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Versions: 00 01 02 03 04 05 06 07                                       
Network Working Group                                     George Swallow
Internet Draft                                       Cisco Systems, Inc.
Category: Standards Track
Expiration Date: August 2004
                                                        Kireeti Kompella
                                                  Juniper Networks, Inc.

                                                              Dan Tappan
                                                     Cisco Systems, Inc.

                                                           February 2004


                    Label Switching Router Self-Test


                  draft-ietf-mpls-lsr-self-test-02.txt

Status of this Memo

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

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

   Abstract

      This document defines a means of self test for a Label-Switching
      Router (LSR) to verify that its dataplane is functioning for
      certain key Multi-Protocol Label Switching (MPLS) applications
      including unicast forwarding based on LDP [LDP] and traffic
      engineering tunnels based on [RSVP-TE].  A new Loopback FEC type



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      is defined to allow an upstream neighbor to assist in the testing
      at very low cost.  MPLS Echo Request and MPLS Echo Reply messages
      [LSP-Ping] are extended to do the actual probing.



Contents

    1      Introduction  ...........................................   3
    1.1    Conventions  ............................................   3
    2      Loopback FEC  ...........................................   4
    2.1    Loopback FEC Element  ...................................   4
    2.2    LDP Procedures  .........................................   5
    3      Data Plane Self Test  ...................................   5
    3.1    Data Plane Verification Request / Reply Messages  .......   7
    3.2    Downstream Verification Object  .........................   8
    3.2.1  IPv4 Downstream Verification Object  ....................   8
    3.2.2  IPv6 Downstream Verification Object  ....................  11
    3.3    Reply-To Object  ........................................  13
    3.3.1  IPv4 Reply-To Object  ...................................  14
    3.3.2  IPv6 Reply-To Object  ...................................  14
    3.4    Sending procedures  .....................................  15
    3.5    Receiving procedures  ...................................  16
    3.6    Upstream Neighbor Verification  .........................  16
    4      Security Considerations  ................................  17
    5      IANA Considerations  ....................................  17
    6      Acknowledgments  ........................................  17
    7      References  .............................................  18
    7.1    Normative References  ...................................  18
    7.2    Informative References  .................................  18
    8      Authors' Addresses  .....................................  18
    9      Intellectual Property Notice  ...........................  19
   10      Full Copyright Statement  ...............................  19


















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1. Introduction

   This document defines a means of self test for a Label-Switching
   Router (LSR) to verify that its dataplane is functioning for certain
   key Multi-Protocol Label Switching (MPLS) applications including
   unicast forwarding based on LDP [LDP] and traffic engineering tunnels
   based on [RSVP-TE].  MPLS Echo Request and MPLS Echo Reply messages
   [LSP-Ping] messages are extended to do the actual probing.  The pings
   are sent to an upstream neighbor, looped back through the LSR under
   test and intercepted, by means of TTL expiration by a downstream
   neighbor.  Extensions to LSP-Ping [LSP-Ping] are defined to allow the
   down stream neighbor to report the test results.

   In order to minimize the load on upstream LSRs a new loopback FEC is
   defined. Receipt of a packet labeled with a loopback label will cause
   the advertising LSR to pop the label off the label stack and send the
   packet out the advertised interface.

   Note that use of a loopback allows an LSR to test label entries for
   which the LSR is not currently some neighbor's next hop.  In this way
   label entries can be verified prior to the occurrence of a routing
   change.

   Some routing protocls, most notably OSPF have no means of exchanging
   the "Link Local Identifiers" used to identify unnumbered links and
   components of bundled links.  These test procedures can be used to
   associate the neighbor's interfaces with the probing LSRs interfaces.
   This is achieved by simply having the TTL of the MPLS Ping expire one
   hop sooner, i.e. at the testing LSR itself.



1.1. Conventions

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














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2. Loopback FEC

   The Loopback FEC type is defined to enable an upstream neighbor to
   assist in LSR self-testing at very low cost.  This FEC causes the
   loopback to occur in the dataplane without control plane involvement
   beyond the initial LDP exchange and dataplane setup.

   An LSR uses the Loopback FEC to selectively advertise loopback labels
   to its neighbor LSRs.  Each loopback label is bound to a particular
   interface.  For multi-access links, a unique label for each neighbor
   is required, since the link-level address is derived from the label
   lookup.  When an MPLS packet with its top label set to a loopback
   label is received from an interface over which that label was
   advertised, the loopback label is popped and the packet is sent on
   the interface to which the loopback label was bound.

