[Docs] [txt|pdf] [draft-ietf-mpls-r...] [IPR] [Errata]


Errata Exist
Internet Engineering Task Force (IETF)                       K. Kompella
Request for Comments: 8029                        Juniper Networks, Inc.
Obsoletes: 4379, 6424, 6829, 7537                             G. Swallow
Updates: 1122                                          C. Pignataro, Ed.
Category: Standards Track                                       N. Kumar
ISSN: 2070-1721                                                    Cisco
                                                               S. Aldrin
                                                                  Google
                                                                 M. Chen
                                                                  Huawei
                                                              March 2017


   Detecting Multiprotocol Label Switched (MPLS) Data-Plane Failures

Abstract

   This document describes a simple and efficient mechanism to detect
   data-plane failures in Multiprotocol Label Switching (MPLS) Label
   Switched Paths (LSPs).  It defines a probe message called an "MPLS
   echo request" and a response message called an "MPLS echo reply" for
   returning the result of the probe.  The MPLS echo request is intended
   to contain sufficient information to check correct operation of the
   data plane and to verify the data plane against the control plane,
   thereby localizing faults.

   This document obsoletes RFCs 4379, 6424, 6829, and 7537, and updates
   RFC 1122.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8029.









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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Conventions . . . . . . . . . . . . . . . . . . . . . . .   5
     1.2.  Structure of This Document  . . . . . . . . . . . . . . .   6
     1.3.  Scope of This Specification . . . . . . . . . . . . . . .   6
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Use of Address Range 127/8  . . . . . . . . . . . . . . .   8
     2.2.  Router Alert Option . . . . . . . . . . . . . . . . . . .  10
   3.  Packet Format . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  Return Codes  . . . . . . . . . . . . . . . . . . . . . .  16
     3.2.  Target FEC Stack  . . . . . . . . . . . . . . . . . . . .  17
       3.2.1.  LDP IPv4 Prefix . . . . . . . . . . . . . . . . . . .  19
       3.2.2.  LDP IPv6 Prefix . . . . . . . . . . . . . . . . . . .  19
       3.2.3.  RSVP IPv4 LSP . . . . . . . . . . . . . . . . . . . .  20
       3.2.4.  RSVP IPv6 LSP . . . . . . . . . . . . . . . . . . . .  20
       3.2.5.  VPN IPv4 Prefix . . . . . . . . . . . . . . . . . . .  21
       3.2.6.  VPN IPv6 Prefix . . . . . . . . . . . . . . . . . . .  22
       3.2.7.  L2 VPN Endpoint . . . . . . . . . . . . . . . . . . .  23
       3.2.8.  FEC 128 Pseudowire - IPv4 (Deprecated)  . . . . . . .  23
       3.2.9.  FEC 128 Pseudowire - IPv4 (Current) . . . . . . . . .  24
       3.2.10. FEC 129 Pseudowire - IPv4 . . . . . . . . . . . . . .  25
       3.2.11. FEC 128 Pseudowire - IPv6 . . . . . . . . . . . . . .  26
       3.2.12. FEC 129 Pseudowire - IPv6 . . . . . . . . . . . . . .  27
       3.2.13. BGP Labeled IPv4 Prefix . . . . . . . . . . . . . . .  28
       3.2.14. BGP Labeled IPv6 Prefix . . . . . . . . . . . . . . .  28
       3.2.15. Generic IPv4 Prefix . . . . . . . . . . . . . . . . .  29
       3.2.16. Generic IPv6 Prefix . . . . . . . . . . . . . . . . .  29
       3.2.17. Nil FEC . . . . . . . . . . . . . . . . . . . . . . .  29
     3.3.  Downstream Mapping (Deprecated) . . . . . . . . . . . . .  30
     3.4.  Downstream Detailed Mapping TLV . . . . . . . . . . . . .  30
       3.4.1.  Sub-TLVs  . . . . . . . . . . . . . . . . . . . . . .  34
       3.4.2.  Downstream Router and Interface . . . . . . . . . . .  40
     3.5.  Pad TLV . . . . . . . . . . . . . . . . . . . . . . . . .  41
     3.6.  Vendor Enterprise Number  . . . . . . . . . . . . . . . .  41
     3.7.  Interface and Label Stack . . . . . . . . . . . . . . . .  42
     3.8.  Errored TLVs  . . . . . . . . . . . . . . . . . . . . . .  43
     3.9.  Reply TOS Octet TLV . . . . . . . . . . . . . . . . . . .  44
   4.  Theory of Operation . . . . . . . . . . . . . . . . . . . . .  44
     4.1.  Dealing with Equal-Cost Multipath (ECMP)  . . . . . . . .  44
     4.2.  Testing LSPs That Are Used to Carry MPLS Payloads . . . .  45
     4.3.  Sending an MPLS Echo Request  . . . . . . . . . . . . . .  46
     4.4.  Receiving an MPLS Echo Request  . . . . . . . . . . . . .  47
       4.4.1.  FEC Validation  . . . . . . . . . . . . . . . . . . .  53







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     4.5.  Sending an MPLS Echo Reply  . . . . . . . . . . . . . . .  54
       4.5.1.  Addition of a New Tunnel  . . . . . . . . . . . . . .  55
       4.5.2.  Transition between Tunnels  . . . . . . . . . . . . .  56
     4.6.  Receiving an MPLS Echo Reply  . . . . . . . . . . . . . .  56
     4.7.  Issue with VPN IPv4 and IPv6 Prefixes . . . . . . . . . .  58
     4.8.  Non-compliant Routers . . . . . . . . . . . . . . . . . .  59
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  59
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  61
     6.1.  TCP and UDP Port Number . . . . . . . . . . . . . . . . .  61
     6.2.  MPLS LSP Ping Parameters  . . . . . . . . . . . . . . . .  61
       6.2.1.  Message Types, Reply Modes, Return Codes  . . . . . .  61
       6.2.2.  TLVs  . . . . . . . . . . . . . . . . . . . . . . . .  62
       6.2.3.  Global Flags  . . . . . . . . . . . . . . . . . . . .  64
       6.2.4.  Downstream Detailed Mapping Address Type  . . . . . .  64
       6.2.5.  DS Flags  . . . . . . . . . . . . . . . . . . . . . .  65
       6.2.6.  Multipath         Types . . . . . . . . . . . . . . .  66
       6.2.7.  Pad Type  . . . . . . . . . . . . . . . . . . . . . .  66
       6.2.8.  Interface and Label Stack Address Type  . . . . . . .  67
     6.3.  IPv4 Special-Purpose Address Registry . . . . . . . . . .  67
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  67
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  67
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  68
   Appendix A.  Deprecated TLVs and Sub-TLVs (Non-normative) . . . .  72
     A.1.  Target FEC Stack  . . . . . . . . . . . . . . . . . . . .  72
       A.1.1.  FEC 128 Pseudowire - IPv4 (Deprecated)  . . . . . . .  72
     A.2.  Downstream Mapping (Deprecated) . . . . . . . . . . . . .  72
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  77
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  77
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  78






















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1.  Introduction

   This document describes a simple and efficient mechanism to detect
   data-plane failures in MPLS Label Switched Paths (LSPs).  It defines
   a probe message called an "MPLS echo request" and a response message
   called an "MPLS echo reply" for returning the result of the probe.
   The MPLS echo request is intended to contain sufficient information
   to check correct operation of the data plane, as well as a mechanism
   to verify the data plane against the control plane, thereby
   localizing faults.

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

   This document makes special use of the address range 127/8.  This is
   an exception to the behavior defined in RFC 1122 [RFC1122], and this
   specification updates that RFC.  The motivation for this change and
   the details of this exceptional use are discussed in Section 2.1
   below.

   This document obsoletes RFC 4379 [RFC4379], RFC 6424 [RFC6424], RFC
   6829 [RFC6829], and RFC 7537 [RFC7537].

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 [RFC2119].

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

   Terminology pertaining to L2 and L3 Virtual Private Networks (VPNs)
   is defined in [RFC4026].

   Since this document refers to the MPLS Time to Live (TTL) far more
   frequently than the IP TTL, the authors have chosen the convention of
   using the unqualified "TTL" to mean "MPLS TTL" and using "IP TTL" for
   the TTL value in the IP header.







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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" (Section 4) first;
   the document is structured the way it is to avoid forward references.

1.3.  Scope of This Specification

   The primary goal of this document is to provide a clean and updated
   LSP ping specification.

   [RFC4379] defines the basic mechanism for MPLS LSP validation that
   can be used for fault detection and isolation.  The scope of this
   document also includes various updates to MPLS LSP ping, including:

   o  Update all references and citations.

      *  Obsoleted RFCs 2434, 2030, and 3036 are respectively replaced
         with RFCs 5226, 5905, and 5036.

      *  Additionally, some informative references were published as
         RFCs: RFCs 4761, 5085, 5885, and 8077.

   o  Incorporate all outstanding RFC errata.

      *  See [Err108], [Err742], [Err1418], [Err1714], [Err1786],
         [Err2978], [Err3399].

   o  Replace EXP with Traffic Class (TC), based on the update from RFC
      5462.

   o  Incorporate the updates from RFC 6829, by adding the pseudowire
      (PW) Forwarding Equivalence Classes (FECs) advertised over IPv6
      and obsoleting RFC 6829.

   o  Incorporate the updates from RFC 7506, by adding the IPv6 Router
      Alert Option (RAO) for MPLS Operations, Administration, and
      Maintenance (OAM).

   o  Incorporate newly defined bits on the Global Flags field from RFCs
      6425 and 6426.

   o  Update the IPv4 addresses used in examples to utilize the
      documentation prefix.  Add examples with IPv6 addresses.





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   o  Incorporate the updates from RFC 6424, by deprecating the
      Downstream Mapping TLV (DSMAP) and adding the Downstream Detailed
      Mapping TLV (DDMAP); updating two new Return Codes; adding the
      motivations of tunneled or stitched LSPs; updating the procedures,
      IANA considerations, and security considerations; and obsoleting
      RFC 6424.

   o  Incorporate the updates from RFC 7537, by updating the IANA
      Considerations section and obsoleting RFC 7537.

   o  Finally, obsolete RFC 4379.

