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Network Working Group                                        K. Kompella
Internet Draft                                          Juniper Networks
Category: Standards Track                                     G. Swallow
Expires: January 2005                                      Cisco Systems
                                                               July 2004

                   Detecting MPLS Data Plane Failures
                    draft-ietf-mpls-lsp-ping-06.txt
                             *** DRAFT ***


Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice

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













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Abstract

   This document describes a simple and efficient mechanism that can be
   used to detect data plane failures in Multi-Protocol Label Switching
   (MPLS) Label Switched Paths (LSPs).  There are two parts to this
   document: information carried in an MPLS "echo request" and "echo
   reply" for the purposes of fault detection and isolation; and
   mechanisms for reliably sending the echo reply.


Changes since last revision

   (This section to be removed before publication.)

   *** Changed the format of an L2 circuit ID FEC back to what it was,
   on demand.  Added a new FEC with sender's PE address field to
   uniquely identify the VC ID  ***

   *** Added a FEC TLV for "Labeled BGP IPv4" ***

   Reformatted section on Downstream Mapping

   Described issue with (and solution to) problem with VPN IPv4/6

   Rephrased section on receiving an LSP ping

   Clarified "Expert Review" allocation policy.
























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1. Introduction

   This document describes a simple and efficient mechanism that can be
   used to detect data plane failures in MPLS LSPs.  There are two parts
   to this document: information carried in an MPLS "echo request" and
   "echo reply"; and mechanisms for transporting the echo reply.  The
   first part aims at providing enough information to check correct
   operation of the data plane, as well as a mechanism to verify the
   data plane against the control plane, and thereby localize faults.
   The second part suggests two methods of reliable reply channels for
   the echo request message, for more robust fault isolation.

   An important consideration in this design is that MPLS echo requests
   follow the same data path that normal MPLS packets would traverse.
   MPLS echo requests are meant primarily to validate the data plane,
   and secondarily to verify the data plane against the control plane.
   Mechanisms to check the control plane are valuable, but are not
   covered in this document.

   To avoid potential Denial of Service attacks, it is recommended to
   regulate the LSP ping traffic going to the control plane.  A rate
   limiter should be applied to the well-known UDP port defined below.

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].

1.2. Structure of this document

   The body of this memo contains four main parts: motivation, MPLS echo
   request/reply packet format, LSP ping operation, and a reliable
   return path.  It is suggested that first-time readers skip the actual
   packet formats and read the Theory of Operation first; the document
   is structured the way it is to avoid forward references.

1.3. Contributors

   The following made vital contributions to all aspects of this
   document, and much of the material came out of debate and discussion
   among this group.

      Ronald P. Bonica, MCI
      Dave Cooper, Global Crossing
      Ping Pan, Ciena
      Nischal Sheth, Juniper Networks, Inc.
      Sanjay Wadhwa, Juniper Networks



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2. Motivation

   When an LSP fails to deliver user traffic, the failure cannot always
   be detected by the MPLS control plane.  There is a need to provide a
   tool that would enable users to detect such traffic "black holes" or
   misrouting within a reasonable period of time; and a mechanism to
   isolate faults.

   In this document, we describe a mechanism that accomplishes these
   goals.  This mechanism is modeled after the ping/traceroute paradigm:
   ping (ICMP echo request [ICMP]) is used for connectivity checks, and
   traceroute is used for hop-by-hop fault localization as well as path
   tracing.  This document specifies a "ping mode" and a "traceroute"
   mode for testing MPLS LSPs.

   The basic idea is to verify that packets that belong to a particular
   Forwarding Equivalence Class (FEC) actually end their MPLS path on an
   LSR that is an egress for that FEC.  This document proposes that this
   test be carried out by sending a packet (called an "MPLS echo
   request") along the same data path as other packets belonging to this
   FEC.  An MPLS echo request also carries information about the FEC
   whose MPLS path is being verified.  This echo request is forwarded
   just like any other packet belonging to that FEC.  In "ping" mode
   (basic connectivity check), the packet should reach the end of the
   path, at which point it is sent to the control plane of the egress
   LSR, which then verifies whether it is indeed an egress for the FEC.
   In "traceroute" mode (fault isolation), the packet is sent to the
   control plane of each transit LSR, which performs various checks that
   it is indeed a transit LSR for this path; this LSR also returns
   further information that helps check the control plane against the
   data plane, i.e., that forwarding matches what the routing protocols
   determined as the path.

   One way these tools can be used is to periodically ping a FEC to
   ensure connectivity.  If the ping fails, one can then initiate a
   traceroute to determine where the fault lies.  One can also
   periodically traceroute FECs to verify that forwarding matches the
   control plane; however, this places a greater burden on transit LSRs
   and thus should be used with caution.












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3. Packet Format

   An MPLS echo request is a (possibly labelled) IPv4 or IPv6 UDP
   packet; the contents of the UDP packet 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                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    TimeStamp Sent (seconds)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  TimeStamp Sent (microseconds)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  TimeStamp Received (seconds)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                TimeStamp Received (microseconds)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            TLVs ...                           |
      .                                                               .
      .                                                               .
      .                                                               .
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Version Number is currently 1.  (Note: the Version Number is to
   be incremented whenever a change is made that affects the ability of
   an implementation to correctly parse or process an MPLS echo
   request/reply.  These changes include any syntactic or semantic
   changes made to any of the fixed fields, or to any TLV or sub-TLV
   assignment or format that is defined at a certain version number.
   The Version Number may not need to be changed if an optional TLV or
   sub-TLV is added.)

