Network Working Group                                   Kireeti Kompella
Internet Draft                                    Juniper Networks, Inc.
Category: Standards Track
Expiration Date: May June 2006
                                                          George Swallow
                                                     Cisco Systems, Inc.

                                                           November

                                                           December 2005

                   Detecting MPLS Data Plane Failures

                    draft-ietf-mpls-lsp-ping-11.txt

                    draft-ietf-mpls-lsp-ping-12.txt

Status of this Memo

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Abstract

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

Contents

 1      Introduction  ..............................................   3   4
 1.1    Conventions  ...............................................   3   4
 1.2    Structure of this document  ................................   3   4
 1.3    Contributors  ..............................................   3   5
 2      Motivation  ................................................   4   5
 2.1    Use of address range 127/8  ................................   6
 3      Packet Format  .............................................   5   7
 3.1    Return Codes  ..............................................   9  12
 3.2    Target FEC Stack  ..........................................  10  13
 3.2.1  LDP IPv4 Prefix  ...........................................  11  14
 3.2.2  LDP IPv6 Prefix  ...........................................  11  14
 3.2.3  RSVP IPv4 LSP  .............................................  12  15
 3.2.4  RSVP IPv6 LSP  .............................................  12  15
 3.2.5  VPN IPv4 Prefix  ...........................................  13  16
 3.2.6  VPN IPv6 Prefix  ...........................................  14  17
 3.2.7  L2 VPN Endpoint  ...........................................  14  17
 3.2.8  FEC 128 Pseudowire (Deprecated)  ...........................  15  18
 3.2.9  FEC 128 Pseudowire (Current)  ..............................  15  18
 3.2.10 FEC 129 Pseudowire  ........................................  16  19
 3.2.11 BGP Labeled IPv4 Prefix  ...................................  16  19
 3.2.12 BGP Labeled IPv6 Prefix  ...................................  17  20
 3.2.13 Generic IPv4 Prefix  .......................................  17  20
 3.2.14 Generic IPv6 Prefix  .......................................  18  21
 3.2.15 Nil FEC  ...................................................  18  21
 3.3    Downstream Mapping  ........................................  19  22
 3.3.1  Multipath Information Encoding  ............................  23  26
 3.3.2  Downstream Router and Interface  ...........................  25  28
 3.4    Pad TLV  ...................................................  25  28
 3.5    Vendor Enterprise Code  ....................................  26 Number  ..................................  29
 3.6    Interface and Label Stack  .................................  26  29
 3.7    Errored TLVs  ..............................................  28  31
 3.8    Reply TOS Byte TLV  ........................................  28  31
 4      Theory of Operation  .......................................  29  32
 4.1    Dealing with Equal-Cost Multi-Path (ECMP)  .................  29  32
 4.2    Testing LSPs That Are Used to Carry MPLS Payloads  .........  30  33
 4.3    Sending an MPLS Echo Request  ..............................  30  33
 4.4    Receiving an MPLS Echo Request  ............................  31  34
 4.4.1  FEC Validation  ............................................  40
 4.5    Sending an MPLS Echo Reply  ................................  34  41
 4.6    Receiving an MPLS Echo Reply  ..............................  35  42
 4.7    Issue with VPN IPv4 and IPv6 Prefixes  .....................  36  42
 4.8    Non-compliant Routers  .....................................  36  43
 5      References  ................................................  37  43
 6      Security Considerations  ...................................  38  44
 7      IANA Considerations  .......................................  38  45
 7.1    Message Types, Reply Modes, Return Codes  ..................  39  46
 7.2    TLVs  ......................................................  40  47
 8      Acknowledgments  ...........................................  41  48

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 cov-
   ered in this document.

   This document makes special use of the address range 127/8.  This is
   an exception to the behavior defined in RFC1122 [RFC1122] and updates
   that RFC.  The motivation for this change and the details of this
   exceptional use are discussed in section 2.1 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].

   The term "Must be Zero" (MBZ) is used in object decriptions descriptions for
   reserved fields.  These fields MUST be set to zero when sent and
   ignored on receipt.

   Terminology pertaining to L2 and L3 VPNs is defined in [RFC4026].

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 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 a
   Label Switching Router (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 isola-
   tion), the packet is sent to the control plane of each transit LSR,
   which performs various checks that it is indeed a transit LSR for
   this path; this LSR also returns further information that helps check
   the control plane against the data plane, i.e., that forwarding
   matches what the routing protocols determined as the path.

   One way these tools can be used is to periodically ping a FEC to
   ensure connectivity.  If the ping fails, one can then initiate a
   traceroute to determine where the fault lies.  One can also 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

2.1. Use of address range 127/8

   As described above, LSP Ping is intended as a diagnostic tool.  It is
   intended to enable providers of an MPLS based service to isolate net-
   work faults.  In particular LSP Ping needs to diagnose situations
   where the control and data planes are out of sync.  It performs this
   by routing an MPLS echo request packet based solely on its label
   stack.  That is the IP destination address is never used in a (possibly labeled) IPv4 or IPv6 UDP packet; for-
   warding decision.  In fact, the contents sender of the UDP an MPLS echo request packet have
   may not know, a priori, the address of the router at the end of the
   LSP.

   Providers of MPLS based services also need the ability to trace all
   of the possible paths that an LSP make take.  Since most MPLS ser-
   vices are based on IP unicast forwarding, these paths are subject to
   equal cost multi-path load sharing (ECMP).

   This leads to 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        |         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 requirements:

   1.  (Note:  Although the Version Number is to LSP in question may be incremented whenever a change is made that affects broken in unknown ways, the ability
       likelihood of
   an implementation a diagnostic packet being delivered to correctly parse or process a user of an
       MPLS echo
   request/reply.  These changes include any syntactic or semantic
   changes made to any of the fixed fields, or to any TLV or sub-TLV
   assignment or format that is defined at a certain version number.
   The Version Number may not need to service MUST be changed if held to an optional TLV or
   sub-TLV is added.)

   The Global Flags field absolute minimum.

   2.  If an LSP is broken in such 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             MBZ             |V|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   One flag is defined for now, the V bit; way that it prematurely terminates,
       the rest diagnostic packet MUST NOT be set to zero
   when sending, and ignored on receipt.

   The V (Validate FEC Stack) flag is set to 1 if the sender wants the
   receiver to perform FEC stack validation; if V is 0, the choice is
   left to IP forwarded.

   3.  A means of varying the receiver.

   The Message Type diagnostic packets such that they exercise
       all ECMP paths is one thus REQUIRED.

   Clearly using general unicast addresses satisfies neither of the following:

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

   The Reply Mode can take one
   first two requirements.  A number of other options for addresses were
   considered, including a portion 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 1 (Do not reply) in private address space (as
   determined by the Reply Mode field
   may be used for one-way connectivity tests; network operator) and the receiving router may
   log gaps in newly designated IPv4
   link local addresses.  Use of the sequence numbers and/or maintain delay/jitter statis-
   tics.  An private address space was deemed
   ineffective since the leading MPLS echo request would normally have 2 (Reply via an
   IPv4/IPv6 UDP packet) based service is IPv4 Virtual Pri-
   vate Networks (VPN).  VPNs often used private addresses.