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 [RFC0792]) 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
   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 isolation), the packet is sent to the
   control plane of each transit LSR, which performs various checks to
   confirm 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.







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   An LSP traceroute may cross a tunneled or stitched LSP en route to
   the destination.  While performing end-to-end LSP validation in such
   scenarios, the FEC information included in the packet by the
   Initiator may be different from the one assigned by the transit node
   in a different segment of a stitched LSP or tunnel.  Let us consider
   a simple case.

   A          B          C           D           E
   o -------- o -------- o --------- o --------- o
     \_____/  | \______/   \______/  | \______/
       LDP    |   RSVP       RSVP    |    LDP
              |                      |
               \____________________/
                       LDP

   When an LSP traceroute is initiated from Router A to Router E, the
   FEC information included in the packet will be LDP while Router C
   along the path is a pure RSVP node and does not run LDP.
   Consequently, node C will be unable to perform FEC validation.  The
   MPLS echo request should contain sufficient information to allow any
   transit node within a stitched or tunneled LSP to perform FEC
   validations to detect any misrouted echo requests.

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

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
   network 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
   forwarding decision.  In fact, the sender of an MPLS echo request
   packet 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 may take.  Since most MPLS services
   are based on IP unicast forwarding, these paths are subject to Equal-
   Cost Multipath (ECMP) load sharing.





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   This leads to the following requirements:

   1.  Although the LSP in question may be broken in unknown ways, the
       likelihood of a diagnostic packet being delivered to a user of an
       MPLS service MUST be held to an absolute minimum.

   2.  If an LSP is broken in such a way that it prematurely terminates,
       the diagnostic packet MUST NOT be IP forwarded.

   3.  A means of varying the diagnostic packets such that they exercise
       all ECMP paths is thus REQUIRED.

   Clearly, using general unicast addresses satisfies neither of the
   first two requirements.  A number of other options for addresses were
   considered, including a portion of the private address space (as
   determined by the network operator) and the IPv4 link-local
   addresses.  Use of the private address space was deemed ineffective
   since the leading MPLS-based service is an IPv4 VPN.  VPNs often use
   private addresses.

   The IPv4 link-local addresses are more attractive in that the scope
   over which they can be forwarded is limited.  However, if one were to
   use an address from this range, it would still be possible for the
   first recipient of a diagnostic packet that "escaped" from a broken
   LSP to have that address assigned to the interface on which it
   arrived and thus could mistakenly receive such a packet.  Older
   deployed routers may not (correctly) implement IPv4 link-local
   addresses and would forward a packet with an address from that range
   toward the default route.

   The 127/8 range for IPv4 and that same range embedded in an
   IPv4-mapped IPv6 address for IPv6 was chosen for a number of reasons.

   RFC 1122 allocates the 127/8 as the "Internal host loopback address"
   and states: "Addresses of this form MUST NOT appear outside a host."
   Thus, the default behavior of hosts is to discard such packets.  This
   helps to ensure that if a diagnostic packet is misdirected to a host,
   it will be silently discarded.

   RFC 1812 [RFC1812] states:

      A router SHOULD NOT forward, except over a loopback interface, any
      packet that 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 provided, it MUST default to
      performing the checks.

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



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   The 127/8 address range provides 16M addresses allowing wide
   flexibility in varying addresses to exercise ECMP paths.  Finally, as
   an implementation optimization, the 127/8 range provides an easy
   means of identifying possible LSP packets.

2.2.  Router Alert Option

   This document requires the use of the RAO set in an IP header in
   order to have the transit node process the MPLS OAM payload.

   [RFC2113] defines a generic Option Value 0x0 for IPv4 RAO that alerts
   the transit router to examine the IPv4 packet.  [RFC7506] defines
   MPLS OAM Option Value 69 for IPv6 RAO to alert transit routers to
   examine the IPv6 packet more closely for MPLS OAM purposes.

   The use of the Router Alert IP Option in this document is as follows:

      In case of an IPv4 header, the generic IPv4 RAO value 0x0
      [RFC2113] SHOULD be used.  In case of an IPv6 header, the IPv6 RAO
      value (69) for MPLS OAM [RFC7506] MUST be used.































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3.  Packet Format

   An MPLS echo request/reply is a (possibly labeled) IPv4 or IPv6 UDP
   packet; the contents of the UDP packet have the following format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Version Number        |         Global Flags          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Message Type |   Reply Mode  |  Return Code  | Return Subcode|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sender's Handle                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sequence Number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    TimeStamp Sent (seconds)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                TimeStamp Sent (seconds fraction)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  TimeStamp Received (seconds)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              TimeStamp Received (seconds fraction)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            TLVs ...                           |
      .                                                               .
      .                                                               .
      .                                                               .
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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












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

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           MBZ           |R|T|V|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   At the time of writing, three flags are defined: the R, T, and V
   bits; the rest MUST 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 the receiver.

   The T (Respond Only If TTL Expired) flag MUST be set only in the echo
   request packet by the sender.  If the T flag is set to 1 in an
   incoming echo request, and the TTL of the incoming MPLS label is more
   than 1, then the receiving node MUST drop the incoming echo request
   and MUST NOT send any echo reply to the sender.  This flag MUST NOT
   be set in the echo reply packet.  If this flag is set in an echo
   reply packet, then it MUST be ignored.  The T flag is defined in
   Section 3.4 of [RFC6425].

   The R (Validate Reverse Path) flag is defined in [RFC6426].  When
   this flag is set in the echo request, the Responder SHOULD return
   reverse-path FEC information, as described in Section 3.4.2 of
   [RFC6426].

   The Message Type is one of the following:

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

   The Reply Mode can take one of the following values:

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






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   An MPLS echo request with 1 (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
   statistics.  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
   Connectivity Verification (VCCV) [RFC5085][RFC5885].  Any application
   that supports an IP control 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 code point is application specific and thus beyond the scope of
   this document.

   Return Codes and Subcodes are described in Section 3.1.

   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 (according to the sender's
   clock) in 64-bit NTP timestamp format [RFC5905] 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 64-bit NTP timestamp
   format in which the corresponding echo request was received.















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   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 4-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 4-octet aligned.

   Two examples of how TLV and sub-TLV lengths are computed, and how
   sub-TLVs are padded to be 4-octet aligned, are 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Type = 1 (LDP IPv4 FEC)    |          Length = 5           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          IPv4 prefix                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |         Must Be Zero                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



















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   The Length for this TLV is 5.  A Target FEC Stack TLV that contains
   an LDP IPv4 FEC sub-TLV and a VPN IPv4 prefix sub-TLV has the
   following format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type = 1 (FEC TLV)       |          Length = 32          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  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 (Deprecated)
               3                  Pad
               4                  Unassigned
               5                  Vendor Enterprise Number
               6                  Unassigned
               7                  Interface and Label Stack
               8                  Unassigned
               9                  Errored TLVs
              10                  Reply TOS Byte
              20                  Downstream Detailed Mapping

   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.





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   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
   implementation does not understand or support them.

   In Sections 3.2 through 3.9 and their various subsections, only the
   Value field of the TLV is included.

3.1.  Return Codes

   The Return Code is set to zero by the sender of an echo request.  The
   receiver of said echo request can set it to one of the values listed
   below in the corresponding echo reply that it generates.  The
   notation <RSC> refers to the Return Subcode.  This field is filled in
   with the stack-depth for those codes that specify that.  For all
   other codes, the Return Subcode 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
      14    See DDMAP TLV for meaning of Return Code and Return
            Subcode (See Note 2)
      15    Label switched with FEC change

   Note 1

      The Return Subcode (RSC) contains the point in the label stack
      where processing was terminated.  If the RSC is 0, no labels were
      processed.  Otherwise, the packet was label switched at depth RSC.





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   Note 2

      The Return Code is per "Downstream Detailed Mapping TLV"
      (Section 3.4).  This Return Code MUST be used only in the message
      header and MUST be set only in the MPLS echo reply message.  If
      the Return Code is set in the MPLS echo request message, then it
      MUST be ignored.  When this Return Code is set, each Downstream
      Detailed Mapping TLV MUST have an appropriate Return Code and
      Return Subcode.  This Return Code MUST be used when there are
      multiple downstreams for a given node (such as Point-to-Multipoint
      (P2MP) or ECMP), and the node needs to return a Return Code/Return
      Subcode for each downstream.  This Return Code MAY be used even
      when there is only one downstream for a given node.

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                    Unassigned
           6         13         VPN IPv4 prefix
           7         25         VPN IPv6 prefix
           8         14         L2 VPN endpoint
           9         10         "FEC 128" Pseudowire - IPv4 (deprecated)
          10         14         "FEC 128" Pseudowire - IPv4
          11        16+         "FEC 129" Pseudowire - IPv4
          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
          24         38         "FEC 128" Pseudowire - IPv6
          25         40+        "FEC 129" Pseudowire - IPv6

   Other FEC types have been defined and 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.







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   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
   [RFC5036] for 192.0.2.1 (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 192.0.2.1/32,
   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 203.0.113.0/24 in VPN foo.  Say further that LSR Y
   with loopback address 192.0.2.1 announced prefix 203.0.113.0/24 with
   Route Distinguisher (RD) RD-foo-Y (which may in general be different
   from the RD that LSR X uses in its own advertisements for VPN foo),
   label 23456, and BGP next hop 192.0.2.1 [RFC4271].  Finally, suppose
   that LSR X receives a label binding of 1001 for 192.0.2.1 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 203.0.113.0/24 and an RD 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 192.0.2.1/32 and the second of type of
   IP VPN with a prefix 203.0.113.0/24 with an RD 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.)

   If, for example, an LSR Y has an LDP mapping for the IPv6 address
   2001:db8::1 (say, label 2001), then to verify that label 2001 does
   reach an egress LSR that announced this prefix via LDP, LSR Y can
   send an MPLS echo request with a FEC Stack TLV with one LDP IPv6
   prefix FEC, with prefix 2001:db8::1/128, and with a label of 2001.

