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Versions: 00 draft-ietf-mpls-lsr-self-test

Network Working Group                                     George Swallow
Internet Draft                                       Cisco Systems, Inc.
Expiration Date: December 2003
                                                              Dan Tappan
                                                     Cisco Systems, Inc.

                                                               June 2003


                             LSR Self-Test


                draft-swallow-mpls-lsr-self-test-00.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
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   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   Abstract

   This document defines a means of self test for a Label-Switched
   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 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] messages are
   extended to do the actually probing.




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Contents

    1      Introduction  ...........................................   4
    1.1    Conventions  ............................................   4
    2      Loopback FEC  ...........................................   5
    2.1    Loopback FEC Element  ...................................   5
    2.2    LDP Procedures  .........................................   6
    3      Data Plane Self Test  ...................................   6
    3.1    Next Hop Verification Object  ...........................   7
    3.2    Additional Error Codes  .................................   9
    3.3    Sending procedures  .....................................   9
    3.4    Receiving procedures  ...................................  10
    3.5    Upstream Neighbor Verification  .........................  11
    4      Security Considerations  ................................  11
    5      IANA Considerations  ....................................  12
    6      Acknowledgments  ........................................  12
    7      References  .............................................  12
    7.1    Normative References  ...................................  12
    7.2    Informative References  .................................  12
    8      Authors' Addresses  .....................................  13



   0. Sub-IP ID Summary

      (This section to be removed before publication.)

      (See Abstract above.)

      RELATED DOCUMENTS

      May be found in the "references" section.

      WHERE DOES IT FIT IN THE PICTURE OF THE SUB-IP WORK

      Fits in the MPLS box.

      WHY IS IT TARGETED AT THIS WG

      MPLS WG is currently looking at MPLS-specific error detection and
      recovery mechanisms.  The mechanisms proposed here are for packet-
      based MPLS LSPs, which is why the MPLS WG is targeted.

      JUSTIFICATION

      The WG should consider this document, as it allows network



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      operators to detect MPLS LSP data plane failures in the network.
      This type of failures have occurred, and are a source of concern
      to operators implementing MPLS networks.
















































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1. Introduction

   This document defines a means of self test for a Label-Switched
   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 actually 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 are defined to allow the
   down stream neighbor to verify 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 label entries for which
   the LSR is not currently its potential upstream 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
   "Link Local Identifiers" used to identify unnumbered links and
   components of bundled links.  These same 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 the LSR self-testing at very low cost.  This FEC allows 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 multiaccess links, one label per 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

         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.

      Address

         An identifier encoded according to the Identifier  Type  field.
         The  length  is  4  bytes for Unnumbered and IPv4, 16 bytes for
         IPv6.


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 neighbors 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.  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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       +------+       +------+       +------+
       |    ,-|-------|-<MERq|       |      |  MERq: MPLS Echo Request
       |    `-|-------|------|-------|->    |
       |      |       |    <-|-------|-<MERp|  MERp: MPLS Echo Reply
       +------+       +------+       +------+
         LSRU           LSRT           LSRD

          Figure 1: Self Test Message Flow

   In order to perform a test on an incoming label stack, LSRT forms an
   MPLS Echo Request.  Included in that is a Next Hop Verification
   Object which describes the interface and label stack that should be
   seen by LSRD.  Optionally a FEC Stack TLV may be included to verify
   LSRD's labels are mapped to the expected FECs.

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

   LSRU receives the packet and performs normal MPLS forwarding, that is
   the loopback label is pop, 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.

   LSRD receives the packet.  It verifies that the packet was received
   on the interface and with the label stack contained in the Next Hop
   Verification TLV.  If a FEC Stack TLV is also included, it verifies
   that the labels in the stack are mapped to those FECs.  The results
   are recorded in an MPLS Echo Reply message and sent to LSRT.



