



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


  Bidirectional Forwarding Detection (BFD) Reverse Path for MPLS Label
                         Switched Paths (LSPs)

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
   transmit BFD control packets over the specified LSP.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see 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
   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.

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   in the Revised BSD License.

Table of Contents

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

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 false
   positive defect notifications (i.e., false 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 [RFC8029] and proposes that it be used to instruct the egress
   BFD system to use an explicit path for its BFD control packets
   associated with a particular BFD session.  IANA has registered this
   TLV in the "TLVs" registry defined by [RFC8029] (see Section 6.1).
   As a special case, forward and reverse directions of the BFD session
   can form a bidirectional co-routed associated channel.

   The LSP ping extension described in this document was developed and
   implemented 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.  Further
   implementation is encouraged to better understand 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 Equivalence 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
   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 BFD Reverse Path

   To bootstrap a BFD session over an MPLS LSP, LSP ping [RFC8029] MUST
   be used with the BFD Discriminator TLV [RFC5884].  This document
   defines a new TLV, the BFD Reverse Path TLV, 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 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 a 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 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:
      This two-octet field has a value of 16384 (see Section 6).

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

   Reverse Path:
      This field contains zero 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 in the "Multiprotocol Label
      Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
      registry are permitted to be used in this field.  Other sub-TLVs
      MUST NOT be used.  (This implies that multicast Target FEC Stack
      sub-TLVs, e.g., the Multicast P2MP LDP FEC Stack sub-TLV and the
      Multicast MP2MP LDP FEC Stack sub-TLV, are not permitted in the
      Reverse Path field.)

   If the egress LSR finds a multicast Target FEC Stack sub-TLV, it MUST
   send an echo reply with the received BFD Reverse Path TLV and BFD
   Discriminator TLV and set the Return Code to 192 ("Inappropriate
   Target FEC Stack sub-TLV present") (see Section 3.2).  The BFD
   Reverse Path TLV includes 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., a BFD
   Reverse Path TLV with no sub-TLVs) is used 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 decision based on
   local policy, i.e., routing over an IP network, as described in
   Section 7 of [RFC5884].

   If the egress peer LSR cannot find the path specified in the BFD
   Reverse Path TLV, it MUST send an echo reply with the received BFD
   Discriminator TLV and BFD Reverse Path TLV and set the Return Code to
   193 ("Failed to establish the BFD session.  The specified reverse
   path was not 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, 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 required by the "MUST" clause
   in this paragraph does not preclude the session from being
   established over a different path as discussed in the "MAY" clause.

   The BFD Reverse Path TLV MAY be used in the process of bootstrapping
   the BFD session 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 with the BFD
   Discriminator TLV and no BFD Reverse Path TLV even though the reverse
   path for the specified BFD session was established according to the
   previously received BFD Reverse Path TLV, the egress BFD peer MUST
   transition to transmitting periodic BFD Control messages as described
   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 an echo response with the Return Code set to 2 ("One
   or more of the TLVs was not understood"), as described in Section 3
   of [RFC8029].

3.2.  Return Codes

   This document defines the following Return Codes for the MPLS LSP
   echo reply:

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

   "Failed to establish the BFD session.  The specified reverse path
   was not found." (193):
      When a specified reverse path is unavailable, the egress BFD peer
      sends an echo reply with the Return Code set to 193 ("Failed to
      establish the BFD session.  The specified reverse path was not
      found.") to the ingress BFD 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.  Upon detection of the network failure, the ingress BFD peer
   MUST transmit the LSP ping echo request with the Reply Path TLV
   [RFC7110] to verify whether the FEC is still valid.  If the failure
   was caused by a change in the FEC used for the reverse direction of
   the BFD session, the ingress BFD peer MUST redirect the reverse path
   of the BFD session using another FEC in the BFD Reverse Path TLV and
   notify an operator.

4.  Use Case Scenario

   In the network presented in Figure 2, ingress LSR peer A monitors two
   tunnels to 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, ingress LSR peer
   A includes a BFD Discriminator TLV with a Discriminator value (e.g.,
   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 includes a BFD Discriminator TLV with a different
   Discriminator value (e.g., foobar-2) 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 ingress
   LSR peer A, e.g., the 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 as either 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 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] 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
   Reply Path TLV, as per [RFC7110], with that sub-TLV.  By using the
   LSP ping with the Reply Path TLV, an operator monitors whether the
   reverse LSP is mapped to the same FEC as the BFD session at the
   egress BFD node.  Selection and control of the rate of the LSP ping
   with the Reply Path TLV follows the recommendation in [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.

   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 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 is 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

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

   +=======+=========+===========+====================================+
   | Type  | TLV     | Reference | Sub-TLV Registry                   |
   |       | Name    |           |                                    |
   +=======+=========+===========+====================================+
   | 16384 | BFD     | RFC 9612  | Only non-multicast sub-TLVs        |
   |       | Reverse |           | (already defined or to be defined  |
   |       | Path    |           | in the future) in the "Sub-TLVs    |
   |       |         |           | for TLV Types 1, 16, and 21"       |
   |       |         |           | registry at                        |
   |       |         |           | <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.  Other sub-TLVs     |
   |       |         |           | MUST NOT be used.                  |
   +-------+---------+-----------+------------------------------------+

                    Table 1: New BFD Reverse Path TLV

6.2.  Return Codes

   IANA has assigned the following Return Code values from the 192-247
   range in the "Return Codes" subregistry within the "Multiprotocol
   Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
   registry.

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

                         Table 2: New Return Codes

7.  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 to 128 (see Section 3.1).  An
   implementation MAY use a mechanism to control that limit.

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>.

Acknowledgments

   The authors greatly appreciate the thorough reviews and 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
