Network Working Group                                        K.                                   Kireeti Kompella
Internet Draft                                    Juniper Networks Networks, Inc.
Category: Standards Track                                     G. Swallow
Expires: April
Expiration Date: August 2005
                                                          George Swallow
                                                     Cisco Systems
                                                            October 2004 Systems, Inc.

                                                           February 2005

                   Detecting MPLS Data Plane Failures
                    draft-ietf-mpls-lsp-ping-07.txt
                             *** DRAFT ***

                    draft-ietf-mpls-lsp-ping-08.txt

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   applicable patent or other IPR claims of which I am we are aware have been
   disclosed, and any of which I we become aware will be disclosed, in
   accordance with RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved. (2005).

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.)

   Added

    o added clarification of TLV lengths, with examples;
    o added a new error code Global Flags field in the header for the
             'validate FEC' flag;
    o fixed the optional vs. mandatory Types wording;
    o added several new FEC sub-TLVs:
         -   12      BGP labeled IPv4 prefix
         +   12      BGP labeled IPv4 prefix
         +   13      BGP labeled IPv6 prefix (TBD)
         +   14      Generic IPv4 prefix
         +   15      Generic IPv6 prefix
         +   16      Nil FEC
    o in Downstream Mapping Mismatch.

   Split TLV space into "mandatory" and "optional"; updated IANA
   allocation policies to reflect this.

   Added two new top-level TLVs for LSR Self Test.

   Added a new optional top-level TLV for "Errored TLVs"

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
         +   added 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 Address Type of the data plane, as well as a mechanism IPv6 Unnumbered;
         +   added DS Flags to verify the
   data plane against the control plane, DS Map, with 2 defined bits;
         +   renamed Hash key type to multipath type and thereby localize faults.
   The second part suggests two methods of reliable reply channels for
   the echo request message, dropped
             codepoints for more robust fault isolation.

   An important consideration which no processing rules have been
             defined;
    o added "Reply TOS byte" TLV;
    o updated processing rules to deal fully deal with implicit
             null labels
    o added text on processing fewer prefixes in this design is that DS maps;
    o added text on "Testing LSPs That Are Used to Carry MPLS echo requests
   follow the same
             Payloads";
    o fixed text on non-compatible routers.

Contents

 1      Introduction  ..............................................   5
 1.1    Conventions  ...............................................   5
 1.2    Structure of this document  ................................   5
 1.3    Contributors  ..............................................   5
 2      Motivation  ................................................   6
 3      Packet Format  .............................................   7
 3.1    Return Codes  ..............................................  10
 3.2    Target FEC Stack  ..........................................  12
 3.2.1  LDP IPv4 Prefix  ...........................................  13
 3.2.2  LDP IPv6 Prefix  ...........................................  13
 3.2.3  RSVP IPv4 Session  .........................................  14
 3.2.4  RSVP IPv6 Session  .........................................  14
 3.2.5  VPN IPv4 Prefix  ...........................................  15
 3.2.6  VPN IPv6 Prefix  ...........................................  15
 3.2.7  L2 VPN Endpoint  ...........................................  15
 3.2.8  FEC 128 Pseudowire (Deprecated)  ...........................  16
 3.2.9  FEC 128 Pseudowire (Current)  ..............................  16
 3.2.10 FEC 129 Pseudowire  ........................................  17
 3.2.11 BGP Labeled IPv4 Prefix  ...................................  17
 3.2.12 Generic IPv4 Prefix  .......................................  18
 3.2.13 Generic IPv6 Prefix  .......................................  18
 3.2.14 Nil FEC  ...................................................  19
 3.3    Downstream Mapping  ........................................  20
 3.3.1  Multipath Information Encoding  ............................  23
 3.3.2  Downstream Router and Interface  ...........................  24
 3.4    Pad TLV  ...................................................  25
 3.5    Error Code  ................................................  25
 3.6    Vendor Enterprise Code  ....................................  25
 3.7    Interface and Label Stack Object  ..........................  26
 3.7.1  IPv4 Interface and Label Stack Object  .....................  26
 3.7.2  IPv6 Interface and Label Stack Object  .....................  27
 3.8    Errored TLVs  ..............................................  28
 3.9    Reply TOS Byte TLV  ........................................  29
 4      Theory of Operation  .......................................  29
 4.1    Dealing with Equal-Cost Multi-Path (ECMP)  .................  29
 4.2    Testing LSPs That Are Used to Carry MPLS Payloads  .........  30
 4.3    Sending an MPLS Echo Request  ..............................  31
 4.4    Receiving an MPLS Echo Request  ............................  31
 4.5    Sending an MPLS Echo Reply  ................................  35
 4.6    Receiving an MPLS Echo Reply  ..............................  36
 4.7    Issue with VPN IPv4 and IPv6 Prefixes  .....................  36
 4.8    Non-compliant Routers  .....................................  37
 5      References  ................................................  37
 6      Security Considerations  ...................................  38
 7      IANA Considerations  .......................................  38
 7.1    Message Types, Reply Modes, Return Codes  ..................  39
 7.2    TLVs  ......................................................  39
 8      Acknowledgments  ...........................................  39
 A      Appendix  ..................................................  40
 A.1    CR-LDP FEC  ................................................  40
 A.2    Downstream Mapping for CR-LDP  .............................  40