3.1. Next Hop Verification Object

   The Next Hop Verification is an optional TLV in an echo request.  The
   Length is 12 + 4*N octets, where N is the number of Downstream
   Labels.  The value field of a Next Hop 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Next Hop IPv4 Router ID                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               | Address Type  |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Next Hop Interface Address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Outgoing Label                 |    Reserved   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Outgoing Label                 |    Reserved   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      The Next Hop Interface Address Type is one of:

                Type #        Address Type
                ------        ------------

                     1        IPv4
                     2        Unnumbered


      Reserved

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


   [Note a format for IPv6 will be added after one is defined in
   [LSP-Ping]]
















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3.2. Additional Error Codes

   The following error codes are defined in addition to those defined in
   [LSP-Ping].

          Value    Meaning
          -----    -------
              8    Incorrect interface
              9    Label Stack mis-match
             10    Label not valid on this interface
             11    Binding for this label is not the given FEC
             12    Not processed



3.3. 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.  These are recorded in a Next Hop Verification
   TLV.

   If binding verification is also to be performed, then beginning at
   the top of the stack, for each label binding received or given to the
   Next Hop router, a corresponding entry in a FEC Stack TLV is
   included.  If there are labels in the stack that are not to be
   processed by the Next Hop Router, corresponding entries MUST NOT be
   included in the FEC Stack TLV.

   The LSR creates an MPLS Echo Request packet with itself as the source
   address 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 itself when the
   packet is looped back; 1 for those labels which will be carried
   through.  Finally the loopback label received for 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.









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3.4. Receiving procedures

   The next hop router performs the following checks on the received
   packet.

   1.  Verify that the Next Hop IPv4(v6) Router ID matches one of its IP
       addresses.

       If this fails, return an error code 5, 'Replying router is not
       one of the Downstream Routers'

   2.  Verify that the Next Hop Interface Address matches the interface
       on which this packet arrived.

       If this fails, return an error code xx1, 'Incorrect interface'.
       A Next Hop Verification TLV SHOULD be included in the reply with
       the received interface and label stack.

   3.  Verify that the label stack on the packet matches the label stack
       in the Next Hop Verification TLV.

       If this fails, return an error code xx2, 'Label Stack mis-match'.
       A Next Hop Verification TLV SHOULD be included in the reply with
       the received interface and label stack.

   4.  Verify that the top label on the stack is valid on this
       interface.

       If this fails, return an error code xx3, 'Label not valid on this
       interface'.  The error sub-code is set to the stack depth of the
       errant label.

   4a. If the label operation in step 4 is a Pop and requires that the
       next label be inspected, repeat step 4 for that label.

   5.  If a FEC Stack TLV is present, then for each FEC verify that the
       corresponding label is the correct binding.  If the label binding
       is incorrect, return an error code of xx4 'Binding for this label
       is not the given FEC'. The error sub-code is set to the stack
       depth of the errant label.

       If a FEC is reached that was not processed in step 4 above,
       return an error code of xx5, 'Not processed'.  The error sub-code
       is set to the stack depth of the errant label.







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3.5. Upstream Neighbor Verification

   To verify that an upstream neighbor is properly echoing packets an
   LSR may send an MPLS Echo 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.


       +------+       +------+       +------+
       |    ,-|-------|-<MERq|       |      |  MERq: MPLS Echo Request
       |    `-|-------|->    |       |      |
       |      |       |      |       |      |
       +------+       +------+       +------+
         LSRU           LSRT           LSRD

          Figure 2: Upstream Neighbor Verification

   No TLVs need to be included in the MPLS Echo 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.
   A Next Hop Verification TLV may be included to assist in
   verification.  This may be particularly useful in a system where
   control is distributed over multiple processor.



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 label be
   filtered from a session except those labels pertaining to interfaces
   directly connected to the neighbor participating in that session.













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5. IANA Considerations

   TBD



6. Acknowledgments

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



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.

















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8. Authors' Addresses

   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


































Swallow & Tappan                                               [Page 13]


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