
                 3034, January 2001.

   [RFC4271]     Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
                 Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
                 2006.

   [RFC4278]      Bellovin, S. and A. Zinin, "Standards Maturity
                 Variance Regarding the TCP MD5 Signature Option (RFC
                 2385) and the BGP-4 Specification", RFC 4278, January
                 2006.

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 [RFC3031] for hop-by-hop routed traffic.
   These procedures are:

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

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

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

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

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

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

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

         .  Request Never.  The LSR never requests a label.

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

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

      -  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 ReleaseOnChange in
            [RFC3031].

         .  Liberal Label retention, called NoReleaseOnChange in
            [RFC3031].

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

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

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

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

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

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

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

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

A.1. Handling Label Distribution Events

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

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

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

A.1.1. Receive Label Request

   Summary:

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

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

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

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

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

   Context:

      -  LSR.  The LSR handling the event.

      -  MsgSource.  The LDP peer that sent the message.

      -  FEC.  The FEC specified in the message.

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

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

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

   Algorithm:

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

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

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

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

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

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

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

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

      LRq.9   Perform LSR Label Distribution procedure:

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

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

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

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

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

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

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

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

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

      LRq.10  Perform LSR Label Request procedure:

         For Request Never

                  1. Goto LRq.13.

         For Request When Needed OR
         For Request On Request

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

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

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

      LRq.12  Perform LSR Label Use procedure.

              For Use Immediate OR For Use If Loop Not Detected

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

      LRq.13  DONE.

      Notes:

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

            The LSR uses the message ID of received Label Request
            messages to detect duplicate requests.  This means that an
            LSR (the upstream peer) may not reuse the message ID used
            for a Label Request until the Label Request transaction has
            completed.

         2. When an LSR sends a label request to a peer, it records that
            the request has been sent and marks it as outstanding.  As
            long as the request is marked outstanding, the LSR SHOULD
            NOT send another request for the same label to the peer.
            Such a second request would be a duplicate.  The
            Send_Label_Request procedure described below obeys this
            rule.

            A duplicate label request is considered a protocol error and
            SHOULD be dropped by the receiving LSR (perhaps with a
            suitable notification returned to MsgSource).

         3. If the LSR is not merge-capable, this test will fail.

         4. The Send_Label procedure may fail due to lack of label
            resources, in which case the LSR SHOULD NOT perform the
            Label Use procedure.

A.1.2. Receive Label Mapping

   Summary:

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

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

      -  Transmission of Label Mapping messages for the FEC to one or
         more LDP peers;

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

   Context:

      -  LSR.  The LSR handling the event.

      -  MsgSource.  The LDP peer that sent the message.

      -  FEC.  The FEC specified in the message.

      -  Label.  The label specified in the message.

      -  PrevAdvLabel.  The label for the FEC, if any, previously
         advertised to an upstream peer.  Assuming no label was
         previously advertised, this is the same label as the one in the
         Label Mapping message being processed.

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

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

      -  SAttributes to be included in the Label Mapping message, if
         any, propagated to upstream peers.

   Algorithm:

      LMp.1   Does the received label mapping match an outstanding label
              request for FEC previously sent to MsgSource?  If not,
              goto LMp.3.

      LMp.2   Delete record of outstanding FEC label request.

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

      LMp.4   Does the LSR have a previously received label mapping for
              FEC from MsgSource?  (See Note 1.)
              If not, goto LMp.8.  (See Note 2.)

      LMp.5   Does the label previously received from MsgSource match
              Label (i.e., the label received in the message)?  (See
              Note 3.)
              If not, goto LMp.8.  (See Note 4.)

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

      LMp.7   Remove Label from forwarding/switching use.  (See Note 5.)

      LMp.8   Execute procedure Send_Message (MsgSource, Label Release,
              FEC, Label, Loop Detected Status code).  Goto LMp.33.

      LMp.9   Does LSR have a previously received label mapping for FEC
              from MsgSource for the LSP in question?  (See Note 6.)
              If not, goto LMp.11.

      LMp.10  Does the label previously received from MsgSource match
              Label (i.e., the label received in the message)?  (See
              Note 3.)
              OR
              Is the received label mapping in response to a previously
              outstanding label request sent to MsgSource?  (See Note
              12.)
              If so, goto LMp.11.

