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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.

Kompella, et al. Standards Track [Page 1] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 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
 material may not have granted the IETF Trust the right to allow
 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.

Kompella, et al. Standards Track [Page 2] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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

Kompella, et al. Standards Track [Page 3] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

   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

Kompella, et al. Standards Track [Page 4] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 5] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 6] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 7] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 8] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 9] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 10] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.)

Kompella, et al. Standards Track [Page 11] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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

Kompella, et al. Standards Track [Page 12] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 13] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kompella, et al. Standards Track [Page 14] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 15] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 16] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 17] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 18] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kompella, et al. Standards Track [Page 19] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kompella, et al. Standards Track [Page 20] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 21] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 22] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 23] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kompella, et al. Standards Track [Page 24] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kompella, et al. Standards Track [Page 25] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 26] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 27] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kompella, et al. Standards Track [Page 28] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 29] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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                         .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kompella, et al. Standards Track [Page 30] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 32] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 33] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 34] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kompella, et al. Standards Track [Page 35] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 36] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 37] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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                               .
 .                                                               .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kompella, et al. Standards Track [Page 38] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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

Kompella, et al. Standards Track [Page 39] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

    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/

Kompella, et al. Standards Track [Page 40] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 41] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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

Kompella, et al. Standards Track [Page 42] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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

Kompella, et al. Standards Track [Page 43] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 44] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 45] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 46] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 47] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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 */

Kompella, et al. Standards Track [Page 48] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

    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 {

Kompella, et al. Standards Track [Page 49] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

             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.
          }
       }

Kompella, et al. Standards Track [Page 50] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

       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.

Kompella, et al. Standards Track [Page 54] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 55] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 56] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

     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.

Kompella, et al. Standards Track [Page 57] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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

Kompella, et al. Standards Track [Page 58] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 59] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 60] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 61] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 62] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 63] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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".

Kompella, et al. Standards Track [Page 64] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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

Kompella, et al. Standards Track [Page 65] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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]

Kompella, et al. Standards Track [Page 66] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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>.

Kompella, et al. Standards Track [Page 67] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 [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.

Kompella, et al. Standards Track [Page 68] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 [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>.

Kompella, et al. Standards Track [Page 69] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 [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>.

Kompella, et al. Standards Track [Page 70] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 [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>.

Kompella, et al. Standards Track [Page 71] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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.

Kompella, et al. Standards Track [Page 72] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 73] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 74] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 75] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 76] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

 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.

Kompella, et al. Standards Track [Page 77] RFC 8029 Detecting MPLS Data-Plane Failures March 2017

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

Kompella, et al. Standards Track [Page 78]

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