   If an end-to-end path comprises of one or more tunneled or stitched
   LSPs, each transit node that is the originating point of a new tunnel
   or segment SHOULD reply back notifying the FEC stack change along
   with the new FEC details, for example, if LSR X has an LDP mapping
   for IPv4 prefix 192.0.2.10 on LSR Z (say, label 3001).  Say further
   that LSR A and LSR B are transit nodes along the path, which also
   have an RSVP tunnel over which LDP is enabled.  While replying back,
   A SHOULD notify that the FEC changes from LDP to <RSVP, LDP>.  If the
   new tunnel is a transparent pipe, i.e., the data-plane trace will not
   expire in the middle of the tunnel, then the transit node SHOULD NOT
   reply back notifying the FEC stack change or the new FEC details.  If
   the transit node wishes to hide the nature of the tunnel from the
   ingress of the echo request, then the transit node MAY notify the FEC
   stack change and include Nil FEC as the new FEC.





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3.2.1.  LDP IPv4 Prefix

   The IPv4 Prefix FEC is defined in [RFC5036].  When an LDP IPv4 prefix
   is encoded in a label stack, the following format is used.  The value
   consists of 4 octets of an IPv4 prefix followed by 1 octet of prefix
   length in bits; the format is given below.  The IPv4 prefix is in
   network byte order; if the prefix is shorter than 32 bits, trailing
   bits SHOULD be set to zero.  See [RFC5036] for an example of a
   Mapping for an IPv4 FEC.

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

3.2.2.  LDP IPv6 Prefix

   The IPv6 Prefix FEC is defined in [RFC5036].  When an LDP IPv6 prefix
   is encoded in a label stack, the following format is used.  The value
   consists of 16 octets of an IPv6 prefix followed by 1 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 [RFC5036] 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                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+













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3.2.3.  RSVP IPv4 LSP

   The value has the format below.  The Value fields are taken from RFC
   3209 [RFC3209], Sections 4.6.1.1 and 4.6.2.1.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 IPv4 Tunnel Endpoint 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 RFC
   3209 [RFC3209], Sections 4.6.1.2 and 4.6.2.2.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 IPv6 Tunnel Endpoint Address                  |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |          Tunnel ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Extended Tunnel ID                      |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   IPv6 Tunnel Sender Address                  |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |            LSP ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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3.2.5.  VPN IPv4 Prefix

   VPN-IPv4 Network Layer Routing Information (NLRI) is defined in
   [RFC4365].  This document uses the term VPN IPv4 prefix for a
   VPN-IPv4 NLRI that has been advertised with an MPLS label in BGP.
   See [RFC3107].

   When a VPN IPv4 prefix is encoded in a label stack, the following
   format is used.  The Value field consists of the RD 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                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The RD is an 8-octet identifier; it does not contain any inherent
   information.  The purpose of the RD is solely to allow one to create
   distinct routes to a common IPv4 address prefix.  The encoding of the
   RD is not important here.  When matching this field to the local FEC
   information, it is treated as an opaque value.























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3.2.6.  VPN IPv6 Prefix

   VPN-IPv6 NLRI is defined in [RFC4365].  This document uses the term
   VPN IPv6 prefix for a VPN-IPv6 NLRI that has been advertised with an
   MPLS label in BGP.  See [RFC3107].

   When a VPN IPv6 prefix is encoded in a label stack, the following
   format is used.  The Value field consists of the RD 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                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The RD is identical to the VPN IPv4 Prefix RD, except that it
   functions here to allow the creation of distinct routes to IPv6
   prefixes.  See Section 3.2.5.  When matching this field to local FEC
   information, it is treated as an opaque value.






















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3.2.7.  L2 VPN Endpoint

   VPLS stands for Virtual Private LAN Service.  The terms VPLS BGP NLRI
   and VPLS Edge Identifier (VE ID) are defined in [RFC4761].  This
   document uses the simpler term L2 VPN endpoint when referring to a
   VPLS BGP NLRI.  The RD is an 8-octet identifier used to distinguish
   information about various L2 VPNs advertised by a node.  The VE ID is
   a 2-octet identifier used to identify a particular node that serves
   as the service attachment point within a VPLS.  The structure of
   these two identifiers is unimportant here; when matching these fields
   to local FEC information, they are treated as opaque values.  The
   encapsulation type is identical to the Pseudowire (PW) Type in
   Section 3.2.9.

   When an L2 VPN endpoint is encoded in a label stack, the following
   format is used.  The Value field consists of an RD (8 octets), the
   sender's (of the ping) 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 - IPv4 (Deprecated)

   See Appendix A.1.1 for details.


















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3.2.9.  FEC 128 Pseudowire - IPv4 (Current)

   FEC 128 (0x80) is defined in [RFC8077], as are the terms PW ID
   (Pseudowire ID) and PW Type (Pseudowire Type).  A PW ID is a non-zero
   32-bit connection ID.  The PW Type is a 15-bit number indicating the
   encapsulation type.  It is carried right justified in the field below
   termed "encapsulation type" with the high-order bit set to zero.

   Both of these fields are treated in this protocol as opaque values.
   When matching these fields to the local FEC information, the match
   MUST be exact.

   When a FEC 128 is encoded in a label stack, the following format is
   used.  The Value field consists of the Sender's Provider Edge (PE)
   IPv4 Address (the source address of the targeted LDP session), the
   Remote PE IPv4 Address (the destination address of the targeted LDP
   session), the PW ID, and the 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 IPv4 Address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE IPv4 Address                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             PW ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            PW Type            |          Must Be Zero         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






















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3.2.10.  FEC 129 Pseudowire - IPv4

   FEC 129 (0x81) and the terms PW Type, Attachment Group Identifier
   (AGI), Attachment Group Identifier Type (AGI Type), Attachment
   Individual Identifier Type (AII Type), Source Attachment Individual
   Identifier (SAII), and Target Attachment Individual Identifier (TAII)
   are defined in [RFC8077].  The PW Type is a 15-bit number indicating
   the encapsulation type.  It is carried right justified in the field
   below PW Type with the high-order bit set to zero.  All the other
   fields are treated as opaque values and copied directly from the FEC
   129 format.  All of these values together uniquely define the FEC
   within the scope of the LDP session identified by the source and
   remote PE IPv4 addresses.

   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 IPv4 Address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE IPv4 Address                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            PW Type            |   AGI Type    |  AGI Length   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                           AGI Value                           ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   AII Type    |  SAII Length  |      SAII Value               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    SAII Value (continued)                     ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   AII Type    |  TAII Length  |      TAII Value               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                    TAII Value (continued)                     ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  TAII (cont.) |  0-3 octets of zero padding                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+









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3.2.11.  FEC 128 Pseudowire - IPv6

   The FEC 128 Pseudowire IPv6 sub-TLV has a structure consistent with
   the FEC 128 Pseudowire IPv4 sub-TLV as described in Section 3.2.9.
   The Value field consists of the Sender's PE IPv6 Address (the source
   address of the targeted LDP session), the Remote PE IPv6 Address (the
   destination address of the targeted LDP session), the PW ID, and the
   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 IPv6 Address                  ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                      Remote PE IPv6 Address                   ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             PW ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            PW Type            |          Must Be Zero         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sender's PE IPv6 Address: The source IP address of the target IPv6
   LDP session. 16 octets.

   Remote PE IPv6 Address: The destination IP address of the target IPv6
   LDP session. 16 octets.

   PW ID: Same as FEC 128 Pseudowire IPv4 in Section 3.2.9.

   PW Type: Same as FEC 128 Pseudowire IPv4 in Section 3.2.9.





















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3.2.12.  FEC 129 Pseudowire - IPv6

   The FEC 129 Pseudowire IPv6 sub-TLV has a structure consistent with
   the FEC 129 Pseudowire IPv4 sub-TLV as described in Section 3.2.10.
   When a FEC 129 is encoded in a label stack, the following format is
   used.  The length of this TLV is 40 + AGI (Attachment Group
   Identifier) length + SAII (Source Attachment Individual Identifier)
   length + TAII (Target Attachment Individual Identifier) 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 IPv6 Address                    ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    Remote PE IPv6 Address                     ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            PW Type            |   AGI Type    |  AGI Length   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           AGI Value                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   AII Type    |  SAII Length  |      SAII Value               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    SAII Value (continued)                     ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   AII Type    |  TAII Length  |      TAII Value               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    TAII Value (continued)                     ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  TAII (cont.) |  0-3 octets of zero padding                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sender's PE IPv6 Address: The source IP address of the target IPv6
   LDP session. 16 octets.

   Remote PE IPv6 Address: The destination IP address of the target IPv6
   LDP session. 16 octets.

   The other fields are the same as FEC 129 Pseudowire IPv4 in
   Section 3.2.10.










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3.2.13.  BGP Labeled IPv4 Prefix

   BGP labeled IPv4 prefixes are defined in [RFC3107].  When a BGP
   labeled IPv4 prefix is encoded in a label stack, the following format
   is used.  The Value field consists of 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                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.14.  BGP Labeled IPv6 Prefix

   BGP labeled IPv6 prefixes are defined in [RFC3107].  When a BGP
   labeled IPv6 prefix is encoded in a label stack, the following format
   is used.  The value consists of 16 octets of an IPv6 prefix followed
   by 1 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                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

















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3.2.15.  Generic IPv4 Prefix

   The value consists of 4 octets of an IPv4 prefix followed by 1 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, the
   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
   signaled by LDP in one Autonomous System (AS), by RSVP-TE [RFC3209]
   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                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.16.  Generic IPv6 Prefix

   The value consists of 16 octets of an IPv6 prefix followed by 1 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                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.17.  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.




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   The Length is 4.  Labels are 20-bit values treated as numbers.

       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 (Deprecated)

   See Appendix A.2 for more details.

3.4.  Downstream Detailed Mapping TLV

   The Downstream Detailed Mapping object is a TLV that MAY be included
   in an MPLS echo request message.  Only one Downstream Detailed
   Mapping object may appear in an echo request.  The presence of a
   Downstream Detailed Mapping object is a request that Downstream
   Detailed Mapping objects be included in the MPLS echo reply.  If the
   replying router is the destination (Label Edge Router) of the FEC,
   then a Downstream Detailed Mapping TLV SHOULD NOT be included in the
   MPLS echo reply.  Otherwise, the replying router SHOULD include a
   Downstream Detailed Mapping object for each interface over which this
   FEC could be forwarded.  For a more precise definition of the notion
   of "downstream", see Section 3.4.2, "Downstream Router and
   Interface".