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 cov-
   ered 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, docu-
   ment, and much of the material came out of debate and discussion
   among this group.

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

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 fur-
   ther 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 periodi-
   cally 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.

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         Global Flags          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  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. at a certain version number.
   The Version Number may not need to be changed if an optional TLV or
   sub-TLV is added.)

   The Global Flags field is a bit vector with the following format:

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             SBZ             |V|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   One flag is defined for now, the V bit; the rest SHOULD be set to
   zero when sending, and ignored on receipt.

   The Version Number may not need V (Validate FEC Stack) flag is set to be changed 1 if an optional TLV or
   sub-TLV the sender wants the
   receiver to perform FEC stack validation; if V is added.) 0, the choice is
   left to the receiver.

   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:

       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 con-
   nectivity 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 con-
   trol entities may set the Reply Mode to 4 to ensure that replies use
   that same channel.  Further definition of this codepoint is
   application applica-
   tion specific and thus beyond the scope of this docuemnt.

   Return Codes and Subcodes are described in the next section.

   The

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

   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.  TLVs may be nested within other
   TLVs, in which case the nested TLVs are called sub-TLVs.

   Two examples follow.  The LDP IPv4 FEC 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Type = 1 (LDP IPv4 FEC)    |          Length = 5           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv4 prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Length for this TLV is 5.  A FEC TLV which contains just an LDP
   IPv4 FEC sub-TLV has the 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 = 1 (FEC TLV)       |          Length = 12          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  sub-Type = 1 (LDP IPv4 FEC)  |          Length = 5           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv4 prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A description of the Types and Values of the top level TLVs for LSP
   ping are given below:

          Type #                  Value Field
          ------                  -----------
               1                  Target FEC Stack
               2                  Downstream Mapping
               3                  Pad
               4                  Error Code
               5                  Vendor Enterprise Code
               6                  TBD
               7                  IPv4 Interface and Label Stack Object
               8                  IPv6 Interface and Label Stack Object
               9                  Errored TLVs
              10                  Reply TOS Byte

   Types less than 32768 (i.e., with the high order bit not set (i.e., 1) equal to 0) are
   mandatory TLVs that MUST either be supported by an implementation or
   result in the return code of 2 ("One or more of the TLVs was not
   understood") being sent in the echo response.

   Types greater than or equal to 32768 (i.e., with the high order bit not set (i.e., 0)
   equal to 1) are optional TLVs that MUST SHOULD be ignored if the implementation implemen-
   tation does not support or understand or support them.

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 Sub-
   code 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    Downstream Mapping Mismatch (See Note 1)

              6    Reserved

              7    Reserved

              8    Label switched at stack-depth <RSC>

              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

Note 1. 1

   The Return Subcode contains the point in the label stack stack" where processing pro-
   cessing was terminated.  If the RSC is 0, no labels were processed.
   Otherwise the packet would have been label switched at depth RSC.

