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

Document Type Active Internet-Draft (mpls WG)
Authors Greg Mirsky , Jeff Tantsura , Ilya Varlashkin , Mach Chen
Last updated 2024-05-17 (Latest revision 2024-05-13)
Replaces draft-mirsky-mpls-bfd-directed
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draft-ietf-mpls-bfd-directed-31
MPLS Working Group                                             G. Mirsky
Internet-Draft                                                  Ericsson
Intended status: Experimental                                J. Tantsura
Expires: 14 November 2024                                         NVIDIA
                                                           I. Varlashkin
                                                                  Google
                                                                 M. Chen
                                                                  Huawei
                                                             13 May 2024

 Bidirectional Forwarding Detection (BFD) Directed Return 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
   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 BFD control packets over the specified LSP.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents 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.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (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 in this document . . . . . . . . . . . .   3
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   3
       1.1.2.  Requirements Language . . . . . . . . . . . . . . . .   3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Control of the Reverse BFD 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
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  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
   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., false alarms, at

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   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 [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 TLV will be allocated
   from the TLV and sub-TLV registry defined in [RFC8029].  As a special
   case, forward and reverse directions of the BFD session can form a
   bi-directional co-routed associated channel.

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

1.1.  Conventions used in this document

1.1.1.  Terminology

   BFD: Bidirectional Forwarding Detection

   FEC: Forwarding Equivalency Class

   LSP: Label Switched Path

   LSR: 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., MPLS-TE 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

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   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 Path

   To bootstrap a BFD session over an MPLS LSP, LSP ping, defined in
   [RFC8029], MUST be used with BFD Discriminator TLV [RFC5884].  This
   document defines a new TLV, 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 BFD Discriminator TLV.

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 message as well.  If the BFD
   Discriminator TLV is not present when the BFD Reverse Path TLV is
   included; then it MUST be treated as malformed 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 has a value of
   TBD1 (to be assigned by IANA as requested in Section 6).

   Length field is two octets long and defines the length in octets of
   the Reverse Path field.

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   Reverse Path field contains none, one, or more sub-TLVs.  Only non-
   multicast Target FEC Stack sub-TLVs (already defined, or to be
   defined in the future) for TLV Types 1, 16, and 21 of MPLS LSP Ping
   Parameters registry are permitted to be used in this field.  Any
   other sub-TLV MUST NOT be used.  (This implies that Multicast Target
   FEC Stack sub-TLVs, i.e., p2mp and mp2mp, are not permitted in the
   Reverse Path field.)  If the egress Label-Switching Router (LSR)
   finds multicast Target Stack sub-TLV, it MUST send echo reply with
   the received Reverse Path TLV, BFD Discriminator TLV and set the
   Return Code to "Inappropriate Target FEC Stack sub-TLV present"
   (Section 3.2).  The BFD Reverse Path TLV includes none, one 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., no sub-TLVs present, is used as withdrawal of
   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 as described in Section 7 of [RFC5884], i.e., routed
   over IP network.

   If the egress peer LSR cannot find the path specified in the Reverse
   Path TLV, it MUST send Echo Reply with the received BFD Discriminator
   TLV, Reverse Path TLV, and set the Return Code to "Failed to
   establish the BFD session.  The specified reverse path was not found"
   (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 Reverse Path TLV
   cannot be found.  For example, optionally, if the egress peer LSR
   cannot find the path specified in the Reverse Path TLV, it MAY
   establish the BFD session over an IP network, as defined in
   [RFC5884].  Note that the return code required by the MUST clause
   does not preclude the session from being established over a different
   path as discussed in the MAY clause.

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

3.2.  Return Codes

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

   *  "Inappropriate Target FEC Stack sub-TLV present" (TBD3).  When
      multicast Target FEC Stack sub-TLV found in the received Echo
      Request, the egress BFD peer sends an Echo Reply with the return
      code set to "Inappropriate Target FEC Stack sub-TLV present" to
      the ingress BFD peer Section 3.1.

   *  "Failed to establish the BFD session.  The specified reverse path
      was not found" (TBD4).  When a specified reverse path is
      unavailable, the egress BFD peer sends an Echo Reply with the
      return code set to "Failed to establish the BFD session.  The
      specified reverse path was not found" to the ingress BFD peer
      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 detection of the network failure,
   MUST transmit the LSP Ping Echo request with Return Path TLV to
   verify whether the FEC is still valid.  If the failure was caused by
   the change in the FEC used for the reverse direction of the BFD
   session, the ingress BFD peer MUST re-direct the reverse path of the
   BFD session using another FEC in BFD Reverse Path TLV, and notify an
   operator.

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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 Discriminator value
   (e.g., foobar-1).  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, 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., H-G-D-C-B-A tunnel, then by looking up the
   list of known Reverse Paths, it MAY find and use the existing BFD
   session.

5.  Operational Considerations

   When an explicit path is set either as Static or RSVP-TE LSP,
   corresponding sub-TLVs, defined in [RFC7110], MAY be used to identify
   the explicit reverse path for the BFD session.  If a particular set
   of sub-TLVs composes the Return 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 defined in [RFC7110] sub-TLVs used in 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
   Path TLV, as per [RFC7110], with that sub-TLV.  By using the LSP Ping
   with Return Path TLV, an operator monitors whether at the egress BFD
   node the reverse LSP is mapped to the same FEC as the BFD session.
   Selection and control of the rate of LSP Ping with Return Path TLV
   follows the recommendation of [RFC5884]: "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."

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   Suppose an operator planned network maintenance activity that
   possibly affects FEC used in the BFD Reverse Path TLV.  In that case,
   the operator can avoid the unnecessary disruption by using the LSP
   Ping with a new FEC in the BFD Reverse Path TLV.  But in some
   scenarios, proactive measures cannot be taken.  Because the frequency
   of LSP Ping messages will be lower than the defect detection time
   provided by the BFD session.  As a result, a change in the 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 value for BFD Reverse Path TLV
   from the 16384-31739 range in the "TLVs" registry of "Multiprotocol
   Label Switching Architecture (MPLS) Label Switched Paths (LSPs) Ping
   Parameters" registry.

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

                    Table 1: New BFD Reverse Type TLV

6.2.  Return Code

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

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

                          Table 2: New Return Code

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.

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   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.

   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.  An implementation
   MAY use a mechanism to control that limit.

9.  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>.

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   [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 thorough review 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

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Internet-Draft   BFD Directed Return Path for MPLS LSPs         May 2024

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

   Ilya Varlashkin
   Google
   Email: imv@google.com

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

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