        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 Address (4 or 16 octets)             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Downstream Interface Address (4 or 16 octets)         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Return Code  | Return Subcode|        Sub-TLV Length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                      List of Sub-TLVs                         .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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   The Downstream Detailed Mapping TLV format is derived from the
   deprecated Downstream Mapping TLV format (see Appendix A.2.)  The key
   change is that variable length and optional fields have been
   converted into sub-TLVs.

   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
      unnumbered.  It also determines the length of the Downstream IP
      Address and Downstream Interface fields.  The Address Type is set
      to one of the following values:

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

   DS Flags

      The DS Flags field is a bit vector of various flags 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.







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          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 that 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 Address and Downstream Interface Address

      IPv4 addresses 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 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
      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 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
      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
      Downstream Detailed 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 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 validation but return Downstream Mapping
      TLVs using the information provided.





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   Return Code

      The Return Code is set to zero by the sender of an echo request.
      The receiver of said echo request can set it in the corresponding
      echo reply that it generates to one of the values specified in
      Section 3.1 other than 14.

      If the receiver sets a non-zero value of the Return Code field in
      the Downstream Detailed Mapping TLV, then the receiver MUST also
      set the Return Code field in the echo reply header to "See DDMAP
      TLV for Return Code and Return Subcode" (Section 3.1).  An
      exception to this is if the receiver is a bud node [RFC4461] and
      is replying as both an egress and a transit node with a Return
      Code of 3 ("Replying router is an egress for the FEC at stack-
      depth <RSC>") in the echo reply header.

      If the Return Code of the echo reply message is not set to either
      "See DDMAP TLV for Return Code and Return Subcode" (Section 3.1)
      or "Replying router is an egress for the FEC at stack-depth
      <RSC>", then the Return Code specified in the Downstream Detailed
      Mapping TLV MUST be ignored.

   Return Subcode

      The Return Subcode is set to zero by the sender.  The receiver can
      set this field to an appropriate value as specified in
      Section 3.1: The Return Subcode is filled in with the stack-depth
      for those codes that specify the stack-depth.  For all other
      codes, the Return Subcode MUST be set to zero.

      If the Return Code of the echo reply message is not set to either
      "See DDMAP TLV for Return Code and Return Subcode" (Section 3.1)
      or "Replying router is an egress for the FEC at stack-depth
      <RSC>", then the Return Subcode specified in the Downstream
      Detailed Mapping TLV MUST be ignored.

   Sub-TLV Length

      Total length in octets of the sub-TLVs associated with this TLV.












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3.4.1.  Sub-TLVs

   This section defines the sub-TLVs that MAY be included as part of the
   Downstream Detailed Mapping TLV.

            Sub-Type    Value Field
           ---------   ------------
             1         Multipath data
             2         Label stack
             3         FEC stack change

3.4.1.1.  Multipath Data Sub-TLV

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Multipath Type |       Multipath Length        |Reserved (MBZ) |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                  (Multipath Information)                      |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The multipath data sub-TLV includes Multipath Information.

   Multipath Type

      The type of the encoding for the Multipath Information.

      The following Multipath Types are defined in this document:

      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 IP         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.






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   Multipath Length

      The length in octets of the Multipath Information.

   MBZ

      MUST be set to zero when sending; MUST be ignored on receipt.

   Multipath Information

      Encoded multipath data (e.g., encoded address or label values),
      according to the Multipath Type.  See Section 3.4.1.1.1 for
      encoding details.

3.4.1.1.1.  Multipath Information Encoding

   The Multipath Information encodes labels or addresses that will
   exercise this path.  The Multipath Information depends on the
   Multipath Type.  The contents of the field are shown in the table
   above.  IPv4 addresses are drawn from the range 127/8; IPv6 addresses
   are drawn from the range 0:0:0:0:0:FFFF:7F00:0/104.  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 more dense encoding of IP addresses.  The IP prefix
   is formatted as a base IP 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 for IPv4 and
   2^(128 - prefix length) bits for IPv6.  Each bit set to 1 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 IPv4 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|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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   Those same addresses embedded in IPv6 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 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 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 more dense encoding of labels.  The label prefix is
   formatted as a base label value with the non-prefix low-order bits
   set to zero.  The maximum prefix (including leading zeros due to
   encoding) length is 27.  Following the prefix is a mask of length
   2^(32 - prefix length) bits.  Each bit set to one represents a valid
   label.  The label is the base label plus the position of the bit in
   the mask where the bits are numbered left to right 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
   Multipath Information is null (i.e., Multipath Length = 0, or for
   Types 8 and 9, a mask of all zeros), the type MUST be set to 0.





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   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
   downstream 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 Detailed Mapping TLVs: 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 Multipath 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 Detailed Mapping 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.

   The encoding of Multipath Information in scenarios where a few LSRs
   apply Entropy-label-based load-balancing while other LSRs are non-EL
   (IP-based) load balanced will be defined in a different document.

   The encoding of Multipath Information in scenarios where LSRs have
   Layer 2 ECMP over Link Aggregation Group (LAG) interfaces will be
   defined in a different document.

3.4.1.2.  Label Stack Sub-TLV

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Label Stack sub-TLV contains 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.  The number of label/protocol pairs present in the sub-TLV
   is determined based on the sub-TLV data length.  When the Downstream
   Detailed Mapping TLV is sent in the echo reply, this sub-TLV MUST be
   included.



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   Downstream Label

      A downstream label is 24 bits, in the same format as an MPLS label
      minus the TTL field, i.e., the MSBit of the label is bit 0, the
      LSBit is bit 19, the TC field [RFC5462] is bits 20-22, and S is
      bit 23.  The replying router SHOULD fill in the TC field and S
      bit; the LSR receiving the echo reply MAY choose to ignore these.

   Protocol

      This specifies the label distribution protocol for the Downstream
      label.  Protocol values are taken from the following table:

      Protocol #        Signaling Protocol
      ----------        ------------------
               0        Unknown
               1        Static
               2        BGP
               3        LDP
               4        RSVP-TE

3.4.1.3.  FEC Stack Change Sub-TLV

   A router MUST include the FEC stack change sub-TLV when the
   downstream node in the echo reply has a different FEC Stack than the
   FEC Stack received in the echo request.  One or more FEC stack change
   sub-TLVs MAY be present in the Downstream Detailed Mapping TLV.  The
   format is as below.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Operation Type | Address Type  | FEC-tlv length|  Reserved     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Remote Peer Address (0, 4, or 16 octets)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                         FEC TLV                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+











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   Operation Type

      The operation type specifies the action associated with the FEC
      stack change.  The following operation types are defined:

            Type #     Operation
            ------     ---------
            1          Push
            2          Pop

   Address Type

      The Address Type indicates the remote peer's address type.  The
      Address Type is set to one of the following values.  The length of
      the peer address is determined based on the address type.  The
      address type MAY be different from the address type included in
      the Downstream Detailed Mapping TLV.  This can happen when the LSP
      goes over a tunnel of a different address family.  The address
      type MAY be set to Unspecified if the peer address is either
      unavailable or the transit router does not wish to provide it for
      security or administrative reasons.

           Type #   Address Type   Address length
           ------   ------------   --------------
           0        Unspecified    0
           1        IPv4           4
           2        IPv6           16

   FEC TLV Length

      Length in octets of the FEC TLV.

   Reserved

      This field is reserved for future use and MUST be set to zero.

   Remote Peer Address

      The remote peer address specifies the remote peer that is the next
      hop for the FEC being currently traced.  If the operation type is
      PUSH, the remote peer address is the address of the peer from
      which the FEC being pushed was learned.  If the operation type is
      pop, the remote peer address MAY be set to Unspecified.

      For upstream-assigned labels [RFC5331], an operation type of pop
      will have a remote peer address (the upstream node that assigned
      the label), and this SHOULD be included in the FEC stack change




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      sub-TLV.  The remote peer address MAY be set to Unspecified if the
      address needs to be hidden.

   FEC TLV

      The FEC TLV is present only when the FEC-tlv length field is non-
      zero.  The FEC TLV specifies the FEC associated with the FEC stack
      change operation.  This TLV MAY be included when the operation
      type is pop.  It MUST be included when the operation type is PUSH.
      The FEC TLV contains exactly one FEC from the list of FECs
      specified in Section 3.2.  A Nil FEC MAY be associated with a PUSH
      operation if the responding router wishes to hide the details of
      the FEC being pushed.

   FEC stack change sub-TLV operation rules are as follows:

   a.  A FEC stack change sub-TLV containing a PUSH operation MUST NOT
       be followed by a FEC stack change sub-TLV containing a pop
       operation.

   b.  One or more pop operations MAY be followed by one or more PUSH
       operations.

   c.  One FEC stack change sub-TLV MUST be included per FEC stack
       change.  For example, if 2 labels are going to be pushed, then
       one FEC stack change sub-TLV MUST be included for each FEC.

   d.  A FEC splice operation (an operation where one FEC ends and
       another FEC starts, MUST be performed by including a pop type FEC
       stack change sub-TLV followed by a PUSH type FEC stack change
       sub-TLV.

   e.  A Downstream Detailed Mapping TLV containing only one FEC stack
       change sub-TLV with pop operation is equivalent to IS_EGRESS
       (Return Code 3, Section 3.1) for the outermost FEC in the FEC
       stack.  The ingress router performing the LSP traceroute MUST
       treat such a case as an IS_EGRESS for the outermost FEC.

3.4.2.  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 consists of the downstream routers/



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   interfaces (and their corresponding labels) for X with respect to L.
   Each pair of downstream router and interface requires a separate
   Downstream Detailed 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
   multicast).  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.

3.5.  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
      -----        -------
          0        Reserved
          1        Drop Pad TLV from reply
          2        Copy Pad TLV to reply
      3-250        Unassigned
    251-254        Reserved for Experimental Use
        255        Reserved

   The Pad TLV can be added to an echo request to create a message of a
   specific length in cases where messages of various sizes are needed
   for troubleshooting.  The first octet allows for controlling the
   inclusion of this additional padding in the respective echo reply.

