Unaffiliated Bidirectional Forwarding Detection (BFD) Echo
draft-ietf-bfd-unaffiliated-echo-13

BFD Working Group                                               W. Cheng
Internet-Draft                                                   R. Wang
Updates: 5880 (if approved)                                 China Mobile
Intended status: Standards Track                             X. Min, Ed.
Expires: 7 June 2025                                           ZTE Corp.
                                                               R. Rahman
                                                                 Equinix
                                                           R. Boddireddy
                                                        Juniper Networks
                                                         4 December 2024

       Unaffiliated Bidirectional Forwarding Detection (BFD) Echo
                  draft-ietf-bfd-unaffiliated-echo-13

Abstract

   This document specifies an extension to the Bidirectional Forwarding
   Detection (BFD) protocol that enables the use of the BFD Echo
   function without the need for an associated BFD control session.
   This "Unaffiliated BFD Echo" mechanism allows rapid detection of
   forwarding path failures in networks where establishing BFD control
   sessions is impractical or undesirable.  By decoupling the Echo
   function from the control plane, network devices can utilize BFD's
   fast failure detection capabilities in a simplified manner, enhancing
   network resiliency and operational efficiency.

   This document updates RFC 5880 by defining a new Unaffiliated BFD
   Echo mechanism.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 7 June 2025.

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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) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   4
   2.  Unaffiliated BFD Echo Procedures  . . . . . . . . . . . . . .   4
   3.  Updates to RFC 5880 . . . . . . . . . . . . . . . . . . . . .   7
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   To minimize the impact of device and link faults on services and to
   improve network availability in single-hop scenarios, a network
   device needs the capability to quickly detect communication faults
   with adjacent devices.  Prompt detection allows for timely remedial
   actions to ensure service continuity.

   BFD [RFC5880] provides a low-overhead, short-interval method for
   detecting faults on the communication path between adjacent
   forwarding engines, which may include interfaces, data links, and the
   forwarding engines themselves.  BFD offers a unified mechanism to
   monitor any media and protocol layers in real time.

   BFD defines two primary modes-Asynchronous mode and Demand mode-to
   accommodate various deployment scenarios.  Additionally, it supports
   an Echo function that reduces the level of BFD support required in
   device implementations, as described in Section 3.2 of [RFC5880].

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   When the Echo function is activated, the local system sends BFD Echo
   packets, and the remote system loops back the received Echo packets
   through the forwarding path, as described in Section 5 of [RFC5880]
   and Section 4 of [RFC5881].  If several consecutive BFD Echo packets
   are not received by the local system, the BFD session is declared
   Down.

   There are two typical scenarios when using the BFD Echo function:

   *  Full BFD protocol capability with adjunct Echo function
      (Affiliated BFD Echo): This scenario requires both the local
      device and the adjacent device to support the full BFD protocol.
      This operation remains unchanged from [RFC5880].

   *  BFD Echo-Only method without full BFD protocol capability
      (Unaffiliated BFD Echo): This scenario requires only the local
      device to support sending and demultiplexing BFD Control packets.
      In this case, BFD Control packets are sent over the BFD Echo port,
      and the processing procedures for Asynchronous mode are used with
      the modifications specified in this document.  Note that this
      method monitors the connectivity to a system over a specific
      interface and does not verify the availability of a specific IP
      address on that system.

   This document specifies the Unaffiliated BFD Echo scenario.

   Section 5 of [RFC5880] indicates that the payload of an Affiliated
   BFD Echo packet is a local matter and, therefore, its contents are
   outside the scope of that specification.  This document, however,
   specifies the contents of the Unaffiliated BFD Echo packet and the
   procedures for handling them.  While this may appear to contravene
   Section 5 of [RFC5880], the core behavior in that RFC states that the
   contents of BFD Echo packets are a local matter; this document is
   defining that "local matter".  Regarding the selection of IP
   addresses, the rules stated in Section 4 of [RFC5881] are applicable
   to the encapsulation of an Unaffiliated BFD Echo packet.

   Section 6.2.2 of [BBF-TR-146] describes a use case for the
   Unaffiliated BFD Echo.

