MPLS Working Group

Internet Engineering Task Force (IETF)                         G. Mirsky
Internet-Draft
Request for Comments: 9612                                      Ericsson
Intended status:
Category: Experimental                                       J. Tantsura
Expires: 14 November 2024
ISSN: 2070-1721                                                   NVIDIA
                                                           I. Varlashkin
                                                                  Google
                                                                 M. Chen
                                                                  Huawei
                                                             13 May
                                                               July 2024

  Bidirectional Forwarding Detection (BFD) Directed Return Reverse Path for MPLS Label
                         Switched Paths (LSPs)
                    draft-ietf-mpls-bfd-directed-31

Abstract

   Bidirectional Forwarding Detection (BFD) is expected to be able to
   monitor a wide variety of encapsulations of paths between systems.
   When a BFD session monitors an explicitly routed unidirectional path path,
   there may be a need to direct the egress BFD peer to use a specific
   path for the reverse direction of the BFD session.  This document
   describes an extension to the MPLS Label Switched Path (LSP) echo
   request that allows a BFD system to request that the remote BFD peer
   transmits
   transmit BFD control packets over the specified LSP.

Status of This Memo

   This Internet-Draft document is submitted in full conformance with the
   provisions of BCP 78 not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and BCP 79.

   Internet-Drafts are working documents
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list  It represents the consensus of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents valid approved by the IESG are candidates for a maximum any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of six months this document, any errata,
   and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 14 November 2024.
   https://www.rfc-editor.org/info/rfc9612.

Copyright Notice

   Copyright (c) 2024 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 (https://trustee.ietf.org/
   license-info)
   (https://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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions used Used in this This document . . . . . . . . . . . .   3
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   3
       1.1.2.  Requirements Language . . . . . . . . . . . . . . . .   3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Control of the Reverse BFD Reverse Path . . . . . . . . . . . . . . .   4
     3.1.  BFD Reverse Path TLV  . . . . . . . . . . . . . . . . . .   4
     3.2.  Return Codes  . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Failure Characterization  . . . . . . . . . . . . . . . .   6
   4.  Use Case Scenario . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  BFD Reverse Path TLV  . . . . . . . . . . . . . . . . . .   8
     6.2.  Return Code . . . . . . . . . . . . . . . . . . . . . . .   8 Codes
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   10. Informative References  . . . . . . . . . . . . . . . . . . .  11
   Appendix A.
   Acknowledgments  . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   [RFC5880], [RFC5881], and [RFC5883] established the Bidirectional
   Forwarding Detection (BFD) protocol for IP networks.  [RFC5884] and
   [RFC7726] set rules for using BFD Asynchronous mode over MPLS Label
   Switched Paths (LSPs), while not defining means to control the path
   that an egress BFD system uses to send BFD control packets towards
   the ingress BFD system.

   When BFD is used to detect defects of the traffic-engineered LSP, the
   path of the BFD control packets transmitted by the egress BFD system
   toward the ingress may be disjoint from the monitored LSP in the
   forward direction.  The fact that BFD control packets are not
   guaranteed to follow the same links and nodes in both forward and
   reverse directions may be one of the factors contributing to
   producing false
   positive defect notifications, i.e., notifications (i.e., false alarms, alarms) at the ingress BFD
   peer.  Ensuring that both directions of the BFD session use co-routed
   paths may, in some environments, improve the determinism of the
   failure detection and localization.

   This document defines the BFD Reverse Path TLV as an extension to LSP
   Ping
   ping [RFC8029] and proposes that it is to be used to instruct the egress
   BFD system to use an explicit path for its BFD control packets
   associated with a particular BFD session.  The  IANA has registered this
   TLV will be allocated
   from in the TLV and sub-TLV "TLVs" registry defined in [RFC8029]. by [RFC8029] (see Section 6.1).
   As a special case, forward and reverse directions of the BFD session
   can form a
   bi-directional bidirectional co-routed associated channel.

   The LSP ping extension, extension described in this document, document was developed and
   implemented resulting from the as a result of an operational experiment.  The lessons
   learned from the operational experiment enabled the use of this
   extension between systems conforming to this specification.  More implementations are  Further
   implementation is encouraged to understand better understand the operational
   impact of the mechanism described in the document.