      LMp.10a Is LSR operating in Downstream Unsolicited mode?  If so,
              delete the label mapping for the label previously received
              from MsgSource and remove it from forwarding/switching
              use.
              Execute procedure Send_Message (MsgSource, Label Release,
              FEC, label previously received from MsgSource).

      LMp.11  Determine the Next Hop for FEC.

      LMp.12  Is MsgSource the Next Hop for FEC?
              If so, goto LMp.14.

      LMp.13  Perform LSR Label Release procedure:

                    For Conservative Label retention:

                       1. Goto LMp.32.

                    For Liberal Label retention:

                       1. Record label mapping for FEC with Label and
                          RAttributes has been received from MsgSource.
                          Goto LMp.33.

      LMp.14  Is LSR an ingress for FEC?
              If not, goto LMp.16.

      LMp.15  Install Label for forwarding/switching use.

      LMp.16  Record label mapping for FEC with Label and RAttributes
              has been received from MsgSource.

      LMp.17  Iterate through LMp.31 for each Peer.  (See Note 7).

      LMp.18  Has LSR previously sent a label mapping for FEC to Peer
              for the LSP in question?  (See Note 8.)
              If so, goto LMp.22.

      LMp.19  Is the Downstream Unsolicited Ordered Control Label
              Distribution procedure being used by LSR?
              If not, goto LMp.28.

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

      LMp.21  Execute procedure Send_Message (Peer, Label Mapping, FEC,
              PrevAdvLabel, SAttributes).  (See Note 13.)
              Goto LMp.28.

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

      LMp.23  Are RAttributes in the received label mapping consistent
              with those previously sent to Peer?  If so, continue
              iteration from LMp.22 for next label mapping.  (See Note
              9.)

      LMp.24  Execute procedure Prepare_Label_Mapping_Attributes (Peer,
              FEC, RAttributes, SAttributes, IsPropagating,
              StoredHopCount).

      LMp.25  Execute procedure Send_Message (Peer, Label Mapping, FEC,
              PrevAdvLabel, SAttributes).  (See Note 10.)

      LMp.26  Update record of label mapping for FEC previously sent to
              Peer to include the new attributes sent.

      LMp.27  End iteration from LMp.22.

      LMp.28  Does LSR have any label requests for FEC from Peer marked
              as pending?
              If not, goto LMp.30.

      LMp.29  Perform LSR Label Distribution procedure:

            For Downstream Unsolicited Independent Control OR For
            Downstream Unsolicited Ordered Control

               1. Execute procedure
                  Prepare_Label_Mapping_Attributes (Peer, FEC,
                  RAttributes, SAttributes, IsPropagating,
                  UnknownHopCount).

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

               3. If no pending requests exist for Peer, goto LMp.30.
                  (See Note 11.)

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

               1. Iterate through Step 5 for each pending label request
                  for FEC from Peer marked as pending.

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

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

               4. Delete record of pending request.

               5. End iteration from Step 1.

               6. Goto LMp.30.

      LMp.30  Perform LSR Label Use procedure:

            For Use Immediate OR For Use If Loop Not Detected

               1. Iterate through Step 3 for each label mapping for FEC
                  previously sent to Peer.

               2. Install label received and label sent to Peer for
                  forwarding/switching use.

               3. End iteration from Step 1.

               4. Goto LMp.31.

      LMp.31  End iteration from LMp.17.
              Go to LMp.33.

      LMp.32  Execute procedure Send_Message (MsgSource, Label Release,
              FEC, Label).

      LMp.33  DONE.

   Notes:

      1.  If the LSR is merging, there should be at most 1 received
          mapping for the FEC for the LSP in question.  In the non-
          merging case, there could be multiple received mappings for
          the FEC for the LSP in question.

      2.  If the LSR has detected a loop and it has not previously
          received a label mapping from MsgSource for the FEC, it simply
          releases the label.

      3.  Does the Label received in the message match any of the 1 or
          more label mappings identified in the previous step (LMp.4 or
          LMp.9)?

      4.  An unsolicited mapping with a different label from the same
          peer would be an attempt to establish multipath label
          switching, which is not supported in this version of LDP.