   The Message Type is one of the following:

       Value    Meaning
       -----    -------
           1    MPLS Echo Request
           2    MPLS Echo Reply

   The Reply Mode can take one of the following values:



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       Value    Meaning
       -----    -------
           1    Do not reply
           2    Reply via an IPv4/IPv6 UDP packet
           3    Reply via an IPv4/IPv6 UDP packet with Router Alert
           4    Reply via application level control channel

   An MPLS echo request with "Do not reply" may be used for one-way
   connectivity tests; the receiving router may log gaps in the sequence
   numbers and/or maintain delay/jitter statistics.  An MPLS echo
   request would normally have "Reply via an IPv4/IPv6 UDP packet"; if
   the normal IP return path is deemed unreliable, one may use "Reply
   via an IPv4/IPv6 UDP packet with Router Alert" (note that this
   requires that all intermediate routers understand and know how to
   forward MPLS echo replies).  The echo reply uses the same IP version
   number as the received echo request, i.e., an IPv4 encapsulated echo
   reply is sent in response to an IPv4 encapsulated echo request.

   Any application which supports an IP control channel between its
   control entities may set the Reply Mode to 4 to ensure that replies
   use that same channel.  Further definition of this codepoint is
   application specific and thus beyond the scope of this docuemnt.

   Return Codes and Subcodes are described in the next section.

   The Sender's Handle is filled in by the sender, and returned
   unchanged by the receiver in the echo reply (if any).  There are no
   semantics associated with this handle, although a sender may find
   this useful for matching up requests with replies.

   The Sequence Number is assigned by the sender of the MPLS echo
   request, and can be (for example) used to detect missed replies.

   The TimeStamp Sent is the time-of-day (in seconds and microseconds,
   wrt the sender's clock) when the MPLS echo request is sent.  The
   TimeStamp Received in an echo reply is the time-of-day (wrt the
   receiver's clock) that the corresponding echo request was received.

   TLVs (Type-Length-Value tuples) 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Type              |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Value                             |
      .                                                               .
      .                                                               .



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

   Types are defined below; Length is the length of the Value field in
   octets.  The Value field depends on the Type; it is zero padded to
   align to a four-octet boundary.

          Type #                           Value Field
          ------                           -----------
               1                           Target FEC Stack
               2                           Downstream Mapping
               3                           Pad
               4                           Error Code
               5                           Vendor Enterprise Code

3.1. Return Codes

   The Return Code is set to zero by the sender.  The receiver can set
   it to one of the values listed below.  The notation <RSC> refers to
   the Return Subcode.  This field is filled in with the stack-depth for
   those codes which specify that.  For all other codes the Return
   Subcode MUST be set to zero.

          Value    Meaning
          -----    -------

              0    No return code or return code contained in the Error
                   Code TLV

              1    Malformed echo request received

              2    One or more of the TLVs was not understood

              3    Replying router is an egress for the FEC at stack
                   depth <RSC>

              4    Replying router has no mapping for the FEC at stack
                   depth <RSC>

              5    Reserved

              6    Reserved

              7    Reserved

              8    Label switched at stack-depth <RSC>




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              9    Label switched but no MPLS forwarding at stack-depth
                   <RSC>

             10    Mapping for this FEC is not the given label at stack
                   depth <RSC>

             11    No label entry at stack-depth <RSC>

             12    Protocol not associated with interface at FEC stack
                   depth <RSC>

             13    Premature termination of ping due to label stack
                   shrinking to a single label

3.2. Target FEC Stack

   A Target FEC Stack is a list of sub-TLVs.  The number of elements is
   determined by the looking at the sub-TLV length fields.

      Sub-Type #       Length              Value Field
      ----------       ------              -----------
               1            5              LDP IPv4 prefix
               2           17              LDP IPv6 prefix
               3           20              RSVP IPv4 Session Query
               4           56              RSVP IPv6 Session Query
               5                           Reserved; see Appendix
               6           13              VPN IPv4 prefix
               7           25              VPN IPv6 prefix
               8           14              L2 VPN endpoint
               9           10              "FEC 128" Pseudowire (old)
              10           14              "FEC 128" Pseudowire (new)
              11          13+              "FEC 129" Pseudowire
              12           10              BGP labeled IPv4 prefix

   Other FEC Types will be defined as needed.

   Note that this TLV defines a stack of FECs, the first FEC element
   corresponding to the top of the label stack, etc.

   An MPLS echo request MUST have a Target FEC Stack that describes the
   FEC stack being tested.  For example, if an LSR X has an LDP mapping
   for 192.168.1.1 (say label 1001), then to verify that label 1001 does
   indeed reach an egress LSR that announced this prefix via LDP, X can
   send an MPLS echo request with a FEC Stack TLV with one FEC in it,
   namely of type LDP IPv4 prefix, with prefix 192.168.1.1/32, and send
   the echo request with a label of 1001.

