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Network Working Group                                      Loa Andersson
Internet Draft                                         Bay                                      Nortel Networks Inc.
Expiration Date: February May 1999
                                                             Paul Doolan
                                                       Ennovate Networks

                                                           Nancy Feldman
                                                                IBM Corp

                                                          Andre Fredette
                                                       Bay
                                                    Nortel Networks Inc.

                                                              Bob Thomas
                                                     Cisco Systems, Inc.

                                                             August

                                                           November 1998


                           LDP Specification


                       draft-ietf-mpls-ldp-01.txt


                       draft-ietf-mpls-ldp-02.txt

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

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

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ftp.ietf.org ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).


Abstract

   An overview of Multi Protocol Label Switching (MPLS) is provided in
   [FRAMEWORK] and a proposed architecture in [ARCH].  A fundamental
   concept in MPLS is that two Label Switching Routers (LSRs) must agree
   on the meaning of the labels used to forward traffic between and



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   through them.  This common understanding is achieved by using the
   Label Distribution Protocol (LDP) referenced in [FRAMEWORK] and [ARCH].  This
   document defines the LDP protocol.



Open Issues

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



Changes from Previous Draft

     - The loop prevention/detection mechanism to be employed by LDP. This spec has retained draft removes the explicit path vector setup mechanism from previous
       drafts.  However, draft-ohba-mpls-loop-prevention-01.txt has been
       proposed as an alternative. the
       spec.

     - Support for explicitly routed LSPs.  The need for this feature
       has been debated at length. This spec refines the previous
       version of draft removes loop prevention from the spec in this area.  However, there remains some
       belief in spec.  The MPLS
       working group will continue to evaluate and compare the WG two
       leading contenders for loop prevention:  loop prevention via path
       vectors and draft-ohba-mpls-loop-prevention-01.txt.  We expect
       that explicitly routed LSPs should one of these methods will be supported
       by enhancements to RSVP selected and not LDP.

       The support for explicitly routed LSPs in the spec is independent added to a later
       version of other LDP features LDP.

     - This draft retains and could, should refines the WG decide to do so,
       be removed without impact path vector mechanism for
       optional loop detection.  In addition, it introduces an upper
       limit on other LDP features. the size of path vectors.

     - Traffic engineering considerations beyond support This draft specifies parameters for explicit
       routing.

     - The need the exponential backup used
       to throttle session setup retry attempts.  It also specifies a
       mechanism for all of resetting the FEC types (called FEC elements backoff parameters in this
       version of response to LSR
       configuration changes by adding an optional parameter to the spec, SMDs in previous versions) is being debated.
       Hello message.

     - This version of the spec defines fewer FEC types than previous
       versions. draft adds Appendix "LDP Label Distribution Procedures".

     - LDP support This draft adds rules for multicast is not defined resolving differences in this version.
       Multicast support will be addressed the Label
       Distribution Discipline and Merge session parameters exchanged in a future version.
       the Initialization message.

     - The This draft modifies message and TLV encodings are slightly by adding
       explicit specification of LSR behavior when an LSR does not
       recognize the message or TLV.

     - This draft modifies the encodings for the Initialization and
       Hello messages to group parameters likely to change in be used together and
       to reduce message sizes.  It defines some minor
       ways in the next new TLVs for use with
       these messages and eliminates some previously defined TLVs.

     - This draft of specifies a procedure for negotiating the spec. maximum PDU
       length to be used for a session.






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Table of Contents

    1          LDP Overview  .......................................   6
    1.1        LDP Peers  ..........................................   6
    1.2        LDP


     - This draft simplifies the encodings for the Label Mapping, Label
       Request, Label Withdraw and Label Release messages by eliminating
       the FEC-Label Mapping, FEC-Request, and FEC-Withdraw-Release
       TLVs.

     - This draft modifies the CoS TLV by specifying that its detailed
       definition is a subject for further study.

     - This draft adds a Return Message Exchange  ...............................   6
    1.3        LDP Error Handling  .................................   7
    1.4        LDP Extensibility Id optional parameter to the
       Label Request message and Future Compatibility  .........   8
    2          LDP Operation  ......................................   8
    2.1        FEC Types  ..........................................   8
    2.2 a Label Request Message Id parameter to
       the Label Mapping packets message to FECs   ...........................   9
    2.3 enable an LSR to match received
       Label Spaces, Identifiers, Sessions Mapping messages with outstanding Label Request messages.

     - This draft refines support for vendor-private protocol extensions
       and Transport  ..  10
    2.4 specifies support for experimental protocol extensions.

     - This draft specifies optional use of the TCP MD5 Signature Option
       to protect against the introduction of spoofed TCP segments into
       LDP Sessions between non-Directly Connected LSRs  ...  11
    2.5 session connection streams.


Open Issues

   The following LDP Discovery   .....................................  12
    2.5.1      Basic Discovery Mechanism  ..........................  12
    2.5.2      Extended Discovery Mechanism  ....................... issues are left unresolved with this version of the
   spec:

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

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

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
















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Table of Contents

    1          LDP Overview  .......................................   7
    1.1        LDP Peers  ..........................................   7
    1.2        LDP Message Exchange  ...............................   7
    1.3        LDP Message Structure  ..............................   8
    1.4        LDP Error Handling  .................................   8
    1.5        LDP Extensibility and Future Compatibility  .........   9
    2          LDP Operation  ......................................   9
    2.1        FECs  ...............................................   9
    2.2        Label Spaces, Identifiers, Sessions and Transport  ..  10
    2.2.1      Label Spaces  .......................................  10
    2.2.2      LDP Identifiers  ....................................  11
    2.2.3      LDP Sessions  .......................................  11
    2.2.4      LDP Transport  ......................................  11
    2.3        LDP Sessions between non-Directly Connected LSRs  ...  12
    2.6
    2.4        LDP Discovery   .....................................  12
    2.4.1      Basic Discovery Mechanism  ..........................  12
    2.4.2      Extended Discovery Mechanism  .......................  13
    2.5        Establishing and Maintaining LDP Sessions  ..........  13
    2.6.1  14
    2.5.1      LDP Session Establishment  ..........................  13
    2.6.2  14
    2.5.2      Transport Connection Establishment  .................  13
    2.6.3  14
    2.5.3      Session Initialization  .............................  14
    2.6.4  15
    2.5.4      Initialization State Machine  .......................  16
    2.6.5  17
    2.5.5      Maintaining Hello Adjacencies  ......................  19
    2.6.6  20
    2.5.6      Maintaining LDP Sessions  ...........................  19
    2.7  20
    2.6        Label Distribution and Management  ..................  20
    2.7.1  21
    2.6.1      Label Distribution Control Mode  ....................  20
    2.7.2  21
    2.6.1.1    Independent Label Distribution Control  .............  21
    2.6.1.2    Ordered Label Distribution Control  .................  21
    2.6.2      Label Retention Mode  ...............................  21
    2.7.3  22
    2.6.2.1    Conservative Label Retention Mode  ..................  22
    2.6.2.2    Liberal Label Retention Mode  .......................  22
    2.6.3      Label Advertisement Mode  ...........................  22
    2.8  23
    2.7        LDP Identifiers and Next Hop Addresses  .............  22
    2.9  23
    2.8        Loop Detection  .....................................  22
    2.10       Loop Prevention via Diffusion  ......................  23
    2.11       Explicitly Routing LSPs  ............................  24
    2.12       ERLSP State Machine  ................................  28
    2.12.1     Loose Segment Peg LSR Transitions:  .................  29
    2.12.2     Loose Segment Non-Peg LSR Transitions:  .............  33
    2.12.2.1   Strict Segment Transitions  .........................  35
    2.12.3     ERLSP Timeouts  .....................................  35
    2.12.4     ERLSP Error Codes  ..................................  35
    2.8.1      Label Request Message  ..............................  24
    2.8.2      Label Mapping Message  ..............................  26
    2.8.3      Discussion  .........................................  27
    3          Protocol Specification  .............................  36  28
    3.1        LDP PDUs  ...........................................  36  28
    3.2        LDP Procedures  .....................................  29
    3.3        Type-Length-Value Encoding  .........................  37
    3.3  30
    3.4        TLV Encodings for Commonly Used TLVs  .................................  38
    3.3.1 Parameters  .........  31
    3.4.1      FEC TLV  ............................................  38
    3.3.1.1  31
    3.4.1.1    FEC Procedures  .....................................  41
    3.3.2  34



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    3.4.2      Label TLVs  .........................................  41
    3.3.2.1  34
    3.4.2.1    Generic Label TLV  ..................................  42
    3.3.2.2  34
    3.4.2.2    ATM Label TLV  ......................................  42
    3.3.2.3  34
    3.4.2.3    Frame Relay Label TLV  ..............................  43



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    3.3.3  35
    3.4.3      Address List TLV  ...................................  43
    3.3.4  36
    3.4.4      COS TLV  ............................................  44
    3.3.5  37
    3.4.5      Hop Count TLV  ......................................  45
    3.3.5.1  37
    3.4.5.1    Hop Count Procedures  ...............................  45
    3.3.6  38
    3.4.6      Path Vector TLV  ....................................  46
    3.3.6.1  38
    3.4.6.1    Path Vector Procedures  .............................  46
    3.3.7  39
    3.4.6.1.1  Label Request Path Vector  ..........................  39
    3.4.6.1.2  Label Mapping Path Vector  ..........................  40
    3.4.7      Status TLV  .........................................  47
    3.4  40
    3.5        LDP Messages  .......................................  48
    3.4.1  42
    3.5.1      Notification Message  ...............................  50
    3.4.1.1  44
    3.5.1.1    Notification Message Procedures  ....................  51
    3.4.1.2  45
    3.5.1.2    Events Signalled Signaled by Notification Messages  ..........  51
    3.4.1.2.1  ...........  45
    3.5.1.2.1  Malformed PDU or Message  ...........................  52
    3.4.1.2.2  46
    3.5.1.2.2  Unknown or Malformed TLV  ...........................  52
    3.4.1.2.3  46
    3.5.1.2.3  Session Hold Timer Expiration  ......................  53
    3.4.1.2.4  47
    3.5.1.2.4  Unilateral Session Shutdown  ........................  53
    3.4.1.2.5  47
    3.5.1.2.5  Initialization Message Events  ......................  53
    3.4.1.2.6  47
    3.5.1.2.6  Events Resulting From Other Messages  ...............  54
    3.4.1.2.7  Explicitly Routed LSP Setup Events  .................  54
    3.4.1.2.8  47
    3.5.1.2.7  Miscellaneous Events  ...............................  54
    3.4.2  48
    3.5.2      Hello Message  ......................................  54
    3.4.2.1  48
    3.5.2.1    Hello Message Procedures  ...........................  55
    3.4.3  50
    3.5.3      Initialization Message  .............................  57
    3.4.3.1  51
    3.5.3.1    Initialization Message Procedures  ..................  61
    3.4.4  58
    3.5.4      KeepAlive Message  ..................................  61
    3.4.4.1  59
    3.5.4.1    KeepAlive Message Procedures  .......................  62
    3.4.5  59
    3.5.5      Address Message  ....................................  62
    3.4.5.1  59
    3.5.5.1    Address Message Procedures  .........................  63
    3.4.6  60
    3.5.6      Address Withdraw Message  ...........................  64
    3.4.6.1  61
    3.5.6.1    Address Withdraw Message Procedures  ................  64
    3.4.7  61
    3.5.7      Label Mapping Message  ..............................  64
    3.4.7.1  61
    3.5.7.1    Label Mapping Message Procedures  ...................  66
    3.4.7.1.1  63
    3.5.7.1.1  Independent Control Mapping  ........................  66
    3.4.7.1.2  63
    3.5.7.1.2  Ordered Control Mapping  ............................  67
    3.4.7.1.3  64
    3.5.7.1.3  Downstream-on-Demand Label Advertisement  ...........  67
    3.4.7.1.4  64
    3.5.7.1.4  Downstream Allocation Unsolicited Label Advertisement  ..........  68
    3.4.8  .........  65
    3.5.8      Label Request Message  ..............................  68
    3.4.8.1  65
    3.5.8.1    Label Request Message Procedures  ...................  69
    3.4.9  66
    3.5.9      Label Withdraw Message  .............................  70
    3.4.9.1  67
    3.5.9.1    Label Withdraw Message Procedures  ..................  71
    3.4.10  68
    3.5.10     Label Release Message  ..............................  72
    3.4.10.1  69
    3.5.10.1   Label Release Message Procedures  ...................  73
    3.4.11     Label Query Message  ................................  73
    3.4.11.1   Label Query Message Procecures  .....................  74
    3.4.12     Explicit Route Request Message  .....................  74
    3.4.12.1   Explicit Route Request Procedures  ..................  78
    3.4.13     Explicit Route Response Message  ....................  78
    3.4.13.1   Explicit Route Response Procedures  .................  79
    3.5  70
    3.6        Messages and TLVs for Extensibility  ................  80  71
    3.6.1      LDP Vendor-private Extensions  ......................  71



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    3.5.1      Procedures for Unknown Messages and TLVs  ...........  80
    3.5.1.1    Unknown Message Types  ..............................  80
    3.5.1.2    Unknown TLV in Known Message Type  ..................  80
    3.5.2      LDP Vendor-Private Extensions  ......................  81
    3.5.2.1


    3.6.1.1    LDP Vendor-Private TLV  .............................  81
    3.5.2.2 Vendor-private TLVs  ............................  71
    3.6.1.2    LDP Vendor-Private Vendor-private Messages  ........................  82
    3.6  72
    3.6.2      LDP Experimental Extensions  ........................  74
    3.7        Message Summary  ....................................  74
    3.8        TLV Summary  ........................................  83
    3.7  75
    3.9        Status Code Summary  ................................  84  76
    3.10       UDP and TCP Ports  ..................................  76
    4          Security  ...........................................  84  77
    4.1        The TCP MD5 Signature Option  .......................  77
    4.2        LDP Use of the TCP MD5 Signature Option  ............  78
    5          Acknowledgments  ....................................  84          Intellectual Property Considerations  ...............  79
    6          Acknowledgments  ....................................  79
    7          References  .........................................  84
    7  79
    8          Author Information  .................................  80

    Appendix.A LDP Label Distribution Procedures  ..................  82
    A.1        Handling Label Distribution Events  .................  84
    A.1.1      Receive Label Request  ..............................  85
    A.1.2      Receive Label Mapping  ..............................  88
    A.1.3      Receive Label Release  ..............................  92
    A.1.4      Receive Label Withdraw  .............................  94
    A.1.5      Recognize New FEC  ..................................  95
    A.1.6      Detect change in FEC next hop  ......................  98
    A.1.7      Receive Notification / No Label Resources  .......... 100
    A.1.8      Receive Notification / No Route  .................... 101
    A.1.9      Receive Notification / Loop Detected  ............... 102
    A.1.10     Receive Notification / Label Resources Available  ... 102
    A.1.11     Detect local label resources have become available  . 103
    A.1.12     LSR decides to no longer label switch a FEC  ........ 104
    A.1.13     Timeout of deferred label request  .................. 104
    A.2        Common Label Distribution Procedures  ............... 105
    A.2.1      Send_Label  ......................................... 105
    A.2.2      Send_Label_Request  ................................. 107
    A.2.3      Send_Label_Withdraw  ................................ 108
    A.2.4      Send_Notification  .................................. 108
    A.2.5      Send_Message  ....................................... 109
    A.2.6      Check_Received_Attributes  .......................... 109
    A.2.7      Prepare_Label_Request_Attributes  ................... 110
    A.2.8      Prepare_Label_Mapping_Attributes  ................... 112












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1. LDP Overview

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

   Note that this document is written with respect to unicast routing
   only. Multicast will be addressed in a future revision.

   Note that this document is written with respect to control-driven
   traffic.  It describes mappings which are initiated for routes in the
   forwarding table, regardless of traffic over those routes.  However,
   LDP does not preclude data-driven support.


1.1. LDP Peers

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


1.2. LDP Message Exchange

   There are four categories of LDP messages:

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

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

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





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

   Discovery messages provide a mechanism whereby LSRs continually indicate their
   presence in a network via by sending the Hello message. message periodically.
   This is transmitted as a UDP packet to the LDP port at the `all LSR



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   routers' group multicast address.  When an LSR chooses to establish a
   session with an another LSR learned via the hello 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
   mappings (although there are circumstances when this second
   requirement could be relaxed).
   messages.  To satisfy these requirements LDP uses the TCP transport
   for adjacency, session, advertisement and notification
   messages. 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 LSR 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 with a peer, session,
         it terminates the peer LDP session by closing the TCP transport
         connection for the session and discarding all label mappings
         learned via the session.

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









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1.4.


1.5. LDP Extensibility and Future Compatibility

   It is likely that functionality will

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



2. LDP Operation

2.1. FEC Types FECs

   It is necessary to precisely define specify which IP packets may be mapped
   to each LSP.  This is done by providing a FEC specification for each
   LSP.  The FEC defines which identifies the set of IP packets which may be mapped to the same LSP, using
   a unique label.

   LDP supports LSP granularity ranging from end-to-end flows to the
   aggregation of all traffic through a common egress node; the choice
   of granularity is determined by the FEC choice.
   that LSP.

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

   Following are the currently defined types of FEC elements.  New
   element types may be added as needed:

      1. IP Address Prefix.  This element provides a list of one or more IP address
         prefixes.  Any is an IP packet whose destination address matches one
         or more prefix of the specified prefixes may be forwarded using the
         associated LSP. any
         length from 0 to 32 bits, inclusive.

      2. Router ID Host Address.  This element provides is a Router ID (ie, 32-bit IP address.

   We say that a 32 bit particular IP address of "matches" a
         router). Any particular IP address
   prefix if and only if that address begins with that prefix.  We also
   say that a particular packet for matches a particular LSP if and only if
   that LSP has an IP Address Prefix FEC element which matches the path to the
   packet's IP destination is
         known address.  With respect to traverse the specified router may be forwarded using
         the associated LSP. This a particular packet
   and a particular LSP, we refer to any IP Address Prefix FEC element allows
   which matches the full set of
         destinations reachable via packet as the "matching prefix".

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




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         in


     - If there is exactly one LSP which has a single Host Address FEC element.

      3. Flow

         This element specifies a set of datagram information, such as
         port, dest-addr, src-addr, etc.  This element provides LDP with
         the ability
       that is identical to support MPLS flows with no aggregation.

   Where a the packet's IP destination address, then
       the packet maps is mapped to more than one that LSP.

     - If there multiple LSPs, each containing a Host Address FEC it
       element that is transmitted on identical to the LSP
   associated with packet's IP destination address,
       then the FEC packet is mapped to which one of those LSPs.  The procedure
       for selecting one of those LSPs is beyond the scope of this
       document.

     - If a packet has matches exactly one LSP, the 'most specific'
   match.


2.2. Mapping packets packet is mapped to FECs

   FEC objects (TLVs) are transmitted in the LDP messages that deal with
   (advertise, request, release ad withdraw) FEC-Label mappings.

   A stream of packets with a given destination network can be
   characterized by
       LSP.

     - If a single Address Prefix FEC Element.  This results
   in each specified address prefix sustaining its own packet matches multiple LSPs, it is mapped to the LSP tree. This
   singular mapping whose
       matching prefix is the longest.  If there is recommended in environments where little or no
   aggregation information one LSP whose
       matching prefix is provided by the routing protocols (such as
   within a simple IGP), or in networks where longest, the number packet is mapped to one of destination
   prefixes those
       LSPs.  The procedure for selecting one of those LSPs is limited.

   In environments where additional aggregation not provided by beyond
       the
   routing protocols scope of this document.

     - If it is desired, an aggregation list may be created.  In
   this, all prefixes known that are to share a common packet must traverse a particular egress point may be
   advertised within the same FEC.  This type of aggregation
       router, and there is
   configured.