   The IPv4 link local addresses are more attractive in the Reply Mode field.  If the normal IP
   return path that scope over
   which they can be forwarded is deemed unreliable, limited.  However, if one may were to use 3 (Reply via
   an
   IPv4/IPv6 UDP address from this range, it would still be possible for the first
   recipient of a diagnostic packet with Router Alert).  Note that this requires "escaped" from a broken LSP to
   have that all intermediate routers understand and know how addressed assigned to forward MPLS
   echo replies.  The echo reply uses the same IP version number as interface on which it arrived and
   thus could mistakenly receive such a packet.  Further, the
   received echo request, i.e., an IPv4 encapsulated echo reply is sent
   in response to link
   local address range has only recently been allocated.  Many deployed
   routers would forward a packet with an address from that range toward
   the default route.

   The 127/8 range for IPv4 encapsulated echo request.

   Any application which supports and that same range embedded in an IP control channel between its con-
   trol entities may set IPv6
   addresses for IPv6 was chosen for a number of reasons.

   RFC1122 allocates the Reply Mode 127/8 as "Internal host loopback address" and
   states that "Addresses of this form MUST NOT appear outside a host."
   Thus the default behavior of hosts is to 4 (Reply via application
   level control channel) discard such packets.  This
   helps to ensure that replies use if a diagnostic packet is mis-directed to a
   host, it will be silently discarded.

   RFC1812 [RFC1812] states that:

      A router SHOULD NOT forward, except over a loopback interface, any
      packet that same channel.
   Further definition of this codepoint has a destination address on network 127.  A router
      MAY have a switch that allows the network manager to disable these
      checks.  If such a switch is application specific and thus
   beyond provided, it MUST default to perform-
      ing the scope of this document.

   Return Codes and Subcodes checks.

   This helps to ensure that diagnostic packets are described never IP forwarded.

   The 127/8 address range provides 16M addresses allowing wide flexi-
   bility in varying addresses to exercise ECMP paths.  Finally, as an
   implementation optimization, the next section.

   the Sender's Handle 127/8 provides an easy means of
   identifying possible LSP Packets.

3. Packet Format

   An MPLS echo request is filled in by a (possibly labeled) IPv4 or IPv6 UDP packet;
   the sender, and returned
   unchanged by contents of 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,
   according to the sender's clock) when the MPLS echo request is sent.
   The TimeStamp Received in an echo reply is the time-of-day (according
   to the receiver's clock) that the corresponding echo request was
   received.

   TLVs (Type-Length-Value tuples) have 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        |         Global Flags          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Message Type |            Length   Reply mode  |  Return Code  | Return Subcode|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Value                        Sender's Handle                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sequence Number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    TimeStamp Sent (seconds)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  TimeStamp Sent (microseconds)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  TimeStamp Received (seconds)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                TimeStamp Received (microseconds)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            TLVs ...                           |
      .                                                               .
      .                                                               .
      .                                                               .
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Types are defined below; Length is the length of the Value field in
   octets.

   The Value field depends on Version Number is currently 1.  (Note: the Type; it Version Number is zero padded to
   align
   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 four-octet boundary.  TLVs certain version number.
   The Version Number may not need to be nested within other
   TLVs, in which case the nested TLVs are called sub-TLVs.  Sub-TLVs
   have independent types and MUST also be four-octet aligned.

   Two examples follow.  The LDP IPv4 FEC changed if an optional TLV or
   sub-TLV has is added.)

   The Global Flags field is a bit vector with the following for-
   mat:

       0                   1                   2                   3 format:

       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             MBZ             |V|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   One flag is 5.  A Target defined for now, the V bit; the rest MUST be set to zero
   when sending, and ignored on receipt.

   The V (Validate FEC Stack TLV which contains
   an LDP IPv4 Stack) flag is set to 1 if the sender wants the
   receiver to perform FEC sub-TLV and a VPN IPv4 prefix sub-TLV has stack validation; if V is 0, the format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 choice is
   left to the receiver.

   The Message Type is one of the following:

       Value    Meaning
       -----    -------
           1    MPLS Echo Request
           2 3 4 5 6 7 8 9 0    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 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type =    Reply via application level control channel

   An MPLS echo request with 1 (FEC TLV)       |          Length = 12          | (Do not reply) in the Reply Mode field
   may be used for one-way connectivity tests; the receiving router may
   log gaps in the sequence numbers and/or maintain delay/jitter statis-
   tics.  An MPLS echo request would normally have 2 (Reply via an
   IPv4/IPv6 UDP packet) in the Reply Mode field.  If the normal IP
   return path is deemed unreliable, one may use 3 (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.

   Some applications support an IP control channel.  One such example is
   the associated control channel defined in Virtual Circuit Connectiv-
   ity Verification [VCCV].  Any application which supports an IP con-
   trol channel between its control entities may set the Reply Mode to 4
   (Reply via application level control channel) to ensure that replies
   use that same channel.  Further definition of this codepoint is
   application specific and thus beyond the scope of this document.

   Return Codes and Subcodes are described in the next section.

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

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

   The TimeStamp Sent is the time-of-day (in seconds and microseconds,
   according to the sender's clock) in NTP format [NTP] when the MPLS
   echo request is sent.  The TimeStamp Received in an echo reply is the
   time-of-day (according to the receiver's clock) in NTP format 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.  Sub-TLVs
   have independent types and MUST also be four-octet aligned.

   Two examples follow.  The LDP IPv4 FEC sub-TLV has the following for-
   mat:

       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 Target FEC Stack TLV which contains
   an LDP IPv4 FEC sub-TLV and a VPN IPv4 prefix 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                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | sub-Type = 6 (VPN IPv4 prefix)|          Length = 13          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                      |
      |                          (8 octets)                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         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                  Not Assigned
               5                  Vendor Enterprise Code Number
               6                  Not Assigned
               7                  Interface and Label Stack
               8                  Not Assigned
               9                  Errored TLVs
              10                  Reply TOS Byte

   Types less than 32768 (i.e., with the high order bit 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
   equal to 1) are optional TLVs that SHOULD be ignored if the implemen-
   tation does not 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 Sub-
   code MUST be set to zero.

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

              0    No return code

              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    Upstream Interface Index Unknown (See Note 1)

              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

   The Return Subcode contains the point in the label stack where 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 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 LSP
             4           56            RSVP IPv6 LSP
             5                         Not Assigned
             6           13            VPN IPv4 prefix
             7           25            VPN IPv6 prefix
             8           14            L2 VPN endpoint
             9           10            "FEC 128" Pseudowire (deprecated)
            10           14            "FEC 128" Pseudowire
            11          16+            "FEC 129" Pseudowire
            12            5            BGP labeled IPv4 prefix
            13           17            BGP labeled IPv6 prefix
            14            5            Generic IPv4 prefix
            15           17            Generic IPv6 prefix
            16            4            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
   [see LDP] 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 [see section
   3.2.5] 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 Distin-
   guisher that LSR X uses in its own advertisements for VPN foo), label
   23456 and BGP nexthop 192.168.1.1 [see BGP].  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 IPv4 Prefix FEC is defined in [LDP].  When a LDP IPv4 prefix is
   encoded in a label stack, the following format is used.  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, trail-
   ing bits SHOULD be set to zero.  See [LDP] for an example of a Map-
   ping 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 IPv6 Prefix FEC is defined in [LDP].  When a LDP IPv6 prefix is
   encoded in a label stack, the following format is used.  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 LSP

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

       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 LSP

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

       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

   VPN-IPv4 NLRI (Network Layer Routing Information) is defined in
   [MPLS-L3-VPN].  This document uses the term VPN IPv4 prefix for a
   VPN-IPv4 NLRI which has been advertised with an MPLS label in BGP.
   See [BGP-LABEL].