   TTL treatment for loopback labels follows the Uniform model.  I.e.
   the TTL carried in the loopback label is decremented and copied to
   the exposed label or IP header as the case may be.



2.1. Loopback FEC Element

   FEC element type 130 is used.   The FEC element is encoded as
   follows: (note: 130 is provisionally assigned, the actual value will
   be assigned by IANA.)


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     130       |      Res      | Interface Type|   Id Length   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Interface Identifier                      |
      |                              "                                |
      |                              "                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Reserved (Res)

         Must be set to zero on transmission and ignored on receipt.








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      Interface Type

            #     Type              Interface Identifier
           ---    ----              --------------------
            0     Unnumbered        A 32 bit Link Identifier as
                                      defined in [RFC3477]
            1     IPv4 Numbered     IPv4 Address
            2     IPv6 Numbered     IPv6 Address


      Identifier Length

         Length of the interface identifier in octets.
         The length is 4 bytes for Unnumbered and IPv4, 16 bytes for IPv6.

      Address

         An identifier encoded according to the Identifier Type field.


2.2. LDP Procedures

   It is RECOMMENDED that loopback labels only be distributed in
   response to a Label Request message, irrespective of the label
   advertisement mode of the LDP session.  However it is recognized that
   in certain cases such as OSPF with unnumbered links, the upstream LSR
   may not have sufficiently detailed information of the neighbor's link
   identifier to form the request.  In these cases, the downstream LSR
   will need to be configured to make unsolicited advertisements.



3. Data Plane Self Test

   A self test operation involves three LSRs, the LSR doing the test, an
   upstream neighbor and a downstream neighbor.  We refer to these as
   LSRs T, U, and D respectively.  In order to minimize the processing
   load on LSRD, two new LSP Ping messages are defined, called the MPLS
   Data Plane Verification Request and the MPLS Data Plane Verification
   Reply.  These messages are used to allow LSRT to obtain the label
   stack, address and interface information of LSRD.

   If FEC verification is required, the MPLS Echo Request and Reply
   messages are used.

   The packet flow is shown below. Although the figure shows LSRD
   adjacent to LSRT it may in some cases be an arbitrary number of hops
   away.



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                 +------+       +------+       +------+
                 |    ,-|-------|<DPVRq|       |      |
                 |    `-|-------|------|-------|->    |
                 |      |       |    <-|-------|<DPVRp|
                 +------+       +------+       +------+
                   LSRU           LSRT           LSRD

               DPVRq: MPLS Data Plane Verification Request
               DPVRp: MPLS Data Plane Verification Reply

                    Figure 1: Self Test Message Flow


   In order to perform a test on an incoming label stack, LSRT forms an
   MPLS Data Plane Verification Request.  Included in that is a Data
   Plane Verification Object which requests that the interface and label
   stack seen by LSRD be returned.  Optionally LSRT could have included
   FEC Stack TLV to verify that LSRD's labels are mapped to the expected
   FECs.  In that case an MPLS Echo Request Message would have been
   used.

   LSRT prepends the packet with the incoming label stack being tested
   and the loopback label received from LSRU.  The TTL values are set
   such that they will expire at LSRD.  LSRT then forwards the packet to
   LSRU.

   LSRU receives the packet and performs normal MPLS forwarding.  That
   is, the loopback label is popped, the TTL is decremented and
   propagated (in this case) to the exposed label.

   LSRT receives the packet and performs normal MPLS forwarding.  If
   everything is functioning as expected this will cause the packet to
   arrive at LSRD with a TTL of 1.

   In this example, we assume that all is working properly.  The TTL
   expires at LSRD causing it to receive the packet LSRD notes the the
   interface and the label stack on which the packet was received and
   records these in a Downstream Verification TLV.  The results are
   recorded in an MPLS Data Plane Verification Reply message and sent to
   LSRT.

   If a FEC Stack TLV had been included, the procedures in [LSP-Ping]
   would be followed.  Additionally, a Downstream Verification TLV would
   be included in the MPLS Echo Reply message.







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3.1. Data Plane Verification Request / Reply Messages

   Two new LSP Ping messages are defined for LSR self test.  The purpose
   of the new messages is two fold.  First the timestamps are removed to
   minimize processing.  Second the message type allows simple
   recognition that minimal processing is necessary to service this
   request.  The definitions of all fields are identical to those found
   in [LSP-PING].