   The router ID FEC type may be used in any environment in an LSP which the
   routing protocols allow routers to determine the egress point for
   specific has an IP packets. For example, the router ID Address Prefix FEC type may be used
   in combination with BGP, OSPF, and/or IS-IS.

   For example, the mapping between IP packets and the router ID may be
   provided via the BGP NEXT_HOP attribute.   When a BGP border LSR
   injects routes into the BGP mesh, it may use its own IP address or
   the
       element (of length 32 bits) which is an address of its external BGP peer as the value of the NEXT_HOP
   attribute.  If the BGP border ISR uses its own IP address as the
   NEXT_HOP attribute, that router,
       then one LSP is created which terminates at the
   BGP border, and the border LSR will forward traffic at layer-3
   towards its external BGP neighbors.  If the BGP border LSR uses the
   external BGP peer as the NEXT_HOP attribute, then a separate LSP may
   be created packet is mapped to that LSP.  The procedure for each external BGP neighbor, thereby allowing
       obtaining this knowledge is beyond the
   border LSR to switch traffic directly to each scope of its external BGP



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   neighbors.

   Similarly, the mapping between IP packet this document.


2.2. Label Spaces, Identifiers, Sessions and router ID may be
   provided by OSPF.  This is comprised Transport

2.2.1. Label Spaces

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

     - Per interface label space.  Interface-specific incoming labels
       are used for interfaces that initiated the link state advertisement.  The Router ID may also
   be the OSPF Area Border Router.

   Note that BGP and OSPF may share the same LSP when a given Router ID
   is found in both protocol's Routing Information Base.

   The Router ID FEC allows aggregation of multiple IP address prefixes
   to the same LSP, without requiring that the prefixes be explicitly
   listed in the FEC.  Also, it allows addresses advertised using OSPF
   and addresses advertised using BGP to be aggregated using the same
   LSP.  Finally, when the set of addresses reachable via a router
   changes, and the changes are announced into the routing protocol
   (BGP, OSPF, and/or IS-IS), use of the routerID FEC eliminates the
   need to explicitly announce the route changes into LDP.


2.3. Label Spaces, Identifiers, Sessions and Transport

   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 which uses VCIs as labels, or a
             frame Relay interface which uses DLCIs as labels. use interface resources for labels.
       An example of such an interface is a label-controlled ATM
       interface that uses VCIs as labels, or a Frame Relay interface
       that uses DLCIs as labels.

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

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





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2.2.2. LDP Identifiers

   An LDP identifier is a six octet quantity used to identify an LSR
   label space.  The first four octets encode an IP address assigned to
   the LSR, and the last two octets identify a specific label space
   within the LSR.  The last two octets of LDP Identif-
        iers Identifiers for
   platform-wide label spaces are always both zero.  This document uses
   the following print representation for LDP Iden-
        tifiers:



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              <IP address> : <Label <label space Id>

        for example, id>

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

   Note that an LSR that manages and advertises more than one multiple label
        space 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 ether-
        net ethernet (and uses per platform lables)
   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 a an LSR must use uses LDP to advertise more than one label space to
      another LSR it uses a separate LDP session for each label space rather than a single space.


2.2.4. LDP session for all the
           label spaces. Transport

   LDP uses TCP as a reliable transport for sessions.

      When multiple LDP sessions are required between two platforms LSRs there is
      one LDP session per TCP connection rather than many session for each LDP sessions per TCP connection.


2.4. session.












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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 LSR
   LSR1 LSRa
   sends traffic matching some criteria via an LSP to non-directly
   connected LSR LSR2 LSRb rather than forwarding the traffic along its nor-
   mally normally
   routed path.

   The path between LSRa and LSRb would include one or more intermediate
   LSRs (LSR1,...LSRn).  An LDP session between LSR1 LSRa and LSR2 enables LSR2 LSRb would
   enable LSRb to label switch traffic arriving on the LSP from LSR1. LSRa by
   providing LSRb means to advertise labels for this purpose to LSRa.

   In this situation LSR1
   applies 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.  First, it

   LSRa first adds the label learned via the its LDP session with LSR2 LSRb to
   the packet label stack (either by replacing the label on top of the
   packet label stack with it if the packet arrives labeled or by
   pushing it if the packet arrives unlabeled).  Next, it pushes the
   label for the LSP learned from LSR1 onto the label stack.



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2.5.


2.4. LDP Discovery

   LDP discovery is a mechanism that enables an LSR to discover poten-
   tial
   potential LDP peers.  Discovery makes it unnecessary to explicitly config-
   ure
   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.


2.5.1.


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 well-known LDP discovery port for the
   "all routers" group multicast address.



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   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.


2.5.2.


2.4.2. Extended Discovery Mechanism

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

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

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

   Extended Discovery differs from Basic Discovery in the following
   ways:



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

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

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

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


2.6.









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2.5. Establishing and Maintaining LDP Sessions

2.6.1.

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.


2.6.2.


2.5.2. Transport Connection Establishment

   The exchange of Hellos results in the creation of a Hello adjacency
   at LSR1 which
   binds 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 an LDP 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:




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          a)

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

          b)

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

         Similarly, address A2 is determined as follows:

          a)

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

          b)

         b.  If LSR2 does not use the Transport Address optional object,
             A2 is the source IP address used for 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



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         unsigned integers.  If A1 > A2, LSR1 plays the active role;
         otherwise it is passive.

     3.  If LSR1 is active, it attempts to establish the LDP TCP con-
          nection
         connection by connecting to the well known 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 well-known LDP port.


2.6.3.


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 distribu-
   tion
   distribution method, timer values, VPI/VCI ranges for label
   controlled ATM, DLCI ranges for label controlled Frame Relay, etc.

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

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


     1.

   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



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   LSR2 to initiate the parameter negotia-
          tion. negotiation.

   In general when there are multiple links between LSR1 and LSR2 and
   multiple label spaces to be advertised by each, the pas-
          sive passive LSR
   cannot know which label space to advertise over a newly established
   TCP connection until it receives the first LDP PDU on the connection.

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

     2.

     1.  When LSR1 plays the passive role:

          a)

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

          b)

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



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

          c)

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

          d)

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

          e)

         e.  If LSR1 receives a Nak an Error Notification message, LSR2 has
             rejected its proposed session parameters and LSR1 closes the TCP con-
               nection.

     3.
             connection.

     2.  When LSR1 plays the active role:

          a)

         a.  If LSR1 receives a Nak an Error Notification message, LSR2 has
             rejected its proposed session parameters and LSR1 closes the TCP con-
               nection.



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          b)
             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 parame-
               ters
             parameters are unacceptable, LSR1 sends a Nak Session
             Rejected/Parameters Error Notification message and closes
             the connection.

          c)

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

          d)

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

       It is possible for a pair of incompatibly configured LSRs that
       disagree on session parameters to engage in an endless sequence
       of messages as each Naks 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



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       are being
     Nak'd. NAK'd.  It is also recommended that an LSR detecting
       such a situa-
     tion situation take action to notify an operator.


2.6.4. Initialization State Machine

   It is convenient to describe LDP

       The session negotiation behavior 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.






















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               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 Nak Error Notification msg
                                 (NAK) and close transport connection

         OPENREC       Receive KeepAlive msg               OPERATIONAL

                       Receive Any other LDP msg           NON EXISTENT
                         Action: Transmit Nak 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 Nak 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






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








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2.6.5.


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 are 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 carries carry the same
   LDP Identifier.

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

   LDP uses the regular receipt of LDP Discovery Hellos to indicate a
   peer's intent to use the label space identified by the Hello.  An LSR
   maintains a hold timer with each Hello adjacency which it restarts
   when it receives a Hello that matches the adjacency.  If the timer
   expires without receipt of a matching Hello from the peer, LDP con-
   cludes
   concludes that the peer no longer wishes to label switch using that
   label space for the that link (or target, in the case of Targeted Hellos)
   in question
   or that the peer has failed, and it failed.  The LSR then deletes the Hello
   adjacency.  When the last Hello adjacency for a LDP session is
   deleted, the LSR terminates the LDP session by closing the transport
   connection.


2.6.6.


2.5.6. Maintaining LDP Sessions

   LDP includes mechanisms to monitor the integrity of the session tran-
   sport connection. LDP session.

   LDP uses the regular receipt of LDP PDUs on the session transport
   connection to monitor the integrity of the connection. session.  An LSR main-
   tains maintains
   a keepalive KeepAlive timer for each peer session which it resets when-
   ever whenever it
   receives an LDP PDU from the session peer.  If the keepalive 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 peer LDP session by closing the transport connection.

   An

   After an LDP session has been established, an LSR must arrange that
   its LDP peer sees receive an LDP PDU from it at least every keepalive KeepAlive time
   period to ensure the peer restarts the session keepalive 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.



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   message.


2.7.


2.6. Label Distribution and Management

2.7.1. Label Distribution Control Mode

   The behavior of the initial setup of LSPs is determined by whether
   the MPLS architecture [ARCH] allows an LSR to distribute a FEC label
   binding in response to an explicit request from another LSR.  This is operating with independent or ordered LSP control.  An LSR
   may support both types of control
   known as a configurable Downstream On Demand label distribution.  It also allows an
   LSR to distribute label bindings to LSRs that have not explicitly
   requested them.  This is known as Downstream Unsolicited label
   distribution.

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


2.6.1. Label Distribution Control Mode

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

2.7.1.1.


2.6.1.1. Independent Label Distribution Control

   When using independent LSP control, each node 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
   allocation
   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.  This can result
   in unlabeled packets being sent to the downstream node.

2.7.1.2. LSR.


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 an 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 for is received before
   mapping the FEC and passing corresponding labels to upstream LSRs.




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   An LSR may be an egress for some FECs, 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 boun-
          dary,
             boundary, such as another area for OSPF summary net-works, networks,
             or another autonomous system for OSPF AS externals and BGP
             routes



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2.7.2.


2.6.2. Label Retention Mode

2.7.2.1.

2.6.2.1. Conservative Label Retention Mode

   In Downstream Allocation Unsolicited advertisement mode, label mapping advertisements adver-
   tisements for all routes may be received from all peer LSRs.  When
   using conservative label retention, advertised label mappings are only
   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 operat-
   ing in Downstream-
   on-Demand Downstream-on-Demand mode, an LSR will request label mappings will
   only be requested of from the
   appropriate next hop LSR according to routing. Since Downstream-on-
   Demand Downstream-
   on-Demand mode is primarily used when label conservation is desired
   (e.g., an ATM switch with limited cross connect space), it is typi-
   cally used with the conservative label retention mode.

   The main advantage of the conservative mode is that the only the labels
   that are required for the forwarding of data are allocated and
   maintained. main-
   tained.  This is particularly important in LSRs where the label space
   is inherently limited, such as in an ATM switch.  A disadvan-
   tage 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.

2.7.2.2.


2.6.2.2. Liberal Label Retention Mode

   In Downstream Allocation Unsolicited advertisement mode, label mapping advertisements adver-
   tisements for all routes may be received from all peer LSRs. LDP peers.  When
   using liberal label retention, advertised every label mappings are retained received from all next hops a
   peer LSR is retained regardless of whether they are valid the LSR is the next hops hop
   for the advertised mapping.  When operating in Downstream-on-Demand mode,
   mode with liberal label retention, an LSR might choose to request



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   label mappings
   are requested of for all known prefixes from all peer LSRs. Note, however, how-
   ever, that Downstream-on-
   Demand Downstream-on-Demand mode is typically associated with used by devices
   such as ATM switch-based LSRs where for which the conservative approach is
   recommended.

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










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2.7.3.


2.6.3. Label Advertisement Mode

   Each interface on an LSR is configured to operate in either Down-
   stream Unsolicited or Downstream-on-Demand allocation advertisement mode.  LSRs
   exchange adver-
   tisement 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 advertise-
   ments


2.8. advertisements.


2.7. LDP Identifiers and Next Hop Addresses

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

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

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

   To enable LSRs to map between a peer LDP identifier and the peer's
   addresses, LSRs advertise their addresses using LDP Address and With-
   draw Address messages.

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



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   advertised addresses from a peer


2.9.


2.8. Loop Detection

   Each LSR MUST support the configurable loop-detection option.  LSRs
   perform loop

   Loop detection via is a configurable option which provides a mechanism
   for finding looping LSPs and for preventing Label Request messages
   from looping in the LSR-path-vector object (TLV) contained
   within each Mapping presence of non-merge capable LSRs.

   The mechanism makes use of Path Vector and Query message.  Upon receiving such 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 mes-
   sage, list of the LSRs that its containing
       message has traversed.  An LSR performs loop detection is identified in a Path Vector
       list by verifying that its unique
   router-id LSR Identifier (Id), which is not already present in the list.  If IP address
       component of its LDP Identifier.  When an LSR propagates a loop is detected, mes-
       sage containing a Path Vector TLV it adds its LSR Id to the Path
       Vector list.  An LSR must transmit that receives a NAK message to with a Path Vector
       that contains its LSR Id detects that the sending node, and does



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   not install message has traversed a
       loop.  LDP supports the mapping or propagate notion of a maximum allowable Path Vector
       length; an LSR that detects a Path Vector has reached the message any further.  In
   addition, maximum
       length behaves as if there is an upstream label spliced to the downstream
   label for containing message has traversed a loop.

     - A Hop Count TLV contains a count of the FEC, LSRS that the containing
       message has traversed.  When an LSR must unsplice the labels. On those mes-
   sages in which no loop is detected, propagates a message contain-
       ing a Hop Count TLV it increments the count.  An LSR must concatenate itself 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 LSR-path-vector before propagating.

   If hop count is unknown.
       Incrementing an unknown hop count value results in an unknown hop
       count value (0).

   The following paragraphs describes LDP loop detection is desired procedures.  In
   these paragraphs, "MUST" means "MUST if configured for loop detec-
   tion".  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 some portion situations when loop detec-
   tion is not configured (see [ATM]).


2.8.1. Label Request Message

   The use of the network, then it
   should be turned on Path Vector TLV and Hop Count TLV prevent Label
   Request messages from looping in ALL LSRs within environments that portion include non-merge
   capable LSRs.

   The rules that govern use of the network,
   else loop detection will not operate properly.


2.10. Loop Prevention via Diffusion Hop Count TLV in Label Request



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   messages by LSR diffusion support R when Loop Detection is enabled are the following:

   - The Label Request message MUST include a configurable option, which permits an LSR
   to verify that a new routed path Hop Count TLV.

   - If R is loop free before installing an
   LSP on that path. An LSR which supports diffusion does not splice an
   upstream label to a new downstream label until it ensures that con-
   catenation of the upstream path with sending the new downstream path will be
   loop free.

   A LSR which detects Label Request because it is a new next hop for an FEC transmits ingress, it
     MUST include a Query mes-
   sage containing its unique router id to each of its upstream peers.
   An LSR that receives such a Query message processes Hop Count TLV with hop count value 1.

   - If R is sending the Query Label Request as fol-
   lows.  (The following procedures are described in terms of Ack and
   Nak messages.  An Ack is a Notification message signalling Success; result of having received a
   Nak is
     Label Request from an upstream LSR, and if the received Label
     Request contains a Notification message signalling Loop Detected)

     o    If Hop Count TLV, R MUST increment the downstream LSR not received hop
     count value by 1 and MUST pass the correct resulting value in a Hop Count
     TLV to its next hop for the given
          FEC, the upstream LSR responds along with an Ack message, indicating the Label Request message;

   The rules that govern use of the downstream Path Vector TLV in Label Request
   messages by LSR may change to R when Loop Detection is enabled are the new path.

     o following:

   - If the downstream LSR R is sending the correct next hop for the given
          FEC, the upstream LSR performs loop detection via the LSR-
          path-vector.

     o    If Label Request because it is a loop FEC ingress, then
     if R is detected, the upstream LSR responds with non-merge capable, it MUST include a Nak
          message that indicates the Path Vector TLV of
     length 1 containing its own LSR Id.

   - If R is to be "pruned, and the LSR
          unsplices all connections for that FEC to sending the downstream node,
          thereby pruning itself off Label Request as a result of having received a
     Label Request from an upstream LSR, then if the tree.

     o    If received Label
     Request contains a loop Path Vector TLV or if R is not detected, the upstream node concatenates non-merge capable:

         R MUST add its
          unique router-id own LSR Id to the LSR-path-vector, Path Vector, and propagates MUST pass the
          Query message
         resulting Path Vector to its upstream peers.

     o    Each LSR which 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 an Ack a Label Request message from its upstream peer
          in response 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 query message, in turn forwards reply, and if R intends to merge
   the newly received Label Request with the ack-
          nowledgement existing outstanding Label
   Request, then R does not propagate the Label Request to the downstream next hop.

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

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






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     o    If an LSR doesn't receive a Ack


2.8.2. Label Mapping Message for a given query
          within a "reasonable" period

   The use of time, it "unsplices" the
          upstream peer that has not responded, Path Vector TLV and responds with a Nak Hop Count TLV in the Label Mapping
   message provide a mechanism to its downstream peer, indicating the pruning of the
          upstream peer.

     o    An find and terminate looping LSPs.  When
   an LSR which receives a new Query Label Mapping message for an FEC before it
          has received responses from all of its upstream peers for a
          previous Query message must concatenate the old and the new
          LSR-path-vector within next hop, the new query advertisement before pro-
          pagating.

     o    The diffusion computation continues until each message
   is propagated upstream path
          responds with as specified below until an acknowledgment. An a ingress LSR that does not have any
          upstream LDP peers must acknowledge the Query message. is
   reached or a loop is found.

   The LSR which began rules that govern the diffusion may splice its upstream label to use of the new downstream label only after receiving Hop Count TLV in Label Mapping
   messages sent by an acknowledge mes-
     sage from the upstream peer.

     As LSR diffusion support R when Loop Detection is enabled are the fol-
   lowing:

   - R MUST include a configurable option, an LSR which
     does not support diffusion will never originate a Query message.
     However, these LSRs must still recognize and process Hop Count TLV.

   - If R is the Query mes-
     sages, as described above.



2.11. Explicitly Routing LSPs

   The need for explicit routing (ER) in MPLS has been explored else-
   where [ARCH] [FRAME].  At egress, the MPLS WG meeting held during hop count value MUST be 1.

   - If the Wash-
   ington IETF there was consensus that LDP should support explicit
   routing of LSPs with provision for indication of associated (forward-
   ing) priority.  This section specifies mechanisms Label Mapping message is being sent to provide that
   support, and provides propagate a means Label
     Mapping message received from the next hop to allow an upstream peer, the
     hop count value MUST be the reservation result of 'resources'
   for incrementing the explicitly routed LSP.

   In this document we propose an end to end setup mechanism that could,
   in principal, be invoked hop count
     value received from either end of the explicitly routed LSP
   (ERLSP).  However we specify it here only for next hop.

   - If the case Label Mapping message is not being sent to propagate a Label
     Mapping message, the hop count value MUST be the result of incre-
     menting R's current knowledge of initiation
   by the ingress in hop count to the belief egress.  Note
     that such a mechanism maps naturally to the setup in hop count to the opposite direction.  We believe that the, inevit-
   able, latency associated with this (end to end) setup mechanism is
   tolerable since most of egress will be unknown if R has not
     received a Label Mapping message from the motivations for ERLSPs, for example
   'traffic engineering' imply next hop.

   Any Label Mapping message MAY contain a Path Vector TLV.  The rules
   that govern the LSPs setup mandatory use of the Path Vector TLV in this manner will
   have a long lifetime (at least Label Mapping
   messages sent by LSR R when compared to those setup in
   response to dynamic routing).