   Say LSR X wanted to verify that a label stack of <1001, 23456> is the



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   right label stack to use to reach a VPN IPv4 prefix of 10/8 in VPN
   foo.  Say further that LSR Y with loopback address 192.168.1.1
   announced prefix 10/8 with Route Distinguisher RD-foo-Y (which may in
   general be different from the Route Distinguisher that LSR X uses in
   its own advertisements for VPN foo), label 23456 and BGP nexthop
   192.168.1.1.  Finally, suppose that LSR X receives a label binding of
   1001 for 192.168.1.1 via LDP.  X has two choices in sending an MPLS
   echo request: X can send an MPLS echo request with a FEC Stack TLV
   with a single FEC of type VPN IPv4 prefix with a prefix of 10/8 and a
   Route Distinguisher of RD-foo-Y.  Alternatively, X can send a FEC
   Stack TLV with two FECs, the first of type LDP IPv4 with a prefix of
   192.168.1.1/32 and the second of type of IP VPN with a prefix 10/8
   with Route Distinguisher of RD-foo-Y.  In either case, the MPLS echo
   request would have a label stack of <1001, 23456>.  (Note: in this
   example, 1001 is the "outer" label and 23456 is the "inner" label.)

3.2.1. LDP IPv4 Prefix

   The value consists of four octets of an IPv4 prefix followed by one
   octet of prefix length in bits; the format is given below.  The IPv4
   prefix is in network byte order; if the prefix is shorter than 32
   bits, trailing bits SHOULD be set to zero.  See [LDP] for an example
   of a Mapping for an IPv4 FEC.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv4 prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.2. LDP IPv6 Prefix

   The value consists of sixteen octets of an IPv6 prefix followed by
   one octet of prefix length in bits; the format is given below.  The
   IPv6 prefix is in network byte order; if the prefix is shorter than
   128 bits, the trailing bits SHOULD be set to zero.  See [LDP] for an
   example of a Mapping for an IPv6 FEC.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv6 prefix                          |
      |                          (16 octets)                          |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.3. RSVP IPv4 Session

   The value has the format below.  The value fields are taken from
   [RFC3209, sections 4.6.1.1 and 4.6.2.1].

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 IPv4 tunnel end point address                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |     Tunnel ID                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Extended Tunnel ID                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   IPv4 tunnel sender address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |            LSP ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.4. RSVP IPv6 Session

   The value has the format below.  The value fields are taken from
   [RFC3209, sections 4.6.1.2 and 4.6.2.2].

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 IPv6 tunnel end point address                 |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |          Tunnel ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Extended Tunnel ID                      |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   IPv6 tunnel sender address                  |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |            LSP ID             |



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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.5. VPN IPv4 Prefix

   The value field consists of the Route Distinguisher advertised with
   the VPN IPv4 prefix, the IPv4 prefix (with trailing 0 bits to make 32
   bits in all) and a prefix length, as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                      |
      |                          (8 octets)                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IPv4 prefix                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |                 Must Be Zero                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.6. VPN IPv6 Prefix

   The value field consists of the Route Distinguisher advertised with
   the VPN IPv6 prefix, the IPv6 prefix (with trailing 0 bits to make
   128 bits in all) and a prefix length, as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                      |
      |                          (8 octets)                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IPv6 prefix                           |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |                 Must Be Zero                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.7. L2 VPN Endpoint

   The value field consists of a Route Distinguisher (8 octets), the
   sender (of the ping)'s CE ID (2 octets), the receiver's CE ID (2
   octets), and an encapsulation type (2 octets), formatted as follows:

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



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      |                      Route Distinguisher                      |
      |                          (8 octets)                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Sender's CE ID        |       Receiver's CE ID        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Encapsulation Type       |         Must Be Zero          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.8. FEC 128 Pseudowire (Deprecated)

   The value field consists of the remote PE address (the destination
   address of the targetted LDP session), a VC ID and an encapsulation
   type, as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE Address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             VC ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Encapsulation Type       |         Must Be Zero          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This FEC will be deprecated, and is retained only for backward
   compatibility.  Implementations of LSP ping SHOULD accept and process
   this TLV, but SHOULD send LSP ping echo requests with the new TLV
   (see next section), unless explicitly asked by configuration to use
   the old TLV.

   An LSR receiving this TLV SHOULD use the source IP address of the LSP
   echo request to infer the Sender's PE Address.

3.2.9. FEC 128 Pseudowire (Current)

   The value field consists of the sender's PE address (the source
   address of the targetted LDP session), the remote PE address (the
   destination address of the targetted LDP session), a VC ID and an
   encapsulation type, as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Sender's PE Address                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE Address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             VC ID                             |



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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Encapsulation Type       |         Must Be Zero          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.10. FEC 129 Pseudowire

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Sender's PE Address                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE Address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            PW Type            |  AGI Length   |  SAII Length  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  TAII Length  | AGI Value ... SAII Value ... TAII Value ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      . ...                                                           .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ...         | 0-3 octets of zero padding                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Length of this TLV is 13 + AGI length + SAII length + TAII
   length.  Padding is used to make the total length a multiple of 4;
   the length of the padding is not included in the Length field.

3.2.11. BGP Labeled IPv4 Prefix

   The value field consists of the BGP Next Hop associated with the NLRI
   advertising the prefix and label, the IPv4 prefix (with trailing 0
   bits to make 32 bits in all), and the prefix length, as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         BGP Next Hop                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv4 Prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |                 Must Be Zero                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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3.3. Downstream Mapping

   The Downstream Mapping object is an optional TLV.  Only one
   Downstream Mapping request may appear in and echo request.  The
   presence of a Downstream Mapping object is a request that Downstream
   Mapping objects be included in the echo reply.  If the replying
   router is the destination of the FEC, then a Downstream Mapping TLV
   SHOULD NOT be included in the echo reply.  Otherwise Downstream
   Mapping objects SHOULD include a Downstream Mapping object for each
   interface over which this FEC could be forwarded.  For a more precise
   definition of the notion of "downstream", see the section named
   "Downstream".