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            9              BGP labeled IPv4 prefix
              13           ??              BGP labeled IPv6 prefix (TBD)
              14            5              Generic IPv4 prefix
              15           17              Generic IPv6 prefix
              16          4*N              Nil FEC

   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
   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)                          |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | 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             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

3.2.6. VPN IPv6 Prefix

3.2.8. FEC 128 Pseudowire (Deprecated)

   The value field consists of the Route Distinguisher advertised with
   the VPN IPv6 prefix, remote PE address (the destination
   address of the IPv6 prefix (with trailing 0 bits to make
   128 bits in all) and targetted LDP session), a prefix length, 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                      |
      |                          (8 octets)                      Remote PE Address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IPv6 prefix                           |
      |                                                               |
      |                                                               |
      |                             VC ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length      Encapsulation Type       |         Must Be Zero          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.7. L2 VPN Endpoint

   This FEC will be deprecated, and is retained only for backward com-
   patibility.  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 a Route Distinguisher (8 octets), the
   sender (of sender's PE address (the source
   address of the ping)'s CE ID (2 octets), targetted LDP session), the receiver's CE remote PE address (the
   destination address of the targetted LDP session), a VC ID (2
   octets), and an
   encapsulation type (2 octets), formatted 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                     Sender's PE Address                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE Address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             VC ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      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                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          (8 octets)                      Remote PE Address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Sender's CE ID            PW Type            |       Receiver's CE ID  AGI Length   |  SAII Length  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Encapsulation Type  TAII Length  |         Must Be Zero AGI Value ... SAII Value ... TAII Value ...   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.8. FEC 128 Pseudowire (Deprecated)
      . ...                                                           .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | ...         | 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 remote PE address (the destination
   address of BGP Next Hop associated with the targetted LDP session), a VC ID NLRI
   advertising the prefix and an encapsulation
   type, 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE Address                         BGP Next Hop                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             VC ID                          IPv4 Prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Encapsulation Type Prefix Length |                 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)

3.2.12. Generic IPv4 Prefix

   The value field consists of the sender's PE address (the source
   address four octets of an IPv4 prefix followed by one
   octet of prefix length in bits; the targetted LDP session), format is given below.  The IPv4
   prefix is in network byte order; if the remote PE address (the
   destination address prefix is shorter than 32
   bits, trailing bits SHOULD be set to zero.  This FEC is used if the
   protocol advertising the label is unknown, or may change during the
   course of the targetted LSP.  An example is an inter-AS LSP that may be sig-
   naled by LDP session), a VC ID in one AS, by RSVP-TE in another AS, and an
   encapsulation type, by BGP between
   the ASes, such as follows: is common for inter-AS VPNs.

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

3.2.10. FEC 129 Pseudowire

3.2.13. Generic 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.

       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                          IPv6 prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE Address                          (16 octets)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            PW Type                                                               |  AGI Length
      |  SAII Length                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  TAII Prefix Length | AGI Value ... SAII Value ... TAII Value ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      . ...                                                           .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ...         | 0-3 octets of zero padding         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.14. Nil FEC

   At times labels from the reserved range, e.g. Router Alert and
   Explicit-null, may be added to the label stack for various diagnostic
   purposes such as influencing load-balancing.  These labels may have
   no explicit FEC associated with them.  The Length of this TLV is 13 + AGI length + SAII length + TAII
   length.  Padding Nil FEC stack is used defined
   to make the total length allow a multiple of 4;
   the length of Target FEC stack subtlv to be added to the padding target FEC
   stack to account for such labels so that proper validation can still
   be performed.

   The Length is 4*N octets, where N is not included in the Length field.

3.2.11. BGP Labeled IPv4 Prefix

   The value field consists number of Labels contained
   in the BGP Next Hop associated Nil FEC stack.

   Labels are 20 bit values treated as numbers.  The first label speci-
   fied correspond with the NLRI
   advertising the prefix and label, label nearest the IPv4 prefix (with trailing 0
   bits to make 32 bits in all), and top of the prefix length, as follows: label stack.