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

      6.  If the received label mapping message matched an outstanding
          label request in LMp.1, then (by definition) the LSR has not
          previously received a label mapping for FEC for the LSP in
          question.  If the LSR is merging upstream labels for the LSP
          in question, there should be at most 1 received mapping.  In
          the non-merging case, there could be multiple received label
          mappings for the same FEC, one for each resulting LSP.

      7.  The LMp.17 iteration includes MsgSource in order to handle the
          case where the LSR is operating in Downstream Unsolicited
          Ordered Control mode.  Ordered Control prevents the LSR from
          advertising a label for the FEC until it has received a label
          mapping from its next hop (MsgSource) for the FEC.

      8.  If the LSR is merging the LSP, it may have previously sent
          label mappings for the FEC LSP to one or more peers.  If the

          LSR is not merging, it may have sent a label mapping for the
          LSP in question to at most one LSR.

      9.  The Loop Detection Path Vector attribute is considered in this
          check.  If the received RAttributes include a Path Vector and
          no Path Vector had been previously sent to the Peer, or if the
          received Path Vector is inconsistent with the Path Vector
          previously sent to the Peer, then the attributes are
          considered to be inconsistent.  Note that an LSR is not
          required to store a received Path Vector after it propagates
          the Path Vector in a mapping message.  If an LSR does not
          store the Path Vector, it has no way to check the consistency
          of a newly received Path Vector.  This means that whenever
          such an LSR receives a mapping message carrying a Path Vector
          it must always propagate the Path Vector.

      10. LMp.22 through LMp.27 deal with a situation that can arise
          when the LSR is using independent control and it receives a
          mapping from the downstream peer after it has sent a mapping
          to an upstream peer.  In this situation, the LSR needs to
          propagate any changed attributes, such as Hop Count, upstream.
          If Loop Detection is configured on, the propagated attributes
          must include the Path Vector.

      11. An LSR operating in Downstream Unsolicited mode MUST process
          any Label Request messages it receives.  If there are pending
          label requests, fall through into the Downstream on Demand
          procedures in order to satisfy the pending requests.

      12. As determined by step LMp.1.

      13. An LSR operating in Ordered Control mode may choose to skip at
          this stage the peer from which it received the advertisement
          that caused it to generate the label-map message.  Doing so
          will in effect provide a form of split-horizon.

A.1.3. Receive Label Abort Request

   Summary:

      When an LSR receives a Label Abort Request message from a peer, it
      checks whether it has already responded to the label request in
      question.  If it has, it silently ignores the message.  If it has
      not, it sends the peer a Label Request Aborted Notification.  In
      addition, if it has a label request outstanding for the LSP in
      question to a downstream peer, it sends a Label Abort Request to
      the downstream peer to abort the LSP.

   Context:

      -  LSR.  The LSR handling the event.

      -  MsgSource.  The LDP peer that sent the message.

      -  FEC.  The FEC specified in the message.

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

      -  Next Hop.  The next hop for the FEC.

   Algorithm:

      LAbR.1  Does the message match a previously received Label Request
              message from MsgSource?  (See Note 1.)
              If not, goto LAbR.12.

      LAbR.2  Has LSR responded to the previously received label
              request?
              If so, goto LAbR.12.

      LAbR.3  Execute procedure Send_Message (MsgSource, Notification,
              Label Request Aborted, TLV), where TLV is the Label
              Request Message ID TLV received in the Label Abort Request
              message.

      LAbR.4  Does LSR have a Label Request message outstanding for FEC?
              If so, goto LAbR.7.

      LAbR.5  Does LSR have a label mapping for FEC?  If not, goto
              LAbR.11.

      LAbR.6  Generate Event: Received Label Release message for FEC
              from MsgSource.  (See Note 2.)
              Goto LAbR.11.

      LAbR.7  Is LSR merging the LSP for FEC?
              If not, goto LAbR.9.

      LAbR.8  Are there outstanding label requests for this FEC?
              If so, goto LAbR.11.

      LAbR.9  Execute procedure Send_Message (Next Hop, Label Abort
              Request, FEC, TLV), where TLV is a Label Request message
              ID TLV containing the Message ID used by the LSR in the
              outstanding Label Request message.

      LAbR.10  Record that a label abort request for FEC is pending.

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

      LAbR.12  DONE.

   Notes:

      1. LSR uses FEC and the Label Request message ID TLV carried by
         the label abort request to locate its record (if any) for the
         previously received label request from MsgSource.