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   This document updates [RFC5880] by defining a new method of BFD Echo-
   only operation which only impacts the BFD Echo packets sender without
   requiring an implementation to support the BFD protocol at the loop-
   back device, such that any IP forwarder can loop-back the BFD Echo
   packets.  It specifies the use of the Unaffiliated BFD Echo over IPv4
   and IPv6 for a single IP hop.  The reason why it cannot be used for
   multihop paths is that the Unaffiliated BFD Echo packets would be
   looped back by the first hop.  A full description of the updates to
   [RFC5880] is provided in Section 3.

1.1.  Conventions Used in This Document

   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.  Unaffiliated BFD Echo Procedures

   This section specifies the Unaffiliated BFD Echo procedures.

           Device A                                     Device B
      +----------------+                           +----------------+
      |                |                           |                |
      |   |------------|                           |                |
      |   |Unaffiliated|                           |                |
      |   | BFD Echo  --->                         |                |
      |   | Session    |                           |                |
      |   |            |   Unaffiliated BFD Echo   |                |
      |   |           -------------------------------| BFD          |
      |   |            |                             | packets      |
      |   |          <-------------------------------| looped       |
      |   |------------|                           |                |
      |                |                           |                |
      |                |                           |                |
      +----------------+                           +----------------+
        BFD supported                               BFD not supported

                  Figure 1: Unaffiliated BFD Echo diagram

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   As shown in Figure 1, device A supports BFD, whereas device B is a
   regular IP forwarder that does not support BFD.  Device A would send
   Unaffiliated BFD Echo packets, and after receiving the Unaffiliated
   BFD Echo packets sent from device A, the one-hop-away BFD peer device
   B immediately loops them back by normal IP forwarding, this allows
   device A to rapidly detect a connectivity loss to device B.  Note
   that device B would not intercept any received Unaffiliated BFD Echo
   packet or parse any BFD protocol field within the Unaffiliated BFD
   Echo packet.

   An Unaffiliated BFD Echo session is not actually a BFD session
   because there is no coordination of BFD protocol state between the
   two link ends: the remote end does not support BFD and so cannot
   engage in a BFD session.  The local end as an initiator may regard
   the Unaffiliated BFD Echo session as a BFD session from its own
   standpoint.

   For the Unaffiliated Echo procedure, an Unaffiliated BFD Echo session
   is established on device A.  The session MUST adhere to the BFD state
   machine specified in Section 6.2 of [RFC5880], with the exception
   that the received state is not derived from BFD Control packets
   originating from the remote system, but rather from packets that are
   generated by the local system and looped back from the remote system.
   Consequently, the AdminDown state is not utilized in Unaffiliated BFD
   Echo.

   BFD Control packets are transmitted and received as Unaffiliated BFD
   Echo packets, using UDP destination port 3785, as defined in
   [RFC5881].  The standard procedures for BFD Asynchronous sessions are
   applied to the looped BFD Control packets, including packet
   validation and authentication, in accordance with [RFC5880].

   Once an Unaffiliated BFD Echo session is created on device A, it
   starts sending Unaffiliated BFD Echo packets.  Unaffiliated BFD Echo
   packets with zeroed "Your Discriminator" field are demultiplexed to
   the proper session based on the source IP address or UDP source port,
   once the remote system loops back the local discriminator, all
   further received packets are demultiplexed based on the "Your
   Discriminator" field only, which is conformed to the procedure
   specified in Section 6.3 of [RFC5880].  An Unaffiliated BFD Echo
   packet follows the same encapsulation rules as for a BFD Echo packet
   as specified in Section 4 of [RFC5881].  All Unaffiliated BFD Echo
   packets for the session MUST be sent with a TTL or Hop Limit value of
   255.  Received packets MUST have a TTL or Hop Limit value of 254
   (similar to Appendix A of [RFC5082] to verify against a configured
   number of hops); otherwise, the received packets MUST be dropped.

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   In the context of an Unaffiliated BFD Echo packet, the "Desired Min
   TX Interval" and "Required Min RX Interval" fields, as defined in
   [RFC5880], MUST be populated with a specific value to prevent the
   potential exposure of uninitialized memory.  It is RECOMMENDED that
   these fields be set to a value of 1 second (1,000,000 microseconds).
   However, upon receipt, these values MUST be ignored and MUST NOT be
   used in the calculation of the Detection Time.