   When a VPN IPv4 prefix is encoded in a label stack, the following
   format is used.  The value field consists of the Route Distinguisher
   advertised with the VPN IPv4 prefix, the IPv4 prefix (with trailing 0
   bits to make 32 bits in all) and a prefix length, as follows:

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

3.2.6. VPN IPv6 Prefix

   VPN-IPv6 NLRI (Network Layer Routing Information) is defined in
   [MPLS-L3-VPN].  This document uses the term VPN IPv6 prefix for a
   VPN-IPv6 NLRI which has been advertised with an MPLS label in BGP.
   See [BGP-LABEL].

   When a VPN IPv6 prefix is encoded in a label stack, the following
   format is used.  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

   VPLS BGP NLRI and VE ID are defined in [VPLS].  This document uses
   the simpler term L2 VPN endpoint when refering referring to a VPLS BGP NLRI.
   When an L2 VPN endpoint is encoded in a label stack, the following
   format is used.  The value field consists of a Route Distinguisher (8
   octets), the sender (of the ping)'s VE ID (2 octets), the receiver's
   VE 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 VE ID        |       Receiver's VE ID        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Encapsulation Type       |         Must Be Zero          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.8. FEC 128 Pseudowire (Deprecated)

   FEC 128 and the term VC ID are defined in [PW-CONTROL].  When a FEC
   128 is encoded in a label stack, the following format is used.  The
   value field consists of the remote PE address (the destination
   address of the targeted LDP session), a VC ID and an encapsulation
   type, as follows:

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

   This FEC is deprecated and is retained only for backward compatibil-
   ity.  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 configured 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)

   FEC 128 and the term VC ID are defined in [PW-CONTROL].  When a FEC
   128 is encoded in a label stack, the following format is used.  The
   value field consists of the sender's PE address (the source address
   of the targeted LDP session), the remote PE address (the destination
   address of the targeted LDP session), a VC ID and an encapsulation
   type, as follows:

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

3.2.10. FEC 129 Pseudowire

   FEC 129 and the terms AII, AGI, SAII, and TAII are defined in [PW-
   CONTROL].  When a FEC 129 is encoded in a label stack, the following
   format is used.  The Length of this TLV is 16 + 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.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Sender's PE Address                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE Address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            PW Type            |   AGI Type    |  AGI Length   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                           AGI Value                           ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   AII Type    |  SAII Length  |      SAII Value               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    SAII Value (contd.) (continued)                     ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   AII Type    |  TAII Length  |      TAII Value               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    TAII Value (contd.) (continued)                     ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Value (cont.)|  TAII (cont.) |  0-3 octets of zero padding                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.11. BGP Labeled IPv4 Prefix

   BGP labeled IPv4 prefixes are defined in [BGP-LABEL].  When a BGP
   labeled IPv4 prefix is encoded in a label stack, the following format
   is used.  The value field consists 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv4 Prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |                 Must Be Zero                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.12. BGP Labeled IPv6 Prefix

   BGP labeled IPv6 prefixes are defined in [BGP-LABEL].  When a BGP
   labeled IPv6 prefix is encoded in a label stack, the following format
   is used.  The value consists of sixteen octets of an IPv6 prefix fol-
   lowed 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv6 prefix                          |
      |                          (16 octets)                          |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.13. Generic 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.  This FEC is used if the
   protocol advertising the label is unknown, or may change during the
   course of the LSP.  An example is an inter-AS LSP that may be sig-
   naled by LDP in one AS, by RSVP-TE [RSVP-TE] in another AS, and by
   BGP between the ASes, such as 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv4 prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.14. 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv6 prefix                          |
      |                          (16 octets)                          |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.15. 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 Nil FEC stack is defined
   to allow a Target FEC stack sub-TLV to be added to the target FEC
   stack to account for such labels so that proper validation can still
   be performed.

   The Length is 4.  Labels are 20 bit values treated as numbers.
   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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Label                 |          MBZ          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Label is the actual label value inserted in the label stack; the MBZ
   fields MUST be zero when sent, and ignored on receipt.

3.3. Downstream Mapping

   The Downstream Mapping object is a TLV which MAY be included in an
   echo request message.  Only one Downstream Mapping object may appear
   in an 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 the replying router 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 sec-
   tion 3.3.2, "Downstream Router and Interface".

   The Length is K + M + 4*N octets, where M is the Multipath Length,
   and N is the number of Downstream Labels.  Values for K are found in
   the description of Address Type below.  The Value field of a 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  |    DS Flags   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Downstream IP Address (4 or 16 octets)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Downstream Interface Address (4 or 16 octets)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Multipath Type| Depth Limit   |        Multipath Length       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                     (Multipath Information)                   .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Maximum Transmission Unit (MTU)

   The MTU is the size in octets of 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 unnum-
   bered.  It also determines the length of the Downstream IP Address
   and Downstream Interface fields.  The resulting total for the initial
   part of the TLV is listed in the table below as "K Octets".  The
   Address Type is set to one of the following values:

             Type #        Address Type           K Octets
             ------        ------------           --------
                  1        IPv4 Numbered                16
                  2        IPv4 Unnumbered              16
                  3        IPv6 Numbered                40
                  4        IPv6 Unnumbered              28

DS Flags

   The DS Flags field 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, and ignored on 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 examination, 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

   IPv4 addresses and and interface indices are encoded in 4 octets,
   IPv6 addresses are encoded in 16 octets.

   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 IPv4 Unnumbered or IPv6 Unnumbered, the Downstream IP
   Address MUST be the downstream LSR's Router ID, and the Downstream
   Interface Address MUST be set to the index assigned by the upstream
   LSR to the interface.

   If an LSR does not know the IP address of its neighbor, then it MUST
   set the Address Type to either IPv4 Unnumbered or IPv6 Unnumbered.
   For IPv4 it must set the Downstream IP Address to 127.0.0.1, for IPv6
   the address is set to 0::1.  In both cases the interface index MUST
   be set to 0.  If an LSR receives an Echo Request packet with either
   of these addresses in the Downstream IP Address field, this indicates
   that it MUST bypass interface verification but continue with label
   validation.

   If the originator of an Echo Request packet wishes to obtain Down-
   stream mapping information but does not know the expected label stack
   then it SHOULD set the Address Type to either IPv4 Unnumbered or IPv6
   Unnumbered.  For IPv4 it MUST set the Downstream IP Address to
   224.0.0.2, for IPv6 the address MUST be set to FF02::2.  In both
   cases the interface index MUST be set to 0.  If an LSR receives an
   Echo Request packet with the all-routers multicast address, then this
   indicates that it MUST bypass both interface and label stack valida-
   tion, but return Downstream Mapping TLVs using the information pro-
   vided.