      2. If LSR has received a label mapping from NextHop, it should
         behave as if it had advertised a label mapping to MsgSource and
         MsgSource has released it.

A.1.4. Receive Label Release

   Summary:

      When an LSR receives a Label Release message for a FEC from a
      peer, it checks whether other peers hold the released label.  If
      none do, the LSR removes the label from forwarding/switching use,
      if it has not already done so, and if the LSR holds a label
      mapping from the FEC next hop, it releases the label mapping.

   Context:

      -  LSR.  The LSR handling the event.

      -  MsgSource.  The LDP peer that sent the message.

      -  Label.  The label specified in the message.

      -  FEC.  The FEC specified in the message.

   Algorithm:

      LRl.1   Does FEC match a known FEC?  If not, goto LRl.14.

      LRl.2   Remove MsgSource from record of peers that hold Label for
              FEC.  (See Note 1.)

      LRl.3   Does message match an outstanding label withdraw for FEC
              previously sent to MsgSource?
              If not, goto LRl.5

      LRl.4   Delete record of outstanding label withdraw for FEC
              previously sent to MsgSource.

      LRl.5   Is LSR merging labels for this FEC?  If not, goto LRl.7.
              (See Note 2.)

      LRl.6   Does LSR have outstanding label advertisements for this
              FEC?
              If so, goto LRl.11.

      LRl.7   Is LSR egress for the FEC?
              If so, goto LRl.11.

      LRl.8   Is there a Next Hop for FEC? AND Does LSR have a
              previously received label mapping for FEC from Next Hop?
              If not, goto LRl.11.

      LRl.9   Is LSR configured to propagate releases?
              If not, goto LRl.11.  (See Note 3.)

      LRl.10  Execute procedure Send_Message (Next Hop, Label Release,
              FEC, Label from Next Hop).

      LRl.11  Remove Label from forwarding/switching use for traffic
              from MsgSource.

      LRl.12  Do any peers still hold Label for FEC?
              If so, goto LRl.14.

      LRl.13  Free the Label.

      LRl.14  DONE.

   Notes:

      1. If LSR is using Downstream Unsolicited label distribution, it
         SHOULD NOT re-advertise a label mapping for FEC to MsgSource
         until MsgSource requests it.

      2. LRl.5 through LRl.9 deal with determining whether where the LSR
         should propagate the Label Release to a downstream peer
         (LRl.9).

      3. If LRl.9 is reached, no upstream LSR holds a label for the FEC,
         and the LSR holds a label for the FEC from the FEC Next Hop.
         The LSR could propagate the Label Release to the Next Hop.  By
         propagating the Label Release, the LSR releases a potentially
         scarce label resource.  In doing so, it also increases the

         latency for re-establishing the LSP should MsgSource or some
         other upstream LSR send it a new Label Request for FEC.

         Whether or not to propagate the release is not a protocol
         issue.  Label distribution will operate properly whether or not
         the release is propagated.  The decision to propagate or not
         should take into consideration factors such as, whether labels
         are a scarce resource in the operating environment, the
         importance of keeping LSP setup latency low by keeping the
         amount of signaling required small, and whether LSP setup is
         ingress-controlled or egress-controlled in the operating
         environment.

A.1.5. Receive Label Withdraw

   Summary:

      When an LSR receives a Label Withdraw message for a FEC from an
      LDP peer, it responds with a Label Release message and it removes
      the label from any forwarding/switching use.  If Ordered Control
      is in use, the LSR sends a Label Withdraw message to each LDP peer
      to which it had previously sent a label mapping for the FEC.  If
      the LSR is using Downstream on Demand label advertisement with
      independent control, it then acts as if it had just recognized the
      FEC.

   Context:

      -  LSR.  The LSR handling the event.

      -  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 iteration for next Peer at LWd.8.

      LWd.10  Does the label previously sent to Peer "map" to the
              withdrawn Label?
              If not, continue iteration for next Peer at LWd.8.  (See
              Note 3.)

      LWd.11  Execute procedure Send_Label_Withdraw (Peer, FEC, Label
              previously sent to Peer).

      LWd.12  End iteration from LWd.8.

      LWd.13  DONE.