   The "Required Min Echo RX Interval" field, as defined in [RFC5880],
   MUST be populated with a specific value to prevent the potential
   exposure of uninitialized memory.  It is RECOMMENDED that this field
   be set to 0.  However, this value MUST be ignored upon receipt.  The
   transmission interval for Unaffiliated BFD Echo packets when in the
   Up state MUST be provisioned on device A.

   The functionality of the Unaffiliated BFD Echo feature is dependent
   on device B performing IP forwarding (specifically, IP redirect
   functionality).  While this capability is typically expected to be
   supported on routers, it may not be enabled by default on hosts.  The
   method for provisioning device B to loop back Unaffiliated BFD Echo
   packets is outside the scope of this document.

   Similar to what's specified in [RFC5880], the Unaffiliated BFD Echo
   session begins with the periodic, slow transmission of Unaffiliated
   BFD Echo packets.  The slow transmission rate should be no less than
   one second per packet, until the session on device A is Up.  After
   the session is Up, the provisioned transmission interval is used.
   When the Unaffiliated BFD Echo session on device A goes Down, the
   slow transmission rate is resumed.  The "Detect Mult" defined in
   [RFC5880] MUST be set to a value provisioned on device A.  When the
   bfd.SessionState is Up and a "Detect Mult" number of Unaffiliated BFD
   Echo packets have not arrived at device A as they should, the device
   A "MUST set bfd.SessionState to Down and bfd.LocalDiag to 2 (Echo
   Function Failed)", as specified in Section 6.8.5 of [RFC5880].

   In summary, the Unaffiliated BFD Echo packet reuses the format of the
   BFD Control packet defined in [RFC5880], and the fields within the
   Unaffiliated BFD Echo packet are populated as follows:

   *  My Discriminator: MUST be set to the provisioned local
      discriminator.

   *  Your Discriminator: MUST initially be set to 0, and then MUST be
      set to the value of "My Discriminator" looped back from the remote
      system.

   *  Desired Min TX Interval: MUST be set to a specific value, with a
      suggested value of 1 second (1,000,000 microseconds).

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   *  Required Min RX Interval: MUST be set to a specific value, with a
      suggested value of 1 second (1,000,000 microseconds).

   *  Required Min Echo RX Interval: MUST be set to a specific value,
      with a suggested value of 0.

   *  Detect Mult: MUST be set to the provisioned maximum allowable
      number of consecutively lost Unaffiliated BFD Echo packets.

3.  Updates to RFC 5880

   The Unaffiliated BFD Echo described in this document reuses the BFD
   Echo function as described in [RFC5880] and [RFC5881], but does not
   require BFD Asynchronous or Demand mode.  In the Unaffiliated BFD
   Echo operation, only the local system has the BFD protocol enabled,
   while the remote system simply loops back the received BFD Echo
   packets as ordinary data packets, without engaging in the BFD
   protocol.

   This document updates [RFC5880] with respect to its descriptions on
   the BFD Echo function as follows.

   The 4th paragraph of Section 3.2 of [RFC5880] is updated as below:

      OLD TEXT
      An adjunct to both modes is the Echo function.

      NEW TEXT
      An adjunct to both modes is the Echo function, which can also be
      running independently.

      OLD TEXT
      Since the Echo function is handling the task of detection, the
      rate of periodic transmission of Control packets may be reduced
      (in the case of Asynchronous mode) or eliminated completely (in
      the case of Demand mode).

      NEW TEXT
      Since the Echo function is handling the task of detection, the
      rate of periodic transmission of Control packets may be reduced
      (in the case of Asynchronous mode) or eliminated completely (in
      the case of Demand mode).  The Echo function may also be used
      independently, with neither Asynchronous nor Demand mode.

   The 3rd and 9th paragraphs of Section 6.1 of [RFC5880] are updated as
   below:

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      OLD TEXT
      Once the BFD session is Up, a system can choose to start the Echo
      function if it desires and the other system signals that it will
      allow it.  The rate of transmission of Control packets is
      typically kept low when the Echo function is active.