   The Length is 16 + M + 4*N octets, where M is the Multipath Length,
   and N is the number of Downstream Labels.  The Value field of a
   Downstream Mapping 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               MTU             | Address Type  |  Resvd (SBZ)  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Downstream IP Address (4 or 16 octets)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Downstream Interface Address (4 or 16 octets)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Hash Key Type | Depth Limit   |        Multipath Length       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                     (Multipath Information)                   .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Maximum Transmission Unit (MTU)

       The MTU is the largest MPLS frame (including label stack) that
       fits on the interface to the Downstream LSR.

   Address Type




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

                 Type #        Address Type
                 ------        ------------
                      1        IPv4
                      2        Unnumbered
                      3        IPv6

   Reserved

       The field marked SBZ SHOULD be set to zero when sending and
       SHOULD be ignored on receipt.

   Downstream IP Address and Downstream Interface Address

       If the interface to the downstream LSR is numbered, then the
       Address Type MUST be set to IPv4 or IPv6, the Downstream IP
       Address MUST be set to either the downstream LSR's Router ID or
       the interface address of the downstream LSR, and the Downstream
       Interface Address MUST be set to the downstream LSR's interface
       address.

       If the interface to the downstream LSR is unnumbered, the Address
       Type MUST be Unnumbered, the Downstream IP Address MUST be the
       downstream LSR's Router ID (4 octets), and the Downstream
       Interface Address MUST be set to the index assigned by the
       upstream LSR to the interface.

   Multipath Length

       The length in octets of the Multipath Information.

   Downstream Label(s)

       The set of labels in the label stack as it would have appeared if
       this router were forwarding the packet through this interface.
       Any Implicit Null labels are explicitly inluded.  Labels are
       treated as numbers, i.e. they are right justified in the field.

       A Downstream Label is 24 bits, in the same format as an MPLS
       label minus the TTL field, i.e., the MSBit of the label is bit 0,
       the LSbit is bit 19, the EXP bits are bits 20-22, and bit 23 is
       the S bit.  The replying router SHOULD fill in the EXP and S
       bits; the LSR receiving the echo reply MAY choose to ignore these
       bits.

   Protocol



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       The Protocol is taken from the following table:

             Protocol #        Signaling Protocol
             ----------        ------------------
                      0        Unknown
                      1        Static
                      2        BGP
                      3        LDP
                      4        RSVP-TE
                      5        Reserved; see Appendix

   Depth Limit

       The Depth Limit is applicable only to a label stack, and is the
       maximum number of labels considered in the hash; this SHOULD be
       set to zero if unspecified or unlimited.

   Multipath Information

       The multipath information encodes labels or addresses which will
       exercise this path.  The multipath informaiton depends on the
       hash key type.  The contents of the field are shown in the table
       above.  IP addresses are drawn from the range 127/8.  Labels are
       treated as numbers, i.e. they are right justified in the field.
       Label and Address pairs MUST NOT overlap and MUST be in ascending
       sequence.

       Hash key 8 allows a denser encoding of IP address.  The IPv4
       prefix is formatted as a base IPv4 address with the non-prefix
       low order bits set to zero.  The maximum prefix length is 27.
       Following the prefix is a mask of length 2^(32-prefix length)
       bits.  Each bit set to one represents a valid address.  The
       address is the base IPv4 address plus the position of the bit in
       the mask where the bits are numbered left to right begining with
       zero.

       Hash key 9 allows a denser encoding of Labels.  The label prefix
       is formatted as a base label value with the non-prefix low order
       bits set to zero.  The maximum prefix (including leading zeros
       due to encoding) length is 27.  Following the prefix is a mask of
       length 2^(32-prefix length) bits.  Each bit set to one represents
       a valid Label.  The label is the base label plus the position of
       the bit in the mask where the bits are numbered left to right
       begining with zero.

       If the received multipath information is non-null, the labels and
       IP addresses MUST be picked from the set provided or the Hash Key
       Type MUST be set to 7.  If the received multipath information is



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       null, the receiver simply returns null.

       For example, suppose LSR X at hop 10 has two downstream LSRs Y
       and Z for the FEC in question.  X could return Hash Key Type 4,
       with low/high IP addresses of 1.1.1.1->1.1.1.255 for downstream
       LSR Y and 2.1.1.1->2.1.1.255 for downstream LSR Z.  The head end
       reflects this information to LSR Y.  Y, which has three
       downstream LSRs U, V and W, computes that 1.1.1.1->1.1.1.127
       would go to U and 1.1.1.128-> 1.1.1.255 would go to V.  Y would
       then respond with 3 Downstream Mappings: to U, with Hash Key Type
       4 (1.1.1.1->1.1.1.127); to V, with Hash Key Type 4
       (1.1.1.127->1.1.1.255); and to W, with Hash Key Type 7.