   The new message types are: (Provisionally; to be assigned)

      Type     Message
      ----     -------
        3      MPLS Data Plane Verification Request
        4      MPLS Data Plane Verification Reply

   The messages have the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Version Number        |         Must Be Zero          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Message Type |   Reply mode  |  Return Code  | Return Subcode|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sender's Handle                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            TLVs ...                           |
   .                                                               .
   .                                                               .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The MPLS Data Plane Verification  Request  message  MAY  contain  the
   following objects:

          Type #            Object
          ------            -----------
            3               Pad
            5               Vendor Enterprise Code
            7 (tba)         IPv4 Downstream Verification Object
            8 (tba)         IPv6 Downstream Verification Object
            9 (tba)         IPv4 Reply-to Object
           10 (tba)         IPv6 Reply-to Object




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   The MPLS Data  Plane  Verification  Reply  message  MAY  contain  the
   following objects:

          Type #            Object
          ------            -----------
            3               Pad
            4               Error Code
            5               Vendor Enterprise Code
            7 (tba)         IPv4 Downstream Verification Object
            8 (tba)         IPv6 Downstream Verification Object



3.2. Downstream Verification Object

   The Downstream Verification Object is an optional TLV in an MPLS Echo
   Request or MPLS Verification Request message.  Only one such object
   may appear.  It's presence signifies a request that a Downstream
   Verification Object be included in the corresponding reply message.
   The purpose of the object is to allow the upstream router to obtain
   the exact interface and label stack information as it appears at the
   replying LSR.  It has two formats, type 7 for IPv4 and type 8 for
   IPv6 (to be assigned by IANA).



3.2.1. IPv4 Downstream Verification Object

   In a request message the Length is always 12.  In a reply message the
   length is 16 + 4*N octets, N is the number of Downstream Labels.  The
   value field of a Downstream Verification TLV has the following
   format:



















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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Flags     | Address Type  |            Reserved           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Downstream IPv4 Address                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Downstream Interface Address                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                          Label Stack                          .
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Flags

         Two flags are defined as shown.  All other flags are reserved
         and MUST be set to zero.

            +-+-+-+-+-+-+-+-+
            |0|0|0|0|0|0|V|R|
            +-+-+-+-+-+-+-+-+

         The R flag can only be set in a request message.  It requests
         that the receiving router verify that the Downstream IPv4
         Address is an address belonging to this router.

         The V flag can only be set in a reply message.  The flag is set
         as a positive verification response to a received R flag.  If
         the R flag was not set in the request this Flag MUST be set
         to zero.
















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      Address Type

         The Address Type indicates if the interface is numbered or
         unnumbered and is set to one of the following values:

            Address Type        Value
            ------------        -----
            No Address            0
            IPv4                  1
            Unnumbered            2

         The value 0, "No Address" is only valid in a Verification
         Request message.


      Reserved

         MUST be set to zero on transmission and ignored on receipt.


      Downstream IPv4 Address

         If the address type is 'No Address', the address field MUST be
         set to zero and ignored on receipt.

         If the address type is 'IPv4', the address field MUST either be
         set to the downstream LSR's Router ID or the downstream LSR's
         interface address.

         If the address type is 'unnumbered', the address field MUST be set
         to the downstream LSR's Router ID.


      Downstream Interface Address

         If the address type is 'IPv4', the interface address field MUST
         MUST be set to the downstream LSR's interface address.

         If the address type is 'unnumbered', interface address field
         MUST be set to the index assigned by the downstream LSR to the
         interface.


      Label Stack

         The label stack of the received echo request message.  If any
         TTL values have been changed by this router, they SHOULD be
         restored.



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3.2.2. IPv6 Downstream Verification Object

   In a request message the Length is always 24.  In a reply message the
   length is 40 + 4*N octets, N is the number of Downstream Labels.  The
   value field of a Downstream Verification TLV has the following
   format:


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Flags     | Address Type  |            Reserved           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Downstream IPv6 Address                    |
      |                Downstream IPv6 Address (Cont.)                |
      |                Downstream IPv6 Address (Cont.)                |
      |                Downstream IPv6 Address (Cont.)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Downstream Interface Address                 |
      |              Downstream Interface Address (Cont.)             |
      |              Downstream Interface Address (Cont.)             |
      |              Downstream Interface Address (Cont.)             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                          Label Stack                          .
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





















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      Flags

         Two flags are defined as shown.  All other flags are reserved
         and MUST be set to zero.