   Notes:

      1. If the Label is not in forwarding/switching use, LWd.1 has no
         effect.

      2. LWd.7 handles the case where the LSR is using Downstream On
         Demand label distribution with independent control.  In this
         situation, the LSR should send a label request to the FEC next
         hop as if it had just recognized the FEC.

      3. LWd.10 handles both label merging (one or more incoming labels
         map to the same outgoing label) and no label merging (one label
         maps to the outgoing label) cases.

A.1.6. Recognize New FEC

   Summary:

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

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

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

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

   Context:

      -  LSR.  The LSR handling the event.

      -  FEC. The newly recognized FEC.

      -  Next Hop.  The next hop for the FEC.

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

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

      -  StoredHopCount.  Hop count associated with FEC label mapping,
         if any, previously received from Next Hop.

   Algorithm:

      FEC.1   Perform LSR Label Distribution procedure:

            For Downstream Unsolicited Independent Control

               1. Iterate through 5 for each Peer.

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

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

               4. Execute procedure Send_Label (Peer, FEC, SAttributes).

               5. End iteration from 1.
                  Goto FEC.2.

            For Downstream Unsolicited Ordered Control

               1. Iterate through 5 for each Peer.

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

               3. Execute procedure Prepare_Label_Mapping_Attributes
                  (Peer, FEC, InitAttributes, SAttributes, Propagating,
                  StoredHopCount).

               4. Execute procedure Send_Label (Peer, FEC, SAttributes).

               5. End iteration from 1.
                  Goto FEC.2.

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

               1. Goto FEC.2.  (See Note 2.)

      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 one that specifies desired LSP characteristics, such as
         Class of Service (CoS).  (Note that while the current version
         of LDP does not specify a CoS attribute, LDP extensions may.)

         The means by which FEC InitAttributes, if any, are specified is
         beyond the scope of LDP.  Note that the InitAttributes will not
         include a known Hop Count or a Path Vector.

      2. An LSR using Downstream On Demand label distribution would send
         a label only if it had a previously received label request
         marked as pending.  The LSR would have no such pending requests
         because it responds to any label request for an unknown FEC by
         sending the requesting LSR a No Route notification and
         discarding the label request; see LRq.3

      3. If the LSR has a label for the FEC from the Next Hop, it should
         behave as if it had just received the label from the Next Hop.
         This occurs in the case of Liberal Label retention mode.

A.1.7. Detect Change in FEC Next Hop

   Summary:

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

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

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

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

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

   Context:

      -  LSR.  The LSR handling the event.

      -  FEC.  The FEC whose next hop changed.

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

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

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

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

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

   Algorithm:

      NH.1   Has LSR previously received and retained a label mapping
             for FEC from Old Next Hop?  If not, goto NH.6.

      NH.2   Remove label from forwarding/switching use.  (See Note 1.)

      NH.3   Is LSR using Liberal Label retention?
             If so, goto NH.6.

      NH.4   Execute procedure Send_Message (Old Next Hop, Label
             Release, OldLabel).

      NH.5   Delete label mapping for FEC previously received from Old
             Next Hop.

      NH.6   Does LSR have a label request pending with Old Next Hop?
             If not, goto NH.10.

      NH.7   Is LSR using Conservative Label retention?
             If not, goto NH.10.

      NH.8   Execute procedure Send_Message (Old Next Hop, Label Abort
             Request, FEC, TLV), where TLV is a Label Request Message ID
             TLV that carries the message ID of the pending label
             request.

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

      NH.10  Is there a New Next Hop for FEC?
             If not, goto NH.16.

      NH.11  Has LSR previously received and retained a label mapping
             for FEC from New Next Hop?
             If not, goto NH.13.

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

      NH.13  Is LSR using Downstream on Demand advertisement? OR Is Next
             Hop using Downstream on Demand advertisement? OR Is LSR
             using Conservative Label retention? (See Note 3.)
             If so, goto NH.14.
             If not, goto NH.20.

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

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

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

      NH.17  Has LSR previously sent a label mapping for FEC to Peer?
             If not, continue iteration for next Peer at NH.16.

      NH.18  Execute procedure Send_Label_Withdraw (Peer, FEC, Label
             previously sent to Peer).

      NH.19  End iteration from NH.16.

      NH.20  DONE.