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   We introduce objects and procedures that provide support for:

     -    Strict and Loose explicit routing

     -    Specification of class of service

     -    Reservation of bandwidth Loop Detection is enabled are the follow-
   ing:

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

   -    ERLSP preemption

     Only unidirectional point-to-point ERLSP If R is specified currently.
     The scheme can be easily extended to accommodate multipoint-to-
     point ERLSPs.  The FEC object (TLV) may be used to determined which
     ERLSPs are "merged" sending the Label Mapping message to form a multipoint-to- point ERLSP.  Alterna-
     tively, propagate a multipoint-to-point ERLSP can be setup Label Map-
     ping message received from the egress by
     completely specifying the multipoint- to-point tree.  Also, tunnel-
     ing ERLSPs within other ERLSPs next hop to an upstream peer, then:

       o If R is for future study.

     To setup merge capable and if R has not previously sent a ERLSP an LSR (that will be Label
         Mapping message to the 'ingress' of upstream peer, then it MUST include a
         Path Vector TLV.

       o If the LSP)
     generates an explicit request.  The explicit request received message contains an
     explicit route object which in turn contains a sequence of explicit
     request next unknown hop objects and count, then R
         MUST include a Path Vector TLV.

       o If R has previously sent a pointer Label Mapping message to the current entry in that
     sequence.  The explicit request next hop objects specify the IP
     address of the LSRs through which the ERLSP should pass.  These LSR
     hops specified in the explicit route are referred to as 'peg LSRs'.

     An explicit request
         upstream peer, then it MUST specify the stream that will be associated
     with the ERLSP by inserting the appropriate FEC value in the
     request.  The FEC value 'opaque tunnel' exists to support ERLSPs
     where the intermediate LSRs on the LSP need know nothing about the
     traffic flowing on the LSP.

     The setup mechanism for ERLSPs employs an end to end protocol.
     Individual ERLSPs are uniquely identified by an ERLSPID associated
     with them by the LSR that initiates their setup.  The ERLSPID is
     generated by the ingress LSR of the LSP.  The ERLSPID has another
     component called Peg ERLSPID which is generated by each peg LSR
     when the next peg LSR from itself is loosely routed.  This is used
     by the intermediate LSRs to identify a loosely routed segment.  The
     Peg ERLSPID is not used in include a segment that is strictly routed.
     Requests travel from the 'ingress' of Path Vector TLV if the
         received message reports an LSP toward what will be
     the 'egress'.  Responses indicating the status of the ERLSP request
     travel back toward the ingress of the ERLSP.  ERLSPID is used in
     both Request and Response messages.

     The addresses specified in the next hop objects count increase, a change in the explicit



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     route object should be those of the LSR's IP address


         hop count from unknown to known, or a change from known to
         unknown.

     If the incom-
     ing interfaces on above rules require R include a Path Vector TLV in the LSRs through which Label
     Mapping message, R computes it as follows:

       o If the LSP should pass.  The
     ERLSPID, FEC, incoming interface (previous hop) and LDP identifier
     of received Label Mapping message included a Path Vector,
         the Path Vector sent upstream MUST be the result of adding R's
         LSR that generated Id to the received Path Vector.

       o If the received message are all stored in an ERLSP
     control block.  Here's had no Path Vector, the Path Vector
         sent upstream MUST be a synopsis path vector of length 1 containing R's
         LSR Id.

   - If the entire mechanism to
     instantiate an ERLSP:

        An ingress node originates a ERLSP request message.  The Label Mapping message
        contains an unique ERLSPID, FEC object, explicit route object,
        and an optional object for resource assignment for the ERLSP.

        At an intermediate node the 'active' ERNH object is identified
        by the pointer in the explicit route object.  On message receipt
        the pointer always points not being sent to propagate a
     received message upstream, the receiving LSR object in the
        explicit route Label Mapping message in case of strict routing.  If MUST include a segment
     Path Vector of ERLSP is loosely routed then pointer always points to the
        upstream peg length 1 containing R's LSR at all the intermediate LSRs in this segment.
        The penultimate Id.

   If R receives a Label Mapping message from its next hop to with a Hop
   Count TLV which exceeds the downstream peg configured maximum value, or with a Path
   Vector TLV containing its own LSR advances the
        pointer to the next ERNH object in the list.

        If Id or which exceeds the ERNH objects subtype indicates 'Strict' maximum
   allowable length, then dependent on R detects that the next ERNH IP address corresponding LSP contains
   a loop.

   When R detects a loop, it MUST stop using the appropriate LDP Identifier label for forwarding,
   drop the
        LDP session with the next hop Label Mapping message. and the appropriate output inter-
        face are discovered (by using the information learnt from the
        address send a Loop Detected Notification
   message see Section "LDP Identifiers").  The outgoing
        interface (next hop) information is also stored in the ERLSP
        control block.  In to the case source of strict ERLSP, the neighbor MUST
        be directly adjacent Label Mapping message.


2.8.3. Discussion

   LSRs which are configured for loop detection are NOT expected to
   store the current LSR.

        If the ERNH object subtype indicates 'Loose' then dependent upon path vectors as part of the next ERNH IP address a next hop is selected as per the FIB
        information for the downstream peg LSR.  This information is
        again maintained LSP state.

   Note that in the ERLSP control block.  Peg a network where only non-merge capable LSRs are
        allowed to change the Explicit Route Object if the path present,
   Path Vectors are passed downstream from ingress to the
        next Peg LSR egress, and are
   not passed upstream.  Even when merge is selected to supported, Path Vectors need
   not be 'loose'.  This allows the Peg
        LSRs to select a specific path to the next Peg LSR.  The default
        path to the next Peg LSR in case the segment is chosen as
        'loose' passed upstream along an LSP which is determined by the hop-by- hop forwarding path known to reach the
        next Peg LSR.  However, Peg
   egress.  When an LSR are allowed only to select experiences a
        path downstream to the change of next Peg LSR, they hop, it need pass
   Path Vectors upstream only when it cannot tell from the hop count
   that the change paths on
        any other segment of next hop does not result in a loop.

   In the ERLSP.

        Bandwidth reservations (if any) case of ordered label distribution, Label Mapping messages are processed.  How this hap-
        pens, i.e. the precise connection admission procedures is out-
        side
   propagated from egress toward ingress, naturally creating the scope of Path
   Vector along the LDP specification.  The admission control
        must also use way.  In the preemption value specified case of independent label distribution,
   an LSR may originate a Label Mapping message for an FEC before
   receiving a Label Mapping message from its downstream peer for that
   FEC.  In this case, the LSP in
        determining if resources are available subsequent Label Mapping message for the LSP.  If a reser-
        vation cannot be accommodated a response indicating that fact is FEC



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        returned


   received from the downstream peer is treated as an update to LSP
   attributes, and the previous hop.  Note Label Mapping message must be propagated
   upstream.  Thus, it is recommended that the resources are only
        reserved at this time.  The LSRs will commit the bandwidth with
        the labels when the response comes back from the egress LSR.

        If the ERLSP can loop detection be accommodated the pointer in the explicit
        request object is incremented to point at the next explicit
        request next hop object configured
   in case of strict routing and the
        request message is sent conjunction with ordered label distribution, to minimize the LDP peer discovered as described
        above.  In case
   number of loose routing, the pointer is incremented
        only if the direct next hop is the next downstream peg LSR. Label Mapping update messages.

   If an LSR finds it impossible to satisfy a Explicit request then
        an 'Explicit response' message is created indicating the reason.
        The ERLSPID from (failed) request loop detection is inserted desired in some portion of the message and network, then it is sent to the LDP peer identified in the associated entry
   should be turned on in
        the ERLSP control block after which the ERLSP block is freed. ALL LSRs receiving Explicit responses indicating failure process
        them in a similar manner.  They create a new Explicit request
        and copy the ERLSPID and Status from within that portion of the Explicit request they
        received into it.  They use network,
   else loop detection will not operate properly.


3. Protocol Specification

   Previous sections that describe LDP operation have discussed
   scenarios that involve the ERLSPID to obtain exchange of messages among LDP peers.
   This section specifies the appropri-
        ate ERLSP control block message encodings and thus identify procedures for pro-
   cessing the messages.

   LDP peer toward
        which the 'new' Explicit response message should be sent.  Hav-
        ing done 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 they free the ERLSP control block.

        When an Explicit request reaches the LSR specified in the last
        ERNH object mes-
   sages in that request and that LSR accedes to the request
        it generates an Explicit response indicating successful setup of
        the ERLSP.  The egress node also includes a label in the
        response message.  The Explicit response is (reverse path) for-
        warded through the LSRs that the original Explicit request
        traversed using the mechanism described above (inspection of
        ERLSP control block).  In this case, of course, the ERLSP con-
        trol block is LDP PDU need not deleted.  An intermediate LSR receiving such be related to one another.  For example,
   a
        response message allocates single PDU could carry a new message advertising FEC-label bindings for
   several FECs, another message requesting label on its incoming interface bindings for several
   other FECs, and creates a connection between the new and the given label in
        the message.  The LSR also commits the previously reserved
        bandwidth to this connection at the appropriate scheduler(s).
        The LSR then forwards the third notification message to its previous hop with the
        new label.  When the successful response reaches 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 ingress LSR version number of the ERLSP is declared in-service.

        There is also support for route pinning for loosely routed seg-
        ments.  When a ERLSP is pinned
     protocol.  This version of the loose path is not changed
        when `better' paths become available.  Once a ERLSP goes in-
        service there is specification specifies LDP protocol support to reassign resources to the
        ERLSP if required.
     version 1.



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2.12. ERLSP State Machine

   The ERLSP control block may contain the following information:
           - ERLSPID/Peg ERLSPID
           - State
           - FEC object
           - Flags
             o Self is Peg Node
             o Pinned path
             o Upstream segment (Strict/Loose) type
             o downstream segment (Strict/Loose) type
           - next peg node
           - preemption level
           - upstream neighbor (next hop/interface)
           - downstream neighbor (next hop/interface)
           - BW information (only at peg LSRs with loose downstream
               segment)
           - Explicit Route Object (only at peg LSRs with loose
               downstream segment)

   For


   PDU Length
     Two octet integer specifying the purpose total length of matching message to existing ERLSP control
   block, both the ERLSPID and Peg ERLSPID in the message are
   matched against the ones this PDU in
     octets, excluding the control block.  Its only Version and PDU Length fields.

     The maximum allowable PDU Length is negotiable when
   both of them match that the message an LDP session
     is considered initialized.  Prior to be for completion of the
   matched control block, otherwise it negotiation the maximum
     allowable length is treated as a new ERLSP
   request. 4096 bytes.

   LDP Identifier
     Six octet field that uniquely identifies the label space for which
     this PDU applies.  The ingress may use first four octets encode an IP address
     assigned to the ERLSPID as LSR.  This address should be the router-id, also
     used to identify the peg ERLSPID.
   At LSR in loop detection Path Vectors.  The last
     two octets identify a label space within the peg nodes, LSR.  For a platform-
     wide label space, these should both be zero.

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


3.2. LDP Procedures

   LDP defines messages, TLVs and Previous
   Peg ERLSDID are compared because Peg ERLSPID contains procedures in the self
   assigned Peg ERLSPID.  Also note following areas:

     - Peer discovery;
     - Session management;
     - Label distribution;
     - Notification of errors and advisory information.

   The sections that follow describe the Request message at
   Peg node is only compared and TLV encodings for ERLSPID
   these areas and the procedures that apply to select a control
   block. them.

   The state tables for peg node and non peg nodes label distribution procedures are given
   separately.  Separate state tables complex and are used only for
   illustrative purposes.  The state engines can be collapsed into
   a single state engine.  Moreover, a completely strict ERLSP can
   be treated difficult to
   describe fully, coherently and unambiguously as a special case collection of loosely routed where every
   neighbor
   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 peg LSR procedure
   described elsewhere in this document conflicts with several of the state transitions
   optimized. Appendix A,
   Appendix A specifies LDP behavior.









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2.12.1. Loose Segment Peg LSR Transitions:

   Peg LSRs in


3.3. Type-Length-Value Encoding

   LDP uses a loosely routed ERLSP segment are those that are expli-
   citly listed in the explicit route object as the starting or ending Type-Length-Value (TLV) encoding scheme to encode much of a loose segment.

      State NULL

          Event      Action                               New State

          Request    Create ERLSP control block; store    Response
                     relevant information from the        Awaited
                     message into the control block;
                     select a new peg ERLSPID; reserve
                     BW specified in the message; obtain
                     next hop (or interface) towards
                     next peg LSR; propagate message
                     towards
   the obtained next hop.

                     If last node information carried in the explicit route   Established
                     object, allocate an upstream label;
                     commit BW; originate LDP messages.

   An LDP TLV is encoded as a Response
                     message upstream.

                     If unable 2 octet field that uses 14 bits to process request for     No change
                     any reason, issue specify
   a NAK message to
                     the sender with appropriate error
                     code.

          Response   Send NAK message Type and 2 bits to the sender.      No change

          Others     Silently ignore event.               No change


      State RESPONSE_AWAITED

          Event      Action                               New State

          Response   Install downstream label in          Established
                     message; choose specify behavior when an upstream label;
                     connect upstream to downstream
                     label; commit BW to LSR doesn't recognize
   the connection;
                     propagate Response upstream with
                     upstream label.

                     If unable to process Response        Null
                     message for any reason then recover
                     resources; originate Type, followed by a Nak message



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                     upstream; originate 2 octet Length Field, followed by a Release
                     message downstream; delete control
                     block.

          Upstream   Release resources; propagate Nak     Null
          lost       downstream; delete control block.

          Downstream Reassign a new Peg ERLSPID.  Start   Retry
          lost       RETRY timer.

          Nak from   Reassign 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 new Peg ERLSPID.  RETRY   Retry
          downstream timer.

                     If error code in Nak notification must be returned to the message originator and
     the entire message must be ignored; if U is severe then  Null
                     propagate set (=1), the Nak upstream; release
                     resources; delete control block.

          Nak from   Release resources; propagate Nak     Null
          upstream   downstream; delete control block.

          New NH     If ERLSP unknown
     TLV is pinned, ignore event.    Retry
                     Otherwise, send a Nak downstream;
                     change NH in silently ignored and the rest of the control block;
                     reassign a new Peg ERLSPID.  Start
                     RETRY timer.

          Others     Silently ignore event.               No change


      State RETRY

          Event      Action                               New State

          Retry      Originate Request message towards    Response
          Timer is processed as
     if the next hop in unknown TLV did not exist.

   F bit
     Forward unknown TLV bit.  This bit applies only when the control block.   Awaited

          New NH     If ERLSP U bit is pinned, ignore
     set and the       No change
                     event.  Otherwise change next hop
                     information in LDP message containing the control block.

          Nak from   Release all resources (BW, label,    Null
          upstream   timer);  delete control block.

          Upstream   Release all resources (BW, label,    Null
          lost       timer);  delete control block.

          Release    Release all resources (BW, label,    Null
                     timer); delete control block.



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          Downstream unknown TLV is to be for-
     warded.  If there F is a new next hop, update   No change
          lost       that in clear (=0), the control block.

                     Otherwise, delete timer; recover     Null
                     resources; send Nak upstream;
                     delete control block.

          Others     Silently ignore event.               No change


      State RECONNECT_AWAITED

          Event      Action                               New State

          Request    Make appropriate changes in the      Established
                     control block; make label
                     connection; send a Response message
                     upstream unknown TLV is not forwarded with upstream label.

                     If unable to process Request         Null
                     message for any reason then send a
                     Release message downstream and a
                     Nak message upstream; release
                     resources; delete control block.

          Reconnect  Release resources; send Release      Null
          Awaited    message downstream; delete control
          Timer      block.

          Upstream   Ignore event.                        No change
          lost

          Downstream Release resources; delete control    Null
          lost       block.

          New NH     Release resources; delete control    Null
                     block.

          Nak from   Release resources; delete control    Null
          downstream block.

          Others     Silently ignore event.               No change


      State ESTABLISHED

          Event      Action                               New State



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          Upstream   Start RECONNECT_AWAITED timer.       Reconnect
          lost                                            Awaited

          Downstream Reassign a new Peg ERLSPID.  Start   Retry
          lost       RETRY timer.

          Nak from   Reassign a new Peg ERLSPID.  Start   Retry
          downstream RETRY timer.

                     If error code in Nak
     the containing message; if F is severe then  Null
                     propagate set (=1), the Nak upstream; release
                     resources; delete control block.

          Nak from   Reassign a new Peg ERLSPID.  Start   Reconnect
          upstream   RECONNECT_AWAITED timer.             Awaited

                     If error code in Nak unknown TLV is severe,      Null
                     then propagate for-
     warded with the Nak downstream;
                     release resources; delete control
                     block.

          New NH     If ERLSP containing message.

   Type
     Encodes how the Value field is pinned, ignore to be interpreted.

   Length
     Specifies the       Retry
                     event.  Otherwise, send a Nak
                     downstream; change next hop length of the Value field in
                     control block; reassign a new Peg
                     ERLSPID.  Start RETRY timer.

          Release    Release resources; propagate         Null
                     message downstream; delete control
                     block.

          Others     Silently ignore event.               No change octets.

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




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2.12.2. Loose Segment Non-Peg LSR Transitions:

   Non-peg LSRs in a loose segment of an ERLSP are the LSRs intermediate
   to two peg LSRs and through which the loose segment


   Note that there is routed using no alignment requirement for the hop-by-hop forwarding path.

      State NULL

          Event      Action                               New State

          Request    Create ERLSP control block; reserve  Response
                     BW specified in first octect of a
   TLV.

   Note that the message; obtain  Awaited
                     next hop (or interface) towards
                     next peg LSR; if penultimate hop to
                     next peg LSR then increment pointer 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 ERNH object; propagate message
                     towards the obtained next hop

                     If unable to process request for     No change
                     any reason, issue an LDP PDU could be encoded as a Nak message TLV.  This specifica-
   tion 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 sender with appropriate error
                     code.

          Response   Send type of a Nak message
   value to the sender.    No change

          Others     Silently ignore event.               No change


      State RESPONSE_AWAITED

          Event      Action                               New State

          Response   Install downstream label be encoded is known, for example by its position in          Established
                     message; choose a mes-
   sage or an upstream label;
                     connect upstream to downstream
                     label; commit BW to connection;
                     propagate Response upstream with
                     upstream label.

                     If unable to process Response        Null
                     message enclosing TLV, and the length of the value is fixed or
   readily derivable from the value encoding itself.

   Some of the TLVs defined for any reason then
                     recovery resources; propagate LDP are similar to one another.  For
   example, there is a Nak
                     message upstream; originate Generic Label TLV, an ATM Label TLV, and a
                     Release message downstream; delete
                     control block.

          Upstream   Originate 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 Nak message downstream;  Null
          lost       delete control block.



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          Downstream Originate TLV type that specifies a Nak message upstream;    Null
          lost       delete control block.

          Nak from   Propagate Nak message upstream;      Null
          downstream release reserved BW; delete control
                     block.

          Nak from   Propagate Nak message downstream;    Null
          upstream   release reserved BW; delete control
                     block;

          New NH     If ERLSP is pinned, ignore the       Null
                     event.  Otherwise, send Nak message
                     upstream and downstream; release
                     reserved BW; delete control block.

          Release    Propagate message downstream;        Null
                     release resources; delete control
                     block.

          Others     Silently ignore event.               No change


      State ESTABLISHED

          Event      Action                               New State

          Upstream   Send Nak message downstream;         Null
          lost       release resources (BW, label);
                     delete control block.

          Downstream Send Nak message upstream; release   Null
          lost       resources; delete control block.

          Nak from   Release resources; propagate Nak     Null
          downstream message upstream; delete control
                     block.

          Nak from   Release resources; propagate         Null
          upstream   message Nak downstream; delete
                     control block.

          New NH     If ERLSP is pinned, ignore the       Null
                     event.  Otherwise, release
                     resources; originate Nak  message
                     upstream; originate Nak message
                     downstream; delete control block.



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          Release    Release resources; propagate         Null
                     message downstream; delete control
                     block.