3.6.  Vendor Enterprise Number

   "Private Enterprise Numbers" [IANA-ENT] are maintained by IANA.  The
   Length of this TLV is always 4; the value is the Structure of
   Management Information (SMI) Private Enterprise Code, in network
   octet order, of the vendor with a Vendor Private extension to any of
   the fields in the fixed part of the message, in which case this TLV
   MUST be present.  If none of the fields in the fixed part of the
   message have Vendor Private extensions, inclusion of this TLV is
   OPTIONAL.  Vendor Private ranges for Message Types, Reply Modes, and
   Return Codes have been defined.  When any of these are used, the
   Vendor Enterprise Number TLV MUST be included in the message.



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3.7.  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 that 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 this TLV has the following format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | 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
      unnumbered.  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
         ------        ------------           --------
              0        Reserved                      4
              1        IPv4 Numbered                12
              2        IPv4 Unnumbered              12
              3        IPv6 Numbered                36
              4        IPv6 Unnumbered              24
          5-250        Unassigned
        251-254        Reserved for Experimental Use
            255        Reserved



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   IP Address and Interface

      IPv4 addresses 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 is 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.

3.8.  Errored TLVs

   The following TLV is a TLV that 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                             |
      .                                                               .
      .                                                               .
      .                                                               .
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+










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3.9.  Reply TOS Octet TLV

   This TLV MAY be used by the originator of the echo request to request
   that an echo reply be sent with the IP header Type of Service (TOS)
   octet 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 a label is mapped to an egress IP address of
   198.51.100.1, the FEC Stack contains a single element, namely, an LDP
   IPv4 prefix sub-TLV with value 198.51.100.1/32.  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 that uniquely
   identifies the LSP.

   FEC Stacks can be more complex.  For example, one may wish to test a
   VPN IPv4 prefix of 203.0.113.0/24 that is tunneled over an LDP LSP
   with egress 192.0.2.1.  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
   verify that the control plane and data plane are both healthy (for
   the FEC Stack being pinged), and that the two planes are in sync.
   The procedures for this are in Section 4.4.

4.1.  Dealing with Equal-Cost Multipath (ECMP)

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





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

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

   To achieve some degree of coverage of alternate paths, there is a
   certain latitude in choosing the destination IP address and source
   UDP port for an MPLS echo request.  This is clearly not sufficient;
   in the case of traceroute, more latitude is offered by means of the
   Multipath Information of the Downstream Detailed Mapping TLV.  This
   is used as follows.  An ingress LSR periodically sends an LSP
   traceroute message to determine whether there are multipaths for a
   given LSP.  If so, each hop will provide some information as to 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
   possible.

4.2.  Testing LSPs That Are Used to Carry MPLS Payloads

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









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4.3.  Sending an MPLS Echo Request

   An MPLS echo request is a UDP packet.  The IP header is set as
   follows: the source IP address is a routable address of the sender;
   the destination IP address is a (randomly chosen) IPv4 address from
   the range 127/8 or an IPv6 address from the range
   0:0:0:0:0:FFFF:7F00:0/104.  The IP TTL is set to 1.  The source UDP
   port is chosen by the sender; the destination UDP port is set to 3503
   (assigned by IANA for MPLS echo requests).  The Router Alert IP
   Option of value 0x0 [RFC2113] for IPv4 or value 69 [RFC7506] for IPv6
   MUST be set in the IP header.

   An MPLS echo request is sent with a label stack corresponding to 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 Traffic Engineered Tunnel [RFC3209].  If all of the FECs in the
   stack correspond to Implicit Null labels, the MPLS echo request is
   considered unlabeled even if further labels will be applied in
   sending the packet.

   If the echo request is labeled, one MAY (depending on what is being
   pinged) set the TTL of the innermost label to 1, to prevent the ping
   request going farther than it should.  Examples of where this SHOULD
   be done include pinging a VPN IPv4 or IPv6 prefix, an L2 VPN
   endpoint, or a pseudowire.  Preventing the ping request from going
   too far can also be accomplished by inserting a Router Alert label
   above this label; however, this may lead to the undesired side effect
   that MPLS echo requests take a different data path than actual data.
   For more information on how these mechanisms can be used for
   pseudowire connectivity verification, see [RFC5085][RFC5885].

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

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

   The TimeStamp Sent is set to the time of day in NTP format that the
   echo request is sent.  The TimeStamp Received is set to zero.

   An MPLS echo request MUST have a FEC Stack TLV.  Also, the Reply Mode
   must be set to the desired Reply Mode; the Return Code and Subcode
   are set to zero.  In the "traceroute" mode, the echo request SHOULD
   include a Downstream Detailed Mapping TLV.



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4.4.  Receiving an MPLS Echo Request

   Sending an MPLS echo request to the control plane is triggered by one
   of the following packet processing exceptions: Router Alert option,
   IP TTL expiration, MPLS TTL expiration, MPLS Router Alert label, or
   the destination address in the 127/8 address range.  The control
   plane further identifies it by UDP destination port 3503.

   For reporting purposes, the bottom of the stack is considered to be a
   stack-depth of 1.  This is to establish an absolute reference for the
   case where the actual stack may have more labels than there are FECs
   in the Target FEC Stack.

   Furthermore, in all the Return Codes listed in this document, a
   stack-depth of 0 means "no value specified".  This allows
   compatibility with existing implementations that 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 set to
       zero.  If there are any TLVs not marked as "Ignore" (i.e., if the
       TLV type is less than 32768, see Section 3) that LSR X does not
       understand, LSR X SHOULD send an MPLS "TLV not understood" (as
       appropriate), and set the Subcode to zero.  In the latter case,
       the misunderstood TLVs (only) are included as sub-TLVs in an
       Errored TLVs TLV in the reply.  The header field's Sender's
       Handle, Sequence Number, and Timestamp Sent are not examined but
       are included in the MPLS echo reply message.

   The algorithm uses the following variables and identifiers:

   Interface-I:        the interface on which the 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 "Label stack
                       sub-TLV" in the Downstream Detailed Mapping TLV
                       (not always present).

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





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

   FEC-stack-depth:    depth of the FEC in the Target FEC Stack that
                       should be used to verify the current actual
                       label.  Requires no initialization.

   Best-return-code:   contains the Return Code for the echo reply
                       packet as currently best known.  As the algorithm
                       progresses, this code may change depending on the
                       results of further checks that it performs.

   Best-rtn-subcode:   similar to Best-return-code, but for the echo
                       reply Subcode.

   FEC-status:         result value returned by the FEC Checking
                       algorithm described in Section 4.4.1.

   /* Save receive context information */

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

   /* The rest of the algorithm iterates over the labels 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 description in the body of the echo
   request, and reports any errors. */

   /* The algorithm iterates as follows. */

   3.  Label Validation:

      If Label-stack-depth is 0 {

      /* The LSR needs to report that it is a tail end for the 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 go to step 5 (Egress Processing).

      }

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



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      Set Label-L to the value extracted from Stack-R at depth
      Label-stack-depth.  Look up Label-L in the Incoming Label Map
      (ILM) to determine if the label has been allocated and an
      operation is associated with it.

      If there is no entry for Label-L {

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

         Set Best-return-code to 11 ("No label entry at stack-depth")
         and Best-rtn-subcode to Label-stack-depth.  Go to step 7 (Send
         Reply Packet).

      }

      Else {

         Retrieve the associated label operation from the corresponding
         Next Hop Label Forwarding Entry (NHLFE), and proceed to step 4
         (Label Operation Check).

      }

   4.  Label Operation Check

      If the label operation is "Pop and Continue Processing" {

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

         Iterate to the next label by decrementing Label-stack-depth,
         and loop back to step 3 (Label Validation).

      }

      If the label operation is "Swap or Pop and Switch based on Popped
      Label" {

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

         If a Downstream Detailed Mapping TLV is present in the received
         echo request {

            If the IP address in the TLV is 127.0.0.1 or 0::1 {





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               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 Detailed 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 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 from the NHLFE entry.

            /* Note: this Return Code is set even if Label-stack-depth
            is one */

            If the output interface is not MPLS enabled {

               Set Best-return-code to Return Code 9, "Label switched
               but no MPLS forwarding at stack-depth" and set
               Best-rtn-subcode to Label-stack-depth and go to step 7
               (Send Reply Packet).

            }

            If a Downstream Detailed Mapping TLV is present {

               A Downstream Detailed Mapping TLV SHOULD be included in
               the echo reply (see Section 3.4) filled in with
               information about the current ECMP path.

            }

         }





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         If no Downstream Detailed Mapping TLV is present, or the
         Downstream IP Address is set to the ALLROUTERS multicast
         address, go to step 7 (Send Reply Packet).

         If the "Validate FEC Stack" flag is not set and the 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 Detailed
         Mapping TLV.  This is done by walking through Stack-D (the
         Downstream labels) from the bottom, decrementing the number of
         labels for each non-Implicit Null label, while incrementing
         FEC-stack-depth for each label.  If the Downstream Detailed
         Mapping TLV contains one or more Implicit Null labels,
         FEC-stack-depth may be greater than Label-stack-depth.  To be
         consistent with the above stack-depths, the bottom is
         considered to be 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 number of FECs in the FEC stack is greater than or equal
         to FEC-stack-depth {
         Perform the FEC Checking procedure (see Section 4.4.1).

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

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

         Go to step 7 (Send Reply Packet).
      }








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   5.  Egress Processing:

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

      If the received echo request contains no Downstream Detailed
      Mapping TLV, or the Downstream IP Address is set to 127.0.0.1 or
      0::1, go to step 6 (Egress FEC Validation).

      Verify that the IP address, interface address, and label stack in
      the Downstream Detailed Mapping TLV match Interface-I and Stack-R.
      If not, set Best-return-code to 5, "Downstream Mapping Mismatch".
      A Received Interface and Label Stack TLV SHOULD be created for the
      echo response packet.  Go to step 7 (Send Reply Packet).

   6.  Egress FEC Validation:

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

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

      Set Best-return-code to FEC-code and Best-rtn-subcode to the value
      in FEC-stack-depth.