   Notes:

      1. If the Label is not in forwarding/switching use, NH.2 has no
         effect.

      2. If the LSR has a label for the FEC from the New Next Hop, it
         should behave as if it had just received the label from the New
         Next Hop.

      3. The purpose of the check on label retention mode is to avoid a
         race with steps LMp.12-LMp.13 of the procedure for handling a
         Label Mapping message where the LSR operating in Conservative
         Label retention mode may have released a label mapping received
         from the New Next Hop before it detected that 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.13 hold.
         Therefore, it executes the Send_Label_Request procedure
         directly rather than perform the LSR Label Request procedure.

A.1.8. Receive Notification / Label Request Aborted

   Summary:

      When an LSR receives a Label Request Aborted notification from an
      LDP peer, it records that the corresponding label request
      transaction, if any, has completed.

   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.

   Note:

      1. The LSR uses the FEC and RequestMessageID to locate its record,
         if any, of the outstanding label request abort.

A.1.9. Receive Notification / No Label Resources

   Summary:

      When an LSR receives a No Label Resources notification from an LDP
      peer, it stops sending label request messages to the peer until it
      receives a Label Resources Available Notification from the peer.

   Context:

      -  LSR.  The LSR handling the event.

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

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

   Algorithm:

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

      NoRes.2 Record that label mapping for FEC from MsgSource is needed
              but that no label resources are available.

      NoRes.3 Set status record indicating it is not OK to send label
              requests to MsgSource.

      NoRes.4 DONE.

A.1.10. Receive Notification / No Route

   Summary:

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

   Context:

      -  LSR.  The LSR handling the event.

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

      -  Attributes.  The attributes associated with the label request.

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

   Algorithm:

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

      NoNH.2  Perform LSR Label No Route procedure.

            For Request No Retry

               1. Goto NoNH.3.

            For Request Retry

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

               2. Start timeout.  Goto NoNH.3.

      NoNH.3  DONE.

A.1.11. Receive Notification / Loop Detected

   Summary:

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

   Context:

      See "Receive Notification / No Route".

   Algorithm:

      See "Receive Notification / No Route".

   Note:

      1. When the Loop Detected notification is in response to a Label
         Request message, it arrives in a Status Code TLV in a
         Notification message.  When it is in response to a Label
         Mapping message, it arrives in a Status Code TLV in a Label
         Release message.

A.1.12. Receive Notification / Label Resources Available

   Summary:

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

   Context:

      -  LSR.  The LSR handling the event.

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

      -  SAttributes.  Attributes stored with the postponed Label
         Request message.

   Algorithm:

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

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

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

      Res.4   Execute procedure Send_Label_Request (MsgSource, FEC,
              SAttributes).  If the procedure fails, terminate
              iteration.

      Res.5   Delete record that no resources are available for a label
              mapping for FEC needed from MsgSource.

      Res.6   End iteration from Res.2.

      Res.7   DONE.

A.1.13. Detect Local Label Resources Have Become Available

   Summary:

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

   Context:

      -  LSR.  The LSR handling the event.

      -  Attributes.  Attributes stored with the postponed Label Mapping
         message.

   Algorithm:

      ResA.1  Iterate through ResA.4 for each Peer to which LSR has
              previously sent a No Label Resources notification.

      ResA.2  Execute procedure Send_Notification (Peer, Label Resources
              Available).

      ResA.3  Delete record that No Label Resources notification was
              previously sent to Peer.

      ResA.4  End iteration from ResA.1.

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

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

      ResA.7  Clear record of FEC label mapping needed for peer but no-
              label-resources.

      ResA.8  End iteration from ResA.5

      ResA.9  DONE.

   Note:

      1. Iteration ResA.5 through ResA.8 handles the situation where the
         LSR is using Downstream Unsolicited label distribution and was
         previously unable to allocate a label for a FEC.

A.1.14. LSR Decides to No Longer Label Switch a FEC

   Summary:

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

   Context:

      -  Peer.  The peer.

      -  FEC.  The FEC.

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

   Algorithm:

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

      NoLS.2  DONE.

   Note:

      1. The LSR may remove the label from forwarding/switching use as
         part of this event or as part of processing the Label Release
         from the peer in response to the label withdraw.  If the LSR
         doesn't wait for the Label Release message from the peer, it
         SHOULD NOT reuse the label until it receives the Label Release.