1.1.  Conventions used Used in this This document

1.1.1.  Terminology

   BFD:   Bidirectional Forwarding Detection

   FEC:   Forwarding Equivalency Equivalence Class

   LSP:   Label Switched Path

   LSR: Label-Switching router   Label Switching Router

1.1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Problem Statement

   When BFD is used to monitor an explicitly routed unidirectional path,
   e.g., path
   (e.g., MPLS-TE LSP, LSP), BFD control packets in the forward direction
   would be in-band using the mechanism defined in [RFC5884].  However,
   the reverse direction of the BFD session would follow the shortest
   path route, which could be completely or partially disjoint from the
   forward path.  This creates the potential for the failure of a
   disjoint resource on the reverse path to trigger a BFD failure
   detection, even though the forward path is unaffected.

   If the reverse path is congruent with the forward path, the potential
   for such false positives is greatly reduced.  For this purpose, this
   specification provides a means for the egress BFD peer to be
   instructed to use a specific path for BFD control packets.

3.  Control of the Reverse BFD Reverse Path

   To bootstrap a BFD session over an MPLS LSP, LSP ping, defined in
   [RFC8029], ping [RFC8029] MUST
   be used with the BFD Discriminator TLV [RFC5884].  This document
   defines a new TLV, the BFD Reverse Path TLV, that MAY contain
   none, one or more sub-TLVs that can be used to
   carry information about the reverse path for the BFD session that is
   specified by the value in the BFD Discriminator TLV.  The BFD Reverse
   Path TLV MAY contain zero or more sub-TLVs.

3.1.  BFD Reverse Path TLV

   The BFD Reverse Path TLV is an optional TLV within the LSP ping
   [RFC8029].  However, if used, the BFD Discriminator TLV MUST be
   included in an Echo Request echo request message as well.  If the BFD
   Discriminator TLV is not present when the BFD Reverse Path TLV is
   included;
   included, then it MUST be treated as a malformed Echo Request, echo request, as
   described in [RFC8029].

   The BFD Reverse Path TLV carries information about the path onto
   which the egress BFD peer of the BFD session referenced by the BFD
   Discriminator TLV MUST transmit BFD control packets.  The format of
   the BFD Reverse Path TLV is as presented in Figure 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   BFD Reverse Path TLV Type   |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Reverse Path                         |
    ~                                                               ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1: BFD Reverse Path TLV

   BFD Reverse Path TLV Type is two octets in length and Type:
      This two-octet field has a value of
   TBD1 (to be assigned by IANA as requested in 16384 (see Section 6).

   Length

   Length:
      This two-octet field is two octets long and defines the length in octets of the Reverse
      Path field.

   Reverse Path Path:
      This field contains none, one, zero or more sub-TLVs.  Only non-
   multicast non-multicast
      Target FEC Stack sub-TLVs (already defined, defined or to be defined in the
      future) for TLV Types 1, 16, and 21 of MPLS LSP in the "Multiprotocol Label
      Switching (MPLS) Label Switched Paths (LSPs) Ping
   Parameters Parameters"
      registry are permitted to be used in this field.  Any
   other sub-TLV  Other sub-TLVs
      MUST NOT be used.  (This implies that Multicast multicast Target FEC Stack
      sub-TLVs, i.e., p2mp e.g., the Multicast P2MP LDP FEC Stack sub-TLV and mp2mp, the
      Multicast MP2MP LDP FEC Stack sub-TLV, are not permitted in the
      Reverse Path field.)

   If the egress Label-Switching Router (LSR) LSR finds a multicast Target FEC Stack sub-TLV, it MUST
   send an echo reply with the received BFD Reverse Path TLV, TLV and BFD
   Discriminator TLV and set the Return Code to "Inappropriate 192 ("Inappropriate
   Target FEC Stack sub-TLV present"
   (Section present") (see Section 3.2).  The BFD
   Reverse Path TLV includes none, one zero or more sub-TLVs.  However, the number
   of sub-TLVs in the Reverse Path field MUST be limited.  The default
   limit is 128 sub-TLV entries, but an implementation MAY be able to
   control that limit.  An empty BFD Reverse Path TLV, i.e., TLV (i.e., a BFD
   Reverse Path TLV with no sub-TLVs present, sub-TLVs) is used as withdrawal of to withdraw any previously
   set reverse path for the BFD session identified in the BFD
   Discriminator TLV.  If no sub-TLVs are found in the BFD Reverse Path
   TLV, the egress BFD peer MUST revert to using the local policy-
   based decision based on
   local policy, i.e., routing over an IP network, as described in
   Section 7 of [RFC5884], i.e., routed
   over IP network. [RFC5884].