      If FEC-status (the result of the check) is 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,
      go to step 7 (Send Reply Packet).

      If FEC-status is 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.
         Loop back to step 6 (Egress FEC Validation).
      }




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   7.  Send Reply Packet:

      Send an MPLS echo reply with a Return Code of Best-return-code and
      a Return Subcode of Best-rtn-subcode.  Include any TLVs created
      during the above process.  The procedures for sending the echo
      reply are found in Section 4.5.

4.4.1.  FEC Validation

   /* This section describes validation of a FEC entry within the Target
   FEC Stack and accepts a FEC, Label-L, and Interface-I.

   If the outermost FEC of the Target FEC stack is the Nil FEC, then the
   node MUST skip the Target FEC validation completely.  This is to
   support FEC hiding, in which the outer hidden FEC can be the Nil FEC.
   Else, the algorithm performs the following steps. */

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

   2.  If the FEC is the Nil FEC {

          If Label-L is either Explicit_Null or Router_Alert, return.

          Else {

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

       }

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






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   5.  This is a protocol check.  Check what protocol would be used to
       advertise the FEC.  If it can be determined that no protocol
       associated with Interface-I would have advertised a FEC of that
       FEC-Type, set FEC-return-code to 12 ("Protocol not associated
       with interface at FEC 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 of value 0x0 [RFC2113] for IPv4 or 69
   [RFC7506] for IPv6.  If the reply is sent over an LSP, the topmost
   label MUST in this case be the Router Alert label (1) (see
   [RFC3032]).

   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 section.

   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
   Detailed Mapping TLVs SHOULD NOT be included in the echo reply.

   If the echo request contains a Downstream Detailed Mapping TLV, and
   the replying router is not the destination of the FEC, the replier
   SHOULD compute its downstream routers and corresponding labels for
   the incoming label and add Downstream Detailed Mapping TLVs for each
   one to the echo reply it sends back.  A replying node should follow
   the procedures defined in Section 4.5.1 if there is a FEC stack
   change due to tunneled LSP.  If the FEC stack change is due to
   stitched LSP, it should follow the procedures defined in
   Section 4.5.2.



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   If the Downstream Detailed Mapping TLV contains Multipath Information
   requiring 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
   Detailed Mapping.  (Note: The originator of the echo request MAY send
   another echo request with the Multipath Information that was not
   included in the reply.)

   Except in the case of Reply Mode 4, "Reply via application-level
   control channel", echo replies are always sent in the context of the
   IP/MPLS network.

4.5.1.  Addition of a New Tunnel

   A transit node knows when the FEC being traced is going to enter a
   tunnel at that node.  Thus, it knows about the new outer FEC.  All
   transit nodes that are the origination point of a new tunnel SHOULD
   add the FEC stack change sub-TLV (Section 3.4.1.3) to the Downstream
   Detailed Mapping TLV in the echo reply.  The transit node SHOULD add
   one FEC stack change sub-TLV of operation type PUSH, per new tunnel
   being originated at the transit node.

   A transit node that sends a Downstream FEC stack change sub-TLV in
   the echo reply SHOULD fill the address of the remote peer, which is
   the peer of the current LSP being traced.  If the transit node does
   not know the address of the remote peer, it MUST set the address type
   to Unspecified.

   The Label Stack sub-TLV MUST contain one additional label per FEC
   being PUSHed.  The label MUST be encoded as defined in
   Section 3.4.1.2.  The label value MUST be the value used to switch
   the data traffic.  If the tunnel is a transparent pipe to the node,
   i.e., the data-plane trace will not expire in the middle of the new
   tunnel, then a FEC stack change sub-TLV SHOULD NOT be added, and the
   Label Stack sub-TLV SHOULD NOT contain a label corresponding to the
   hidden tunnel.

   If the transit node wishes to hide the nature of the tunnel from the
   ingress of the echo request, then it MAY not want to send details
   about the new tunnel FEC to the ingress.  In such a case, the transit
   node SHOULD use the Nil FEC.  The echo reply would then contain a FEC
   stack change sub-TLV with operation type PUSH and a Nil FEC.  The
   value of the label in the Nil FEC MUST be set to zero.  The remote
   peer address type MUST be set to Unspecified.  The transit node
   SHOULD add one FEC stack change sub-TLV of operation type PUSH, per
   new tunnel being originated at the transit node.  The Label Stack
   sub-TLV MUST contain one additional label per FEC being PUSHed.  The
   label value MUST be the value used to switch the data traffic.



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4.5.2.  Transition between Tunnels

   A transit node stitching two LSPs SHOULD include two FEC stack change
   sub-TLVs.  One with a pop operation for the old FEC (ingress) and one
   with the PUSH operation for the new FEC (egress).  The replying node
   SHOULD set the Return Code to "Label switched with FEC change" to
   indicate change in the FEC being traced.

   If the replying node wishes to perform FEC hiding, it SHOULD respond
   back with two FEC stack change sub-TLVs, one pop followed by one
   PUSH.  The pop operation MAY either exclude the FEC TLV (by setting
   the FEC TLV length to 0) or set the FEC TLV to contain the LDP FEC.
   The PUSH operation SHOULD have the FEC TLV containing the Nil FEC.
   The Return Code SHOULD be set to "Label switched with FEC change".

   If the replying node wishes to perform FEC hiding, it MAY choose to
   not send any FEC stack change sub-TLVs in the echo reply if the
   number of labels does not change for the downstream node and the FEC
   type also does not change (Nil FEC).  In such case, the replying node
   MUST NOT set the Return Code to "Label switched with FEC change".

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 ensure that it is well-formed,
   then attempt to match up the echo reply with an echo request that it
   had previously sent, using the destination UDP port and the Sender's
   Handle.  If no match is found, then X jettisons the echo reply;
   otherwise, it checks the Sequence Number to see if it matches.

   If the echo reply contains Downstream Detailed Mappings, and X wishes
   to traceroute further, it SHOULD copy the Downstream Detailed
   Mapping(s) into its next echo request(s) (with TTL incremented by
   one).

   If one or more FEC stack change sub-TLVs are received in the MPLS
   echo reply, the ingress node SHOULD process them and perform some
   validation.

   The FEC stack changes are associated with a downstream neighbor and
   along a particular path of the LSP.  Consequently, the ingress will
   need to maintain a FEC stack per path being traced (in case of
   multipath).  All changes to the FEC stack resulting from the
   processing of a FEC stack change sub-TLV(s) should be applied only
   for the path along a given downstream neighbor.  The following
   algorithm should be followed for processing FEC stack change
   sub-TLVs.



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       push_seen = FALSE
       fec_stack_depth = current-depth-of-fec-stack-being-traced
       saved_fec_stack = current_fec_stack

       while (sub-tlv = get_next_sub_tlv(downstream_detailed_map_tlv))

           if (sub-tlv == NULL) break

           if (sub-tlv.type == FEC-Stack-Change) {

               if (sub-tlv.operation == POP) {
                   if (push_seen) {
                       Drop the echo reply
                       current_fec_stack = saved_fec_stack
                       return
                   }

                   if (fec_stack_depth == 0) {
                       Drop the echo reply
                       current_fec_stack = saved_fec_stack
                       return
                   }

                   Pop FEC from FEC stack being traced
                   fec_stack_depth--;
               }

               if (sub-tlv.operation == PUSH) {
                   push_seen = 1
                   Push FEC on FEC stack being traced
                   fec_stack_depth++;
               }
            }
        }


        if (fec_stack_depth == 0) {
            Drop the echo reply
            current_fec_stack = saved_fec_stack
            return
        }

   The next MPLS echo request along the same path should use the
   modified FEC stack obtained after processing the FEC stack change
   sub-TLVs.  A non-Nil FEC guarantees that the next echo request along
   the same path will have the Downstream Detailed Mapping TLV validated
   for IP address, interface address, and label stack mismatches.




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   If the top of the FEC stack is a Nil FEC and the MPLS echo reply does
   not contain any FEC stack change sub-TLVs, then it does not
   necessarily mean that the LSP has not started traversing a different
   tunnel.  It could be that the LSP associated with the Nil FEC
   terminated at a transit node, and at the same time, a new LSP started
   at the same transit node.  The Nil FEC would now be associated with
   the new LSP (and the ingress has no way of knowing this).  Thus, it
   is not possible to build an accurate hierarchical LSP topology if a
   traceroute contains Nil FECs.

   A reply from a downstream node with Return Code 3, may not
   necessarily be for the FEC being traced.  It could be for one of the
   new FECs that was added.  On receipt of an IS_EGRESS reply, the LSP
   ingress should check if the depth of Target FEC sent to the node that
   just responded was the same as the depth of the FEC that was being
   traced.  If it was not, then it should pop an entry from the Target
   FEC stack and resend the request with the same TTL (as previously
   sent).  The process of popping a FEC is to be repeated until either
   the LSP ingress receives a non-IS_EGRESS reply or until all the
   additional FECs added to the FEC stack have already been popped.
   Using an IS_EGRESS reply, an ingress can build a map of the
   hierarchical LSP structure traversed by a given FEC.

   When the MPLS echo reply Return Code is "Label switched with FEC
   change", the ingress node SHOULD manipulate the FEC stack as per the
   FEC stack change sub-TLVs contained in the Downstream Detailed
   Mapping TLV.  A transit node can use this Return Code for stitched
   LSPs and for hierarchical LSPs.  In case of ECMP or P2MP, there could
   be multiple paths and Downstream Detailed Mapping TLVs with different
   Return Codes (see Section 3.1, Note 2).  The ingress node should
   build the topology based on the Return Code per ECMP path/P2MP
   branch.

4.7.  Issue with VPN IPv4 and IPv6 Prefixes

   Typically, an 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
   certain circumstances, the label stack can shrink to a single label
   before the ping hits the egress PE; this will result in the ping
   terminating prematurely.  One such scenario is a multi-AS Carrier's
   Carrier VPN.