Multipath Type

   The following Multipath Types are defined:

      Key   Type                  Multipath Information
      ---   ----------------      ---------------------
       0    no multipath          Empty (Multipath Length = 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 exercise 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 included.  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 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

3.3.1. Multipath Information Encoding

   The multipath information encodes labels or addresses which will
   exercise this path.  The multipath information depends on the 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.  For Type 4,
   ranges indicated by Address pairs MUST NOT overlap and MUST be in
   ascending sequence.

   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 beginning with zero.  For example the IP addresses
   127.2.1.0, 127.2.1.5-127.2.1.15, and 127.2.1.20-127.2.1.29 would be
   encoded 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 1 1 1 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type 9 allows a denser encoding of Labels.  The label prefix is 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 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 beginning with
   zero.  Label values of all the odd numbers between 1152 and 1279
   would be encoded 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If the received multipath information is non-null, the labels and IP
   addresses MUST be picked from the set provided.  If none of these
   labels or addresses map to a particular downstream interface, then
   for that interface, the type MUST be set to 0.  If the received mul-
   tipath information is null, (i.e. Multipath Length = 0, or for Types
   8 and 9 a mask of all zeroes) the receiver the type MUST be set to 0.

   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 Multipath Type
   4, with low/high IP addresses of 127.1.1.1->127.1.1.255 for down-
   stream LSR Y and 127.2.1.1->127.2.1.255 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 127.1.1.1->127.1.1.127
   would go to U and 127.1.1.128-> 127.1.1.255 would go to V.  Y would
   then respond with 3 Downstream Mappings: to U, with Multipath Type 4
   (127.1.1.1->127.1.1.127); to V, with Multipath Type 4
   (127.1.1.127->127.1.1.255); and to W, with Multipath Type 0.

   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 a Downstream Map with 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 and TTL=1 at LSR X, X
   must be able to compute which LSRs could receive the packet if it was
   originated with TTL=n+1, over which interface the request would
   arrive and what label stack those LSRs would see.  (It is outside the
   scope of this document to specify how this computation is done.)  The
   set of these LSRs/interfaces are the downstream routers/interfaces
   (and their corresponding labels) for X with respect to L.  Each pair
   of downstream router and interface requires a separate Downstream
   Mapping to be added to the reply.

   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 mul-
   ticast).  In the former case, the Multipath Information is used as a
   hint to the sender as to how it may influence the choice of these
   alternatives.

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. Vendor Enterprise Code Number

   SMI Private Enterprise Numbers are maintained by IANA.  The Length is
   always 4; the value is the SMI Private Enterprise code, in net-
   work network
   octet order, of the vendor with a Vendor Private extension to any of
   the fields in the fixed part of the message, in which case this TLV
   MUST be present.  If none of the fields in the fixed part of the message mes-
   sage have vendor private extensions, inclusion of this this TLV in is
   OPTIONAL.  Vendor private ranges for Message Types, Reply Modes, and
   Return Codes have been defined.  When any of these are used the Vendor Ven-
   dor Enterprise Code Number TLV MUST be included in the message.

3.6. Interface and Label Stack

   The Interface and Label Stack TLV MAY be included in a reply message
   to report the interface on which the request message 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.

   The Length is K + 4*N octets, N is the number of labels in the Label
   Stack.  Values for K are found in the description of Address Type
   below.  The Value field of a Downstream Mapping has the following
   format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Address Type  |             Must be Zero                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   IP Address (4 or 16 octets)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Interface (4 or 16 octets)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                          Label Stack                          .
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Address Type

   The Address Type indicates if the interface is numbered or unnum-
   bered.  It also determines the length of the IP Address and Interface
   fields.  The resulting total for the initial part of the TLV is
   listed in the table below as "K Octets".  The Address Type is set to
   one of the following values:

             Type #        Address Type           K Octets
             ------        ------------           --------
                  1        IPv4 Numbered                12
                  2        IPv4 Unnumbered              12
                  3        IPv6 Numbered                36
                  4        IPv6 Unnumbered              24

IP Address and Interface

   IPv4 addresses and and interface indices are encoded in 4 octets,
   IPv6 addresses are encoded in 16 octets.

   If the interface upon which the echo request message was received is
   numbered, then the Address Type MUST be set to IPv4 or IPv6, the IP
   Address MUST be set to either the LSR's Router ID or the interface
   address, and the Interface MUST be set to the interface address.

   If the interface unnumbered, the Address Type MUST be either IPv4
   Unnumbered or IPv6 Unnumbered, the IP Address MUST be the LSR's
   Router ID, and the Interface MUST be set to the index assigned 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.

3.7. Errored TLVs

   The following TLV is a TLV which MAY be included in an echo reply to
   inform the sender of an echo request of Mandatory TLVs either not
   supported by an implementation, or parsed and found to be in error.

   The Value field contains the TLVs that were not understood, encoded
   as sub-TLVs.

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

3.8. Reply TOS Byte TLV

       This TLV MAY be used by the originator of used by the originator of the echo request to
   request
       that a echo reply be sent with the IP header TOS byte set to
       the value specified in the 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | 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
   bottom being a VPN IPv4 prefix, and the top being an LDP IPv4 prefix.
   If the underlying (LDP) tunnel were not known, or was considered
   irrelevant, the FEC stack could be a single element with just the VPN
   IPv4 sub-TLV.

   When an MPLS echo request is received, the receiver is expected to
   request
   verify that a echo reply the control plane and data plane are both healthy (for
   the FEC stack being pinged), and that the two planes are in sync.
   The procedures for this are in section 4.4 below.

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

   LSPs need not be sent 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 IP header TOS byte set
   desired endpoint.  Finally, LSPs may have backup paths, detour paths
   and other alternative paths to take should the value specified in primary LSP go down.

   To deal with the TLV.  This TLV has a length 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 4 with probing 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Reply-TOS Byte|                 Must various flavors of backup paths that will typ-
   ically not be used for forwarding data unless the primary LSP is down
   will not be zero                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4. Theory of Operation

   An 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 request is used to test requests can exercise all
   possible paths.  This, while desirable, may not be practical, because
   the algorithms that a particular LSP.  The LSP given LSR uses to distribute packets over
   alternative paths may be
   tested proprietary.

   To achieve some degree of coverage of alternate paths, there is identified by a
   certain latitude in choosing the "FEC Stack"; destination IP address and source
   UDP port for example, if an MPLS echo request.  This is clearly not sufficient;
   in the LSP was
   set up via LDP, and case of traceroute, more latitude is to an egress IP address offered by means 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
   Multipath Information of the LSP being tested Downstream Mapping TLV.  This is used as
   follows.  An ingress LSR periodically sends an RSVP LSP, the
   FEC stack consists of MPLS traceroute mes-
   sage to determine whether there are multipaths for a single element that captures the RSVP Session
   and Sender Template which uniquely identifies the given LSP.