          Others     Silently ignore event.               No change


2.12.2.1. Strict Segment Transitions

      A LSR whose upstream TLV class and downstream segment of an ERLSP is
      strict has a state transition exactly similar to the non-peg
      LSR (only different being TLV subtype that
   specifies a particular kind of TLV within that class, this specifica-
   tion does not handle formalize the case notion of pinned
      down option).



2.12.3. ERLSP Timeouts

   The following timeouts are used in the state transition:


     RETRY
          Default value TBD.  This timer is set by the peg LSR to ori-
          ginate a Request message downstream on TLV subtype.

   The specification assigns type values for related TLVs, such as the elapse
   label TLVs, from of the timer
          when a  downstream loose segment is lost.

     RECONNECT
          Default value TBD.  This timer is set by contiguous block in the peg LSR to dein-
          stall an ERLSP on 16-bit TLV type number
   space.

   Section "TLV Summary" lists the elapse TLVs defined in this version of the timer when a upstream
          loose segment is lost.


2.12.4. ERLSP Error Codes

   NOTE*NOTE*NOTE*NOTE*NOTE*NOTE:

     To be supplied.

     This subsection should be moved to Section 3.

   END NOTE * END NOTE * END NOTE:









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3. Protocol Specification

   Previous sections
   protocol and the section in this document that describe LDP operation have discussed
   scenarios that involve the exchange of messages among LDP peers.
   This section specifies the message encodings and procedures describes each.


3.4. TLV Encodings for pro-
   cessing the messages.

   LDP message exchanges Commonly Used Parameters

   There are accomplished several parameters used by sending LDP protocol data
   units (PDUs) over LDP session TCP connections.

   Each LDP PDU can carry one or more than one LDP messages.  Note that the mes-
   sages message.  The
   TLV encodings for these commonly used parameters are specified in an LDP PDU need not be related
   this section.


3.4.1. FEC TLV

   Labels are bound 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 Forwarding Equivalence Classes (FECs).  a third notification message signalling some event.



3.1. LDP PDUs

   Each LDP PDU FEC is
   a fixed LDP header followed by list of one or more LDP mes-
   sages. FEC elements.  The fixed LDP header FEC TLV encodes FEC items.

   Its encoding is:





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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|F| FEC (0x0100)              |  Version                      |         PDU      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         LDP Identifier                        FEC Element 1                          |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |          Res
   ~                                                               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

   PDU Length
     Two FEC element.

     A FEC Element value is encoded as a 1 octet integer specifying field that specifies
     the total element type, and a variable length of this PDU in bytes,
     excluding the Version and PDU Length fields.

   LDP Identifier
     Six octet field that uniquely identifies is the label space for which
     this PDU applies. 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 first four octets encode an FEC Element value encoding is:

         FEC Element       Type      Value
         type name

           Wildcard        0x01      No value; i.e., 0 value octets;
                                         see below.
           Prefix          0x02      See below.
           Host Address    0x03      4 octet full IP address
     assigned address; see below.


     Wildcard FEC Element
       To be used only in the Label Withdraw and Label Release Messages.
       Indicates the withdraw/release is to be applied to all FECs asso-
       ciated with the LSR.  This address should label within the following label TLV.  Must be
       the router-id, also
     used only FEC Element in LSR Path Vector used by loop detection and loop prevention the FEC TLV.

     Prefix FEC Element value encoding:








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     procedures.  The last two octets identify a label space within the
     LSR.  For a platform-wide label space, these should both be zero.

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



3.2. Type-Length-Value Encoding

   LDP uses a Type-Length-Value (TLV) encoding scheme to encode much of
   LDP message contents.  An LDP TLV is encoded as a 2 octet Type field,
   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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type                      |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Prefix (2)   |     Address Family            |                         Value     PreLen    |
   ~                                                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Prefix                                    |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type
     Encodes how
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Address Family
         Two octet quantity containing a value from ADDRESS FAMILY
         NUMBERS in [rfc1700] that encodes the Value field is to be interpreted.

   Length
     Specifies address family for the
         address prefix in the Prefix field.

       PreLen
         One octet unsigned integer containing the length in bits of the Value field
         address prefix that follows.

       Prefix
         An address prefix encoded according to the Address Family
         field, whose length, in octets.

   Value
     Octet string of Length octets bits, was specified in the PreLen
         field, padded to a byte boundary.


     Host Address FEC Element encoding:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Host Addr (3) |     Address Family            | Host Addr Len |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                     Host Addr                                 |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Address Family
         Two octet quantity containing a value from ADDRESS FAMILY
         NUMBERS in [rfc1700] that encodes information the
     interpretation of which is specfied by address family for the Type
         address prefix in the Prefix field.

   Note that

       Host Addr Len
         Length of 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 Host address in an LDP PDU could be octets.

       Host Addr
         An address encoded as a TLV.  This specifica-
   tion does not use the TLV scheme according 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 Address Family field.




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   value to be encoded is known, for example by its position


3.4.1.1. FEC Procedures

   If in decoding a mes-
   sage or FEC TLV an enclosing TLV, and the length of LSR encounters a FEC Element type it can-
   not decode, it should stop decoding the value is fixed or
   readily derivable from FEC TLV, abort processing the value encoding itself.

   Some of
   message containing the TLVs defined for LDP are similar to one another.  For
   example, there is a Generic Label TLV, and send an ATM Notification message to its
   LDP peer signaling an error.


3.4.2. Label TLV, and a Frame
   Relay TLV; see Sections "Generic TLVs

   Label TLV", "ATM TLVs encode labels.  Label TLV", and
   "Frame Relay TLV".

   While is possible to think about TLVs related in this way in terms of
   a TLV type that specifies a TLV class are carried by the messages
   used to advertise, request, release and a TLV subtype that speci-
   fies a particular kind withdraw label mappings.

   There are several different kinds of TLV within Label TLVs which can appear in
   situations that class, this specification
   does not formalize the notion of require a Label TLV.


3.4.2.1. Generic Label TLV subtype.

   The specification assigns type values

   An LSR uses Generic Label TLVs to encode labels for related TLVs, such as the use on links for
   which label TLVs, from values are independent of a contiguous block in the 16-bit TLV type number
   space.

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



3.3. Commonly Used TLVs

   There such links are several TLV encodings used by more than one LDP message.
   The encodings for these commonly used TLVs are specified in this sec-
   tion.



3.3.1. FEC TLV

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

   Its encoding is:














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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     FEC (0x0100)
   |U|F| Generic Label (0x0200)    |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        FEC Element 1     Label                                                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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


3.4.2.2. ATM Label TLV

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|F| ATM Label (0x0201)        |                                                               |
   ~                                                               ~
   |         Length                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Res| V |                        FEC Element n          VPI          |         VCI                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   FEC Element 1



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   Res
     This field is reserved. It must be set to FEC Element n
     There are several types of FEC elements; see Section "FEC Types".
     The FEC element encoding depends zero on the type of FEC element.  Note
     that while the representation of the FEC element value transmission and
     must be ignored on receipt.

   V-bits
     Two-bit switching indicator.  If V-bits is type-
     dependent that 00, both the value encoding itself VPI and VCI
     are significant.  If V-bits is one where standard LDP
     TLV encoding 01, only the VPI field is not used.

     A FEC Element value signifi-
     cant.  If V-bit is encoded as a 1 octet field that specifies 10, only the element type, and a variable length field that 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 type-
     dependent element value.

     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 Prefix value encoding below.
           Router Id       0x03      4 octet full IP address.
           Flow            0x04      See Flow value encoding below.


     Wildcard FEC Element
       To VCI is less than 16- bits, it
     should be used only right justified in the Label Withdraw field and Label Release Messages.
       Indicates the withdraw/release is to preceding bits must
     be applied set to all FECs asso-
       ciated with the label within 0. If Virtual Path switching is indicated in the following label TLV.  Must V-bits
     field, then this field must be ignored by the only FEC Element in receiver and set to 0
     by the FEC TLV.

     Prefix FEC Element value encoding:






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3.4.2.3. Frame Relay Label TLV

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|F| Frame Relay Label (0x0202)|       Length                  |     Address Family            |    PreLen     |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
     |                                                               |
     |                            Prefix                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Reserved    |Len|                     DLCI                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Address Family
         Two octet quantity containing a value from ADDRESS FAMILY
         NUMBERS in Assigned Numbers [ref] that encodes the address fam-
         ily for the address prefix in the Prefix field.

       PreLen
         One octet unsigned integer containing the length in bits of


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

   Len
     This field specifies the
         address prefix that follows.

       Prefix
         An address prefix encoded according number of bits of the DLCI. The following
     values are supported:

        0 = 10 bits DLCI
        1 = 17 bits DLCI
        2 = 23 bits DLCI




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


3.4.3. Address Family
         field, whose length, in bits, was specified List TLV

   The Address List TLV appears in the PreLen
         field, padded to a byte boundary.

     Flow FEC Element value encoding: Address and Address Withdraw mes-
   sages.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Network Source
   |U|F| Address List (0x0101)     |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Network Destination     Address Family            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |         Source Port
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |        Dest Port
   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                                               |    Protocol
   |   Direction                        Addresses                              |
   ~                                                               ~
   |        Reserved                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Network Source


   Address
         Four Family
     Two octet source IPv4 address.

       Network Destination quantity containing a value from ADDRESS FAMILY NUMBERS
     in [rfc1700] that encodes the addresses contained in the Addresses
     field.

   Addresses
     A list of addresses from the specified Address Family.  The encod-
     ing of the individual addresses depends on the Address Family.

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

         Address Family      Address
         Four Encoding

         IPv4                4 octet destination full IPv4 address.

       NOTE*NOTE*NOTE*NOTE*NOTE*NOTE: address










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         For generality the address encodings here should include an
         Address Family field, etc.

       END NOTE * END NOTE * END NOTE:


       Source Port
         Two octet source port.

       Destination Port
         Two octet destination port.

       Protocol
         Protocol type.

       Direction
         One octet indicating the direction


3.4.4. COS TLV

   The COS (Class of the LSP.  Field is set to
         1 on Downstream; Service) TLV may appear as an optional field is set to 2 on Upstream.

       NOTE*NOTE*NOTE*NOTE*NOTE*NOTE:

         Use of this FEC is not fully specified in this version of the
         protocol

       END NOTE * END NOTE * END NOTE:



3.3.1.1. FEC Procedures

   If in decoding a FEC TLV an LSR encounters a FEC Element type it can-
   not decode, it should stop decoding the FEC TLV, abort processing the
   message containing the TLV, and send an Ack/Nack message to its LSR
   peer signalling an error.



3.3.2. Label TLVs

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

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







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3.3.2.1. Generic Label TLV

   An LSR uses Generic Label TLVs  It is used to encode labels for use on links for
   which label values are independent of the underlying link technology.
   Examples of such links are PPP
   request and Ethernet. advertise (Label, FEC, class of service) bindings.  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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Generic Label (0x0200)
   |U|F| COS (0x0102)              |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Label                                                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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



3.3.2.2. 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     ATM Label (0x0201)                                                               |      Length
   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Res| V     COS Value                                                 |          VPI
   |              VCI                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Res
     This


   COS Value
     The value field for this TLV is reserved. It must be set to zero on transmission and
     must be ignored on receipt.

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

   VPI
     Virtual Path Identifier. If VPI a subject for further study.

     One possibility is less than 12-bits it should be
     right justified in this field and preceding bits should be to define a set of CoS values that map to
     0.




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   VCI
     Virtual Connection Identifier. If the VCI is less than 16- bits, it
     should Dif-
     ferentiated Services [DIFFSERV] code points.  Other CoS values
     could be right justified supported in the field and the preceding bits must
     be set addition to 0. If Virtual Path switching is indicated or in place of the V-bits
     field, then this Differentiated
     Services code points.


3.4.5. Hop Count TLV

   The Hop Count TLV appears as an optional field must be ignored by the receiver and in messages that set to 0
     by
   up LSPs.  It calculates the sender.



3.3.2.3. Frame Relay Label TLV

   An number of LSR uses Frame Relay Label TLVs to encode labels hops along an LSP as the
   LSP is being setup.

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

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Frame Relay Label (0x0202)
   |U|F| Hop Count (0x0103)        |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Reserved        |Len|                 DLCI     HC Value  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

   Len
     This field specifies the number of bits of the DLCI. The following
     values are supported:
        0 = 10 bits DLCI
   +-+-+-+-+-+-+-+-+


   HC Value
     1 = 17 bits DLCI
        2 = 23 bits DLCI

   DLCI
     The Data Link Connection Identifier.  Refer to
     draft-ietf-mpls-fr-01.txt [FR] for the label values and formats.




3.3.3. Address List TLV

   The Address List TLV appears in Address and Address Withdraw mes-
   sages.

   Its encoding is: octet unsigned integer hop count value.




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


   Address Family
     Two octet quantity containing


3.4.5.1. Hop Count Procedures

   During setup of an LSP an LSR may receive a value from ADDRESS FAMILY NUMBERS
     in Assigned Numbers [ref] Label Mapping or Label
   Request message for the LSP that encodes contains the addresses contained in Hop Count TLV.  If it
   does, it should record the Addresses field.

   Addresses
     A list of addresses from hop count value.  If the specified Address Family.  The encod-
     ing of LSR then pro-
   pagates the individual addresses depends on Label Mapping message for the Address Family. LSP to an upstream peer or
   the Label Request message to a downstream peer to continue the LSP
   setup, it must increment the recorded hop count value and include it
   in a Hop Count TLV in the message.  The following address encodings are defined by this version of first LSR in the
     protocol:

         Address Family      Address Encoding

         IPv4                4 octet full IPv4 address




3.3.4. COS TLV LSP should
   set the hop count value to 1.

   By convention a value of 0 indicates an unknown hop count.  The COS (Class
   result of Service) TLV may appear as incrementing an optional field unknown hop count is itself an unknown hop
   count (0).

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

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


3.4.6. Path Vector TLV

   The Path Vector TLV is used with the Hop Count TLV in Label Request
   and Label Mapping messages that carry to implement the optional LDP loop detec-
   tion 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 mappings.
   advertisement has traversed to setup an LSP.

   Its encoding is:















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     COS (0x0102)
   |U|F| Path Vector (0x0104)      |        Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            LSR Id 1                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     COS Value                                                               |
   ~                                                               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   COS Value
     The COS Value may be one
   |                            LSR Id n                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   One or more LSR Ids
     A list of several types, encoded as a 1 octet
     type followed by a variable length, type-dependent value.  Note
     that router-ids indicating the encoding path of LSRs the COS value message has
     traversed.  Each LSR Id is not the standard LDP TLV
     encoding.  Note also that the length IP address (router-id) component of
     the type-dependent value
     can be derived from LDP identifier for the length of corresponding LSR.  This ensures it is
     unique within the COS TLV. LSR network.


3.4.6.1. Path Vector Procedures

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

         COS Name     Type code    Value

         IP Prec      0x01         1 octet IP Precedence


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


3.4.6.1.1. Label Request Path Vector

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

   An LSR encounters that receives a COS type it cannot
   decode, it should stop decoding Path Vector in a Label Request message must
   perform the COS TLV, abort processing procedures described in Section "Loop Detection".

   If the
   message containing LSR detects a loop, it must reject the TLV, and send an Ack/Nack Label Request message.
   The LSR must:

      1. Transmit a Notification message to its the sending LSR
   peer signalling an error.



3.3.5. Hop Count signaling
         "Loop Detected".

      2. Not propagate the Label Reqeust message further.

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





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      1. A loop is found,

      2. The LSP egress is reached,

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


3.4.6.1.2. Label Mapping Path Vector

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

   An LSR that set
   up LSPs.  It calculates receives a Path Vector in a Label Mapping message must
   perform the procedures described in Section "Loop Detection".

   If the number of LSR hops along detects a loop, it must reject the Label Mapping message
   in order to prevent a forwarding loop.  The LSR must:

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

      2. Not propagate the message further.

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

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

      1. A loop is found,

      2. An LSP ingress is reached, or

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


3.4.7. Status TLV

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

   The encoding for the Status TLV is:






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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Hop Count (0x0103)
   |U|F| Status (0x0300)           |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     HC Value                     Status Code                               |
   +-+-+-+-+-+-+-+-+


   HC Value
     1 octet
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Message Type             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Status Code
     32-bit unsigned integer hop count value.



3.3.5.1. Hop Count Procedures

   During setup of an LSP an LSR 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 the LSR then passes a
   Label Mapping message for encoding the LSP to an upstream peer or event being signaled.  The
     structure of a Label



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   Request to a downstream peer to continue the LSP setup, it must
   increment the recorded hop count value and include it in a Hop Count
   TLV in the message.  The first LSR in the LSP should set the hop
   count value to 1.

   If an LSR receives a Label Mapping message containing a Hop Count
   TLV, it must check the hop count value to determine whether the hop
   count has wrapped (hop count value = 0).  If so, it must reject the
   Label Mapping message in order to prevent a forwarding loop.



3.3.6. Path Vector TLV

   The Path Vector TLV is used in messages that implement LDP loop
   detection and prevention.  It records the path of LSRs a label adver-
   tisement has traversed to setup an LSP.  Its encoding 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Path Vector (0x0104)        |      Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            LSR Id 1                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                                                               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            LSR Id n
     |E|F|                 Status Data                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   One or more LSR Ids
     A list of router-identifiers indicating the path of LSRs

     E bit
       Fatal error bit.  If set (=1), this is a fatal error notifica-
       tion.  If clear (=0), this is an advisory notification.

     F bit
       Forward bit.  If set (=1), the map-
     ping message has traversed.  Each router-id must notification should be forwarded
       to the router-id
     component of the LDP identifier LSR for the corresponding LSR.  This
     ensures it is unique within the LSR network.



3.3.6.1. Path Vector Procedures

   During setup of an LSP an LSR may receive a Label Mapping message next-hop or previous-hop for the LSP that contains LSP, if any,
       associated with the Path Vector TLV. event being signaled.  If it does, clear (=0), the LSR must
   pass a Label Mapping message for
       notification should not be forwarded.

     Status Data
       30-bit unsigned integer which specifies the LSP to status information.

     This specification defines Status Codes (32-bit unsigned integers
     with the upstream peer(s) to
   continue above encoding).

     A Status Code of 0 signals success.

   Message ID
     If non-zero, 32-bit value that identifies the LSP setup.  This peer message must include a Path Vector to which
     the Status TLV
   in refers.  If zero, no specific peer message is being
     identified.

   Message Type
     If non-zero, the message.  The value type of the path vector in peer message to which the Path Vector Status TLV



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   must be the received path vector with the LSRs own LSR Id appended to
   it.

   If an LSR receives a Label Mapping message containing a Path Vector
   TLV, it must check the path vector value to determine whether the
   vector contains its own LSR-id.


     refers.  If so, it must reject the Label Map-
   ping message in order to prevent a forwarding loop.

   The Path Vector TLV is also used in zero, the Label Query message.  See
   Sections "Loop Detection" and "Loop Prevention via Diffusion" for
   more details.



3.3.7. Status TLV

   Notification messages carry Status TLVs does not refer to specify events being sig-
   nalled.

   The encoding for any specific
     peer message.


3.5. LDP Messages

   All LDP messages have the Status TLV is: 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              |     Status (0x0300)           |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Status Code                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                                               |
   +                                                               +
   |                     Mandatory Parameters                      |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Message Type                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Status Code
     32-bit unsigned integer encoding the event being signalled.  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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |F|E|                 Status Data
   +                                                               +
   |                     Optional Parameters                       |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     F


   U bit
       Fatal error
     Unknown message bit.  If set (=1), this is a fatal error notifica-
       tion.  If clear (=0), this is  Upon receipt of an advisory notification.




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     E bit
       End-to-end bit.  If set (=1), the notification should be for-
       warded to the LSR for the next-hop or previous-hop for the LSP, unknown message, if any, associated with the event being signalled.  If U is
     clear (=0), the a notification should not be forwarded.

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

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

     A Status Code of 0 signals success.