   If the egress peer LSR cannot find the path specified in the BFD
   Reverse Path TLV, it MUST send Echo Reply an echo reply with the received BFD
   Discriminator
   TLV, TLV and BFD Reverse Path TLV, TLV and set the Return Code to "Failed
   193 ("Failed to establish the BFD session.  The specified reverse
   path was not found"
   (Section found.") (see Section 3.2).  If an implementation
   provides additional configuration options, these can control actions
   at the egress BFD peer, including when the path specified in the BFD
   Reverse Path TLV cannot be found.  For example, optionally, if the egress peer
   LSR cannot find the path specified in the BFD Reverse Path TLV, it
   MAY establish the BFD session over an IP network, as defined in
   [RFC5884].  Note that the return code Return Code required by the MUST "MUST" clause
   in this paragraph does not preclude the session from being
   established over a different path as discussed in the MAY "MAY" clause.

   The BFD Reverse Path TLV MAY be used in the bootstrapping process of a bootstrapping
   the BFD session process as described in Section 6 of [RFC5884].  A system
   that supports this specification MUST support using the BFD Reverse
   Path TLV after the BFD session has been established.  If a system
   that supports this specification receives an LSP Ping ping with the BFD
   Discriminator TLV and no BFD Reverse Path TLV even though the reverse
   path for the specified BFD session has been was established according to the
   previously received BFD Reverse Path TLV, the egress BFD peer MUST
   transition to transmitting periodic BFD Control messages as
   defined described
   in Section 7 of [RFC5884].  If a BFD system that received an LSP Ping ping
   with the BFD Reverse Path TLV does not support this specification, it
   will "result result in an echo response with the Return Code of set to 2 ("One
   or more of the TLVs was not understood") being sent understood"), as described in the echo response"
   (Section Section 3
   of [RFC8029]). [RFC8029].

3.2.  Return Codes

   This document defines the following Return Codes for the MPLS LSP Echo
   Reply:

   *
   echo reply:

   "Inappropriate Target FEC Stack sub-TLV present" (TBD3). (192):
      When a multicast Target FEC Stack sub-TLV is found in the received Echo
      Request,
      echo request, the egress BFD peer sends an Echo Reply echo reply with the return
      code
      Return Code set to "Inappropriate 192 ("Inappropriate Target FEC Stack sub-TLV present"
      present") to the ingress BFD peer peer, as described in Section 3.1.

   *

   "Failed to establish the BFD session.  The specified reverse path
   was not found" (TBD4). found." (193):
      When a specified reverse path is unavailable, the egress BFD peer
      sends an Echo Reply echo reply with the
      return code Return Code set to "Failed 193 ("Failed to
      establish the BFD session.  The specified reverse path was not found"
      found.") to the ingress BFD peer peer, as described in Section 3.1.

3.3.  Failure Characterization

   A failure detected by a BFD session that uses the BFD Reverse Path
   TLV could be due to a change in the FEC used in the BFD Reverse Path
   TLV.  The ingress BFD peer, upon  Upon detection of the network failure, the ingress BFD peer
   MUST transmit the LSP Ping Echo ping echo request with Return the Reply Path TLV
   [RFC7110] to verify whether the FEC is still valid.  If the failure
   was caused by
   the a change in the FEC used for the reverse direction of
   the BFD session, the ingress BFD peer MUST re-direct redirect the reverse path
   of the BFD session using another FEC in the BFD Reverse Path TLV, TLV and
   notify an operator.

4.  Use Case Scenario

   In the network presented in Figure 2, ingress LSR peer A monitors two
   tunnels to the egress LSR peer H: A-B-C-D-G-H and A-B-E-F-G-H.  To
   bootstrap a BFD session to monitor the first tunnel, the ingress LSR peer
   A includes a BFD Discriminator TLV with a Discriminator value (e.g., foobar-1).  Peer
   foobar-1) [RFC7726].  Ingress LSR peer A includes a BFD Reverse Path
   TLV referencing the H-G-D-C-B-A tunnel to control the path from the
   egress LSR.  To bootstrap a BFD session to monitor the second tunnel,
   ingress LSR peer A, A includes a BFD Discriminator TLV with a different
   Discriminator value (e.g., foobar-2) [RFC7726] and a BFD Reverse Path TLV that
   references the H-G-F-E-B-A tunnel.