   FEC stacks  If
   so, each hop will provide some information how each of its downstream
   paths can be exercised.  The ingress can then send MPLS echo requests
   that exercise these paths.  If several transit LSRs have ECMP, the
   ingress may attempt to compose these to exercise all possible paths.
   However, full coverage may not be more complex.  For example, one possible.

4.2. Testing LSPs That Are Used to Carry MPLS Payloads

   To detect certain LSP breakages, it may wish be necessary to test a
   VPN IPv4 prefix of 10.1/8 that is tunneled over encapsulate
   an LDP LSP MPLS echo request packet with
   egress 10.10.1.1.  The FEC stack would then contain two sub-TLVs, the
   bottom being a VPN IPv4 prefix, at least one additional label when
   testing LSPs that are used to carry MPLS payloads (such as LSPs used
   to carry L2VPN and the top being an L3VPN traffic.  For example, when testing LDP IPv4 prefix.
   If the underlying (LDP) tunnel were not known, or was considered
   irrelevant, the FEC stack could be a single element with
   RSVP-TE LSPs, just the VPN
   IPv4 sub-TLV.

   When sending an MPLS echo request is received, the receiver is expected to
   verify that packet may not detect
   instances where the control plane and data plane are both healthy (for router immediately upstream of the FEC stack being pinged), and that destination of
   the two planes are in sync.
   The procedures for this are in section 4.4 below.

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

   LSPs need not be simple point-to-point tunnels.  Frequently, a single LSP ping 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 forward the MPLS echo request successfully over an ingress,
   there may also be several different LSPs to choose from to get
   interface not configured to carry MPLS payloads because of the use of
   penultimate hop popping.  Since the
   desired endpoint.  Finally, LSPs may have backup paths, detour paths
   and other alternative paths receiving router has no means to take should
   differentiate whether the primary LSP go down.

   To deal with IP packet was sent unlabeled or implicitly
   labeled, the last two first: it is assumed that addition of labels shimmed above the LSR sourcing MPLS echo requests can force 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 into any desired LSP,
   so choosing among multiple LSPs at the ingress is not an issue. a UDP packet.  The
   problem of probing IP header is set as fol-
   lows: the various flavors source IP address is a routable address of backup paths that will typ-
   ically not be used for forwarding data unless the primary LSP is down
   will not be addressed here.

   Since sender; the actual LSP and path that a given packet may take may not be
   known
   destination IP address is a priori, it (randomly chosen) address from 127/8; the
   IP TTL is useful if 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 can exercise all
   possible paths.  This, while desirable, may not
   requests).  The Router Alert option MUST be practical, because set in the algorithms that IP header.

   An MPlS Echo Request is sent with a given LSR uses label stack corresponding to distribute packets over
   alternative paths may be proprietary.

   To achieve some degree of coverage of alternate paths, there the
   FEC stack being tested.  Note that further labels could be applied
   if, for example, the normal route to the topmost FEC in the stack is
   via a
   certain latitude Traffic Engineered Tunnel [RSVP-TE].  If all of the FECs in choosing the destination IP address and source
   UDP port for an
   stack correspond to Implicit Null Labels the MPLS echo request.  This request is clearly not sufficient;
   considered unlabeled even if further labels will be applied in send-
   ing the case of traceroute, more latitude packet.

   If the echo request is offered by means of labeled, one MAY (depending on what is being
   pinged) set the
   Multipath Information TTL of the Downstream Mapping TLV.  This is used as
   follows.  An ingress LSR periodically sends an MPLS traceroute mes-
   sage innermost label to determine whether there are multipaths for a given LSP.  If
   so, each hop will provide some information how each 1, to prevent the ping
   request going farther than it should.  Examples of its downstream
   paths can where this SHOULD
   be exercised.  The ingress can then send MPLS echo requests
   that exercise these paths.  If several transit LSRs have ECMP, done include pinging a VPN IPv4 or IPv6 prefix, an L2 VPN end
   point or a pseudowire.  Preventing the
   ingress may attempt to compose these ping request from going to exercise all possible paths.
   However, full coverage may not far
   can also be possible.

4.2. Testing LSPs That Are Used to Carry MPLS Payloads

   To detect certain LSP breakages, it accomplished by inserting a router alert label above this
   label; however, this may be necessary lead to encapsulate
   an the undesired side effect that MPLS
   echo request packet with at least one additional label when
   testing LSPs that are requests take a different data path than actual data.  For more
   information on how these mechanisms can be used for pseudowire con-
   nectivity verification, see [VCCV].

   In "ping" mode (end-to-end connectivity check), the TTL in the outer-
   most label is set to carry MPLS payloads (such as LSPs used 255.  In "traceroute" mode (fault isolation
   mode), the TTL is set successively to carry L2VPN 1, 2, ....

   The sender chooses a Sender's Handle, and L3VPN traffic.  For example, when testing LDP or
   RSVP-TE LSPs, just a Sequence Number.  When
   sending an subsequent MPLS echo request packet may not detect
   instances where the router immediately upstream of the destination of requests, the LSP ping may forward sender SHOULD increment
   the MPLS echo request successfully over an
   interface not configured sequence number by 1.  However, a sender MAY choose to carry MPLS payloads because of the use send a
   group of
   penultimate hop popping.  Since echo requests with the receiving router has no means same sequence number to
   differentiate whether improve the IP
   chance of arrival of at least one packet was sent unlabeled or implicitly
   labeled, with that sequence number.

   The TimeStamp Sent is set to the addition of labels shimmed above time-of-day (in seconds and
   microseconds) that 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 is sent.  The TimeStamp Received
   is set to zero.

   An MPLS echo request is MUST have a (possibly) labeled UDP packet.  The IP
   header is FEC Stack TLV.  Also, the Reply Mode
   must be set as follows: to the desired reply mode; the Return Code and Subcode
   are set to zero.  In the "traceroute" mode, the echo request SHOULD
   include a Downstream Mapping TLV.

4.4. Receiving an MPLS Echo Request

   Sending An MPLS Echo Request to the source IP address control plane is a routable address triggered by
   one of the sender; following packet processing exceptions: Router Alert
   Option, IP TTL expiration, MPLS TTL expiration, MPLS Router Alert
   Label, or the destination IP address is a (randomly chosen) address from 127/8; in the IP TTL is set to 1. 127/8 address range. The source UDP port is
   chosen
   control plane further identifies it by the sender; the destination UDP destination port 3503.

   For reporting purposes the bottom of stack is set considered to 3503
   (assigned by IANA for MPLS echo requests).  The Router Alert option be
   stack-depth of 1. This is set in to establish an absolute reference for
   the IP header.

   If case where the echo request is labeled, one actual stack may (depending on what is being
   pinged) set the TTL of have more labels than there are
   FECs in the innermost label to 1, to prevent Target FEC Stack.

   Further, in all the ping
   request going farther than it should.  Examples of error codes listed in this include ping-
   ing a VPN IPv4 or IPv6 prefix, an L2 VPN end point or document a pseudowire.
   stack-depth of 0 means "no value specified". This can also be accomplished by inserting a router alert label above
   this label; however, this may lead allows
   compatibility with existing implementations which do not use the
   Return Subcode field.