3.3.1. "Downstream"

   The notion of "downstream router" and "downstream interface" should
   be explained.  Consider an LSR X.  If a packet that was originated
   with TTL n>1 arrived with outermost label L at LSR X, X must be able
   to compute which LSRs could receive the packet if it was originated
   with TTL=n+1, over which interface the request would arrive and what
   label stack those LSRs would see.  (It is outside the scope of this
   document to specify how this computation is done.)  The set of these
   LSRs/interfaces are the downstream routers/interfaces (and their
   corresponding labels) for X with respect to L.  Each pair of
   downstream router and interface requires a separate Downstream
   Mapping to be added to the reply.  (Note that there are multiple
   Downstream Label fields in each TLV as the incoming label L may be
   swapped with a label stack.)

   The case where X is the LSR originating the echo request is a special
   case.  X needs to figure out what LSRs would receive the MPLS echo
   request for a given FEC Stack that X originates with TTL=1.

   The set of downstream routers at X may be alternative paths (see the
   discussion below on ECMP) or simultaneous paths (e.g., for MPLS
   multicast).  In the former case, the Multipath sub-field is used as a
   hint to the sender as to how it may influence the choice of these
   alternatives.  The "No of Multipaths" is the number of IP
   Address/Next Label fields.  The Hash Key Type is taken from the
   following table:

      Key   Type                  Multipath Information
      ---   ----------------      ---------------------
       0    no multipath          (empty; M = 0)
       1    label                 labels
       2    IP address            IP addresses
       3    label range           low/high label pairs
       4    IP address range      low/high address pairs



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       5    no more labels        (empty; M = 0)
       6    All IP addresses      (empty; M = 0)
       7    no match              (empty; M = 0)
       8    Bit-masked IPv4       IP address prefix and bit mask
              address set
       9    Bit-masked label set  Label prefix and bit mask

   Type 0 indicates that all packets will be forwarded out this one
   interface.

   Types 1, 2, 3, 4, 8 and 9 specify that the supplied Multipath
   Information will serve to execise this path.

   Types 5 and 6 are TBD.

   Type 7 indicates that no matches are possible given the Multipath
   Information in the received DS mapping information.

3.4. Pad TLV

   The value part of the Pad TLV contains a variable number (>= 1) of
   octets.  The first octet takes values from the following table; all
   the other octets (if any) are ignored.  The receiver SHOULD verify
   that the TLV is received in its entirety, but otherwise ignores the
   contents of this TLV, apart from the first octet.

              Value        Meaning
              -----        -------
                  1        Drop Pad TLV from reply
                  2        Copy Pad TLV to reply
              3-255        Reserved for future use

3.5. Error Code

   The Error Code TLV is currently not defined; its purpose is to
   provide a mechanism for a more elaborate error reporting structure,
   should the reason arise.

3.6. Vendor Enterprise Code

   The Length is always 4; the value is the SMI Enterprise code, in
   network octet order, of the vendor with a Vendor Private extension to
   any of the fields in the fixed part of the message, in which case
   this TLV MUST be present.  If none of the fields in the fixed part of
   the message have vendor private extensions, this TLV is OPTIONAL.






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4. Theory of Operation

   An MPLS echo request is used to test a particular LSP.  The LSP to be
   tested is identified by the "FEC Stack"; for example, if the LSP was
   set up via LDP, and is to an egress IP address of 10.1.1.1, the FEC
   stack contains a single element, namely, an LDP IPv4 prefix sub-TLV
   with value 10.1.1.1/32.  If the LSP being tested is an RSVP LSP, the
   FEC stack consists of a single element that captures the RSVP Session
   and Sender Template which uniquely identifies the LSP.

   FEC stacks can be more complex.  For example, one may wish to test a
   VPN IPv4 prefix of 10.1/8 that is tunneled over an LDP LSP with
   egress 10.10.1.1.  The FEC stack would then contain two sub-TLVs, the
   first being a VPN IPv4 prefix, and the second being an LDP IPv4
   prefix.  If the underlying (LDP) tunnel were not known, or was
   considered irrelevant, the FEC stack could be a single element with
   just the VPN IPv4 sub-TLV.

   When an MPLS echo request is received, the receiver is expected to do
   a number of tests that verify that the control plane and data plane
   are both healthy (for the FEC stack being pinged), and that the two
   planes are in sync.

4.1. Dealing with Equal-Cost Multi-Path (ECMP)

   LSPs need not be simple point-to-point tunnels.  Frequently, a single
   LSP may originate at several ingresses, and terminate at several
   egresses; this is very common with LDP LSPs.  LSPs for a given FEC
   may also have multiple "next hops" at transit LSRs.  At an ingress,
   there may also be several different LSPs to choose from to get to the
   desired endpoint.  Finally, LSPs may have backup paths, detour paths
   and other alternative paths to take should the primary LSP go down.

   To deal with the last two first: it is assumed that the LSR sourcing
   MPLS echo requests can force the echo request into any desired LSP,
   so choosing among multiple LSPs at the ingress is not an issue.  The
   problem of probing the various flavors of backup paths that will
   typically not be used for forwarding data unless the primary LSP is
   down will not be addressed here.

   Since the actual LSP and path that a given packet may take may not be
   known a priori, it is useful if MPLS echo requests can exercise all
   possible paths.  This, while desirable, may not be practical, because
   the algorithms that a given LSR uses to distribute packets over
   alternative paths may be proprietary.