       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                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               Label 1                 |                          IPv4 Prefix          SBZ          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length               Label 2                 |                 Must Be Zero          SBZ          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Label 1, Label 2, ... are the actual labels inserted in the label
   stack; the SBZ fields SHOULD be zero when sent, and ignored on
   receipt.

3.3. Downstream Mapping

   The Downstream Mapping object is an optional TLV.  Only one
   Downstream Down-
   stream 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 Down-
   stream 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)    DS Flags   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Downstream IP Address (4 or 16 octets)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Downstream Interface Address (4 or 16 octets)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Hash Key Type | Multipath 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

   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 Numbered
                  2        IPv4 Unnumbered
                  3        IPv6

   Reserved Numbered
                  4        IPv6 Unnumbered

DS Flags

   The DS Flags field marked SBZ SHOULD is a bit vector with the following format:

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      | Rsvd(MBZ) |I|N|
      +-+-+-+-+-+-+-+-+

   Two flags are defined currently, I and N.  The remaining flags MUST
   be set to zero when sending sending, and
       SHOULD be ignored on receipt. receipt.

       Flag  Name and Meaning
       ----  ----------------

          I  Interface and Label Stack Object Request

             When this flag is set, it indicates that the replying
             router should include an Interface and Label Stack
             Object in the Echo-Reply message

          N  Treat as a Non-IP Packet

             Echo-Request messages will be used to diagnose non-IP
             flows.  However, these messages are carried in IP
             packets.  For a router which alters its ECMP algorithm
             based on the FEC or deep packet examinition, this flag
             requests that the router treat this as it would if the
             determination of an IP payload had failed.

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 down-
   stream 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 Type

   The follow Mutipath Types are defined:

      Key   Type                  Multipath Information
      ---   ----------------      ---------------------
       0    no multipath          (empty; M = 0)
       2    IP address            IP addresses
       4    IP address range      low/high address pairs
       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 2, 4, 8 and 9 specify that the supplied Multipath Information
   will serve to execise this path.

Depth Limit

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

Multipath Length

   The length in octets of the Multipath Information.

Multipath Information

   Address or label values encoded according to the Multipath Type.  See
   the next section below for encoding details.

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

   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.

3.3.1. Multipath Information Encoding

   The multipath information encodes labels or addresses which will
   exercise this path.  The multipath informaiton depends on the
       hash key multi-
   path 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

   Type 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

   Type 9 allows a denser encoding of Labels.  The label prefix is formatted for-
   matted as a base label value with the non-prefix low order bits set
   to zero.  The maximum prefix (including leading zeros due to encoding) encod-
   ing) 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 provided.  If none of these
   labels or addresses map to a particular downstream interface, then
   for that interface, the Hash Key
       Type type MUST be set to 7. 0.  If the received multipath mul-
   tipath information is 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.  The received 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 V, with Hash Key Type 4 (1.1.1.127->1.1.1.255); and to W, with
   Hash Key Type 7.

   Note that computing multi-path information may impose a significant
   processing burden on the receiver.  A receiver MAY thus choose to
   process a subset of the received prefixes.  The sender, on receiving
   a reply to W, a Downstream Map with Hash Key Type 7.

3.3.1. "Downstream" partial information, SHOULD assume
   that the prefixes missing in the reply were skipped by the receiver,
   and MAY re-request information about them in a new echo request.

3.3.2. Downstream Router and Interface

   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 down-
   stream 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
       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 label L may be forwarded 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 this one
   interface.

   Types 1, 2, 3, 4, 8 and 9 specify 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 mul-
   ticast).  In the former case, the supplied Multipath
   Information will serve sub-field is used as a
   hint to execise this path.

   Types 5 and 6 are TBD.

   Type 7 indicates that no matches are possible given the Multipath
   Information in sender as to how it may influence the received DS mapping information. choice of these
   alternatives.  The "No of Multipaths" is the number of IP
   Address/Next Label fields.

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 pro-
   vide 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 net-
   work 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.