A.1.15. Timeout of Deferred Label Request

   Summary:

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

   Context:

      -  LSR.  The LSR handling the event.

      -  FEC.  The FEC associated with the timeout event.

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

      -  Attributes.  Attributes stored with the deferred Label Request
         message.

   Algorithm:

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

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

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

      TO.4    DONE.

A.2. Common Label Distribution Procedures

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

A.2.1. Send_Label

   Summary:

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

   Parameters:

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

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

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

   Additional Context:

      -  LSR.  The LSR executing the procedure.

      -  Label.  The label allocated and sent to Peer.

   Algorithm:

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

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

      SL.3   Install Label for forwarding/switching use.

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

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

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

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

      SL.8   Return success.

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

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

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

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

      SL.13  Record label mapping needed for FEC and Attributes for
             Peer, but no-label-resources.  (See Note 1.)

      SL.14  Return failure.

   Note:

      1. SL.13 handles the case of Downstream Unsolicited label
         distribution when the LSR is unable to allocate a label for a
         FEC to send to a Peer.

A.2.2. Send_Label_Request

   Summary:

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

   Parameters:

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

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

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

   Additional Context:

      -  LSR.  The LSR executing the procedure.

   Algorithm:

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

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

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

      SLRq.4  Record that label request for FEC has been sent to Peer
              and mark it as outstanding.

      SLRq.5  Return success.

      SLRq.6  Postpone the label request by recording that label mapping
              for FEC and Attributes from Peer is needed but that no
              label resources are available.

      SLRq.7  Return failure.

   Note:

      1. If the LSR is a non-merging LSR, it must distinguish between
         attempts to send label requests for a FEC triggered by
         different upstream LDP peers from duplicate requests.  This
         procedure will not send a duplicate label request.

A.2.3. Send_Label_Withdraw

   Summary:

      An LSR uses the Send_Label_Withdraw procedure to withdraw a label
      for a FEC from an LDP peer.  To do this, the LSR sends a Label
      Withdraw message to the peer.

   Parameters:

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

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

      -  Label.  The label being withdrawn.

   Additional Context:

      -  LSR.  The LSR executing the procedure.

   Algorithm:

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

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

A.2.4. Send_Notification

   Summary:

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

   Parameters:

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

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

   Additional Context:

      None.

Algorithm:

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

A.2.5. Send_Message

   Summary:

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

   Parameters:

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

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

      -  Additional message contents . . .  .

   Additional Context:

      None.

   Algorithm:

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

A.2.6. Check_Received_Attributes

   Summary:

      Check the attributes received in a Label Mapping or Label Request
      message.  If the attributes include a Hop Count or Path Vector,
      perform a Loop Detection check.  If a loop is detected, cause a
      Loop Detected Notification message to be sent to MsgSource.

   Parameters:

      -  MsgSource.  The LDP peer that sent the message.

      -  MsgType.  The type of message received.

      -  RAttributes.  The attributes in the message.

   Additional Context:

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

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

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

   Algorithm:

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

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

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

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

      CRa.5   Return No Loop Detected.

      CRa.6   Is MsgType LabelMapping?
              If so, goto CRa.8.  (See Note 1.)

      CRa.7   Execute procedure Send_Notification (MsgSource, Loop
              Detected).

      CRa.8   Return Loop Detected.

      CRa.9   DONE.

   Note:

      1. When the attributes being checked were received in a Label
         Mapping message, the LSR sends the Loop Detected notification
         in a Status Code TLV in a Label Release message.  (See Section
         "Receive Label Mapping".)

A.2.7. Prepare_Label_Request_Attributes

   Summary:

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

   Parameters:

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

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

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

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

   Additional Context:

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

   Algorithm:

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

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

      PRqA.3  Include Hop Count of 1 in SAttributes.

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

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

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

      PRqA.7  Increment RAttributes Hop Count and copy the resulting Hop
              Count to SAttributes.  (See Note 2.)
              Goto PRqA.9.

      PRqA.8  Include Hop Count of unknown (0) in SAttributes.

      PRqA.9  Is Loop Detection configured on LSR?
              If not, goto PRqA.14.

      PRqA.10 Do RAttributes have a Path Vector?
              If so, goto PRqA.12.