   Message ID
     If non-zero, 32-bit value that identifies is returned to the peer message to which originator;
     if U is set (=1), the Status TLV refers.  If zero, no specific peer unknown message is being
     identified. silently ignored.

   Message Type
     If non-zero,
     Identifies the type of the peer message to which

   Message Length
     Specifies the Status TLV
     refers.  If zero, cumulative length in octets of the Status TLV does not refer Message ID, Manda-
     tory Parameters, and Optional Parameters.

   Message Id
     32-bit value used to any specific
     peer identify this message.


3.4. LDP Messages

   All LDP  Used by the sending
     LSR to facilitate identifying notification messages have that may apply
     to this message.  An LSR sending a notification message in response
     to this message should include this Message Id in the following TLV 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Message Type              |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                     Mandatory Parameters                      |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                     Optional Parameters                       |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ notification
     message; see Section "Notification Message".




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   Message Type
     Identifies the type of message

   Message Length
     Specifies the length of the message value component (Mandatory plus
     Optional Parameters) in octets

   Message Id
     Four octet 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".


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

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

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

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


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

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

       Message Name            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           0x0401           "Label Mapping Message"
       Label Request           0x0402           "Label Request Message"
       Label Withdraw          0x0403          "Label Withdraw Message"
       Label Release           0x0404           "Label Release Message"
       Label Query             0x0405   "Label Query Message"
       Explicit Route Request  0x0500   "Explicit Route Request Message"
       Explicit Route Response 0x0501   "Explicit Route Response Message"




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

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

   While 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.




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   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.



3.4.1.


3.5.1. Notification Message

   An LSR sends a Notification message to inform an LDP peer of a signi-
   ficant event.  A Notification message signals a fatal error or pro-
   vides advisory information regarding an item such as the processing outcome of processing an LDP messages
   message or the state of the LDP session.

   The encoding for the Notification Message is:

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


   Message Id
     Four octet integer
     32-bit value used to identify this message.

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




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

         Optional Parameter     Type     Length  Value

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


     Other Optional Parameters, specific to the particular event being
     signalled
     signaled by the Notification Messages may appear.  These are



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     described elsewhere.

       Extended Status
         The 4 octet value is an Extended Status Code that encodes addi-
         tional information that supplements the status information con-
         tained in the Notification Status Code.



3.4.1.1. Notification Message Procedures

   If an

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

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


3.5.1.1. Notification Message Procedures

   If an LSR encounters a condition requiring it to notify its peer with
   advisory or error information it sends the peer a Notification mes-
   sage containing a Status TLV that encodes the information and option-
   ally additional TLVs that provide more information about the event.

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

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



3.4.1.2.


3.5.1.2. Events Signalled Signaled by Notification Messages

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





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3.4.1.2.1.


3.5.1.2.1. Malformed PDU or Message

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

   An LDP PDU received on a TCP connection for an LDP session is mal-
   formed 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 session.  This is a fatal error signalled signaled by
       the Bad LDP Identifier Status Code.

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

     - The PDU Length field is too short (< 20) or too long
       (> TBD). 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.  See Section "Unknown Message Types"
       for more detail.

       If the Message Type is < 0x80000000 0x8000 (high order bit = 0) it is a
       fatal error signalled signaled by the Unknown Message Type Status Code.

       If the Message Type is >= 0x8000000 0x8000 (high order bit = 1) it is
       silently discarded.

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



3.4.1.2.2.


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:





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     - 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 signalled signaled by the Bad TLV Length Status Code.

     - The TLV type is unknown.  See Section "Unknown TLV in Known Mes-
       sage Type" for more detail.

       If the TLV type is < 0x80000000 0x8000 (high order bit 0) it is a fatal
       error signalled signaled by the Unknown TLV Status Code.

       If the TLV type is >= 0800000000 08000 (high order bit 1) the TLV is
       silently dropped.  Section "Unknown TLV in Known Message Type"
       elaborates on this behavior.

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



3.4.1.2.3.


3.5.1.2.3. Session Hold Timer Expiration

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


3.4.1.2.4.


3.5.1.2.4. Unilateral Session Shutdown

   This is a non-fatal fatal event signalled signaled by the Shutdown Status Code.  The
   Notification Message may optionally include an Extended Status TLV to
   provide a reason for the Shutdown.  Note that although this is a
   "non-fatal" event, the  The sending LSR terminates the
   session immediately after sending the Notification.


3.4.1.2.5.


3.5.1.2.5. Initialization Message Events

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






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3.4.1.2.6. Message".


3.5.1.2.6. Events Resulting From Other Messages

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


3.4.1.2.7. Explicitly Routed LSP Setup Events

   Establishment of an Explicitly Routed LSP may fail for a variety of
   reasons.  All such failures are considered non-fatal conditions and
   they are signalled by the Explicit Response Message.


3.4.1.2.8.



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3.5.1.2.7. Miscellaneous Events

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



3.4.2.


3.5.2. Hello Message

   LDP Hello Messages are exchanged as part of the LDP Discovery Mechan-
   ism; see Section "LDP Discovery".

   The encoding for the Hello Message is:

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


   Message Id
     Four octet integer
     32-bit value used to identify this message.

   Optional

   Common Hello Parameters
     This variable length field contains 0 or more parameters, each
     encoded as a TLV.  The optional TLV
     Specifies parameters defined by this version
     of common to all Hello messages.  The encoding
     for the protocol are




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         Optional Parameter    Type     Length  Value

         Targeted Common Hello        0x0400 Parameters TLV is:

      0      --
         Send Targeted Hello   0x0401                   1                   2                   3
      0      --
         Transport Address     0x0402 1 2 3 4      See below
         Hello Hold Time       0x0403 5 6 7 8 9 0 1 2 3 4      See below


     Targeted Hello
       This Hello is a Targeted Hello.  Without this optional parameter
       the Hello is a Link Hello.

     Send Targeted Hello
       Requests the receiver to send periodic Targeted Hellos to the
       source of this Hello.  An LSR initiating Extended Discovery uses
       this option.

     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. 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |U|F| 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 below) When present, this parameter Section "Hello Message Pro-
       cedures").  Hello Hold Time specifies the time in
       seconds the sending LSR
       will maintain its record of Hellos from the receiving LSR without
       receipt of another Hello.  When not
       present,



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

       A value of 0 means use the default.  There are interface type
       specific defaults for Link Hellos as well as a default for Targeted Tar-
       geted Hellos.



3.4.2.1.  A value of 0xfffff means infinite.

     T, Targeted Hello Message Procedures

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

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

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

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

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

           Optional Parameter    Type     Length  Value

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


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



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         sending LSR.


3.5.2.1. Hello Message Procedures

   An LSR receiving Hellos from another LSR maintains a Hello adjacency
   for
   corresponding to the Hellos.  The LSR maintains a hold timer with the
   Hello adja-
   cency 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 "Maintain-
   ing Hello Adjacencies" and "Maintaining LDP Sessions".

   A LSR processes

   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:







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      1. The LSR checks whether the Hello is acceptable.  The criteria
         for determining whether a Hello is acceptable are implementa-
         tion 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 common PDU header for the
         Hello, it attempts to establish a session for follows the label space;
         see section procedures of Section "LDP Session Establishment". Estab-
         lishment".

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

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

       A Targeted Hello from IP source address a.b.c.d is acceptable if
       either:







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

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

       The following describes how an LSR processes Hello optional TLVs:

           Targeted Hello
             No special processing required.

           Send Targeted Hello
             If the Send Targeted Hello option is carried by the Hello,
             the LSR checks whether it has been configured to send Tar-
             geted Hellos to the Hello source in response to Hellos with
             this option.  If not, it ignores the option.  If so, it
             initiates periodic transmission of Targeted Hellos to the
             Hello source.

       Transport Address
         The LSR associates the specified transport address with the



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         Hello adjacency.

           Hello Hold Time
             A pair of LSRs negotiate the hold times they use for Hellos
             from each other.  Each LSR proposes a hold time in its Hel-
             los either explicitly by including the Hold Time

       Configuration Sequence Number
         The Configuration Sequence Number optional
             TLV or implicitly by omitting it.  The hold time parameter is used by
         the LSRs is sending LSR to signal configuration changes to the minimum of receiv-
         ing LSR.  When a receiving LSR playing the hold times proposed active role in LDP
         session establishment detects a change in their
             Hellos.

       We recommend that the interval between Hello transmissions be at
       most one third of sending LSR con-
         figuration, it may clear the Hello hold time.



3.4.3. Initialization Message

   The LDP Initialization Message is exchanged as part of session setup backoff delay, if
         any, associated with the LDP ses-
   sion establishment procedure; see sending LSR (see Section "LDP Session Establish-
   ment".

   The encoding for "Session Ini-
         tialization").

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


3.5.3. Initialization Message

   The LDP Initialization Message is exchanged as part of the LDP ses-
   sion establishment procedure; see Section "LDP Session Establish-
   ment".

   The encoding for the Initialization Message is:

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





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   Message Id
     Four octet integer
     32-bit value used to identify this message.

   Common Session Parameters TLV
     Specifies values proposed by the sending LSR for parameters common
     to all LDP sessions.

     The encoding for the Basic Common Session Parameters TLV is:







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


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

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

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

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












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         Optional Parameter       Type     Length  Value

         Label Allocation         0x0501     1     See below
            Discipline
         Loop Detection           0x0502     0      --
         Merge                    0x0503     1     See below
         ATM Null Encapsulation   0x0504     0      --
         ATM Label Range          0x0600     8     See below
         Frame Relay Label Range  0x0601     8     See below

       A, Label Allocation Advertisement Discipline
         Indicates the type of Label allocation. advertisement.  A value of 0 is means
         Downstream allocation, A Unsolicited advertisement; a value of 1 is Downstream means Down-
         stream On Demand.

         If this optional parameter is not specfied, one LSR proposes Downstream alloca-
         tion is used.

       Loop Detection Unsolicted and the other pro-
         poses Downstream on Demand, the rules for resolving this
         difference is:



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           - If present, indicates that Loop Detection the session is enabled.  If
         absent, Loop Detection is disabled.

       Merge
         Specifies the merge capabilities of an for a label-controlled ATM link or a
             label-controlled Frame Relay
         switch.  The following values are supported link, then Downstream on
             Demand must be used.

           - Otherwise, Downstream Unsolicted must be used.

         If the label advertisement discipline determined in this version of way is
         unacceptable to an LSR, it must send a Session
         Rejected/Parameters Advertisement Mode Notification message in
         response to the specification:

                   Value          Meaning

                     0            Merge Initialization message and not supported

                   For ATM Merge: establish the
         session.

       D, Loop Detection
         Indicates whether loop detection based on path vectors is
         enabled.  A value of 0 means loop detection is disabled; a
         value of 1            VP Merge supported
                     2            VC Merge supported
                     3            VP & VC Merge supported

                   For Frame Relay Merge:
                     Non-zero     Merge supported


       ATM Null Encapsulation
         If present, specifies means that loop detection is enabled.

       PVLim, Path Vector Limit
         The configured maximum path vector length.  Must be 0 if loop
         detection is disabled (D = 0).  If the loop detection pro-
         cedures would require the LSR supports to send a path vector that
         exceeds this limit, the null
         encapsulation of [rfc1483] LSR will behave as if a loop had been
         detected for its data VCs on the ATM link
         managed by 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 LDP session.  In this case IP packets are
         carried directly inside AAL5 frames.  If absent, network be
         configured with the null
         encapsulation is same path vector limit.  Although
         knowledege of a peer's path vector limit will not supported.


       ATM Label Range



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         Used when change an LDP session manages label exchange for
         LSR's behavior, it does enable the LSR to alert an ATM link.
         The ATM Label Range TLV contains 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 may NOT establish an adjacency with neighbors whose
         intersection range is NULL.

        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 operator to
         a possible misconfiguration.

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

         Minimum VPI (12 bits)
           This 12 bit field specifies the lower bound of a block of
           Virtual Path Identifiers

       Max PDU Length
         Two octet unsigned integer that is supported on proposes the originating
           switch.  If maximum allowable
         length for LDP PDUs for the VPI is session.  A value of 255 or 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 default maximum length of
           Virtual Connection Identifiers that is supported on 4096 octets.

         The receiving LSR MUST calculate the ori-
           ginating switch. 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 VCI maximum PDU length determined this way is less than 16-bits it should
           be right justified in this field and preceding bits should be
           set unacceptable
         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 an LSR, 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 must send a block of
           Virtual Connection Identifiers that is supported on the ori-
           ginating switch.  If the VCI is less than 16-bits it should
           be right justified Session Rejected/Parameters Max PDU
         Length Notification message in this field and preceding bits should be
           set response to 0. the Initialization



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       Frame Relay Label Range
         Used when an


         message and not establish the session.

       Receiver LDP session manages label exchange for a Frame
         Relay link.  The Frame Relay Label Range TLV contains Identifer
         Identifies the receiver's label
         range supported by space.  This LDP Identifier,
         together with the transmitting LSR.  A receiving LSR MUST
         calculate sender's LDP Identifier in the intersection between PDU header
         enables the received range and receiver to match the Initialization message with
         one of its
         own supported label range.  The intersection Hello adjacencies; see Section "Hello Message Pro-
         cedures".

         If there is no matching Hello adjacency, the range LSR must send a
         Session Rejected/No Hello Notification message in
         which response to
         the LSR may allocate Initialization message and accept labels.  LSRs may NOT not establish an adjacency with neighbors whose intersection range
         is NULL. the session.

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

         Optional Parameter       Type     Length  Value

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


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

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |U|F|   ATM Sess Parms (0x0501) | Reserved        |Len|                 Minimum DLCI      Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | M |   N   |E|                        Reserved                 |                     Maximum DLCI
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 ATM Label Range Component 1                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         Len
           This field specifies
       |                                                               |
       ~                                                               ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 ATM Label Range Component N                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       M, ATM Merge Capabilities
         Specifies the number of bits merge capabilities of the DLCI. an ATM switch.  The
           following follow-
         ing values are supported:

                Len    DLCI bits

                0       10
                1       17
                2       23




3.4.3.1. Initialization Message Procedures

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



3.4.4. KeepAlive Message

   An LSR sends KeepAlive Messages as part of a mechanism that monitors
   the integrity supported in this version of the LDP session transport connection.

   The encoding for the KeepAlive Message is: specification:



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                   Value          Meaning

                     0            Merge not supported
                     1            VP Merge supported
                     2            VC Merge supported
                     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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     KeepAlive (0x0201)        |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Message Id
     Four octet integer used to identify this message.

   Optional Parameters
     No optional parameters are defined for            VP & VC Merge supported

         If the KeepAlive message.



3.4.4.1. KeepAlive Message Procedures merge capabilities of the LSRs differ, then:

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

           - The Hold Timer mechanism described in Section "Maintaining LDP Ses-
   sions" resets interoperability of VP-merge-capable switches with
             non-VPN-merge-capable switches is a seesion hold timer every time an LDP PDU subject for future
             study.

         Note that if VP merge is received.
   The KeepAlive Message used, it is provided to allow reset the responsibility of the Hold Timer in
   circumstances where an LSR has no other information to communicate
         ingress node to
   an LDP peer.

   An ensure that the chosen VCI is unique within the
         LSR must arrange domain.

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

       E, ATM Null Encapsulation
         A value of 1 specifies specifies that the LSR supports the null
         encapsulation of [rfc1483] for its peer sees an data VCs on the ATM link
         managed by the LDP Message from it at
   least every Hold Time period. That message may be any other from session.  In this case IP packets are car-
         ried directly inside AAL5 frames.  A value of 0 specifies that
         the
   protocol or, in circumstances where there null encapsulation is no need to send one of
   them, it not supported.

       Reserved
         This field is reserved.  It must be KeepAlive Message.



3.4.5. Address Message

   An set to zero on transmission
         and ignored on receipt.

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

         A receiving LSR sends MUST calculate the Address Message to an LDP peer to advertise intersection between the
         received range and its
   interface addresses. own supported label range.  The encoding for inter-
         section is the Address Message is:











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         labels.  LSRs MUST NOT establish a session with neighbors for
         which the intersection of ranges is NULL.  In this case, the
         LSR must send a Session Rejected/Parameters Label Range Notifi-
         cation message in response to the Initialization message and
         not establish the session.

         The encoding for an ATM Label Range Component is:



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


   Message Id
     Four octet integer used


         Res
           This field is reserved. It must be set to identify this message.

   Address List TLV
     The list zero on transmis-
           sion and must be ignored on receipt.

         Minimum VPI (12 bits)
           This 12 bit field specifies the lower bound of interface addresses being advertised by a block of
           Virtual Path Identifiers that is supported on the sending
     LSR.  The encoding for originating
           switch.  If the Address List TLV VPI is specified less than 12-bits it should be right
           justified in Section
     "Address List TLV".

   Optional Parameters
     No optional parameters are defined for this field and preceding bits should be set to
           0.

         Minimum VCI (16 bits)
           This 16 bit field specifies the Address message.



3.4.5.1. Address Message Procedures

   An LSR lower bound of a block of
           Virtual Connection Identifiers that receives an Address Message message uses is supported on the addresses ori-
           ginating switch.  If the VCI is less than 16-bits it
   learns should
           be right justified in this field and preceding bits should be
           set to maintain a database for mapping between peer LDP Identif-
   iers and next hop addresses; see section "LDP Identifiers and Next
   Hop Addresses".

   When 0.

         Maximum VPI (12 bits)
           This 12 bit field specifies the upper bound of a new LDP session block of
           Virtual Path Identifiers that is initialized and before sending Label Map-
   ping or Label Request messages and LSR should advertise its interface
   addresses with one or more Address messages.

   Whenever an LSR "activates" a new interface address, supported on the originating
           switch.  If the VPI is less than 12-bits it should adver-
   tise be right
           justified in this field and preceding bits should be set to
           0.

         Maximum VCI (16 bits)
           This 16 bit field specifies the new address with an Address message.

   Whenever an LSR "de-activates" upper bound of a previously advertised address, block of
           Virtual Connection Identifiers that is supported on the ori-
           ginating switch.  If the VCI is less than 16-bits it should withdraw the address with
           be right justified in this field and preceding bits should be
           set to 0.

     Frame Relay Session Parameters
       Used when an Address Withdraw message; see
   Section "Address Withdraw Message". LDP session manages label exchange for a Frame Relay
       link to specify Frame Relay-specific session parameters.







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3.4.6. Address Withdraw Message

   An LSR sends the Address Message to an LDP peer to withdraw previ-
   ously advertised interface addresses.

   The encoding for the Address Withdraw Message is:


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |U|F|   FR Sess Parms (0x0502)  |     Address Withdraw (0x0301) |      Message      Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Message ID M |   N   |                          Reserved                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Frame Relay Label Range Component 1               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Address List TLV                                                               |
       ~                                                               ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Optional Parameters             Frame Relay Label Range Component N               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Message Id
     Four octet integer used to identify this message.

   Address list TLV


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

                   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.

       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 interface addresses being withdrawn Frame Relay Label Range Components which together
         specify the Label range supported by the sending transmitting LSR.
     The encoding for

         A receiving LSR MUST calculate the Address list TLV intersection between the
         received range and its own supported label range.  The inter-
         section is specified the range in Section
     "Address List TLV".

   Optional Parameters
     No optional parameters are defined 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 Address Withdraw mes-
     sage.



3.4.6.1. Address Withdraw Message Procedures

   See Section "Address Message Procedures"



3.4.7. Label Mapping Message

   An
         LSR sends must send a Session Rejected/Parameters Label Mapping Range Notifi-
         cation message in response to an LDP peer to advertise
   FEC-label bindings to the peer.

   The encoding for the Label Mapping Message is: Initialization message and



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         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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |     Label Mapping (0x0400)    |      Message Length Reserved    |Len|                     Minimum DLCI            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Reserved        |                     FEC-Label Mapping TLV 1                     Maximum DLCI            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                                                               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC-Label Mapping TLV n                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Message Id
     Four octet integer used


         Reserved
           This field is reserved.  It must be set to identify this message.