            +-+-+-+-+-+-+-+-+
            |0|0|0|0|0|0|V|R|
            +-+-+-+-+-+-+-+-+

         The R flag can only be set in a request message.  It requests
         that the receiving router verify that the Downstream IPv6
         Address is an address belonging to this router.

         The V flag can only be set in a reply message.  The flag is set
         as a positive verification response to a received R flag.  If
         the R flag was not set in the echo request this Flag MUST be set
         to zero.


      Address Type

         The Address Type indicates if the interface is numbered or
         unnumbered and is set to one of the following values:

            Address Type        Value
            ------------        -----
            No Address            0
            IPv6                  1
            Unnumbered            2

         The value 0, "No Address" is only valid in a Verification
         Request message.


      Reserved

         MUST be set to zero on transmission and ignored on receipt.














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      Downstream IPv6 Address

         If the address type is 'No Address', the address field MUST be
         set to zero and ignored on receipt.

         If the address type is 'IPv6', the address field MUST either be
         set to the downstream LSR's Router ID or the downstream LSR's
         interface address.

         If the address type is 'unnumbered', the address field MUST be set
         to the downstream LSR's Router ID.


      Downstream Interface Address

         If the address type is 'IPv6', the interface address field MUST
         MUST be set to the downstream LSR's interface address.

         If the address type is 'unnumbered', first four octets of
         interface address field MUST be set to the index assigned by the
         downstream LSR to the interface.  The remaining 12 octets MUST
         be set to zero.


      Label Stack

         The label stack of the received echo request message.  If any
         TTL values have been changed by this router, they SHOULD be
         restored.



3.3. Reply-To Object

   In order to perform detailed diagnostics of a particular failing flow
   in the face of ECMP, it is useful to be able to use the exact source
   and destination addresses of that flow.  The Reply-To Object is an
   optional TLV in a MPLS Data Plane Verification Request message.  The
   Object has two formats, type 9 for IPv4 and type 10 for IPv6 (to be
   assigned by IANA).











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3.3.1. IPv4 Reply-To Object

   The length of an IPv4 Reply-To Object is 5 octets; the value field
   has the following format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Reply-to IPv4 Address                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    DS-Byte    |
      +-+-+-+-+-+-+-+-+


      Reply-to IPv4 Address

         The address to which the MPLS Data Plane Verification Reply
         message is to be sent.


      DS-Byte

         The DS-Byte to be used in the MPLS Data Plane Verification Reply
         packet.



3.3.2. IPv6 Reply-To Object

   The length of an IPv6 Reply-To Object is 17 octets; the value field
   has the following format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Reply-to IPv6 Address                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Reply-to IPv6 Address (Cont.)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Reply-to IPv6 Address (Cont.)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Reply-to IPv6 Address (Cont.)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    DS-Byte    |
      +-+-+-+-+-+-+-+-+






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      Reply-to IPv6 Address

         The address to which the MPLS Data Plane Verification Reply
         message is to be sent.


      DS-Byte

         The DS-Byte to be used in the MPLS Data Plane Verification Reply
         packet.



3.4. Sending procedures

   In order to perform a test on an incoming label stack, an LSR first
   determines the expected outgoing label stack, next hop router and
   next hop interface.

   The LSR creates an MPLS Data Plane Verification Request message and
   includes a Data Plane Verification Object.  Optionally a FEC Stack
   TLV may be included.  In this case an MPLS Echo Request Message MUST
   be used.

   In normal use, the source address is set to an address belonging to
   the LSR and the destination set to an address in the range of 127/8.
   The IP TTL SHOULD be set to 1.  The incoming label stack is prepended
   to the packet.  The TTL of these labels SHOULD be set to appropriate
   values - 2 for those labels which will be process by this  when the
   packet is looped back; 1 for those labels which will be carried
   through.  Finally the loopback label bound to the incoming interface
   is prepended to the packet.  The TTL is set such that it will have
   the value of 3 on the wire.

   The packet is sent to the upstream neighbor on an interface for which
   the loopback label is valid.