3.7. Interface and Label Stack Object

   The Interface and Label Stack Object is an optional TLV.  It is used
   in a Reply message to report the interface on which the Request
   Message Mes-
   sage was received and the label stack which was on the packet when it
   was received.  Only one such object may appear.  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).

3.8.

3.7.1. IPv4 Interface and Label Stack Object

   The Length is 8 + 4*N octets, N is the number of Downstream Labels.
   The value field of a Interface and Label Stack 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Downstream IPv4 Address                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Downstream Interface Address                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                          Label Stack                          .
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      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.

3.9.

3.7.2. IPv6 Interface and Label Stack Object

   The Length is 32 + 4*N octets, N is the number of Downstream Labels.
   The value field of a Interface and Label Stack 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                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                          .
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      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.

3.10.

3.8. Errored TLVs

   The following TLV is an optional TLV defined to be sent back to back to the
   sender of an Echo Request to inform it of Mandatory TLVs either not
   supported by an implementation, or parsed and found to be in error.

   The Value field contains the TLVs not understood encoded as subtlvs.

       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 = 9          |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Value                             |
      .                                                               .
      .                                                               .
      .                                                               .
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.9. Reply TOS Byte TLV

       This TLV is used by the originator of the echo request to request
       that a echo reply be sent with the
   sender of an Echo Request to inform it of Mandatory TLVs either not
   supported by an implementation, or parsed and found IP header TOS byte set to be
       the value specified in error.

   The Value field contains the TLVs not understood encoded as subtlvs. TLV.  This TLV has a length of 4 with
       the following value field.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Type = 32678      |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Value                             |
      .                                                               .
      .                                                               .
      .                                                               .
      | Reply-TOS Byte|                 Must be zero                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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. pre-
   fix.  If the underlying (LDP) tunnel were not known, or was
   considered consid-
   ered 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 typ-
   ically 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
   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 tracer-
   oute 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 possi-
   ble paths.  However, full coverage may not be possible.

4.2. Testing LSPs That Are Used to Carry MPLS Payloads

   To detect certain LSP breakages, it may be necessary to encapsulate
   an MPLS echo request packet with at least one additional label when
   testing LSPs that are used to carry MPLS payloads (such as LSPs used
   to carry L2VPN and L3VPN traffic.  For example, when testing LDP or
   RSVP-TE LSPs, just sending an MPLS echo request packet may not detect
   instances where the router immediately upstream of the destination of
   the LSP ping may forward the MPLS echo request successfully over an
   interface not configured to carry MPLS payloads because of the use of
   penultimate hop popping.  Since the receiving router has no means to
   differentiate whether the IP packet was sent unlabeled or implicitly
   labeled, the addition of labels shimmed above the MPLS echo request
   (using the Nil FEC) will prevent a router from forwarding such a
   packet out unlabeled interfaces.

4.3. 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 ping-
   ing 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 outer-
   most 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.

   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 set to zero.  In the "traceroute" mode, the echo request with TTL=n; the sender MAY choose to reduce the
   size of SHOULD a "Downstream Multipath
   Downstream 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. TLV.

4.4. 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,  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 two variables, called FEC-stack-depth and Label-stack-depth,
   to the number of labels in the received label stack.  If the label-
   stack-depth is 0, assume there is one implicit null label and set
   label-stack-depth to 1.  Processing now continues with the following
   steps:

   Label_Validation:

     If the label at Label-stack-depth is valid, goto Label_Operation.
     If not, set Best-return-code to 11, "No label entry at stack-depth"
     and Best-return-subcode to Label-stack-depth.  Goto
     Send_Reply_Packet.

   Label_Operation:

     Switch on label operation.

     Case:  Pop and Continue Processing (Note: this includes
                Explicit_Null and Router_Alert)

       If Label-stack-depth is greater than 1, decrement Label-stack-
       depth and goto Label_Validation.  Otherwise, set FEC-stack-depth
       to 1, set Best-return-code to 3 "Replying router is an egress for
       the FEC at stack depth", set Best-return-subcode to 1 and goto
       Egress_Processing.