      NEW TEXT
      When a system is running with Asynchronous or Demand mode, once
      the BFD session is Up, it can choose to start the Echo function if
      it desires and the other system signals that it will allow it.
      The rate of transmission of Control packets is typically kept low
      for Asynchronous mode or eliminated completely for Demand mode
      when the Echo function is active.

      OLD TEXT
      If the session goes Down, the transmission of Echo packets (if
      any) ceases, and the transmission of Control packets goes back to
      the slow rate.

      NEW TEXT
      In Asynchronous mode or Demand mode, if the session goes Down, the
      transmission of Echo packets (if any) ceases, and the transmission
      of Control packets goes back to the slow rate.

   The 2nd paragraph of Section 6.4 of [RFC5880] is updated as below:

      OLD TEXT
      When a system is using the Echo function, it is advantageous to
      choose a sedate reception rate for Control packets, since liveness
      detection is being handled by the Echo packets.  This can be
      controlled by manipulating the Required Min RX Interval field (see
      section 6.8.3).

      NEW TEXT
      When a system is using the Echo function with Asynchronous mode,
      it is advantageous to choose a sedate reception rate for Control
      packets, since liveness detection is being handled by the Echo
      packets.  This can be controlled by manipulating the Required Min
      RX Interval field (see section 6.8.3).  Note that a system
      operating in Demand mode would direct the remote system to cease
      the periodic transmission of BFD Control packets, by setting the
      Demand (D) bit in its BFD Control packets.

   The 2nd paragraph of Section 6.8 of [RFC5880] is updated as below:

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      OLD TEXT
      When a system is said to have "the Echo function active" it means
      that the system is sending BFD Echo packets, implying that the
      session is Up and the other system has signaled its willingness to
      loop back Echo packets.

      NEW TEXT
      When a system in Asynchronous or Demand mode is said to have "the
      Echo function active" it means that the system is sending BFD Echo
      packets, implying that the session is Up and the other system has
      signaled its willingness to loop back Echo packets.

   The 7th paragraph of Section 6.8.3 of [RFC5880] is updated as below:

      OLD TEXT
      When the Echo function is active, a system SHOULD set
      bfd.RequiredMinRxInterval to a value of not less than one second
      (1,000,000 microseconds).  This is intended to keep received BFD
      Control traffic at a negligible level, since the actual detection
      function is being performed using BFD Echo packets.

      NEW TEXT
      When the Echo function is active with Asynchronous mode, a system
      SHOULD set bfd.RequiredMinRxInterval to a value of not less than
      one second (1,000,000 microseconds).  This is intended to keep
      received BFD Control traffic at a negligible level, since the
      actual detection function is being performed using BFD Echo
      packets.  While a system operating in Demand mode would not
      receive BFD Control traffic.

   The 1st and 2nd paragraphs of Section 6.8.9 of [RFC5880] are updated
   as below:

      OLD TEXT
      BFD Echo packets MUST NOT be transmitted when bfd.SessionState is
      not Up.  BFD Echo packets MUST NOT be transmitted unless the last
      BFD Control packet received from the remote system contains a
      nonzero value in Required Min Echo RX Interval.

      NEW TEXT
      When a system is using the Echo function with either Asynchronous
      or Demand mode, BFD Echo packets MUST NOT be transmitted when
      bfd.SessionState is not Up, and BFD Echo packets MUST NOT be
      transmitted unless the last BFD Control packet received from the
      remote system contains a nonzero value in Required Min Echo RX
      Interval.

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      OLD TEXT
      BFD Echo packets MAY be transmitted when bfd.SessionState is Up.
      The interval between transmitted BFD Echo packets MUST NOT be less
      than the value advertised by the remote system in Required Min
      Echo RX Interval...

      NEW TEXT
      When a system is using the Echo function with either Asynchronous
      or Demand mode, BFD Echo packets MAY be transmitted when
      bfd.SessionState is Up, and the interval between transmitted BFD
      Echo packets MUST NOT be less than the value advertised by the
      remote system in Required Min Echo RX Interval...

4.  Operational Considerations

   All Operational Considerations from [RFC5880] apply, except that the
   Unaffiliated BFD Echo can only be used across one hop, which result
   in unneccessity of a congestion control mechanism.