           C---------D
           |         |
   A-------B         G-----H
           |         |
           E---------F

                Figure 2: Use Case for BFD Reverse Path TLV

   If an operator needs egress LSR peer H to monitor a path to the ingress
   LSR peer A, e.g., the H-G-D-C-B-A tunnel, then by looking up the list
   of known Reverse Paths, reverse paths, it MAY find and use the existing BFD session.

5.  Operational Considerations

   When an explicit path is set either as either Static or RSVP-TE LSP,
   corresponding sub-TLVs, defined sub-TLVs (defined in [RFC7110], [RFC7110]) MAY be used to identify
   the explicit reverse path for the BFD session.  If a particular set
   of sub-TLVs composes the Return Reply Path TLV [RFC7110] and does not
   increase the length of the Maximum Transmission Unit for the given
   LSP, that set can be safely used in the BFD Reverse Path TLV.  If any
   of the sub-TLVs defined in [RFC7110] sub-TLVs are used in the BFD Reverse Path
   TLV, then the periodic verification of the control plane against the
   data plane, as recommended in Section 4 of [RFC5884], MUST use the Return
   Reply Path TLV, as per [RFC7110], with that sub-TLV.  By using the
   LSP Ping ping with Return the Reply Path TLV, an operator monitors whether at the egress BFD
   node the
   reverse LSP is mapped to the same FEC as the BFD session. session at the
   egress BFD node.  Selection and control of the rate of the LSP Ping ping
   with Return the Reply Path TLV follows the recommendation of in [RFC5884]: "The

   |  The rate of generation of these LSP Ping Echo request messages
   |  SHOULD be significantly less than the rate of generation of the
   |  BFD Control packets.  An implementation MAY provide configuration
   |  options to control the rate of generation of the periodic LSP Ping
   |  Echo request messages." messages.

   Suppose an operator planned a network maintenance activity that
   possibly affects the FEC used in the BFD Reverse Path TLV.  In that
   case, the operator can avoid the unnecessary disruption by using the LSP
   Ping
   ping with a new FEC in the BFD Reverse Path TLV.  But in some
   scenarios, proactive measures cannot be taken.  Because taken because the frequency
   of LSP Ping ping messages will be is lower than the defect detection time provided
   by the BFD session.  As a result, a change in the reverse-
   path reverse-path FEC
   will first be detected as the BFD session's failure.  An operator
   will be notified as described in Section 3.3.

6.  IANA Considerations

6.1.  BFD Reverse Path TLV

   The

   IANA is requested to assign a new has assigned the following value for the BFD Reverse Path TLV
   from the 16384-31739 range in the "TLVs" registry of subregistry within the
   "Multiprotocol Label Switching Architecture (MPLS) Label Switched Paths (LSPs)
   Ping Parameters" registry.

   +=======+=======+=============+====================================+
   |Type   |TLV    |Reference

   +=======+=========+===========+====================================+
   | Type  | TLV     | Reference | Sub-TLV Registry                   |
   |       |Name       | Name    |           |                                    |
   +=======+=========+===========+====================================+
   | 16384 |
   +=======+=======+=============+====================================+ BFD     | RFC 9612  | (TBD1)|BFD    |This document| Only non-multicast sub-TLV sub-TLVs        |
   |       |       |Reverse| Reverse |           | (already defined or to be defined  |
   |       |Path       | Path    |           | in the future) at                  |
   |       |TLV    |             | [https://www.iana.org/assignments/ |
   |       |       |             | mpls-lsp-ping-parameters/mpls-lsp- in the "Sub-TLVs    |
   |       |         |           | ping-parameters.xml#sub-tlv- for TLV Types 1, 16, and 21"       |
   |       |         |           | 1-16-21] registry at                        |
   |       |         |           | (https://www.iana.org/assignments/ <https://www.iana.org/assignments/ |
   |       |         |           | mpls-lsp-ping-parameters/mpls-lsp- |
   |       |         |           | ping-parameters.xml#sub-tlv-       |
   |       |         |           | 1-16-21) 1-16-21> are permitted to be used  |
   |       |         |           | in this field.  Any other sub-TLV  Other sub-TLVs     |
   |       |         |           | MUST NOT be used.                  |
   +-------+-------+-------------+------------------------------------+
   +-------+---------+-----------+------------------------------------+

                    Table 1: New BFD Reverse Type Path TLV

6.2.  Return Code

   The Codes

   IANA is requested to assign new has assigned the following Return Code values from the range 192-247 of
   range in the "Return Codes" registry of subregistry within the "Multi-Protocol "Multiprotocol
   Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters", as in
   Table 2.