   To get around this problem, one approach is for the LSR that receives
   such a ping to realize that the ping terminated prematurely and to
   send back Return Code 13.  In that case, the initiating LSR can retry




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   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 LSP
   ping, then no reply will be sent, resulting in possible "false
   negatives".  When in "traceroute" mode, if 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 the Downstream Detailed Mapping TLV "Downstream
   IP Address" field set to the ALLROUTERs multicast address until a
   reply is received with a Downstream Detailed Mapping TLV.  The label
   Stack TLV MAY be omitted from the Downstream Detailed Mapping TLV.
   Furthermore, the "Validate FEC Stack" flag SHOULD NOT be set until an
   echo reply packet with a Downstream Detailed Mapping TLV is received.

5.  Security Considerations

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

   There are at least three approaches to attacking LSRs using the
   mechanisms defined here.  One is a Denial-of-Service (DoS) attack, by
   sending MPLS echo requests/replies to LSRs and thereby increasing
   their workload.  The 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 DoS 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 in Section 6.1.

   Unsophisticated replay and spoofing attacks involving faking or
   replaying MPLS echo reply messages are unlikely to be effective.
   These replies would have to match the Sender's Handle and Sequence
   Number of an outstanding MPLS echo request message.  A non-matching
   replay would be discarded as the sequence has moved on, thus a spoof
   has only a small window of opportunity.  However, to provide a
   stronger defense, an implementation MAY also validate the TimeStamp
   Sent by requiring an exact match on this field.




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   To protect against unauthorized sources using MPLS echo request
   messages to obtain network information, it is RECOMMENDED that
   implementations provide a means of checking the source addresses of
   MPLS echo request messages against an access list before accepting
   the 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 is not working as it should.

   It does not 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.
   Implementations SHOULD, however, provide a means of filtering the
   addresses to which echo reply messages may be sent.

   The value part of the Pad TLV contains a variable number of octets.
   With the exception of the first octet, these contents, if any, are
   ignored on receipt, and can therefore serve as a clandestine channel.

   When MPLS LSP ping is used within an administrative domain, a
   deployment can increase security by using border filtering of
   incoming LSP ping packets as well as outgoing LSP ping packets.

   Although this document makes special use of 127/8 addresses, these
   are used only in conjunction with the UDP port 3503.  Furthermore,
   these packets are only processed by routers.  All other hosts MUST
   treat all packets with a destination address in the range 127/8 in
   accordance to RFC 1122.  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 RFC 1812.  In particular,
   the default behavior is to treat packets destined to a 127/8 address
   as "martians".

   If a network operator wants to prevent tracing inside a tunnel, one
   can use the Pipe Model [RFC3443], i.e., hide the outer MPLS tunnel by
   not propagating the MPLS TTL into the outer tunnel (at the start of
   the outer tunnel).  By doing this, LSP traceroute packets will not
   expire in the outer tunnel, and the outer tunnel will not get traced.

   If one doesn't wish to expose the details of the new outer LSP, then
   the Nil FEC can be used to hide those details.  Using the Nil FEC
   ensures that the trace progresses without false negatives and all
   transit nodes (of the new outer tunnel) perform some minimal
   validations on the received MPLS echo requests.






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6.  IANA Considerations

6.1.  TCP and UDP Port Number

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

6.2.  MPLS LSP Ping Parameters

   IANA maintains the "Multiprotocol Label Switching (MPLS) Label
   Switched Paths (LSPs) Ping Parameters" registry at
   [IANA-MPLS-LSP-PING].

   The following subsections detail the name spaces managed by IANA.
   For some 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
   [RFC5226]), "Specification Required", and "Vendor Private Use".

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

6.2.1.  Message Types, Reply Modes, Return Codes

   IANA has created and will 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 "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 Number TLV MUST be present in the message.








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   Message Types defined in this document are the following:

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

   Reply Modes defined in this document are the following:

      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.

   IANA has updated the reference for each these values to this
   document.

6.2.2.  TLVs

   IANA has created and maintains a registry for the Type field of top-
   level TLVs as well as for any associated sub-TLVs.  Note that 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
   ranges 0-16383 and 32768-49161 are made via Standards Action as
   defined in [RFC5226]; assignments in the ranges 16384-31743 and
   49162-64511 are made via "Specification Required"; values in the
   ranges 31744-32767 and 64512-65535 are for Vendor Private Use and
   MUST NOT be allocated.

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












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   TLVs and sub-TLVs defined in this document are the following:

      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        Unassigned
                      6        VPN IPv4 prefix
                      7        VPN IPv6 prefix
                      8        L2 VPN endpoint
                      9        "FEC 128" Pseudowire - IPv4 (Deprecated)
                     10        "FEC 128" Pseudowire - IPv4
                     11        "FEC 129" Pseudowire -  IPv4
                     12        BGP labeled IPv4 prefix
                     13        BGP labeled IPv6 prefix
                     14        Generic IPv4 prefix
                     15        Generic IPv6 prefix
                     16        Nil FEC
                     24        "FEC 128" Pseudowire - IPv6
                     25        "FEC 129" Pseudowire - IPv6
         2                     Downstream Mapping (Deprecated)
         3                     Pad
         4                     Unassigned
         5                     Vendor Enterprise Number
         6                     Unassigned
         7                     Interface and Label Stack
         8                     Unassigned
         9                     Errored TLVs
              Any value        The TLV not understood
        10                     Reply TOS Byte
        20                     Downstream Detailed Mapping

   IANA has updated the reference for each of these values to this
   document.














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6.2.3.  Global Flags

   IANA has created a "Global Flags" subregistry of the "Multiprotocol
   Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
   registry.

   This registry tracks the assignment of 16 flags in the Global Flags
   field of the MPLS LSP ping echo request message.  The flags are
   numbered from 0 (most significant bit, transmitted first) to 15.

   New entries are assigned by Standards Action.

   Initial entries in the registry are as follows:

      Bit number  |  Name                      | Reference
      ------------+----------------------------+--------------
        15        |  V Flag                    | [RFC8029]
        14        |  T Flag                    | [RFC6425]
        13        |  R Flag                    | [RFC6426]
        12-0      |  Unassigned                | [RFC8029]

6.2.4.  Downstream Detailed Mapping Address Type

   This document extends RFC 4379 by defining a new address type for use
   with the Downstream Mapping and Downstream Detailed Mapping TLVs.
   IANA has established a registry to assign address types for use with
   the Downstream Mapping and Downstream Detailed Mapping TLVs, which
   initially allocates the following assignments:

      Type #     Address Type      K Octets    Reference
      ------     ------------      --------    ---------
           1     IPv4 Numbered           16    [RFC8029]
           2     IPv4 Unnumbered         16    [RFC8029]
           3     IPv6 Numbered           40    [RFC8029]
           4     IPv6 Unnumbered         28    [RFC8029]
           5     Non IP                  12    [RFC6426]

             Downstream Detailed Mapping Address Type Registry

   Because the field in this case is an 8-bit field, the allocation
   policy for this registry is "Standards Action".










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6.2.5.  DS Flags

   This document defines the Downstream Mapping (DSMAP) TLV and the
   Downstream Detailed Mapping (DDMAP) TLV, which have Type 2 and Type
   20, respectively, assigned from the "TLVs" subregistry of the
   "Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs)
   Ping Parameters" registry.

   DSMAP has been deprecated by DDMAP, but both TLVs share a field: DS
   Flags.

   IANA has created and now maintains a registry entitled "DS Flags".

   The registration policy for this registry is Standards Action
   [RFC5226].

   IANA has made the following assignments:

    Bit Number Name                                         Reference
    ---------- -------------------------------------------  ---------
          7    N: Treat as a Non-IP Packet                  [RFC8029]
          6    I: Interface and Label Stack Object Request  [RFC8029]
          5    E: ELI/EL push indicator                     [RFC8012]
          4    L: Label-based load balance indicator        [RFC8012]
        3-0    Unassigned


























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6.2.6.  Multipath Types

   IANA has created and now maintains a registry entitled "Multipath
   Types".

   The registration policy [RFC5226] for this registry is Standards
   Action.

   IANA has made the following assignments:

    Value      Meaning                                  Reference
    ---------- ---------------------------------------- ---------
          0    no multipath                             [RFC8029]
          1    Unassigned
          2    IP address                               [RFC8029]
          3    Unassigned
          4    IP address range                         [RFC8029]
        5-7    Unassigned
          8    Bit-masked IP address set                [RFC8029]
          9    Bit-masked label set                     [RFC8029]
         10    IP and label set                         [RFC8012]
     11-250    Unassigned
    251-254    Reserved for Experimental Use            [RFC8029]
        255    Reserved                                 [RFC8029]

6.2.7.  Pad Type

   IANA has created and now maintains a registry entitled "Pad Types".

   The registration policy [RFC5226] for this registry is Standards
   Action.

   IANA has made the following initial assignments:

   Registry Name: Pad Types

    Value      Meaning                                  Reference
    ---------- ---------------------------------------- ---------
          0    Reserved                                 [RFC8029]
          1    Drop Pad TLV from reply                  [RFC8029]
          2    Copy Pad TLV to reply                    [RFC8029]
      3-250    Unassigned
    251-254    Experimental Use                         [RFC8029]
        255    Reserved                                 [RFC8029]







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6.2.8.  Interface and Label Stack Address Type

   IANA has created and now maintains a registry entitled "Interface and
   Label Stack Address Types".

   The registration policy [RFC5226] for this registry is Standards
   Action.

   IANA has made the following initial assignments:

   Registry Name: Interface and Label Stack Address Types

    Value      Meaning                                  Reference
    ---------- ---------------------------------------- ---------
          0    Reserved                                 [RFC8029]
          1    IPv4 Numbered                            [RFC8029]
          2    IPv4 Unnumbered                          [RFC8029]
          3    IPv6 Numbered                            [RFC8029]
          4    IPv6 Unnumbered                          [RFC8029]
      5-250    Unassigned
    251-254    Experimental Use                         [RFC8029]
        255    Reserved                                 [RFC8029]

6.3.  IPv4 Special-Purpose Address Registry

   IANA has updated the reference in Note 1 of the "IANA IPv4 Special-
   Purpose Address Registry" [IANA-SPECIAL-IPv4] to point to this
   document.

7.  References

7.1.  Normative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <http://www.rfc-editor.org/info/rfc1122>.

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <http://www.rfc-editor.org/info/rfc1812>.