   An LSR X that receives an MPLS Echo Request then processes it as
   follows.

   1. General packet sanity is verified. If the packet is not
      well-formed, LSR X SHOULD send an MPLS Echo Reply with the
      Return Code set to "Malformed echo request received" and the
      Subcode to the undesired side effect zero. If there are any TLVs not marked as "Ignore"
      that LSR X does not understand, LSR X SHOULD send an MPLS echo requests take a different data path than actual data.

   In "ping" mode (end-to-end connectivity check), the TTL in "TLV
      not understood" (as appropriate), and the outer-
   most label is Subcode set to 255.
      zero. In "traceroute" mode (fault isolation
   mode), the TTL is set successively to 1, 2, .... latter case, the misunderstood TLVs (only) are
      included as sub-TLVs in an Errored TLVs TLV in the reply. The sender chooses a
      header fields Sender's Handle, and a Sequence Number.  When
   sending subsequent Number, and Timestamp
      Sent are not examined, but are included in the MPLS echo requests, Echo Reply
      message.

   The algorithm uses the sender SHOULD increment following variables and identifiers:

   Interface-I:       the sequence number by 1.  However, a sender MAY choose to send a
   group of echo requests with interface on which the same sequence number MPLS Echo Request was
                      received.

   Stack-R:           the label stack on the packet as it was
                      received.

   Stack-D:           the label stack carried in the Downstream
                      Mapping TLV (not always present)

   Label-L:           the label from the actual stack currently being
                      examined.  Requires no initialization.

   Label-stack-depth: the depth of label being verified. Initialized
                      to improve the
   chance number of arrival labels in the received label
                      stack S.

   FEC-stack-depth:   depth of at least one packet with that sequence number.

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

   An MPLS echo request MUST have a Target FEC Stack TLV.  Also, the Reply Mode
   must that
                      should be set to the desired reply mode; the Return Code and Subcode
   are set used to zero.  In verify the "traceroute" mode, current actual
                      label. Requires no initialization.

   Best-return-code:  contains the echo request SHOULD
   include a Downstream Mapping TLV.

4.4. Receiving an MPLS Echo Request

   An LSR X that receives an MPLS echo request first parses return code for the Echo Reply
                      packet
   to ensure that it is a well-formed packet, and that as currently best known. As algorithm
                      progresses, this code may change depending on
                      the TLVs results of further checks 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 it performs.

   Best-rtn-subcode:  similar to zero.  In Best-return-code, but for the latter case, Echo
                      Reply Subcode.

   FEC-status:        result value returned by the misunderstood TLVs (only) are
   included FEC Checking
                      algorithm described in the reply. section 4.4.1.

   /* Save receive context information */

   2. If the echo request is good, LSR X notes stores the interface I over
      which the echo was received, received in Interface-I, and the label stack
      with which it came.

   For reporting purposes the bottom of stack is considered to be stack-
   depth came in Stack-R.

   /* The rest of 1.  This is to establish an absolute reference for the case
   where algorithm iterates over the stack may have more labels than are in Stack-R,
      verifies validity of label values, reports associated label
      switching operations (for traceroute), verifies correspondence
      between the Stack-R and the Target FEC stack.  Fur-
   ther, Stack description in all the error codes listed in this document a stack-depth
      body of the Echo Request, and reports any errors. */

   /* The algorithm iterates as follows. */

   3. Label Validation:

      If Label-stack-depth is 0 means "no value specified".  This allows compatibility with exist-
   ing implementations which do not use {

      /* The LSR needs to report its being a tail-end for the Return Subcode field.

   X employs two variables, called LSP */

         Set FEC-stack-depth to 1, set Label-L to 3 (Implicit Null).
         Set Best-return-code to 3 ("Replying router is an egress for
         the FEC at stack depth"), set Best-rtn-subcode to the
         value of FEC-stack-depth (1) and Label-stack-
   depth.  X sets Label-stack-depth go to step 5 (Egress
         Processing).
      }

      /* This step assumes there's always an entry for well-known
         label values */

      Set Label-L to the number of labels value extracted from Stack-R at depth
      Label-stack-depth.  Lookup Label-L in the
   received label stack.  If Incoming Label Map
      (ILM) to determine if the label-stack-depth is 0, assume there is
   one implicit null label has been allocated and set label-stack-depth to 1.  FEC-stack-
   depth an
      operation is used later and need not be initialized.  Processing now con-
   tinues associated with the following steps:

   Label_Validation: it.

      If the label at Label-stack-depth there is valid, goto Label_Operation.
     If not, set no entry for L {

      /* Indicates a temporary or permanent label synchronization
         problem the LSR needs to report an error */

         Set Best-return-code to 11, "No 11 ("No label entry at stack-depth" stack-depth")
         and Best-return-subcode Best-rtn-subcode to Label-stack-depth.  Goto
     Send_Reply_Packet.

   Label_Operation:

     Switch on Go to step 7
         (Send Reply Packet).
      }
      Else {

         Retrieve the associated label operation.

     Case:  Pop and Continue Processing (Note: this includes
                Explicit_Null operation from the
         corresponding NLFE and Router_Alert) proceed to step 4 (Label Operation).
      }

   4. Label Operation Check

      If Label-stack-depth the label operation is greater than 1, decrement Label-stack-
       depth "Pop and goto Label_Validation.  Otherwise, set FEC-stack-depth
       to 1, set Best-return-code Continue Processing" {

      /* Includes Explicit Null and Router Alert label cases */

         Iterate 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 next label by decrementing Label-stack-depth
         and Switch based on Popped Label loop back to step 3 (Label Validation).
      }

      If the label operation is either swap "Swap or pop Pop and switch Switch based on
       the popped label, Popped
         Label" {

         Set Best-return-code to 8, "Label 8 ("Label switched at
       stack-depth" stack-depth")
         and Best-return-subcode Best-rtn-subcode to Label-stack-depth. Label-stack-depth to report transit
         switching.

         If a Downstream Mapping TLV is present, a present in the received Echo
         Request {

            If the IP address in the TLV is 127.0.0.1 or 0::1: {
               Set Best-return-code to 6 ("Upstream Interface Index
               Unknown").  An Interface and Label Stack TLV SHOULD be
               included in the reply and filled with Interface-I and
               Stack-R.
            }

            Else {

               Verify that the IP address, interface address and label
               stack in the Downstream mapping TLVs Mapping TLV match Interface-I
               and Stack-R.  If there is a mismatch, set
               Best-return-code to 5, "Downstream Mapping Mismatch".
               An Interface and Label Stack TLV SHOULD be created for each multipath.

       Determine included in
               the reply and filled in based on Interface-I and
               Stack-R.  Go to step 7 (Send Reply Packet).
            }
         }

         For each available downstream ECMP path {
            Retrieve output interface. interface from the NHLFE entry.