   FEC-Label Mapping TLV
     Each zero on transmis-
           sion and ignored on receipt.

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

                Len    DLCI bits

                0       10
                1       17
                2       23


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

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

   Note that contains a FEC TLV, a Label TLV,
     an optional COS TLF, an optional Hop Count TLV, there is no Generic Session Parameters TLV for sessions
   which advertise Generic Labels.


3.5.3.1. Initialization Message Procedures

   See Section "LDP Session Establishment" and an optional
     Path Vector TLV: particularly Section
   "Session Initialization" for general procedures for handling the



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   Initialization Message.


3.5.4. KeepAlive Message

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

   The encoding for the KeepAlive Message is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|   KeepAlive (0x0201)        |   FEC-label Mapping (0x0700)  |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC TLV                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Label TLV                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     COS TLV (optional)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Hop Count TLV (optional)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Path Vector TLV (optional)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

   NOTE*NOTE*NOTE*NOTE*NOTE*NOTE:




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     Need to add multipath possibility to above by allowing multiple
     label TLVs


   Message Id
     32-bit value used to the FEC-label Mapping TLV.  This will be done with
     the addition:

              Label TLV2 (optional)
              ...
              Label TLVn (optional)

     with discussion.

   END NOTE * END NOTE * END NOTE: identify this message.

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



3.4.7.1. Label Mapping


3.5.4.1. KeepAlive Message Procedures

   The Mapping message Hold Timer mechanism described in Section "Maintaining LDP Ses-
   sions" resets a session hold timer every time an LDP PDU is used by received.
   The KeepAlive Message is provided to allow reset of the Hold Timer in
   circumstances where an LSR has no other information to distribute a label mapping
   for a FEC communicate to its LDP peers.  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 peer.

   An LSR must arrange that mapping to all its
   peers, or whether peer receive an LDP Message from it uses at
   least every Hold Time period.  Any LDP protocol message will do but,
   in circumstances where no other LDP protocol messages have been sent
   within the period, a different mapping for each of its peers. KeepAlive message must be sent.


3.5.5. Address Message

   An LSR is always responsible for the consistency of sends the label map-
   pings it has distributed, and that its peers have these mappings.



3.4.7.1.1. Independent Control Mapping

   If Address Message to an LSR is configured for independent control, a mapping message is
   transmitted by an LSR LDP peer to peers upon any of the following conditions:

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

      2. advertise its
   interface addresses.

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

      3. The next hop for an FEC changes to another LDP peer, and loop
         detection is configured. Address Message is:



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      4.


    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|   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 attributes list of a mapping change.

      5. interface addresses being advertised by the sending
     LSR.  The receipt of a mapping from encoding for the downstream next hop  AND
            a) no upstream mapping has been created  OR
            b) loop detection Address List TLV is configured  OR
            c) the attributes of specified in Section
     "Address List TLV".

   Optional Parameters
     No optional parameters are defined for the mapping have changed.



3.4.7.1.2. Ordered Control Mapping

   If an Address message.


3.5.5.1. Address Message Procedures

   An LSR is doing ordered control, a Mapping that receives an Address Message message is transmitted
   by downstream LSRs upon any of uses the following conditions:

      1. The LSR recognizes addresses it
   learns to maintain a new FEC via the forwarding table, and is
         the egress database for that FEC.

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

      3. The next hop for an FEC changes to another LDP peer, addresses; see Section "LDP Identifiers and loop
         detection is configured.

      4. The attributes of a mapping change.

      5. The receipt of Next
   Hop Addresses".

   When a mapping from the downstream next hop  AND
            a) no upstream mapping has been created   OR
            b) loop detection new LDP session is configured   OR
            c) the attributes of the mapping have changed.


3.4.7.1.3. Downstream-on-Demand initialized and before sending Label Advertisement

   In general, the upstream Map-
   ping or Label Request messages an LSR is responsible for requesting label map-
   pings when operating in Downstream-on-Demand mode.  However, unless
   some rules are followed, it is possible for neighboring LSRs should advertise its interface
   addresses 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 one or more Address messages.

   Whenever an LSR
   and Rd is "activates" a new interface address, it should adver-
   tise the downstream new address with an Address message.

   Whenever an LSR for "de-activates" a particular FEC.  In this example,
   Ru is using Downstream allocation 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 adver-
   tise a label if previously advertised address, it wants Ru to use one.  If Rd and Ru operate as sug-
   gested, no labels will be distributed and packets must be routed at
   layer-3.
   should withdraw the address with an Address Withdraw message; see
   Section "Address Withdraw Message".







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   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.
   However, if all interfaces on an LSR are configured to operate in
   Downstream- on-Demand mode the LSR can wait to issue a request until
   a corresponding request has been sent from an upstream LSR.


3.4.7.1.4. Downstream Allocation Label Advertisement

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



3.4.8. Label Request


3.5.6. Address Withdraw Message

   An LSR sends the Label Request Address Message to an LDP peer to request a
   binding (mapping) for one or more specific FECs. withdraw previ-
   ously advertised interface addresses.

   The encoding for the Label Request 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Label Request (0x0401)
   |U    Address Withdraw (0x0301) |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC-Request TLV 1                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                                                               ~
   |                     Address List TLV                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC-Request TLV n                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Message Id
     Four octet integer
     32-bit value used to identify this message.

   FEC-Request

   Address list TLV



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     Each specifies an FEC for which a label mapping is requested.  A
     FEC-Request
     The list of interface addresses being withdrawn by the sending LSR.
     The encoding for the Address list TLV is a nested TLV that contains a FEC TLV, an specified in Section
     "Address List TLV".

   Optional Parameters
     No optional COS TLV, and parameters are defined for the Address Withdraw mes-
     sage.


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 optional Hop Count TLV. LDP peer to advertise
   FEC-label bindings to the peer.

   The encoding for the Label Mapping Message is:





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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|   Label Mapping (0x0400)    |   FEC-Request (0x0701)        |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC TLV                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     COS                     Label TLV (optional)                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

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

   Label TLV
     Specifies the Label component of the FEC-Label mapping.  See Sec-
     tion "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
         COS TLV               1            See below
         Hop Count TLV (optional)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         1            See below
         Path Vector TLV       variable     See below

     The encodings for the FEC, COS, and Hop Count Count, and Path Vector TLVs are specified can be
     found in Section "Commonly Used TLVs".

 Optional Parameters
   No optional parameters are defined "TLV Encodings for the Label Request message.


3.4.8.1. Commonly Used Parameters".

       Label Request Message Procedures

   The Request Id
         If this Label Mapping message is used by an upstream LSR a response to explicitly request a Label Request
         message that carried the downstream LSR assign and advertise a label for an FEC.

   An LSR transmits a Return Message Id optional parameter
         (see Section "Label Request Message") the Label Mapping message under any of
         must include the following condi-
   tions:

      1. Request Message Id optional parameter.  The LSR recognizes a new FEC via the forwarding table, and the
         next hop
         value of this optional parameter is an Operational LDP peer, and the LSR doesn't
         already have a mapping from Message Id of the next hop for
         corresponding Label Request Message.



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       COS
         Specifies the given FEC.

      2. The  next hop Class of Service (COS) to be associated with the FEC changes, and
         FEC-Label mapping.  If not present, the LSR doesn't already
         have a mapping from that next hop should use its
         default COS for IP packets as the given FEC.

   If a request cannot be satisfied by the downstream LSR, the request-
   ing LSR may optionally choose to request again at a later time, or,
   if COS.

       Hop Count
         Specifies the downstream LSR is configured for Downstream Allo- cation, running total of the
   requesting number of LSR may wait for hops along the mapping, assuming that
         LSP being setup by the downstream
   LSR will provide Label Message.  Section "Hop Count Pro-
         cedures" describes how to handle this TLV.

       Path Vector
         Specifies the mapping automatically when it is available.

   NOTE*NOTE*NOTE*NOTE*NOTE*NOTE:

     In LSRs along the case where LSP being setup by the downstream LSR is doing DoD, Label Mes-
         sage.  Section "Path Vector Procedures" describes how does the



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     requesting LSR decide when to make its request?

     TDP addresses handle
         this issue TLV.


3.5.7.1. Label Mapping Message Procedures

   The Mapping message is used by having an LSR to distribute a "now I have label resources"
     message which it sends to downwstream peers whose requests it has
     denied.  This serves as mapping
   for a signal to them FEC to re-issue their
     requests. an LDP should probably have this.  Without such a signal,
     the denied requester has no recourse but to periodically retry.

   END NOTE * END NOTE * END NOTE:



3.4.9. Label Withdraw Message

   An peer.  If an LSR sends distributes a Label Withdraw Message mapping for a FEC
   to an multiple LDP peer peers, it is a local matter whether it maps a single
   label to signal the
   peer FEC, and distributes that the peer may not continue mapping to use specific FEC-label all its peers, or
   whether it uses a different mapping for each of its peers.

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

   See Appendx A "LDP Label Distribution Procedures" for more details.


3.5.7.1.1. Independent Control Mapping

   If an LSR is configured for independent control, a mapping message is
   transmitted by the LSR had previously advertised.  This breaks upon any of the mapping
   between following conditions:

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

      2. The encoding LSR receives a Request message from an upstream peer for a
         FEC present in 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Label Withdraw (0x0402)   |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC-Withdraw-Release TLV 1                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                                                               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC-Withdraw-Release TLV n                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Message Id
     Four octet integer used to identify this message.

   FEC-Withdraw-Release TLV
     Each TLV specifies a FEC-label mapping being withdrawn.  A FEC-
     Withdraw-Release TLV is a nested TLV that contains LSR's forwarding table.

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







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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | FEC-Withdraw-Release (0x0702) |      Length                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     FEC TLV                                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Label TLV (optional)                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      4. The encodings for the FEC and Label TLVs are specified in Section
     "Commonly Used TLVs".

     NOTE*NOTE*NOTE*NOTE*NOTE*NOTE:

       Need to add multipath possibility to above by allowing multiple
       label TLVs to attributes of a mapping change.

      5. The receipt of a mapping from the FEC-label Mapping TLV.  This will be done with downstream next hop  AND
            a) no upstream mapping has been created  OR
            b) loop detection is configured  OR
            c) the addition:

                Label TLV2 (optional)
                ...
                Label TLVn (optional)

       with discussion.

     END NOTE * END NOTE * END NOTE:


 Optional Parameters
   No optional parameters are defined for attributes of the Label Withdraw message.



3.4.9.1. Label Withdraw Message Procedures

   An mapping have changed.


3.5.7.1.2. Ordered Control Mapping

   If an LSR transmits is doing ordered control, a Withdraw Mapping message under is transmitted
   by downstream LSRs upon any of the following condition: conditions:

      1. The LSR no longer recognizes a previously known new FEC via the forwarding table, and is
         the egress for that FEC.

      2. Optionally, the The LSR has unspliced receives a Request message from an upstream label from the
         downstream label.

   The peer for a
         FEC present in the FEC-Withdraw-Release TLV 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 which requesting label map-
   pings 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 be withdrawn. use one.  If Rd and Ru
   operate as suggested, no label TLV follows the FEC, all labels associ-
   ated with the FEC are will be distributed from Rd to Ru.

   This livelock situation can be withdrawn, else only the labels specified
   in avoided if the following Label TLV are to rule is
   observed: an LSR operating in Downstream-on-Demand mode should not be withdrawn.



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3.4.10. Label Release Message

   An


   expected to send unsolicited mapping advertisements.  Therefore, if
   the downstream LSR sends is operating in Downstream-on-Demand mode, the
   upstream LSR is responsible for requesting label mappings as needed.


3.5.7.1.4. Downstream Unsolicited Label Advertisement

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


3.5.8. Label Release message Request Message

   An LSR sends the Label Request Message to an LDP peer to signal the
   peer that the LSR no longer needs specific FEC-label mappings previ-
   ously requested of and/or advertised by the peer. request a
   binding (mapping) for a FEC.

   The encoding for the Label Release Request Message is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   |U|   Label Release (0x0403) Request (0x0401)    |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC-Withdraw-Release TLV 1                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                                                               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC-Withdraw-Release                     FEC TLV n                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Optional Parameters                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Message Id
     Four octet integer
     32-bit value used to identify this message.

   FEC-Withdraw-Release TLVs
     Each

   FEC TLV specifies a FEC-label mapping being released.
     The encod-
     ing FEC for the FEC-Withdraw-Release TLV which a label is specified in being requested.  See Section "With-
     draw Message".

     NOTE*NOTE*NOTE*NOTE*NOTE*NOTE:

       Need to add multipath possibility to above by allowing multiple
       label TLVs to the FEC-label Mapping TLV. "FEC
     TLV" for encoding.

   Optional Parameters
     This will be done with
       the addition:

                Label TLV2 (optional)
                ...
                Label TLVn (optional)

       with discussion.

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

         Optional Parameter    Length       Value

         Return Message Id     0            See below
         COS TLV               1            See below
         Hop Count TLV         1            See below



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   Optional Parameters
     No optional parameters are defined


         Path Vector TLV       variable     See below

     The encodings for the Label Release message.



3.4.10.1. Label Release Message Procedures

   An LSR transmits a Release message to a COS, Hop Count, and Path Vector TLVs can be
     found in Section "TLV Encodings for Commonly Used Parameters".

       Return Message Id
         Requests the LDP peer when it is no longer
   needs include the Message Id of this Label
         Request message in its Label Mapping message response.  If an
         LDP peer receives a label previously received from or Label Request message with the Return Mes-
         sage Id optional parameter, its Label Mapping message response
         must contain a Label Request Message Id optional parameter with
         the Message Id of the Label Request message.  See Section
         "Label Mapping Message".

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

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

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


3.5.8.1. Label Request Message Procedures

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

   An LSR transmits may transmit a Release Request message under any of the following condi-
   tions: con-
   ditions:

      1. The LSR which sent recognizes a new FEC via the label mapping is no longer forwarding table, and the
         next hop
         for the mapped FEC, is an LDP peer, and the LSR is configured doesn't already have a
         mapping from the next hop for conservative
         operation. the given FEC.

      2. The next hop to the FEC changes, and the LSR determines that doesn't already
         have a previously received label is no
         longer valid, as mapping from that next hop for the downstream given FEC.







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      3. The LSR receives a Label Request for a FEC from which it was received
         is no longer an upstream LDP
         peer, the FEC next hop for the FEC, is an LDP peer, and the LSR is config-
         ured for conservative operation.

      3. doesn't
         already have a mapping from the next hop.

   The receiving LSR has received should respond to a Withdraw Label Request message for with a previously
         received label.

   Note that if an LSR is configured
   Label Mapping for "liberal mode", the requested label or with a release mes-
   sage will never be transmitted in Notification message
   indicating why it cannot satisfy the case request.

   This version 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 protocol defines the next hop following Status Codes for
   the FEC. Notification message that signals a request cannot be satisfied:

     No Route
       The FEC in the FEC-Withdraw-Release TLV for which a label was requested is for a Prefix FEC Ele-
       ment, and the LSR does not have a route for which labels are
   to be released.  If no that prefix.

     No Label Resources
       The LSR cannot provide a label TLV follows the FEC TLV, all labels
   associated with because of resource limitations.
       When resources become available the FEC are to be released, else only LSR must notify the labels
   specified in request-
       ing LSR by sending a Notification message with the following Label TLV are to be released.



3.4.11. Label Query Message
       Resources Available Status Code.

       An LSR sends that receives a No Label Query message Resources response to an LDP peer when performing a Label
       Request message must not issue further Label Request messages
       until it receives a Notification message with the
   loop prevention diffusion algorithm on Label Resources
       Available Status code.

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

   See Appendx A "LDP Label Distribution Procedures" for more details.


3.5.9. Label Withdraw Message

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

   The encoding for the Label Query Withdraw Message is:











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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   |U|   Label Query (0x0405) Withdraw (0x0402)   |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC TLV                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Path Vector                     Label TLV (optional)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Message Id
     Four octet integer
     32-bit value used to identify this message.

   The encodings for

   FEC TLV
     Identifies the FEC and Path Vector TLVs can be found in Sec-
   tion "Commonly Used TLVs". for which the FEC-label mapping is being with-
     drawn.

   Optional Parameters
     No
     This variable length field contains 0 or more parameters, each
     encoded as a TLV.  The optional parameters are defined are:

         Optional Parameter    Length       Value

         Label TLV             variable     See below

     The encoding for the Label Query message.



3.4.11.1. TLVs are found in Section "Label TLVs".

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


3.5.9.1. Label Query Withdraw Message Procecures

   See Section "Loop Prevention Procedures

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

      1. The LSR no longer recognizes a previously known FEC.

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

   The FEC TLV specifies the FEC for handling which labels are to be withdrawn.
   If no Label TLV follows the Query Message.



3.4.12. Explicit Route Request Message

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       ER Request (0x0500)     |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Message ID                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC-ER TLV 1                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                                                               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     FEC-ER TLV n                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ FEC, all labels associated with the FEC



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   Message Id
     Four octet integer used


   are to identify this message.

   FEC-ER TLV
     Each specifies a binding between an FEC and a label.  A FEC-ER be withdrawn; otherwise only the label specified in the
   optional Label TLV is a nested to be withdrawn.

   The FEC TLV that contains a may contain the Wildcard FEC TLV, a Element; if so, it may con-
   tain no other FEC Elements.  In this case, if the Label TLV, Withdraw mes-
   sage contains an explicit-
     route identifier (ERLSPID) optional Label TLV, then the explict-route TLV, label is to be with-
   drawn from all FECs to which it is bound.  If there is not an
   optional
     COS TLF, and Label TLV in the Label Withdraw message, then the sending
   LSR is withdrawing all label mappings previously advertised to the
   receiving LSR.

   See Appendx A "LDP Label Distribution Procedures" for more details.


3.5.10. Label Release Message

   An LSR sends a Label Release message to an optional Bandwith Reservation TLV: LDP peer to signal the
   peer that the LSR no longer needs specific FEC-label mappings previ-
   ously requested of and/or advertised by the peer.

   The encoding for the Label Release Message is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|   Label Release (0x0403)   |         FEC-ER TLV  (0x0703)  |      Message Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    FEC TLV                                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    ERLSPID TLV                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Explicit Route TLV                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    COS                     FEC TLV (optional)                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Bandwidth Reservation                     Label TLV (optional)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     The encodings for


   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




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         Label TLV             variable     See below

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

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


3.5.10.1. Label Release Message Procedures

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

   An LSR must transmit a Label Release message under any of the follow-
   ing conditions:

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

      2. The LSR receives a label mapping from an LSR which is not the
         next hop for the FEC, and the LSR is configured for conserva-
         tive operation.

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

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

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

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

   See Appendx A "LDP Label Distribution Procedures" for more details.




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3.6. Messages and TLVs for Extensibility

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

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


3.6.1. LDP Vendor-private Extensions

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


3.6.1.1. LDP Vendor-private TLVs

   The Type range 0x2F00 through 0x2FFF is reserved for vendor-private
   TLVs.

   The encoding for a vendor-private TLV is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|F|    Type (0x2F00-0x2FFF)   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Vendor ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                           Data....                            |
   ~                                                               ~
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

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




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

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

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

   Vendor Id
     802 Vendor ID as assigned by the IEEE.

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


3.6.1.2. LDP Vendor-private Messages

   The Message Type range 0x2F00 through 0x2FFF is reserved for vendor-
   private Messages.























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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |U|    Msg Type (0x2F00-0x2FFF) |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Vendor ID                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +                                                               +
   |                     Remaining Mandatory Parameters            |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                     Optional Parameters                       |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

     The determination as to whether a vendor-private message is under-
     stood is based on the Msg Type and the Vendor ID parameter.

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



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

   Optional Parameters
     Variable length set of optional message parameters.


3.6.2. LDP Experimental Extensions

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

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

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

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

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

       Administration of Experiment IDs is the responsiblity of the
       experimenters.