   To achieve some degree of coverage of alternate paths, there is a
   certain lattitude in choosing the destination IP address and source



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   UDP port for an MPLS echo request.  This is clearly not sufficient;
   in the case of traceroute, more lattitude is offered by means of the
   "Multipath Exercise" sub-TLV of the Downstream Mapping TLV.  This is
   used as follows.  An ingress LSR periodically sends an MPLS
   traceroute message to determine whether there are multipaths for a
   given LSP.  If so, each hop will provide some information how each of
   its downstreams can be exercised.  The ingress can then send MPLS
   echo requests that exercise these paths.  If several transit LSRs
   have ECMP, the ingress may attempt to compose these to exercise all
   possible paths.  However, full coverage may not be possible.

4.2. Sending an MPLS Echo Request

   An MPLS echo request is a (possibly) labelled UDP packet.  The IP
   header is set as follows: the source IP address is a routable address
   of the sender; the destination IP address is a (randomly chosen)
   address from 127/8; the IP TTL is set to 1.  The source UDP port is
   chosen by the sender; the destination UDP port is set to 3503
   (assigned by IANA for MPLS echo requests).  The Router Alert option
   is set in the IP header.

   If the echo request is labelled, one may (depending on what is being
   pinged) set the TTL of the innermost label to 1, to prevent the ping
   request going farther than it should.  Examples of this include
   pinging a VPN IPv4 or IPv6 prefix, an L2 VPN end point or a
   pseudowire.  This can also be accomplished by inserting a router
   alert label above this label; however, this may lead to the undesired
   side effect that MPLS echo requests take a different data path than
   actual data.

   In "ping" mode (end-to-end connectivity check), the TTL in the
   outermost label is set to 255.  In "traceroute" mode (fault isolation
   mode), the TTL is set successively to 1, 2, ....

   The sender chooses a Sender's Handle, and a Sequence Number.  When
   sending subsequent MPLS echo requests, the sender SHOULD increment
   the sequence number by 1.  However, a sender MAY choose to send a
   group of echo requests with the same sequence number to improve the
   chance of arrival of at least one packet with that sequence number.

   The TimeStamp Sent is set to the time-of-day (in seconds and
   microseconds) that the echo request is sent.  The TimeStamp Received
   is set to zero.

   An MPLS echo request MUST have a FEC Stack TLV.  Also, the Reply Mode
   must be set to the desired reply mode; the Return Code and Subcode
   are set to zero.




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   In the "traceroute" mode, the echo request SHOULD contain one or more
   Downstream Mapping TLVs.  For TTL=1, all the downstream routers (and
   corresponding labels) for the sender with respect to the FEC Stack
   being pinged SHOULD be sent in the echo request.  For n>1, the
   Downstream Mapping TLVs from the echo reply for TTL=(n-1) are copied
   to the echo request with TTL=n; the sender MAY choose to reduce the
   size of a "Downstream Multipath Mapping TLV" when copying into the
   next echo request as long as the Hash Key Type matching the label or
   IP address used to exercise the current MP is still present.

4.3. Receiving an MPLS Echo Request

   An LSR X that receives an MPLS echo request first 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.

   X matches up the labels in the received label stack with the FECs
   contained in the FEC stack.  The matching is done beginning at the
   bottom of both stacks, and working up.  For reporting purposes the
   bottom of stack is consided to be stack-depth of 1.  This is to
   establish an absolute reference for the case where the stack may have
   more labels than are in the FEC stack.

   If there are more FECs than labels, the extra FECs are assumed to
   correspond to Implicit Null Labels.  That is, extra Implicit Null
   Labels are added to the top of the received label stack and the stack
   depth is set to the depth of the FEC stack.  Thus for the processing
   below, there is never the case where there is a FEC with no
   corresponding label.  Further, the label operation associated with an
   assumed Null Label is 'pop and continue processing'.

   Note: in all the error codes listed in this draft a stack-depth of 0
   means "no value specified".  This allows compatibility with existing
   implementations which do not use the Return Subcode field.

   X sets a variable, call it current-stack-depth, to the number of
   labels in the received label stack.  Processing now continues with
   the following steps:

   1.  Check if there is a FEC corresponding to the current-stack-
       depth.  If there is, go to step 2.  If not, check if the label is



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       valid on interface I.  If it is, continue with step 4.  Otherwise
       X MUST send an MPLS echo reply with a Return Code 11, "No label
       entry at stack-depth" and a Return Subcode set to current-stack-
       depth.

   2.  Check the FEC at the current-stack-depth to determine what
       protocol would be used to advertise it.  If it can determine that
       no protocol associated with interface I, would have advertised a
       FEC of that FEC-Type, X MUST send an MPLS echo reply with a
       Return Code 12, "Protocol not associated with interface at FEC
       stack-depth" and a Return Subcode set to current-stack-depth.

   3.  Check that the mapping for the FEC at the current-stack-depth is
       the corresponding label.

       If no mapping for the FEC exists, X MUST send an MPLS echo reply
       with a Return Code 4, "Replying router has no mapping for the FEC
       at stack-depth" and a Return Subcode set to current- stack-depth.

       If a mapping is found, but the mapping is not the corresponding
       label, X MUST send an MPLS echo reply with a Return Code 10,
       "Mapping for this FEC is not the given label at stack-depth" and
       a Return Subcode set to current-stack-depth.