            /* Note: this return code is set even if Label-stack-depth
               is one */

            If it the output interface is not valid to forward a
       labeled packet on this interface, MPLS-enabled {

               set Best-return-code to Return Code 9, "Label switched
               but no MPLS forwarding at stack-depth" and set Best-return-subcode
               Best-rtn-subcode to Label-stack-depth and goto
               Send_Reply_Packet.  (Note: this return code is set even if Label-
       stack-depth
            }

            If a Downstream Mapping TLV is one.) present {

              A Downstream mapping TLV SHOULD be included in the Echo
              Reply (see section 3.3) filled in with information about
              the current ECMP path.
            }
         }

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

       Verify that the IP address, interface address and label stack
       match the received interface and label stack.  If the IP address
       is either 127.0.0.1 or 0::1 bypass the interface check, and set
       Best-return-code to 6, "Upstream Interface Index Unknown".  For
       any other error, set Best-return-code
               Go to 5, "Downstream Mapping
       Mis-match".  For either error, an Interface and Label Stack TLV
       SHOULD be created.  If Best-return-code equals 5, goto
       Send_Reply_Packet. step 7 (Send Reply Packet).

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

       Locate set and the label at Label-stack-depth LSR is not
         configured to perform FEC checking by default, go to step 7
         (Send Reply Packet).

      /* Validate the Target FEC Stack in the received Echo Request.
         First determine FEC-stack-depth from the Downstream Labels Mapping
         TLV.  This is done by
       counting walking through Stack-D (the Downstream
         Labels) from the bottom of bottom, decrementing the stack, skipping over, but count-
       ing Implicit Null number of labels and set
         for each non-Implicit Null label, while incrementing
         FEC-stack-depth to that depth.
       (Note: for each label. If the Downstream Labels contain Mapping TLV
         contains one or more Implicit Null labels, this FEC-stack-depth
         may be at a depth greater than Label-stack-depth.) Label-stack-depth.  To be consistent with
         the above stack-depths, the bottom is considered to entry 1.
         */

         Set FEC-stack-depth to 0.  Set i to Label-stack-depth.

         While (i > 0 ) do {
            ++FEC-stack-depth.
            if Stack-D[FEC-stack-depth] != 3 (Implicit Null)
               --i.
         }
         If the depth number of labels in the FEC stack is greater
            than or equal to FEC-
       stack-depth, FEC-stack-depth {

            Perform the FEC Checking. Checking procedure (see subsection 4.4.1
            below).

            If FEC-status is 2, 2 set Best-
       return-code Best-return-code to 10, "Mapping 10 ("Mapping
            for this FEC is not the given label at stack-depth". stack-depth").

            If the return code is 1 set Best-return-code to
            FEC-return-code and Best-return-subcode Best-rtn-subcode to FEC-stack-depth.

       Goto Send_Reply_Packet.

   Egress_Processing:
         }

         Go to step 7 (Send Reply Packet).
      }

   5. Egress Processing:

      /* These steps are performed by the LSR that identified itself
         as the tail-end LSR for an LSP. */

      If received Echo Request contains no Downstream Mapping TLV TLV, or
         the Downstream IP Address is present, goto Egress_FEC_Valida-
     tion. set to 127.0.0.1 or 0::1:
            Go t0 step 6 (Egress FEC Validation).

      Verify that the IP address, interface address and label stack match in
      the received interface Downstream mapping TLV match Interface-I and label stack. Stack-R. If
      not, set Best-return-
     code 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
      created for the Echo Response packet. Go to step 7 (Send Reply
      Packet).

   6. Egress FEC checking.  If FEC-status Validation:

      /* This is 1, set a loop for all entries in the Target FEC Stack
         starting with FEC-stack-depth. */

      Perform FEC checking by following the algorithm described in
      subsection 4.4.1 for Label-L and the FEC at FEC-stack-depth.

      Set Best-return-code to FEC-code and Best-return-subcode Best-rtn-subcode to FEC-stack-depth.  Goto
     Send_Reply_Packet.

     Increment the
      value in FEC-stack-depth.

      If FEC-stack-depth FEC-status (the result of the check) is greater than 1,
         Go to step 7 (Send Reply Packet).

      /* Iterate to the next FEC entry */
      ++FEC-stack-depth.

      If FEC-stack-depth > the number of FECs in the FEC-stack, goto Send_Reply_Packet.
         Go to step 7 (Send Reply Packet).

      If FEC-status is 0, increment 0 {
         ++Label-stack-depth.
         If Label-stack-depth > the number of labels in Stack-R,
            Go to step 7 (Send Reply Packet).

         Label-L = extracted label from Stack-R at depth
            Label-stack-depth.  Goto
     Egress_FEC_Validation.

   Send_Reply_Packet:
         Loop back to step 6 (Egress FEC Validation).
      }

   7. Send Reply Packet:

      Send an MPLS echo reply Echo Reply with a Return Code of Best-return-code,
      and a Return Subcode of Best-return-subcode. Best-rtn-subcode.  Include any TLVs
      created during the above process.  The procedures for sending
      the
     echo reply Echo Reply are found in the next subsection below.

   FEC_Checking: 4.4.1.

4.4.1. FEC Validation

   /* This routine subsection describes validation of a FEC entry within the
      Target FEC Stack and accepts a FEC, Label, Label-L and Interface.  It returns two Interface-I.
      The algorithm performs the following steps. */

   1. Two return values, FEC-status and FEC-return-code, both of which are initialized
      to 0.

   2. If the FEC is the Nil FEC, check that Label FEC {
         If Label-L is either Explicit_Null or Router_Alert.  If so Router_Alert, return.

         Else
     set {
            Set FEC-return-code to 10, "Mapping 10 ("Mapping for this FEC is not
            the given label at stack-depth". stack-depth").
            Set FEC-status to 1 and return.
            Return.
         }
      }

   3. Check that the FEC label mapping for FEC. that describes how traffic received
      on the LSP is further switched or which application it is
      associated with. If no mapping exists, set FEC-return-code to
      Return 4, "Replying router has no mapping for the FEC at
      stack-depth". Set FEC-status to 1. Return.

   4. If the label mapping for FEC is Implicit Null, set FEC-status to
     2.  Goto Check_Protocol.

     If
      2 and proceed to step 5. Otherwise, if the label mapping for FEC
      is Label, goto Check_Protocol.  Else Label-L, proceed to step 5. Otherwise, set FEC-return-code to 10, "Mapping
      10 ("Mapping for this FEC is not the given label at stack-depth".  Set
      stack-depth"), set FEC-status to 1 and return.

   Check_Protocol:

   5. This is a protocol check. Check what protocol would be used to
      advertise FEC.  If it can be determined that no protocol
      associated with interface I Interface-I would have advertised a FEC of that
      FEC-Type, set FEC-return-code to
     12, "Protocol 12 ("Protocol not associated
      with interface at FEC stack-depth". stack-depth"). Set FEC-status to 1.

   6. 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 requester 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 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.

   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.) reply.)

   Except in the case of Reply Mode 4, "Reply via application level con-
   trol channel", Echo Replies are always sent in the context of the
   IP/MPLS network.

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; 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 Mapping(s) into its
   next echo request(s) (with TTL incremented by one).

4.7. Issue with VPN IPv4 and IPv6 Prefixes

   Typically, a LSP ping for a VPN IPv4 prefix or VPN 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 cer-
   tain circumstances, the label stack can shrink to a single label
   before the ping hits the egress PE; this will result in the ping ter-
   minating prematurely.  One such scenario is a multi-AS Carrier's Car-
   rier 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.