   [RFC2113]  Katz, D., "IP Router Alert Option", RFC 2113,
              DOI 10.17487/RFC2113, February 1997,
              <http://www.rfc-editor.org/info/rfc2113>.






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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <http://www.rfc-editor.org/info/rfc3032>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <http://www.rfc-editor.org/info/rfc4271>.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              DOI 10.17487/RFC4379, February 2006,
              <http://www.rfc-editor.org/info/rfc4379>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.

   [RFC6424]  Bahadur, N., Kompella, K., and G. Swallow, "Mechanism for
              Performing Label Switched Path Ping (LSP Ping) over MPLS
              Tunnels", RFC 6424, DOI 10.17487/RFC6424, November 2011,
              <http://www.rfc-editor.org/info/rfc6424>.

   [RFC7506]  Raza, K., Akiya, N., and C. Pignataro, "IPv6 Router Alert
              Option for MPLS Operations, Administration, and
              Maintenance (OAM)", RFC 7506, DOI 10.17487/RFC7506, April
              2015, <http://www.rfc-editor.org/info/rfc7506>.

7.2.  Informative References

   [Err108]   RFC Errata, Erratum ID 108, RFC 4379.

   [Err742]   RFC Errata, Erratum ID 742, RFC 4379.

   [Err1418]  RFC Errata, Erratum ID 1418, RFC 4379.




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   [Err1714]  RFC Errata, Erratum ID 1714, RFC 4379.

   [Err1786]  RFC Errata, Erratum ID 1786, RFC 4379.

   [Err2978]  RFC Errata, Erratum ID 2978, RFC 4379.

   [Err3399]  RFC Errata, Erratum ID 3399, RFC 4379.

   [IANA-ENT] IANA, "PRIVATE ENTERPRISE NUMBERS",
              <http://www.iana.org/assignments/enterprise-numbers>.

   [IANA-MPLS-LSP-PING]
              IANA, "Multiprotocol Label Switching (MPLS) Label Switched
              Paths (LSPs) Ping Parameters",
              <http://www.iana.org/assignments/
              mpls-lsp-ping-parameters>.

   [IANA-SPECIAL-IPv4]
              IANA, "IANA IPv4 Special-Purpose Address Registry",
              <http://www.iana.org/assignments/
              iana-ipv4-special-registry>.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <http://www.rfc-editor.org/info/rfc792>.

   [RFC3107]  Rekhter, Y. and E. Rosen, "Carrying Label Information in
              BGP-4", RFC 3107, DOI 10.17487/RFC3107, May 2001,
              <http://www.rfc-editor.org/info/rfc3107>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <http://www.rfc-editor.org/info/rfc3209>.

   [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
              in Multi-Protocol Label Switching (MPLS) Networks",
              RFC 3443, DOI 10.17487/RFC3443, January 2003,
              <http://www.rfc-editor.org/info/rfc3443>.

   [RFC4026]  Andersson, L. and T. Madsen, "Provider Provisioned Virtual
              Private Network (VPN) Terminology", RFC 4026,
              DOI 10.17487/RFC4026, March 2005,
              <http://www.rfc-editor.org/info/rfc4026>.







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   [RFC4365]  Rosen, E., "Applicability Statement for BGP/MPLS IP
              Virtual Private Networks (VPNs)", RFC 4365,
              DOI 10.17487/RFC4365, February 2006,
              <http://www.rfc-editor.org/info/rfc4365>.

   [RFC4461]  Yasukawa, S., Ed., "Signaling Requirements for Point-to-
              Multipoint Traffic-Engineered MPLS Label Switched Paths
              (LSPs)", RFC 4461, DOI 10.17487/RFC4461, April 2006,
              <http://www.rfc-editor.org/info/rfc4461>.

   [RFC4761]  Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
              <http://www.rfc-editor.org/info/rfc4761>.

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <http://www.rfc-editor.org/info/rfc5036>.

   [RFC5085]  Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual
              Circuit Connectivity Verification (VCCV): A Control
              Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085,
              December 2007, <http://www.rfc-editor.org/info/rfc5085>.

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, DOI 10.17487/RFC5331, August 2008,
              <http://www.rfc-editor.org/info/rfc5331>.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
              2009, <http://www.rfc-editor.org/info/rfc5462>.

   [RFC5885]  Nadeau, T., Ed. and C. Pignataro, Ed., "Bidirectional
              Forwarding Detection (BFD) for the Pseudowire Virtual
              Circuit Connectivity Verification (VCCV)", RFC 5885,
              DOI 10.17487/RFC5885, June 2010,
              <http://www.rfc-editor.org/info/rfc5885>.

   [RFC6425]  Saxena, S., Ed., Swallow, G., Ali, Z., Farrel, A.,
              Yasukawa, S., and T. Nadeau, "Detecting Data-Plane
              Failures in Point-to-Multipoint MPLS - Extensions to LSP
              Ping", RFC 6425, DOI 10.17487/RFC6425, November 2011,
              <http://www.rfc-editor.org/info/rfc6425>.






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   [RFC6426]  Gray, E., Bahadur, N., Boutros, S., and R. Aggarwal, "MPLS
              On-Demand Connectivity Verification and Route Tracing",
              RFC 6426, DOI 10.17487/RFC6426, November 2011,
              <http://www.rfc-editor.org/info/rfc6426>.

   [RFC6829]  Chen, M., Pan, P., Pignataro, C., and R. Asati, "Label
              Switched Path (LSP) Ping for Pseudowire Forwarding
              Equivalence Classes (FECs) Advertised over IPv6",
              RFC 6829, DOI 10.17487/RFC6829, January 2013,
              <http://www.rfc-editor.org/info/rfc6829>.

   [RFC7537]  Decraene, B., Akiya, N., Pignataro, C., Andersson, L., and
              S. Aldrin, "IANA Registries for LSP Ping Code Points",
              RFC 7537, DOI 10.17487/RFC7537, May 2015,
              <http://www.rfc-editor.org/info/rfc7537>.

   [RFC8012]  Akiya, N., Swallow, G., Pignataro, C., Malis, A., and S.
              Aldrin, "Label Switched Path (LSP) and Pseudowire (PW)
              Ping/Trace over MPLS Networks Using Entropy Labels (ELs)",
              RFC 8012, DOI 10.17487/RFC8012, November 2016,
              <http://www.rfc-editor.org/info/rfc8012>.

   [RFC8077]  Martini, L., Ed., and G. Heron, Ed., "Pseudowire Setup and
              Maintenance Using the Label Distribution Protocol (LDP)",
              STD 84, RFC 8077, DOI 10.17487/RFC8077, February 2017,
              <http://www.rfc-editor.org/info/rfc8077>.

























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Appendix A.  Deprecated TLVs and Sub-TLVs (Non-normative)

   This appendix describes deprecated elements, which are non-normative
   for an implementation.  They are included in this document for
   historical and informational purposes.

A.1.  Target FEC Stack

A.1.1.  FEC 128 Pseudowire - IPv4 (Deprecated)

   FEC 128 (0x80) is defined in [RFC4447], as are the terms PW ID
   (Pseudowire ID) and PW Type (Pseudowire Type).  A PW ID is a non-zero
   32-bit connection ID.  The PW Type is a 15-bit number indicating the
   encapsulation type.  It is carried right justified in the field below
   termed encapsulation type with the high-order bit set to zero.  Both
   of these fields are treated in this protocol as opaque values.

   When a FEC 128 is encoded in a label stack, the following format is
   used.  The Value field consists of the Remote PE IPv4 Address (the
   destination address of the targeted LDP session), the PW ID, and the
   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 IPv4 Address                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             PW ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            PW Type            |          Must Be Zero         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This FEC is deprecated and is retained only for backward
   compatibility.  Implementations of LSP ping SHOULD accept and process
   this TLV, but SHOULD send LSP ping echo requests with the new TLV
   (see Section 3.2.9), 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.

A.2.  Downstream Mapping (Deprecated)

   The Downstream Mapping object is a TLV that 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.



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   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
   Section 3.4.2, "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
   Downstream Mapping has the following format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               MTU             | Address Type  |    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.













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   Address Type

      The Address Type indicates if the interface is numbered or
      unnumbered.  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
            5        Non IP                       12

   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 that 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.







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   Downstream IP Address and Downstream Interface Address

      IPv4 addresses 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
      Downstream 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 validation, but return Downstream
      Mapping TLVs using the information provided.














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   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 IP         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
      maximum 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 Section 3.4.1.1.1 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 is bit 19, the TC bits are bits 20-22, and bit 23 is the S
      bit.  The replying router SHOULD fill in the TC and S bits; the
      LSR receiving the echo reply MAY choose to ignore these bits.





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   Protocol

      The protocol is taken from the following table:

      Protocol #        Signaling Protocol
      ----------        ------------------
               0        Unknown
               1        Static
               2        BGP
               3        LDP
               4        RSVP-TE

Acknowledgements

   The original acknowledgements from RFC 4379 state the following:

      This document is the outcome of many discussions among many
      people, including 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
      Downstream Mapping TLV was adapted from text suggested by Curtis
      Villamizar.

   We would like to thank Loa Andersson for motivating the advancement
   of this specification.

   We also would like to thank Alexander Vainshtein, Yimin Shen, Curtis
   Villamizar, David Allan, Vincent Roca, Mirja Kuhlewind, and Elwyn
   Davies for their review and useful comments.

Contributors

   A mechanism used to detect data-plane failures in MPLS LSPs was
   originally published as RFC 4379 in February 2006.  It was produced
   by the MPLS Working Group of the IETF and was jointly authored by
   Kireeti Kompella and George Swallow.

   The following made vital contributions to all aspects of the original
   RFC 4379, 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.



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Authors' Addresses

   Kireeti Kompella
   Juniper Networks, Inc.

   Email: kireeti.kompella@gmail.com


   George Swallow
   Cisco Systems, Inc.

   Email: swallow.ietf@gmail.com


   Carlos Pignataro (editor)
   Cisco Systems, Inc.

   Email: cpignata@cisco.com


   Nagendra Kumar
   Cisco Systems, Inc.

   Email: naikumar@cisco.com


   Sam Aldrin
   Google

   Email: aldrin.ietf@gmail.com


   Mach(Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com















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