   4.  X determines the label operation.  If the operation is to pop and
       continue processing, X checks the current-stack-depth.  If it is
       one, X MUST send an MPLS echo reply with a Return Code 3,
       "Replying router is an egress for the FEC at stack depth" and a
       Return Subcode set to one. Otherwise, X decrements current-stack-
       depth and goes back to step 1.

       If the label operation is pop and switch based on the popped
       label, X then checks if it is valid to forward a labelled packet.
       If it is, X MUST send an MPLS echo reply with a Return Code 8,
       "Label switched at stack-depth" and a Return Subcode set to
       current-stack-depth.  If it is not valid to forward a labelled
       packet, X MUST send an MPLS echo reply with a Return Code 9,
       "Label switched but no MPLS forwarding at stack-depth" and a
       Return Subcode set to current-stack-depth.  This return code is
       sent even if current-stack-depth is one.

       If the label operation is swap, X MUST send an MPLS echo reply
       with a Return Code 8, "Label switched at stack-depth" and a
       Return Subcode set to current-stack-depth.

   If the MPLS echo request contains a downstream mapping TLV, and the
   MPLS echo reply has either a Return Code of 8, or a Return Code of 9
   with a Return Subcode of 1 then Downstream mapping TLVs SHOULD be



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   included for each multipath.

   X uses the procedure in the next subsection to send the echo reply.

4.4. Sending an MPLS Echo Reply

   An MPLS echo reply is a UDP packet.  It MUST ONLY be sent in response
   to an MPLS echo request.  The source IP address is a routable address
   of the replier; the source port is the well-known UDP port for LSP
   ping.  The destination IP address and UDP port are copied from the
   source IP address and UDP port of the echo request.  The IP TTL is
   set to 255.  If the Reply Mode in the echo request is "Reply via an
   IPv4 UDP packet with Router Alert", then the IP header MUST contain
   the Router Alert IP option.  If the reply is sent over an LSP, the
   topmost label MUST in this case be the Router Alert label (1) (see
   [LABEL-STACK]).

   The format of the echo reply is the same as the echo request.  The
   Sender's Handle, the Sequence Number and TimeStamp Sent are copied
   from the echo request; the TimeStamp Received is set to the time-of-
   day that the echo request is received (note that this information is
   most useful if the time-of-day clocks on the requestor and the
   replier are synchronized).  The FEC Stack TLV from the echo request
   MAY be copied to the reply.

   The replier MUST fill in the Return Code and Subcode, as determined
   in the previous subsection.

   If the echo request contains a Pad TLV, the replier MUST interpret
   the first octet for instructions regarding how to reply.

   If the echo request contains a Downstream Mapping TLV, the replier
   SHOULD compute its downstream routers and corresponding labels for
   the incoming label, and add Downstream Mapping TLVs for each one to
   the echo reply it sends back.

4.5. Receiving an MPLS Echo Reply

   An LSR X should only receive an MPLS Echo Reply in response to an
   MPLS Echo Request that it sent.  Thus, on receipt of an MPLS Echo
   Reply, X should parse the packet to assure that it is well-formed,
   then attempt to match up the Echo Reply with an Echo Request that it
   had previously sent, using the destination UDP port and the Sender's
   Handle.  If no match is found, then X jettisons the Echo Reply;
   otherwise, it checks the Sequence Number to see if it matches.  Gaps
   in the Sequence Number MAY be logged and SHOULD be counted.  Once an
   Echo Reply is received for a given Sequence Number (for a given UDP
   port and Handle), the Sequence Number for subsequent Echo Requests



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   for that UDP port and Handle SHOULD be incremented.

   If the Echo Reply contains Downstream Mappings, and X wishes to
   traceroute further, it SHOULD copy the Downstream Mappings into its
   next Echo Request (with TTL incremented by one).

4.6. Issue with VPN IPv4 and IPv6 Prefixes

   Typically, a LSP ping for a VPN IPv4 or IPv6 prefix is sent with a
   label stack of depth greater than 1, with the innermost label having
   a TTL of 1.  This is to terminate the ping at the egress PE, before
   it gets sent to the customer device.  However, under certain
   circumstances, the label stack can shrink to a single label before
   the ping hits the egress PE; this will result in the ping terminating
   prematurely.  One such scenario is a multi-AS Carrier's Carrier VPN.

   To get around this problem, one approach is for the LSR that receives
   such a ping to realize that the ping terminated prematurely, and send
   back error code 13.  In that case, the initiating LSR can retry the
   ping after incrementing the TTL on the VPN label.  In this fashion,
   the ingress LSR will sequentially try TTL values until it finds one
   that allows the VPN ping to reach the egress PE.

4.7. Non-compliant Routers

   If the egress for the FEC Stack being pinged does not support MPLS
   ping, then no reply will be sent, resulting in possible "false
   negatives".  If in "traceroute" mode, a transit LSR does not support
   LSP ping, then no reply will be forthcoming from that LSR for some
   TTL, say n.  The LSR originating the echo request SHOULD try sending
   the echo request with TTL=n+1, n+2, ..., n+k in the hope that some
   transit LSR further downstream may support MPLS echo requests and
   reply.  In such a case, the echo request for TTL>n MUST NOT have
   Downstream Mapping TLVs, until a reply is received with a Downstream
   Mapping.
















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Normative References

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

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

   [LABEL-STACK] Rosen, E., et al, "MPLS Label Stack Encoding", RFC
       3032, January 2001.