   Some devices that would benefit from the use of BFD may be unable to
   support the full BFD protocol.  Examples of such devices include
   servers running virtual machines, or Internet of Things (IoT)
   devices.  By using Unaffiliated BFD Echo, these devices only need to
   support a basic loopback function.

   As specified in Section 2 of this document, some configurations are
   needed to make the Unaffiliated BFD Echo work, although the
   configurations won't go beyond the scope of [RFC5880].  At a BFD-
   enabled local system, the Unaffiliated BFD Echo session can coexist
   with other type of BFD session, in which scenario the remote system
   for the Unaffiliated BFD Echo session must be different from the
   remote system for other type of BFD session, and the local system's
   discriminators for different BFD sessions must be different, at the
   same time it's not necessary for the local system to differentiate
   the Unaffiliated BFD Echo session from other type of BFD session.

5.  Security Considerations

   All Security Considerations from [RFC5880] and [RFC5881] apply.

   Unaffiliated BFD Echo requires the remote device to loop Unaffiliated
   BFD Echo packets.  In order to provide this service, the remote
   device cannot make use of Unicast Strict Reverse Path Forwarding
   (RPF) [RFC3704], otherwise the Unaffiliated BFD Echo packets might
   not pass the RPF check at the remote device.

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   As specified in Section 5 of [RFC5880], BFD Echo packets may be
   spoofed.  Specifically for Unaffiliated BFD Echo, a DoS attacker may
   send spoofed Unaffiliated BFD Echo packets to the loop-back device,
   so some form of authentication SHOULD be included.  Considering the
   Unaffiliated BFD Echo packets in this document are also BFD Control
   packets, the "Authentication Section" as defined in [RFC5880] for BFD
   Control packet is RECOMMENDED to be included within the Unaffiliated
   BFD Echo packet.

   As stated in Section 2, in order to avoid unset values being a
   potential vector for disclosure of uninitialized memory, all fields
   of the Unaffiliated BFD Echo packet MUST be populated with a certain
   value, even if some of the fields are ignored on receipt.

6.  IANA Considerations

   This document has no IANA action requested.

7.  Acknowledgements

   The authors would like to acknowledge Ketan Talaulikar, Greg Mirsky,
   Santosh Pallagatti, Aijun Wang, Eric Vyncke, Adrian Farrel, Tim
   Wicinski, Dhruv Dhody, Stephen Farrell, Gunter Van de Velde, Gyan
   Mishra, Brian Trammell, Gorry Fairhurst, Mahesh Jethanandani, John
   Scudder, Murray Kucherawy, and Zaheduzzaman Sarker for their careful
   review and very helpful comments.

   The authors would like to acknowledge Jeff Haas for his guidance,
   insightful review, and very helpful comments.

   The authors would like to acknowledge Erik Auerswald for his
   insightful comments during the discussion of this document.

   The authors would like to acknowledge Detao Zhao for the very helpful
   discussion.

8.  Contributors

   Liu Aihua
   ZTE
   Email: liu.aihua@zte.com.cn

   Qian Xin
   ZTE
   Email: qian.xin2@zte.com.cn

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   Zhao Yanhua
   ZTE
   Email: zhao.yanhua3@zte.com.cn

9.  References

9.1.  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>.

   [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>.

9.2.  Informative References

   [BBF-TR-146]
              Broadband Forum, "BBF Technical Report - Subscriber
              Sessions Issue 1", 2013, <https://www.broadband-
              forum.org/technical/download/TR-146.pdf>.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <https://www.rfc-editor.org/info/rfc3704>.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
              <https://www.rfc-editor.org/info/rfc5082>.

Authors' Addresses

   Weiqiang Cheng
   China Mobile
   Beijing
   China

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   Email: chengweiqiang@chinamobile.com

   Ruixue Wang
   China Mobile
   Beijing
   China
   Email: wangruixue@chinamobile.com

   Xiao Min (editor)
   ZTE Corp.
   Nanjing
   China
   Email: xiao.min2@zte.com.cn

   Reshad Rahman
   Equinix
   Ottawa
   Canada
   Email: reshad@yahoo.com

   Raj Chetan Boddireddy
   Juniper Networks
   Email: rchetan@juniper.net

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