      PRqA.11 Is LSR merge-capable?
              If so, goto PRqA.14.
              If not, goto PRqA.13.

      PRqA.12 Add LSR Id to beginning of Path Vector from RAttributes
              and copy the resulting Path Vector into SAttributes.
              Goto PRqA.14.

      PRqA.13 Include Path Vector of length 1 containing LSR Id in
              SAttributes.

      PRqA.14 DONE.

   Notes:

      1. The link with Peer may require that Hop Count be included in
         Label Request messages; for example, see [RFC3035] and
         [RFC3034].

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

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
         Mapping message.

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

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

   Additional Context:

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

   Algorithm:

      PMpA.1  Do the RAttributes include any unknown TLVs?
              If not, goto PMpA.4.

      PMpA.2  Do the settings of the U- and F-bits require forwarding of
              these TLVs?
              If not, goto PMpA.4.

      PMpA.3  Copy the unknown TLVs in SAttributes.

      PMpA.4  Is Hop Count required for this Peer?  (see Note 1.) OR Do
              RAttributes include a Hop Count? OR Is Loop Detection
              configured on LSR?
              If not, goto PMpA.24.

      PMpA.5  Is LSR egress for FEC?
              If not, goto PMpA.7.

      PMpA.6  Include Hop Count of 1 in SAttributes.  Goto PMpA.24.

      PMpA.7  Do RAttributes have a Hop Count?
              If not, goto PMpA.11.

      PMpA.8  Is LSR a member of the edge set for an LSR domain whose
              LSRs do not perform TTL decrement AND Is Peer in that
              domain?  (See Note 2.)  If not, goto PMpA.10.

      PMpA.9  Include Hop Count of 1 in SAttributes.  Goto PMpA.12.

      PMpA.10 Increment RAttributes Hop Count and copy the resulting Hop
              Count to SAttributes.  (See Note 2.)  Goto PMpA.12.

      PMpA.11 Include Hop Count of unknown (0) in SAttributes.

      PMpA.12 Is Loop Detection configured on LSR?
              If not, goto PMpA.24.

      PMpA.13 Do RAttributes have a Path Vector?
              If so, goto PMpA.22.

      PMpA.14 Is LSR propagating a received Label Mapping?
              If not, goto PMpA.23.

      PMpA.15 Does LSR support merging?
              If not, goto PMpA.17.

      PMpA.16 Has LSR previously sent a Label Mapping for FEC to Peer?
              If not, goto PMpA.23.

      PMpA.17 Do RAttributes include a Hop Count?
              If not, goto PMpA.24.

      PMpA.18 Is Hop Count in RAttributes unknown(0)?
              If so, goto PMpA.23.

      PMpA.19 Has LSR previously sent a Label Mapping for FEC to Peer?
              If not, goto PMpA.24.

      PMpA.20 Is Hop Count in RAttributes different from PrevHopCount?
              If not, goto PMpA.24.

      PMpA.21 Is the Hop Count in RAttributes > PrevHopCount? OR Is
              PrevHopCount unknown(0)?
              If not, goto PMpA.24.

      PMpA.22 Add LSR Id to beginning of Path Vector from RAttributes
              and copy the resulting Path Vector into SAttributes.
              Goto PMpA.24.

      PMpA.23 Include Path Vector of length 1 containing LSR Id in
              SAttributes.

      PMpA.24 DONE.

   Notes:

      1. The link with Peer may require that Hop Count be included in
         Label Mapping messages; for example, see [RFC3035] and
         [RFC3034].

      2. If the LSR is at the edge of a cloud of LSRs that do not
         perform TTL-decrement and it is propagating the Label Mapping
         message upstream into the cloud, it sets the Hop Count to 1 so
         that Hop Count across the cloud is calculated properly.  This
         ensures proper TTL management for packets forwarded across the
         part of the LSP that passes through the cloud.

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

Editors' Addresses

   Loa Andersson
   Acreo AB
   Isafjordsgatan 22
   Kista, Sweden
   EMail: loa.andersson@acreo.se
          loa@pi.se

   Ina Minei
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA 94089
   EMail: ina@juniper.net

   Bob Thomas
   Cisco Systems, Inc.
   1414 Massachusetts Ave
   Boxborough, MA 01719
   EMail: rhthomas@cisco.com

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