3.7. Message Summary

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
















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       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"
       Vendor-Private          0x2F00-0x2FFF
       Experimental            0x3F00-0x3FFF



3.8. TLV Summary

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

       TLV                      Type      Section Title

       FEC                      0x0100    "FEC TLV"
       Address List             0x0101    "Address List TLV"
       COS                      0x0102    "COS TLV"
       Hop Count                0x0103    "Hop Count TLV"
       Path Vector              0x0104    "Path Vector TLV"
       Generic Label            0x0200    "Generic Label TLV"
       ATM Label                0x0201    "ATM Label TLV"
       Frame Relay Label        0x0202    "Frame Relay Label TLV"
       Status                   0x0300    "Status TLV"
       Extended Status          0x0301    "Notification Message"
       Returned PDU             0x0302    "Notification Message"
       Returned Message         0x0303    "Notification Message"
       Common Hello             0x0400    "Hello Message"
          Parameters
       Transport Address        0x0401    "Hello Message"
       Configuration            0x0402    "Hello Message"
          Sequence Number
       Common Session           0x0500    "Initialization Message"
          Parameters
       ATM Session Parameters   0x0501    "Initialization Message"
       Frame Relay Session      0x0502    "Initialization Message"
          Parameters
       Vendor-Private           0x2F00-0x2FFF
       Experimental             0x3F00-0x3FFF




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3.9. Status Code Summary

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

       Status Code               Type          Section Title

       Success                   0x00000000    "Status TLV"
       Bad LDP Identifer         0x80000001    "Events Signaled by ..."
       Bad Protocol Version      0x80000002    "Events Signaled by ..."
       Bad PDU Length            0x80000003    "Events Signaled by ..."
       Unknown Message Type      0x80000004    "Events Signaled by ..."
       Bad Message Length        0x80000005    "Events Signaled by ..."
       Unknown TLV               0x80000006    "Events Signaled by ..."
       Bad TLV length            0x80000007    "Events Signaled by ..."
       Malformed TLV Value       0x80000008    "Events Signaled by ..."
       Hold Timer Expired        0x80000009    "Events Signaled by ..."
       Shutdown                  0x8000000A    "Events Signaled by ..."
       Loop Detected             0x0000000B    "Loop Detection"
       Unknown FEC               0x0000000C    "FEC Procedures"
       No Route                  0x0000000D    "Label Request Mess ..."
       No Label Resources        0x0000000E    "Label Request Mess ..."
       Label Resources Available 0x0000000F    "Label Request Mess ..."
       Session Rejected/         0x80000010    "Session Initialization"
          No Hello
       Session Rejected/         0x80000011    "Session Initialization"
          Parameters Advertisement Mode
       Session Rejected/         0x80000012    "Session Initialization"
          Parameters Max PDU Length
       Session Rejected/         0x80000013    "Session Initialization"
          Parameters Label Range




3.10. UDP and TCP Ports

   The UDP port for LDP Hello messages is 646.

   The TCP port for establishing LDP session connections is 646.











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4. Security

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

   It is based on use of the TCP MD5 Signature Option specified in
   [rfc2385] for use by BGP.  See [rfc1321] for a specification of the
   MD5 hash function.


4.1. The TCP MD5 Signature Option

   The following quotes from [rfc2385] outline the security properties
   achieved by using the TCP MD5 Signature Option and summarizes its
   operation:

      "IESG Note

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

      "Abstract

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

      "Introduction

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

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



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         TCP implementations with changing passwords).

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

      "MD5 as a Hashing Algorithm

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

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

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

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

   End of quotes from [rfc2385].


4.2. LDP Use of the TCP MD5 Signature Option

   LDP uses the TCP MD5 Signature Option as follows:

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

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

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



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

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


5. Intellectual Property Considerations

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


6. Acknowledgments

   The ideas and text in this document have been collected from a number
   of sources. We would like to thank Rick Boivie, Ross Callon, Alex
   Conta, Eric Gray, Yoshihiro Ohba, Eric Rosen, Bernard Suter, Yakov
   Rekhter, and Arun Viswanathan.


7. References

   [ARCH] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol Label
   Switching Architecture", draft-ietf-mpls-arch-02.txt, July 1998

   [ATM] B. Davie, J. Lawrence, K. McCloghrie, Y. Rekhter, E. Rosen, G.
   Swallow, P. Doolan, "Use of Label Switching With ATM", draft-ietf-
   mpls-atm-00.txt, September, 1998

   [DIFFSERV] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang, W.
   Weiss, "An Architecture for Differentiated Services", draft-ietf-
   diffserv-arch-02.txt, October, 1998

   [ENCAP] E. Rosen, Y. Rekhter, D. Tappan, D. Farinacci, G. Fedorkow,
   T. Li, A. Conta, "MPLS Label Stack Encoding" draft-ietf-mpls-label-
   encaps-02.txt, July, 1998

   [FR] A. Conta, P. Doolan, A. Malis, "Use of Label Switching on Frame
   Relay Networks" draft-ietf-mpls-fr-02.txt, October, 1998




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   [FRAMEWORK] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swal-
   low, A. Viswanathan, "A Framework for Multiprotocol Label Switching"
   draft-ietf-mpls-framework-02.txt, November 1997

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

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

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

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

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

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


8. Author Information

   Loa Andersson
   Nortel Networks Inc
   Kungsgatan 34, PO Box 1788
   111 97 Stockholm
   Sweden
   Phone: +46 8 441 78 34,  Mobile: +46 70 522 78 34
   email: loa_andersson@baynetworks.com

   Paul Doolan
   Ennovate Networks
   330 Codman Hill Rd
   Marlborough MA 01719
   Phone: 978-263-2002
   email: pdoolan@ennovatenetworks.com

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

   Andre Fredette
   Nortel Networks Inc
   3 Federal Street



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   Billerica, MA  01821
   Phone:  978-916-8524
   email: fredette@baynetworks.com

   Bob Thomas
   Cisco Systems, Inc.
   250 Apollo Dr.
   Chelmsford, MA 01824
   Phone:  978-244-8078
   email: rhthomas@cisco.com









































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

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

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

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

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

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

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

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

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





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

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

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

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

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

         . Request Never. The LSR never requests a label.

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

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

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

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

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

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

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





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

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

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

     - Label No Route procedure (called Label Not Available procedure in
       [ARCH]), which is performed by an upstream LSR to determine how
       to respond to a No Route notification from a downstream LSR in
       response to a request for a FEC label mapping.  The architecture
       specification defines two Label No Route procedures:

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

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


A.1. Handling Label Distribution Events

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










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A.1.1. Receive Label Request

 Summary:

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

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

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

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

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

 Context:

     - LSR. The LSR handling the event.

     - MsgSource. The LDP peer that sent the message.

     - FEC. The FEC specified in the message.

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

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

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

 Algorithm:

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

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

   LRq.3   Execute procedure Send_Notification (MsgSource, No Route).
           Goto LRq.11.

   LRq.4   Has LSR previously received a label request for FEC from
           MsgSource?
           If not, goto LRq.6.  (See Note 1.)



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   LRq.5   Is the label request a duplicate request?
           If so, Goto LRq.11.  (See Note 2.)

   LRq.6   Record label request for FEC received from MsgSource and mark
           it pending.

   LRq.7   Perform LSR Label Distribution procedure:

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

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

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

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

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

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

               1.  Is LSR egress for FEC? OR
                   Has LSR previously received and retained a label map-
                   ping for FEC from Next Hop?
                   If not, goto LRq.8.

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

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

   LRq.8   Perform LSR Label Request procedure:

             For Request Never




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               1.  Goto LRq.11.

             For Request When Needed OR
             For Request On Request

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

               2.  Execute procedure Send_Label_Request (Next Hop, FEC,
                   SAttributes).
                   Goto LRq.11.

   LRq.9   Has LSR successfully sent a label for FEC to MsgSource?
           If not, goto LRq.11.  (See Note 3.)

   LRq.10  Perform LSR Label Use procedure.

             For Use Immediate OR
             For Use If Loop Not Detected

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

   LRq.11  DONE

 Notes:

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

      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).







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      3. The Send_Label procedure may fail due to lack of label
         resources, in which case the LSR should not perform the Label
         Use procedure.


A.1.2. Receive Label Mapping

 Summary:

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

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

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

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

 Context:

     - LSR. The LSR handling the event.

     - MsgSource. The LDP peer that sent the message.

     - FEC. The FEC specified in the message.

     - Label. The label specified in the message.

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

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

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

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

 Algorithm:

   LMp.1   Does the received label mapping match an outstanding label
           request for FEC previously sent to MsgSource.
           If not, goto LMp.9.




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   LMp.2   Delete record of outstanding FEC label request.

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

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

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

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

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

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

   LMp.9   Determine the Next Hop for FEC.

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

   LMp.11  Perform LSR Label Release procedure:

             For Conservative Label retention:

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

             For Liberal Label retention:

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

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

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



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

   LMp.15  Install Label for forwarding/switching use.

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

   LMp.17  Iterate through for LMp.25 for each Peer, other than
           MsgSource.

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

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

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

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

   LMp.22  Update record of label mapping for FEC previously sent to
           Peer to include the new attributes sent.
           Goto LMp.24.

   LMp.23  Perform LSR Label Distribution procedure:

             For Downstream Unsolicited Independent Control OR
             For Downstream Unsolicited Ordered Control

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

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

               3.  Goto LMp.24.

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

               1.  Does LSR have a label request for FEC from Peer



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                   marked as pending?
                   If not, continue iteration for next Peer at LMp.17.

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

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

               4.  Goto LMp.24.

   LMp.24  Perform LSR Label Use procedure:

             For Use Immediate OR
             For Use If Loop Not Detected

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

   LMp.25  End iteration from LMp.17.

   LMp.26  DONE.

 Notes:

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

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

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

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



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         has no way to check the consistency of a newly received Path
         Vector.  This means that whenever such an LSR receives a map-
         ping message carrying a Path Vector it must always propagate
         the Path Vector.

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


A.1.3. Receive Label Release

 Summary:

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

 Context:

     - LSR. The LSR handling the event.

     - MsgSource. The LDP peer that sent the message.

     - Label. The label specified in the message.

     - FEC. The FEC specified in the message.

 Algorithm:

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

   LRl.2   Does message match an outstanding label withdraw for FEC pre-
           viously sent to MsgSource?
           If not, goto LRl.4

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

   LRl.4   Is LSR merging labels for this FEC?
           If not, goto LRl.6.  (See Note 2.)



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   LRl.5   Has LSR previously advertised a label for this FEC to other
           peers?
           If so, goto LRl.10.

   LRl.6   Is LSR egress for the FEC?
           If so, goto LRl.10

   LRl.7   Is there a Next Hop for FEC? AND
           Does LSR have a previously received label mapping for FEC
           from Next Hop?
           If not, goto LRl.10.

   LRl.8   Is LSR configured to propagate releases?
           If so, goto LRl.10.  (See Note 3.)

   LRl.9   Execute procedure Send_Message (Next Hop, Label Release, FEC,
           Label from Next Hop).

   LRl.10  Remove Label from forwarding/switching use for traffic from
           MsgSource.

   LRl.11  Do any peers still hold Label for FEC?
           If so, goto LRl.13.

   LRl.12  Free the Label.

   LRl.13  DONE.

 Notes:

      1. If LSR is using Downstream Unsolicted label distribution, it
         should not re-advertise a label mapping for FEC to MsgSource
         until MsgSource requests it.

      2. LRl.4 through LRl.8 deal with determining whether where the LSR
         should propagate the label release to a downstream peer
         (LRl.9).

      3. If LRl.8 is reached, no upstream LSR holds a label for the FEC,
         and the LSR holds a label for the FEC from the FEC Next Hop.
         The LSR could propagate the Label Release to the Next Hop. By
         propagating the Label Release the LSR releases a potentially
         scarce label resource. In doing so, it also increases the
         latency for re-establishing the LSP should MsgSource or some
         other upstream LSR send it a new Label Request for FEC.

         Whether or not to propagate the release is not a protocol
         issue. Label distribution will operate properly whether or not



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         the release is propagated. The decision to propagate or not
         should take into consideration factors such as: whether labels
         are a scarce resource in the operating environment; the impor-
         tance of keeping LSP setup latency low by keeping the amount of
         signalling required small; whether LSP setup is ingress-
         controlled or egress-controlled in the operating environment.


A.1.4. Receive Label Withdraw

 Summary:

     When an LSR receives a label withdraw message for a FEC from an LDP
     peer, it responds with a label release message and it removes the
     label from any forwarding/switching use. If ordered control is in
     use, the LSR sends a label withdraw message to each LDP peer to
     which it had previously sent a label mapping for the FEC. If the
     LSR is using Downstream on Demand label advertisement with indepen-
     dent 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?



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           If not, goto LWd.13.

   LWd.7   Generate Event: Recognize New FEC for FEC.
           Goto LWd.13.  (See Note 2.)

   LWd.8   Iterate through LWd.12 for each Peer, other than MsgSource.

   LWd.9   Has LSR previously sent a label mapping for FEC to Peer?
           If not, continue interation for next Peer at LWd.8.

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

   LWd.11  Execute procedure Send_Label_Withdraw (Peer, FEC, Label pre-
           viously sent to Peer).

   LWd.12  End iteration from LWd.8.

   LWd.13  DONE

 Notes:

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

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

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


A.1.5. Recognize New FEC

 Summary:

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

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





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     - 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 map-
                   ping 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.




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

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

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

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

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

               1.  Goto FEC.2.  (See Note 2.)

   FEC.2   Has LSR previously received and retained a label mapping for
           FEC from Next Hop?
           If so, goto FEC.5

   FEC.3   Is Next Hop an LDP peer?
           If not, Goto FEC.6

   FEC.4   Perform LSR Label Request procedure:

             For Request Never

               1.  Goto FEC.6

             For Request When Needed OR
             For Request On Request

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

               2.  Execute procedure Send_Label_Request (Next Hop, FEC,
                   SAttributes).
                   Goto FEC.6.

   FEC.5   Generate Event: Received Label Mapping from Next Hop.  (See
           Note 3.)

   FEC.6   DONE.





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 Notes:

      1. An example of an attribute that might be part of InitAttributes
         is CoS. The means by which FEC InitAttributes, if any, are
         specified is beyond the scope of LDP. Note that the InitAttri-
         butes 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 COS TLVs discard-
         ing the label request; see LRq.3

      3. If the LSR has a label for the FEC from the Next Hop, it should
         behave as if it had just received the label from the Next Hop.
         This occurs in the case of Liberal label retention mode.


A.1.6. 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 mappping messages for the FEC to one or
       more LDP peers;

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

     - Any of the actions that can be found in Section
     "Commonly Used TLVs".

     ERLSPID TLV occur when the LSR receives a label
       mapping from the FEC's new next hop.

 Context:

     - LSR. The globally unique value that identifies LSR handling the explicit route. event.

     - FEC. The encoding FEC whose next hop changed.

     - New Next Hop. The current next hop for the ERLSPID 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         ERLSPID (0x0801)      |      Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Explicit Identifier                     |
       +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                     Peg Explicit Identifier                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Explicit Identifier
         A 6-octet globally unique value that identifies the explicit FEC.







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         route LSP.  It is generated by


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

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

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

     - SAttributes. Attributes to be included in Label Label Request message.  The first four octets
       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,
           OldLlabel).

    NH.5   Delete label mapping for FEC previously received from Old
           Next Hop.

    NH.6   Has LSR previously received and retained a label mapping for
           FEC from New Next Hop?
           If not, goto NH.8.

    NH.7   Generate Event: Received Label Mapping from New Next Hop.
           Goto NH.11.  (See Note 2.)

    NH.8   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.9. If not, goto NH.11.

    NH.9   Execute procedure Prepare_Label_Request_Attributes (Next Hop,
           FEC, CurAttributes, SAttributes)

    NH.10  Execute procedure Send_Label_Request (New Next Hop, FEC, SAt-
           tributes).
           (See Note 4.)

    NH.11  DONE.




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 Notes:

      1. If Label is not in forwarding/switching use, NH.2 has no
         effect.

      2. If the LSR IP
         Address.  The last two octets contain has a `Local identifier'
         value.  It is incumbent on an LSR that originates an Explicit
         Request message to choose an unused value label for the Local Iden-
         tifier.

       Peg Explicit Identifier
         A 6-octet globally unique value that identifies a loose segment
         of an explicit route LSP.  It is generated by FEC from the upstream peg
         LSR that creates New Next Hop, it
         should behave as if it had just received the loose segment.  The first four octets is label from the LSR IP Address. New
         Next Hop.

      3. The last two octets contain a 'Local iden-
         tifier' value.  It is incumbent purpose of the check on a peg LSR that creates a
         loose segment label retention mode is to choose an unused value for the Local Identif-
         ier every time avoid a
         race with steps LMp.10-LMp.11 of the segment is reestablished.  When procedure for handling a segment is
         strictly routed this field is set to zero by
         Label Mapping message where the sender and
         ignored by LSR operating in Conservative
         Label retention mode may have released a label mapping received
         from the receiver.

     Explicit Route TLV
       The sequence of ER New Next Hop (ERNH) TLVs and a pointer to before it detected the one
       that should be processed by FEC next hop had
         changed.

      4. Regardless of the LSR that receives this ER TLV.
       The encoding for Label Request procedure in use by the Explicit Route 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Explicit Route TLV  (0x0800) |      Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Next ERNH TLV Pointer     |     Reserved        |P|Preempt|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 ERNH TLV  (Variable length)                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Next ERNH TLV Pointer
         This 16 bit unsigned integer points to LSR, it
         must send a label request if the offset conditions in octets of
         the next ERNH TLV to be processed.  The first octet after NH.8 hold.
         Therefore it executes the
         two reserved octets that follow this pointer is defined to have Send_Label_Request procedure directly
         rather than perform LSR Label Request procedure.


A.1.7. Receive Notification / No Label Resources

 Summary:

     When an offset value of zero.  For example LSR receives a No Label Resources notification from an ERNH TLV Pointer value
         of zero would point LDP
     peer, it stops sending label request messages to the first ERNH TLV in peer until it
     receives a Label Resources Available Notification from the sequence of
         ERNH Objects.

       P bit
         when set indicates peer.

 Context:

     - LSR. The LSR handling the event.

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

     - MsgSource. The LDP peer that sent the loosely routed segments must remain
         pinned-down.  ERLSP must be rerouted only when adjacency Notification message.

 Algorithm:

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

   NoRes.2 Record label mapping for FEC from MsgSource is
         lost along the segment.  When not set indicates loose segment needed but
           that no label resources are available.

   NoRes.3 Set status record indicating it is not pinned down and must be changed OK to match the underlying
         hop-by-hop path. send label
           requests to MsgSource.



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       Preempt
         A 16 level preemption is provided to facilitate placement of
         ERLSP when resources aren't available.  Each LSR maintains this
         value in the ERLSP control block.  A higher preemption value
         can preempt LSPs with lower value.

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

       ERNH TLV
         This TLV contains the four octet IP address of


   NoRes.4 DONE.


A.1.8. Receive Notification / No Route

 Summary:

     When an LSR through
         which the Explicit receives a No Route LSP is to pass and notification from an (optional)
         reservation (RES) TLV LDP peer in
     response to be processed by that LSR. a Label Request message, the Label No Route procedure
     in use dictates its response. The strict TLV indicates that LSR either will take no further
     action, or it will defer the ER LSP setup must be routed
         directly via label request by starting a timer and
     send another Label Request message to the LSR indicated in peer when the ERNH object; i.e. that
         that timer later
     expires.

 Context:

     - LSR. The LSR must be handling the next hop in event.

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

     - Attributes. The attibutes associated with the Explicit Route LSP's path. label request.

     - MsgSource. The loose TLV indicates LDP peer that sent the LSP may be routed in any way;
         i.e. via other unspecified LSRs, so long as it (eventually)
         reaches the Notification message.