4.8. 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 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 to probe LSRs further down
   the path.  In such a case, the echo request for TTL > n SHOULD be
   sent with Downstream Mapping TLV "Downstream IP Address" field set to
   the ALLROUTERs multicast address until a reply is received with a
   Downstream 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 reply packet with a Downstream Mapping TLV
   is received.

5. References

Normative References

   [BGP]          Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
                  (BGP-4)", RFC 1771, March 1995.

   [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.

   [NTP]          Mills, D., "Simple Network Time Protocol (SNTP)
                  Version 4 for IPv4, IPv6 and OSI", RFC 2030, October
                  1996.

   [RFC1122]      Braden, R., "Requirements for Internet Hosts -
                  Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1812]      Almquist, P. and F. Kastenholz, "Towards Requirements
                  for IP Routers", RFC 1716, November 1994.

   [RFC4026]      Andersson, L. and T. Madsen, "Provider Provisioned
                  Virtual Private Network (VPN) Terminology", RFC 4026,
                  March 2005.

Informative References

   [BGP-LABEL]    Rekhter, Y. and E. Rosen, "Carrying Label Information
                  in BGP-4", RFC 3107, May 2001.

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

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

   [MPLS-L3-VPN]  Rekhter, Y. & Rosen, E., "BGP/MPLS IP VPNs",
                  draft-ietf-l3vpn-rfc2547bis-03.txt, work-in-progress.

   [PW-CONTROL]   Martini, L. et al., "Pseudowire Setup and Maintenance
                  using the Label Distribution Protocol",
                  draft-ietf-pwe3-control-protocol-17.txt,
                  work-in-progress.

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

   [VCCV]         Nadeau, T. & Aggarwal, R., "Pseudo Wire Virtual
                  Circuit Connectivity Verification (VCCV),
                  draft-ietf-pwe3-vccv-07.txt, August 2005,
                  work-in-progress.

   [VPLS]         Kompella, K. and Rekhter, Y., "Virtual Private LAN
                  Service", draft-ietf-l2vpn-vpls-bgp-05,
                  work-in-progress.

6. Security Considerations

   Overall, the security needs for LSP Ping are are similar to those of
   ICMP Ping.

   There are at least two three approaches to attacking LSRs using the mecha-
   nisms mech-
   anisms defined here.  One is a Denial of Service attack, by sending
   MPLS echo requests/replies to LSRs and thereby increasing their work-
   load.  The other second is obfuscating the state of the MPLS data plane
   liveness by spoofing, hijacking, replaying or otherwise tampering
   with MPLS echo requests and replies.  The third is an unauthorized
   source using an LSP Ping to obtain information about the network.

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

   Replay

   Unsophisticated replay and spoofing attacks involving faking or
   replaying MPLS Echo Reply Messages are unlikely to be effective given that effective.
   These replies would have to match the the Sender's Handle and
   Sequence Number need to be valid.  Thus a of an outstanding MPLS Echo Request Message.  A non-
   matching replay would be discarded as the sequence has moved on.  A on, thus
   a spoof has only a small window of opportunity, however opportunity.  However to provide a
   stronger defence, an implementation MAY
   provide a validation on also validate the TimeStamp
   Sent by requiring and exact match on this field.

   To protect against unauthorized sources using MPLS Echo Request mes-
   sages to limit the window to obtain network information, it is RECOMMENDED that implemen-
   tations provides a means of checking the
   resolution source addresses of MPLS
   Echo Request messages against an access list before accepting the system clock.
   message.

   It is not clear how to prevent hijacking (non-delivery) of echo
   requests or replies; however, if these messages are indeed hijacked,
   LSP ping will report that the data plane isn't working as it should.

   It doesn't seem vital (at this point) to secure the data carried in
   MPLS echo requests and replies, although knowledge of the state of
   the MPLS data plane may be considered confidential by some.  Imple-
   mentations SHOULD however provide a means of filtering the addresses
   to which Echo Reply messages may be sent.

   Although this document makes special use of 127/8 address, these are
   used only in conjunction with the UDP port 3503.  Further these pack-
   ets are only processed by routers.  All other hosts MUST treat all
   packets with a destination address in the range 127/8 in accordance
   to RFC1122.  Any packet received by a router with a destination
   address in the range 127/8 without a destination UDP port of 3503
   MUST be treated in accordance to RFC1812.

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" "Specification Required" and "Vendor Private
   Use".

   Values from "Expert Review" "Specification Required" ranges MUST be registered with
   IANA.  The request MUST be made via 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 Private Network Management Private Enterprise Codes. Num-
   bers.  For each name space that has a Vendor Private range, it must
   be specified where exactly the SMI Private Enterprise Code Number resides;
   see below for exam-
   ples. examples.  In this way, several enterprises (vendors)
   can use the same code point without fear of collision.

7.1. Message Types, Reply Modes, Return Codes

   It is requested that IANA maintain registries for Message Types,
   Reply Modes, and Return Codes.  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; "Specification
   Required"; 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 Number TLV MUST be present in the message.

   Message Types defined in this document are:

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

   Reply Modes defined in this document are:

       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

   Return Codes defined in this document are listed in section 3.1.

7.2. TLVs

   It is requested that IANA maintain a registry for the Type field of
   top-level TLVs as well as for any associated sub-TLVs.  Note the
   meaning of a sub-TLV is scoped by the TLV.  The number spaces for the
   sub-TLVs of various TLVs are independent.

   The valid range for TLVs and sub-TLVs 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 "Specification Required" as defined above;
   values in the range 31744-32767 and 64512-65535 are for Vendor Private Pri-
   vate 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 Private Enterprise Code, Number, in network octet
   order.  The rest of the Value field is private to the vendor.

   TLVs and sub-TLVs defined in this document are:

          Type       Sub-Type        Value Field
          ----       --------        -----------
             1                       Target FEC Stack
                          1          LDP IPv4 prefix
                          2          LDP IPv6 prefix
                          3          RSVP IPv4 LSP
                          4          RSVP IPv6 LSP
                          5          Not Assigned
                          6          VPN IPv4 prefix
                          7          VPN IPv6 prefix
                          8          L2 VPN endpoint
                          9          "FEC 128" Pseudowire (Deprecated)
                         10          "FEC 128" Pseudowire
                         11          "FEC 129" Pseudowire
                         12          BGP labeled IPv4 prefix
                         13          BGP labeled IPv6 prefix
                         14          Generic IPv4 prefix
                         15          Generic IPv6 prefix
                         16          Nil FEC
             2                       Downstream Mapping
             3                       Pad
             4                       Not Assigned
             5                       Vendor Enterprise Code Number
             6                       Not Assigned
             7                       Interface and Label Stack
             8                       Not Assigned
             9                       Errored TLVs
                     Any value       The TLV not understood
            10                       Reply TOS Byte

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 Vanson
   Lim.

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

Authors' Addresses

   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

Copyright Notice

   Copyright (C) The Internet Society (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

Expiration Date

   May

   June 2006

Disclaimer of Validity

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