     Case:  Swap or Pop and Switch based on Popped Label

       If the label operation is either swap or pop and switch based on
       the popped label, Best-return-code to 8, "Label switched at
       stack-depth" and Best-return-subcode to Label-stack-depth.

       If a variable, call Downstream Mapping TLV is present, a Downstream mapping TLVs
       SHOULD be created for each multipath.

       Determine the output interface.  If it current-stack-depth, is not valid to forward a
       labelled packet on this interface, set Best-return-code to Return
       Code 9, "Label switched but no MPLS forwarding at stack-depth"
       and set Best-return-subcode to Label-stack-depth and goto
       Send_Reply_Packet.  (Note: this return code is set even if Label-
       stack-depth is one.)

       If no Downstream Mapping TLV is present, or the Downstream IP
       Address is set to the number of
   labels in All-Routers multicast address goto
       Send_Reply_Packet.

       Verify that the IP address, interface address and label stack
       match the received interface and 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 not, set Best-
       return-code to step 2. 5, "Downstream Mapping Mis-match".  A Received
       Interface and Label Stack TLV SHOULD be created.  Goto
       Send_Reply_Packet.

       If not, check if the label "Validate FEC Stack" flag is
       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 not set, goto
       Send_Reply_Packet.

       Locate the label
       entry at stack-depth" Label-stack-depth in the Downstream Labels
       and a Return Subcode set FEC-stack-depth to current-stack- that depth.

   2.  Check (Note: If the FEC Downstream
       Labels contain one or more Implicit Null labels, this may be at a
       depth greater than Label-stack-depth.

       If the current-stack-depth to determine what
       protocol would be used depth of the FEC stack is greater than or equal to advertise it. FEC-
       stack-depth, Perform FEC Checking.  If it can determine that
       no protocol associated with interface I, would have advertised a FEC-status is 2, set Best-
       return-code to 10, "Mapping for this FEC of that FEC-Type, X MUST send an MPLS echo reply with a
       Return Code 12, "Protocol is not associated with interface the given label
       at FEC
       stack-depth" and a Return Subcode stack-depth".

       If the return code is 1 set Best-return-code to current-stack-depth.

   3.  Check FEC-return-code
       and Best-return-subcode to FEC-stack-depth.

       Goto Send_Reply_Packet.

   Egress_Processing:

     If no Downstream Mapping TLV is present, goto
     Egress_FEC_Validation.

     Verify that the mapping for IP address, interface address and label stack match
     the received interface and label stack.  If not, set Best-return-
     code to 5, "Downstream Mapping Mis-match".  A Received Interface
     and Label Stack TLV SHOULD be created.  Goto Send_Reply_Packet.

   Egress_FEC_Validation:

     Perform FEC at the current-stack-depth checking.  If FEC-status is
       the corresponding label. 1, set Best-return-code
     to FEC-code and Best-return-subcode to FEC-stack-depth.  Goto
     Send_Reply_Packet.

     Increment FEC-stack-depth.  If no mapping for FEC-stack-depth is greater than
     the FEC exists, X MUST send number of FECs in the FEC-stack, goto Send_Reply_Packet.  If
     FEC-status is 0, increment Label-stack-depth.  Goto
     Egress_FEC_Validation.

   Send_Reply_Packet:

     Send an MPLS echo reply with a Return Code 4, "Replying router has no mapping for the FEC
       at stack-depth" of Best-return-code,
     and a Return Subcode set to current- stack-depth.

       If a mapping is found, but of Best-return-subcode.  Include any TLVs
     created during the mapping is not above process.  The procedures for sending the corresponding
       label, X MUST send an MPLS
     echo reply with are found in the next subsection below.

   FEC_Checking:

     This routine accepts a Return Code FEC, Label, and Interface.  It returns two
     values, FEC-status and FEC-return-code, both of which are
     initialized to 0.

     If the FEC is the Nil FEC, check that Label is either
     Explicit_Null or Router_Alert.  If so return.  Else
     set FEC-return-code to 10, "Mapping for this FEC is not the given
     label at stack-depth" and
       a Return Subcode set stack-depth".  Set FEC-status to current-stack-depth.