   [RSVP]  Braden, R. (Editor), et al, "Resource ReSerVation protocol
       (RSVP) -- Version 1 Functional Specification," RFC 2205,
       September 1997.

   [RSVP-REFRESH]  Berger, L., et al, "RSVP Refresh Overhead Reduction
       Extensions", RFC 2961, April 2001.

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


Informative References

   [ICMP]  Postel, J., "Internet Control Message Protocol", RFC 792.

   [LDP]  Andersson, L., et al, "LDP Specification", RFC 3036, January
       2001.


Security Considerations

   There are at least two approaches to attacking LSRs using the
   mechanisms defined here.  One is a Denial of Service attack, by
   sending MPLS echo requests/replies to LSRs and thereby increasing
   their workload.  The other is obfuscating the state of the MPLS data
   plane liveness by spoofing, hijacking, replaying or otherwise
   tampering with MPLS echo requests and replies.

   Authentication will help reduce the number of seemingly valid MPLS
   echo requests, and thus cut down the Denial of Service attacks;
   beyond that, each LSR must protect itself.

   Authentication sufficiently addresses spoofing, replay and most
   tampering attacks; one hopes to use some mechanism devised or
   suggested by the RPSec WG.  It is not clear how to prevent hijacking
   (non-delivery) of echo requests or replies; however, if these
   messages are indeed hijacked, LSP ping will report that the data



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   plane isn't working as it should.

   It doesn't seem vital (at this point) to secure the data carried in
   MPLS echo requests and replies, although knowledge of the state of
   the MPLS data plane may be considered confidential by some.


5. IANA Considerations

   The TCP and UDP port number 3503 has been allocated by IANA for LSP
   echo requests and replies.

   The following sections detail the new name spaces to be managed by
   IANA.  For each of these name spaces, the space is divided into
   assignment ranges; the following terms are used in describing the
   procedures by which IANA allocates values: "Standards Action" (as
   defined in [IANA]); "Expert Review" and "Vendor Private Use".

   Values from "Expert Review" ranges MUST be registered with IANA, and
   MUST be accompanied by an Experimental RFC that describes the format
   and procedures for using the code point; the actual assignment is
   made during the IANA actions for the RFC.

   Values from "Vendor Private" ranges MUST NOT be registered with IANA;
   however, the message MUST contain an enterprise code as registered
   with the IANA SMI Network Management Private Enterprise Codes.  For
   each name space that has a Vendor Private range, it must be specified
   where exactly the SMI Enterprise Code resides; see below for
   examples.  In this way, several enterprises (vendors) can use the
   same code point without fear of collision.

5.1. Message Types, Reply Modes, Return Codes

   It is requested that IANA maintain registries for Message Types,
   Reply Modes, Return Codes and Return Subcodes.  Each of these can
   take values in the range 0-255.  Assignments in the range 0-191 are
   via Standards Action; assignments in the range 192-251 are made via
   Expert Review; values in the range 252-255 are for Vendor Private
   Use, and MUST NOT be allocated.

   If any of these fields fall in the Vendor Private range, a top-level
   Vendor Enterprise Code TLV MUST be present in the message.









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5.2. TLVs

   It is requested that IANA maintain registries for the Type field of
   top-level TLVs as well as for sub-TLVs.  The valid range for each of
   these is 0-65535.  Assignments in the range 0-32767 are made via
   Standards Action as defined in {IANA]; assignments in the range
   32768-64511 are made via Expert Review (see below); values in the
   range 64512-65535 are for Vendor Private Use, and MUST NOT be
   allocated.

   If a TLV or sub-TLV has a Type that falls in the range for Vendor
   Private Use, the Length MUST be at least 4, and the first four octets
   MUST be that vendor's SMI Enterprise Code, in network octet order.
   The rest of the Value field is private to the vendor.


Acknowledgments

   This document is the outcome of many discussions among many people,
   that include Manoj Leelanivas, Paul Traina, Yakov Rekhter, Der-Hwa
   Gan, Brook Bailey, Eric Rosen, Ina Minei and Shivani Aggarwal.

   The description of the Multipath Information sub-field of the
   Downstream Mapping TLV was adapted from text suggested by Curtis
   Villamizar.


Appendix

   This appendix specifies non-normative aspects of detecting MPLS data
   plane liveness.

5.1. CR-LDP FEC

   This section describes how a CR-LDP FEC can be included in an Echo
   Request using the following FEC subtype:

      Sub-Type #       Length              Value Field
      ----------       ------              -------------
               5            6              CR-LDP LSP ID

   The value consists of the LSPID of the LSP being pinged.  An LSPID is
   a four octet IPv4 address (a local address on the ingress LSR, for
   example, the Router ID) plus a two octet identifier that is unique
   per LSP on a given ingress LSR.

       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



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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Ingress LSR Router ID                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |            LSP ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.2. Downstream Mapping for CR-LDP

   If a label in a Downstream Mapping was learned via CR-LDP, the
   Protocol field in the Mapping TLV can use the following entry:

         Protocol #        Signaling Protocol
         ----------        ------------------
                  5        CR-LDP


Authors' Address

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

   George Swallow
   Cisco Systems
   1414 Massachusetts Ave,
   Boxborough, MA 01719
   Phone:  +1 978 936 1398
   Email:  swallow@cisco.com



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   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.



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