   In diagnostic situations, the source and destination addresses MAY be
   set to any value.  In this case, a Reply-to IPv4 or IPv6 Object MUST
   be included.  The IP TTL MUST be set to 1.  The TTL of labels other
   than the loopback label MUST be set to appropriate values - 2 for
   those labels which will be process by this LSR when the packet is
   looped back; 1 for those labels which will be carried through.








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3.5. Receiving procedures

   An LSR X that receives an MPLS Verification Request message formats a
   MPLS Verification Reply message.  The Sender's Handle and Sequence
   Number are copied from the Request message.

   X then parses the packet to ensure that it is a well-formed packet,
   and that the TLVs that are not marked "Ignore" are understood.  If
   not, X SHOULD send an MPLS echo reply with the Return Code set to
   "Malformed echo request received" or "TLV not understood" (as
   appropriate), and the Subcode set to zero.  In the latter case, the
   misunderstood TLVs (only) are included in the reply.

   If the echo request is good, X notes the interface I over which the
   echo was received, and the label stack with which it came. If the
   MPLS echo request contained a Downstream Verification object, then X
   must format this information as a Downstream Verification object and
   include it in its MPLS echo reply message.

   The source address of the Reply message MUST be an address of the
   replying LSR.  If the request included a Reply-to IPv4 or IPv6
   Object, the MPLS Data Plane Verification Reply message MUST be sent
   to that address.  Otherwise the Reply message is sent to the source
   address of the Verification Request message.

   An LSR MUST be capable of filtering addresses that are to be replied
   to.  If a filter has been invoked (i.e. configured) and an address
   does not pass the filter, then a reply MUST NOT be sent, and the
   event SHOULD be logged.


3.6. Upstream Neighbor Verification

   To verify that an upstream neighbor is properly echoing packets an
   LSR may send an MPLS Data Plane Verification Request packet with the
   TTL set so that the packet will expire upon reaching reaching itself.
   This procedure not only tests that the neighbor is correctly
   processing the loopback label, it also allow the node to verify the
   neighbor's interface mapping.












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           +------+       +------+    DPVRq: MPLS Data Plane
           |    ,-|-------|<DPVRq|           Verification Request
           |    `-|-------|->    |
           |      |       |      |
           +------+       +------+
             LSRU           LSRT

              Figure 2: Upstream Neighbor Verification

   No TLVs need to be included in the MPLS Data Plane Verification
   Request.  By noting the Sender's Handle and Sequence Number, as well
   as the loopback label, LSRT is able to detect that a) the packet was
   looped, and b) determine (or verify) the interface on which the
   packet was received.



4. Security Considerations

   Were loopback labels widely known, they might be subject to abuse.
   It is therefore RECOMMENDED that loopback labels only be shared
   between trusted neighbors.  Further, if the loopback labels are drawn
   from the Global Label Space, or any other label space shared across
   multiple LDP sessions, it is RECOMMENDED that all loopback labels be
   filtered from a session except those labels pertaining to interfaces
   directly connected to the neighbor participating in that session.



5. IANA Considerations

   TBD



6. Acknowledgments

   The authors would like to thank Vanson Lim, Tom Nadeau, and Bob
   Thomas for their comments and suggestions.












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7. References

7.1. Normative References

   [RFC3036]  Andersson, L. et al., "LDP Specification", January 2001.

   [LSP-Ping] Bonica, R. et al., "Detecting MPLS Data Plane Liveness",
              work-in-progress.

   [RFC3477]  Kompella, K. & Y. Rekhter, "Signalling Unnumbered Links
              in Resource ReSerVation Protocol - Traffic Engineering
              (RSVP-TE)", January 2003.

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


7.2. Informative References

   [RSVP-TE]  Awduche, D., et al, "RSVP-TE: Extensions to RSVP for LSP
              tunnels", RFC 3209, December 2001.



8. Authors' Addresses

      Kireeti Kompella
      Juniper Networks, Inc.
      1194 N. Mathilda Ave.
      Sunnyvale, CA 94089
      Email:  kireeti@juniper.net



      George Swallow
      Cisco Systems, Inc.
      1414 Massachusetts Ave
      Boxborough, MA 01719

      Email:  swallow@cisco.com


      Dan Tappan
      Cisco Systems, Inc.
      1414 Massachusetts Ave
      Boxborough, MA 01719

      Email:  tappan@cisco.com



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9. Intellectual Property Notice

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10. Full Copyright Statement

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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

















































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