   4.  X determines 1 and return.

     Check that the label operation. mapping for FEC.  If the operation is no mapping exists, set
     FEC-return-code 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, 4, "Replying router is an egress has no mapping for
     the FEC at stack depth" and a
       Return Subcode set to one. Otherwise, X decrements current-stack-
       depth and goes back stack-depth".  Set FEC-status to step 1.  Return.

     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 mapping for FEC 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 Implicit Null, set FEC-status to current-stack-depth.  This return code is
       sent even if current-stack-depth is one.
     2.  Goto Check_Protocol.

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

   If 10, "Mapping for this FEC is not the MPLS echo request contains a downstream mapping TLV, given
     label at stack-depth".  Set FEC-status to 1 and the
   MPLS echo reply has either a Return Code of 8, or a Return Code of 9 return.

   Check_Protocol:

     Check what protocol would be used to advertise FEC.  If it can be
     determined that no protocol associated with interface I would
     have advertised a Return Subcode FEC of 1 then Downstream mapping TLVs SHOULD be
   included for each multipath.

   X uses the procedure in the next subsection that FEC-Type, set FEC-return-code to send the echo reply.

4.4.
     12, "Protocol not associated with interface at FEC stack-depth".
     Set FEC-status to 1.  Return.

4.5. 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 replying router is the destination of the FEC, then Downstream
   Mapping TLVs SHOULD NOT be included in the echo reply.

   If the echo request contains a Downstream Mapping TLV, and the reply-
   ing router is not the destination of the FEC, the replier SHOULD compute com-
   pute 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.

   If the Downstream Mapping TLV contains multipath information requir-
   ing more processing than the receiving router is willing to perform,
   the responding router MAY choose to respond with only a subset of
   multipaths contained in the Echo Request Downstream Map.  (Note: The
   originator of the echo request MAY send another echo request with the
   multipath information that was not included in the reply.)

4.6. 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, oth-
   erwise, 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
   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.

4.7. 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, circum-
   stances, the label stack can shrink to a single label before the ping
   hits the egress PE; this will result in the ping terminating
   prematurely. prema-
   turely.  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.

4.8. 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". nega-
   tives".  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 to probe LSRs further downstream may support MPLS echo requests and
   reply. down
   the path.  In such a case, the echo request for TTL>n MUST NOT have TTL > n SHOULD be
   sent with Downstream Mapping TLVs, TLV "Downstream IP Address" field set to
   the ALLROUTERs multicast address until a reply is received with a
   Downstream
   Mapping. Mapping TLV.  The Label Stack MAY be omitted from the
   Downstream Mapping TLV.  Further the "Validate FEC Stack" flag SHOULD
   NOT be set until an ECHO REQUEST packet with a Downstream Mapping TLV
   is received.

5. References

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
                  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.

6. Security Considerations

   There are at least two approaches to attacking LSRs using the
   mechanisms mecha-
   nisms defined here.  One is a Denial of Service attack, by sending
   MPLS echo requests/replies to LSRs and thereby increasing their workload. work-
   load.  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 tam-
   pering attacks; one hopes to use some mechanism devised or suggested
   by the RPSec WG.  It is not clear how to prevent hijacking
   (non-delivery) (non-
   delivery) of echo requests or replies; however, if these messages are
   indeed hijacked, LSP ping will report that the data plane isn't working work-
   ing 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.

7. 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. exam-
   ples.  In this way, several enterprises (vendors) can use the same
   code point without fear of collision.

5.1.

7.1. 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.

5.2.

7.2. 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 0-16383 and 32768-49161
   are made via Standards Action as defined in [IANA]; assignments in
   the range 16384-31743 and 49162-64511 are made via Expert Review (see
   below); values in the range 31744-32746 and 64512-65535 are for
   Vendor Ven-
   dor 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.

8. 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, Shivani Aggarwal and Vansom Vanson
   Lim.

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

A. Appendix

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

5.1.

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

5.2.

A.2. Downstream Mapping for CR-LDP

   If a label in a Downstream Mapping was learned via CR-LDP, the
   Protocol Proto-
   col 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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