         +=========+=============================+===============+ Parameters"
   registry.

     +=======+===========================================+===========+
     | Value | Description Meaning                                   | Reference |
         +=========+=============================+===============+
     +=======+===========================================+===========+
     |  (TBD3) 192   | Inappropriate Target FEC Stack sub-TLV    | This document RFC 9612  |
     |       | Stack sub-TLV present. present                                   |           |
         +---------+-----------------------------+---------------+
     +-------+-------------------------------------------+-----------+
     |  (TBD4) 193   | Failed to establish the BFD | This document |
         |         | session.  The specified | RFC 9612  |
     |       | specified reverse path was not found.     |           |
         +---------+-----------------------------+---------------+
     +-------+-------------------------------------------+-----------+

                         Table 2: New Return Code Codes

7.  Implementation Status

   Note to RFC Editor: This section MUST be removed before publication
   of the document.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   - The organization responsible for the implementation: ZTE
   Corporation.

   - The implementation's name ROSng empowers commonly used routers,
   e.g., ZXCTN 6000.

   - A brief general description: A Return Path can be specified for a
   BFD session over RSVP tunnel or LSP.  The same can be specified for a
   backup RSVP tunnel/LSP.

   The implementation's level of maturity: production.

   - Coverage: RSVP LSP (no support for Static LSP)

   - Version compatibility: draft-ietf-mpls-bfd-directed-10.

   - Licensing: proprietary.

   - Implementation experience: simple once you support RFC 7110.

   - Contact information: Qian Xin qian.xin2@zte.com.cn

   - The date when information about this particular implementation was
   last updated: 12/16/2019

8.  Security Considerations

   Security considerations discussed in [RFC5880], [RFC5884], [RFC7726],
   [RFC8029], and [RFC7110] apply to this document.

   The BFD Reverse Path TLV may be exploited as an attack vector by
   inflating the number of included sub-TLVs.  The number of sub-TLVs
   MUST be limited to mitigate that threat.  The default limit for the
   number of sub-TLVs is set in Section 3.1 to 128. 128 (see Section 3.1).  An
   implementation MAY use a mechanism to control that limit.

9.

8.  Normative References

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

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              DOI 10.17487/RFC5881, June 2010,
              <https://www.rfc-editor.org/info/rfc5881>.

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
              June 2010, <https://www.rfc-editor.org/info/rfc5883>.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
              June 2010, <https://www.rfc-editor.org/info/rfc5884>.

   [RFC7110]  Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord,
              "Return Path Specified Label Switched Path (LSP) Ping",
              RFC 7110, DOI 10.17487/RFC7110, January 2014,
              <https://www.rfc-editor.org/info/rfc7110>.

   [RFC7726]  Govindan, V., Rajaraman, K., Mirsky, G., Akiya, N., and S.
              Aldrin, "Clarifying Procedures for Establishing BFD
              Sessions for MPLS Label Switched Paths (LSPs)", RFC 7726,
              DOI 10.17487/RFC7726, January 2016,
              <https://www.rfc-editor.org/info/rfc7726>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.  Informative References

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

Appendix A.

Acknowledgments

   The authors greatly appreciate a the thorough review reviews and the most helpful
   comments from Eric Gray and Carlos Pignataro.  The authors much
   appreciate the help of Qian Xin, who provided information about the
   implementation of this specification.

Authors' Addresses

   Greg Mirsky
   Ericsson
   Email: gregimirsky@gmail.com

   Jeff  Tantsura
   NVIDIA
   Email: jefftant.ietf@gmail.com

   Ilya Varlashkin
   Google
   Email: imv@google.com

   Mach(Guoyi) Chen
   Huawei
   Email: mach.chen@huawei.com