 Algorithm:

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

   NoNH.2  Perform LSR specified in the ERNH object.  This TLV may 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
         followed by the optional Reservation TLV.

         The ERNH encodings are:
          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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |      ER Strict TLV  (0x0802)  |      Length                   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                             IPv4 Address                      |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


          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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |      ER Loose TLV  (0x0803)   |      Length                   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                             IPv4 Address                      |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Ipv4 Address
         The IP address of the next sent to MsgSource.

               2.  Start timeout. Goto NoNH.3.

   NoNH.3  DONE.








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A.1.9. Receive Notification / Loop Detected

 Summary:

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

     Bandwidth Reservation TLV
       Specifies notification.

     Context:

         See "Receive Notification / No Route".

     Algorithm:

         See "Receive Notification / No Route"


A.1.10. 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 bandwidth reservation required at each peer.

 Context:

     - LSR. The LSR hop. handling the event.

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

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

 Algorithm:

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

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

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

   Res.4   Execute procedure Send_Label_Request (MsgSource, FEC, SAttri-
           butes).  If the procedure fails, terminate iteration.




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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Bandwidth TLV  (0x0804)  |      Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      BW requirement                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     BW Requirement
       Unsigned 32 bit integer representing


   Res.5   Delete record that no resources are available for a label
           mapping for FEC needed from MsgSource.

   Res.6   End iteration from Res.2

   Res.7   DONE.


A.1.11. Detect local label resources have become available

 Summary:

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

 Context:

     - LSR. The LSR handling the event.

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

 Algorithm:

   ResA.1  Iterate through ResA.4 for each Peer to which LSR has previ-
           ously sent a No Label Resources notification.

   ResA.2  Execute procedure Send_Notification (Peer, Label Resources
           Available)

   ResA.3  Delete record that No Label Resources notification was previ-
           ously 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 bandwidth, in units
           procedure fails, terminate iteration.

   ResA.7  Clear record of
       kilo bps, that must be reserved FEC label mapping needed for peer but no-
           label-resources.

   ResA.8  End iteration from ResA.5




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   ResA.9  DONE.

 Notes:

      1. Iteration ResA.5 through ResA.8 handles the LSP at every LSR identi-
       fied in situation where the ERNH Object.  The bandwidth
         LSR is guaranteed within using Downstream Unsolicited label distribution and was
         previously unable to allocate a
       coarser time period allowing label for simpler implementations.  The
       specified bandwidth is guaranteed within several milliseconds or a few seconds time period.  Nodes FEC.


A.1.12. LSR decides to no longer label switch a FEC

 Summary:

     An LSR may also use this as unilaterally decide to no longer label switch a minimal
       bandwidth guarantee within the same time period.


3.4.12.1. Explicit Route Request Procedures

   See Sections "Explicitly Routing LSPs" and "ERLSP State Machine" FEC for
   general procedures
     an LDP peer. An LSR that does so must send a label withdraw message
     for handling the Explicit Route Request Message.


3.4.13. Explicit Route Response Message

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      ER Response (0x0501)     |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Message ID                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    ERLSPID TLV                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Label TLV                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Status TLV                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ FEC to the peer.

 Context:

     - Peer. The encodings peer.

     - FEC. The FEC.

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

 Algorithm:

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

   NoLS.2  DONE.

 Notes:

      1. The LSR may remove the Label, and Status TLVs can be found label from forwarding/switching use as
         part of this event or as part of processing the label release
         from the peer in response to the label withdraw.


A.1.13. Timeout of deferred label request

 Summary:

     Label requests are deferred in Section
   3.3.3 ("Commonly Used TLVs").

   Message Id
     Four octet integer used response to identify this message. No Route and Loop
     Detected notifications.  When a deferred FEC label request for a
     peer times out, the LSR sends the label request.





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   ERLSPID TLV


 Context:

     - LSR. The globally unique value used for ERLSPID in LSR handling the event.

     - FEC. The FEC associated with the timeout event.

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

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

 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 elicited this Response message.
   handle label distribution events.


A.2.1. Send_Label

 Summary:

     The encoding Send_Label procedure allocates a label for the
     ERLSPID (shown above a FEC for an LDP
     peer, if possible, and repeated here sends a label mapping for convenience) 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         ERLSPID (0x0801)      |      Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Explicit Identifier                     |
       +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                     Peg Explicit Identifier                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     Explicit Identifier
       A 6-octet globally unique value that identifies the explicit
       route LSP.  It is generated by FEC to the LSR that creates
     peer. If the Explicit
       Request message.  The first four octets LSR is unable to allocate the LSR IP Address.
       The last two octets contain label and if it has a `Local identifier' value.  It
     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
       incumbent on an LSR that originates an Explicit Request message to choose an unused value be sent.

     - FEC. The FEC for the Local Identifier.

     Peg Explicit Identifier
       A 6-octet globally unique value that identifies which a loose segment
       of an explicit route LSP.  It label mapping is generated by to be sent.







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

 Additional Context:

     - LSR. The LSR that creates executing the loose segment. procedure.

     - Label. The first four octets is label allocated and sent to Peer.

 Algorithm:

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

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

    SL.3   Install Label for forwarding/switchng use.

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

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

    SL.6   Does LSR IP Address.  The last two octets contain have a 'Local identifier'
       value.  It is incumbent on record of a peg 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 that creates have a loose segment
       to choose an unused value label request for the Local Identifier every time the
       segment is reestablished.  When a segment is strictly routed this
       field is set 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 zero by the sender and ignored by the receiver.



3.4.13.1. Explicit Route Response Procedures

   See Sections "Explicitly Routing LSPs" and "ERLSP State Machine" Peer.
           Goto SL.14.

    SL.13  Record label mapping needed for
   general procedures FEC and Attributes for handling the Explicit Response Request Mes-
   sage. Peer,
           but no-label-resources. (See Note 1.)

    SL.14  Return failure.




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3.5. Messages and TLVs for Extensibility

   The procedures to provide for LDP extensiblity include rules for han-
   dling unknown messages and TLVs.  The rules described in


 Notes:

      1. SL.13 handles the sections
   that follow make use case of Downstream Unsolicited label distri-
         bution when the high order bits in the message or TLV
   type field.  In these rules, "b" represents an arbitray bit value in LSR is unable to allocate a label for a FEC to
         send to a message or TLV type.


3.5.1. Procedures Peer.


A.2.2. Send_Label_Request

 Summary:

     An LSR uses the Send_Label_Request procedure to send a request for Unknown Messages and TLVs

3.5.1.1. Unknown Message Types

   When
     a message with an unknown Message Type is received, there are
   two possibilities as described below.  The choice label for how a FEC to handle an unknown Message Type is determined by the high-order bit of the
   Message Type field. LDP peer if currently permitted to do so.

 Parameters:

     - Message Type = 0bbbbbbbbbbbbbbb Peer. The entire message must be rejected and the event signalled by a
       Notification Message with LDP peer to which the Unknown Message Type Status Code. label request is to be sent.

     - Message Type = 1bbbbbbbbbbbbbbb FEC. The entire message must be dropped silently (i.e., it should FEC for which a label request is to be
       ignored and no error should sent.

     - Attributes. Attributes to be returned).

       In either case described above, an included in the label request. E.g.,
       Hop Count, Path Vector, CoS.

 Additional Context:

     - LSR. The LSR that does not understand executing the message type must not attempt procedure.

 Algorithm:

   SLRq.1  Has a label request for FEC previously been sent to process the message.


3.5.1.2. Unknown TLV in Known Message Type

   When an unknown TLV Peer and
           is found in a known Message Type, there are three
   possibilities it marked as described below.  The choice outstanding?
           If so, Return success.  (See Note 1.)

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

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

   SLRq.4  Record label request for how FEC has been sent to handle an
   unknown TLV is determined by the high-order two bits of the TLV Type
   field.

     - TLV Type = 0bbbbbbbbbbbbbbb

       The entire message must be rejected Peer and the event signalled by a
       Notification Message with the Unknown TLV Status Code.

     - TLV Type = 10bbbbbbbbbbbbbb

       The TLV must be dropped silently (i.e., mark
           it should be ignored as outstanding.

   SLRq.5  Return success.

   SLRq.6  Postpone the label request by recording label mapping for FEC
           and Attributes from Peer is needed but that no error should be returned).  If the semantics of the including label
           resources are available.



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       Message Type dictate that message be forwarded to other nodes,


   SLRq.7  Return failure.

 Notes:

      1. If the TLV LSR is a non-merging LSR it must distinguish between
         attempts to send label requests for a FEC triggered by dif-
         ferent upstream LDP peers from duplicate requests. This pro-
         cedure will not be forwarded with the message.

     - TLV Type = 11bbbbbbbbbbbbbb

       The TLV must be silently ignored (i.e., no error should be
       returned). If send a duplicate label request.


A.2.3. Send_Label_Withdraw

 Summary:

     An LSR uses the semantics of Send_Label_Withdraw procedure to withdraw a label
     for a FEC from an LDP peer. To do this the including Message Type dictate
       that LSR sends a Label With-
     draw message be forwarded to other nodes, the TLV must be for-
       warded unmodified with the message.


3.5.2. peer.

 Parameters:

     - Peer. The LDP Vendor-Private Extensions

   Both Vendor-Private Messages and Vendor-Private Objects are defined peer to convey vendor-private information or LDP extensions between LDP
   nodes. These extensions may also be useful for experimentation in
   existing networks.


3.5.2.1. LDP Vendor-Private TLV

   The following three Vendor-Private TLV classes are defined which the label withdraw is to be used
   in any message: sent.

     - Vendor Private TLV Class 1.  TLV type values:

       0x3FXX (boolean 00111111bbbbbbbb) FEC. The FEC for which a label is being withdrawn.

     - Vendor Private TLV Class 2.  TLV type values:

       0xBFXX (boolean 10111111bbbbbbbb) Label. The label being withdrawn

 Additional Context:

     - Vendor Private TLV Class 3,  TLV type values:

       0xFFXX (boolean 11111111bbbbbbbb)

   These TLVs are LSR. The LSR executing the procedure.

 Algorithm:

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

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


A.2.4. Send_Notification

 Summary:

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






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 Parameters:

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

     - Status. Status code to be included in the high order bit(s) of 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 TLV type. Send_Message procedure to send an LDP peer an LDP
     message.

 Parameters:

     - Peer. The unspecified part of LDP peer to which the TLV message is to be sent.

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

     - Additional message contents . . .  .

 Additional Context:

     None.

 Algorithm:

     This procedure is assigned by the vendor and should be interpreted means by a receiving which an LSR only if it
   understands sends an LDP message of
     the Vendor ID encoded 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 TLV Value field.

   The Value field of attributes include a Vendor Private TLV Hop Count or Path Vector, per-
     form a loop detection check. If a loop is defined as follows: detected, send a Loop
     Detected Notification message to MsgSource.



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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Vendor ID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Data....                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     Vendor


 Parameters:

     - MsgSource. The LDP peer that sent the message.

     - RAttributes. The attributes in the message.

 Additional Context:

     - LSR Id. The unique LSR Id
          802 Vendor ID as assigned by of this LSR.

     - Hop Count. The Hop Count, if any, in the IEEE.

     Data received attributes.

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

 Algorithm:

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

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

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

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

   CRa.5   Return No Loop Detected.

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

   CRa.7   Return Loop Detected.

   CRa.8   DONE


A.2.7. Prepare_Label_Request_Attributes

 Summary:

     This procedure is used whenever a Label Request is to be sent to a
     Peer to compute the Vendor ID Hop Count and Path Vector, if any, to include
     in the Value field
          are optional vendor-dependent data.



3.5.2.2. LDP Vendor-Private Messages message.





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 Parameters:

     - Peer. The LDP Vendor-Private Message peer to which the message is carried in LDP PDUs to convey
   vendor-private information or LDP extensions between LSRs.

   The following two Vendor-Private Message classes are defined:

     - Vendor Private Message Class 1.  Message type values:

               0x7FXX (boolean 01111111bbbbbbbb) be sent.

     - Vendor Private Message Class 2.  Message type values:

               0xFFXX (boolean 11111111bbbbbbbb) FEC. The first TLV in FEC for which a vendor private message must label request is to be sent.

     - RAttributes. The attributes this LSR associates with the Vendor
       Private ID TLV, a Vendor Private Class 3 TLV, encoded as shown
       below:

            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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |      0xFF     |      0x00     |               0x04            |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                          Vendor ID                            |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Vendor-Private messages are LSP for
       FEC.

     - SAttributes. The attributes to be handled according to included in the high
       order bit 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 message type number.  The determination as 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?



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       whether


           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 Vendor-Private message resulting Path Vector into SAttributes.
           Goto PRqA.14.

   PRqA.13 Include Path Vector of length 1 containing LSR Id in SAttri-
           butes.

   PRqA.14 DONE.

         Notes:

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

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


A.2.8. Prepare_Label_Mapping_Attributes

 Summary:

     This procedure is understood used whenever a Label Mapping is based on to be sent to a
     Peer to compute the
       Vendor ID in first TLV Hop Count and Path Vector, if any, to include
     in the message.

 Parameters:

     - Peer. The LDP peer to which the message body.


3.6. TLV Summary is to be sent.

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

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

     - SAttributes. The attributes to be included in this version of the protocol.

       TLV                    Type      Section Title

       FEC                      0x0100    "FEC TLV"
       Address List             0x0101    "Address List TLV"
       COS                      0x0102    "COS TLV"
       Hop Count                0x0103    "Hop Count TLV"
       Path Vector              0x0104    "Path Vector TLV"
       Generic Label            0x0200    "Generic Label TLV"
       ATM Label                0x0201    "ATM Label TLV"
       Frame Relay Label        0x0202    "Frame Relay Label TLV"
       Status                   0x0300    "Status TLV"
       Extended Status          0x0301    "Notification Message"
       Targeted Hello           0x0400    "Hello Message"
       Send Targeted Hello      0x0401    "Hello Message"
       Transport Address        0x0402    "Hello Message"
       Hello Hold Time          0x0403    "Hello Message"
       Common Session           0x0500    "Initialization Message"
          Parameters
       Label Allocation         0x0501    "Initialization Message"
          Discipline
       Loop Detection           0x0502    "Initialization Message"
       Merge                    0x0503    "Initialization Message"
       ATM Null Encapsulation   0x0504    "Initialization Message"
       ATM Label Range          0x0600    "Initialization Message"
       Frame Relay Request
       message.

     - IsPropagating. The LSR is sending the Label Range  0x0601    "Initialization Message"
       FEC-Label Mapping        0x0700    "Label Mapping Message"
       FEC-Request              0x0701    "Label Request Message"
       FEC-Withdraw-Release     0x0702    "Label Withdraw Message"
       FEC-ER TLV               0x0703    "Explicit Request Message"
       Explicit Route           0x0800    "Explicit Request Message"
       ERLSPID                  0x0801    "Explicit Request Message"
       ER Strict                0x0802    "Explicit Request Message"
       ER Loose                 0x0803    "Explicit Request Message"
       Bandwidth                0x0804    "Explicit Request Message" message to
       propagate one received from the FEC next hop.





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3.7. Status Code Summary


     - PrevHopCount. The following are the Status Codes defined in Hop Count, if any, this version of LSR associates with the
   protocol.

       Status Code                   Type      Section Title

       Success                       0x0000    "Status TLV"
       Bad LDP Identifer             0x8001    "Events Signalled by ..."
       Bad Protocol Version          0x8002    "Events Signalled by ..."
       Bad PDU Length                0x8003    "Events Signalled by ..."
       Unknown Message Type          0x8004    "Events Signalled by ..."
       Bad Message Length            0x8005    "Events Signalled by ..."
       Unknown TLV                   0x8006    "Events Signalled by ..."
       Bad TLV length                0x8007    "Events Signalled by ..."
       Malformed TLV Value           0x8008    "Events Signalled by ..."
       Hold Timer Expired            0x8009    "Events Signalled by ..."
       Shutdown                      0x000A    "Events Signalled by ..."
       LSP for the FEC.

 Additional Context:

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

 Algorithm:

   PMpA.1  Is Hop Count required for this Peer (see Note 1.) ? OR
           Do RAttributes include a Hop Count? OR
           Is Loop Detected                 0x000B    "Loop Detection Via Diffusion"



4. Security

   Security considerations will be addressed in a future revision configured on LSR?
           If not, goto PMpA.19.

   PMpA.2  Is LSR egress for FEC?
           If not, goto PMpA.4.

   PMpA.3  Include Hop Count of
   this document.


5. Acknowledgments

   The ideas and text 1 in this document SAttributes. Goto PMpA.19.

   PMpA.4  Do RAttributes have been collected from a number
   of sources. We would like to thank Rick Boivie, Ross Callon, Alex
   Conta, Eric Rosen, Bernard Suter, Yakov Rekhter, and Arun
   Viswanathan.


6. References

   [FRAMEWORK] Callon et al, "A Framework for Multiprotocol Label
   Switching" draft-ietf-mpls-framework-01.txt, July 1997

   [ARCH] Rosen et al, "A Proposed Architecture for MPLS" draft-ietf-
   mpls-arch-02.txt, July 1998

   [ENCAP] Farinacci et al, "MPLS Label Stack Encoding" draft-ietf-
   mpls-label-encaps-02.txt, July, 1998

   [FR] Conta et al, "Use Hop Count?
           If not, goto PMpA.6.

   PMpA.5  Increment RAttributes Hop Count and copy the resulting Hop
           Count to SAttributes. See Note 2. Goto PMpA.7.

   PMpA.6  Include Hop Count of Label Switching unknown (0) in SAttributes.

   PMpA.7  Is Loop Detection configured on Frame Relay Networks" LSR?
           If not, goto PMpA.19.

   PMpA.8  Do RAttributes have a Path Vector?
           If so, goto PMpA.17.

   PMpA.9  Is LSR propagating a received Label Mapping?
           If not, goto PMpA.18.

   PMpA.10 Does LSR support merging?
           If not, goto PMpA.12.

   PMpA.11 Has LSR previously sent a Label Mapping for FEC to Peer?
           If not, goto PMpA.18.

   PMpA.12 Do RAttributes include a Hop Count?
           If not, goto PMpA.19.

   Res.13 Is Hop Count in Rattributes unknown(0)?
           If so, goto PMpA.18.




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   draft-ietf-mpls-fr-01.txt, August,         draft-ietf-mpls-ldp-02.txt          November 1998

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

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

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


7. Author Information

   Loa Andersson
   Bay Networks Inc
   3 Federal Street
   Billerica, MA  01821
   email: Loa_Andersson@baynetworks.com

   Paul Doolan
   Ennovate Networks
   330 Codman Hill Rd
   Marlborough MA 01719
   Phone: 978-263-2002
   email: pdoolan@ennovatenetworks.com

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

   Andre Fredette
   Bay Networks Inc
   3 Federal Street
   Billerica, MA  01821
   Phone:  978-916-8524
   email: fredette@baynetworks.com

   Bob Thomas
   Cisco Systems, Inc.
   250 Apollo Dr.
   Chelmsford, MA 01824
   Phone:  978-244-8078
   email: rhthomas@cisco.com


   PMpA.14 Has LSR previously sent a Label Mapping for FEC to Peer?
           If not goto PMpA.19.

   PMpA.15 Is Hop Count in RAttributes different from PrevHopCount ?
           If not goto PMpA.19.

   PMpA.16 Is the Hop Count in RAttributes > PrevHopCount? OR
           Is PrevHopCount unknown(0)
           If not, goto PMpA.19.

   PMpA.17 Add LSR Id to beginning of Path Vector from RAttributes and
           copy the resulting Path Vector into SAttributes. Goto
           PMpA.19.

   PMpA.18 Include Path Vector of length 1 containing LSR Id in SAttri-
           butes.

   PMpA.19 DONE.

 Notes:

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

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


























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----