Unaffiliated BFD Echo
draft-ietf-bfd-unaffiliated-echo-10

BFD Working Group                                               W. Cheng
Internet-Draft                                                   R. Wang
Updates: 5880 (if approved)                                 China Mobile
Intended status: Standards Track                             X. Min, Ed.
Expires: 31 March 2024                                         ZTE Corp.
                                                               R. Rahman
                                                                 Equinix
                                                           R. Boddireddy
                                                        Juniper Networks
                                                       28 September 2023

                         Unaffiliated BFD Echo
                  draft-ietf-bfd-unaffiliated-echo-10

Abstract

   Bidirectional Forwarding Detection (BFD) is a fault detection
   protocol that can quickly determine a communication failure between
   two forwarding engines.  This document proposes a use of the BFD Echo
   where the local system supports BFD but the neighboring system does
   not support BFD.  BFD Control packet and its processing procedures
   can be executed over the BFD Echo port where the neighboring system
   only loops packets back to the local system.

   This document updates RFC 5880.

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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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 31 March 2024.

Copyright Notice

   Copyright (c) 2023 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
   2.  Unaffiliated BFD Echo Procedures  . . . . . . . . . . . . . .   3
   3.  Updates to RFC 5880 . . . . . . . . . . . . . . . . . . . . .   6
   4.  Unaffiliated BFD Echo Applicability . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   To minimize the impact of device/link faults on services and improve
   network availability, in the single-hop cases a network device needs
   to be able to quickly detect faults in communication with adjacent
   devices.  Measures can then be taken to promptly rectify the faults
   to ensure service continuity.

   BFD [RFC5880] is a low-overhead, short-duration method to detect
   faults on the communication path between adjacent forwarding engines.
   The faults can be on interfaces, data link(s), and even the
   forwarding engines.  It is a single, unified mechanism to monitor any
   media and protocol layers in real time.

   BFD defines Asynchronous and Demand modes to satisfy various
   deployment scenarios.  It also supports an Echo function to reduce
   the device requirement for BFD.  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.  If
   several consecutive BFD Echo packets are not received by the local
   system, then the BFD session is declared to be Down.

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   When using BFD Echo function, there are two typical scenarios as
   below:

   *  Full BFD protocol capability with affiliated Echo function.  This
      scenario requires both the local device and the neighboring device
      to support the full BFD protocol.

   *  BFD Echo-Only method without full BFD protocol capability.  This
      scenario requires only the local device to support sending and
      demultiplexing BFD Control packets.  In this scenario, the BFD
      Control packets are sent over the BFD Echo port, but that the
      processing procedures for Asynchronous mode are used with the
      modifications described 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 at
      that system.

   The former scenario is referred to as affiliated BFD Echo, which is
   not changed by this document in any way.  The latter scenario is
   referred to as Unaffiliated BFD Echo, which is specified in this
   document.

   Section 5 of [RFC5880] indicates that the payload of an affiliated
   BFD Echo packet is a local matter and hence its contents are outside
   the scope of that specification.  This document, on the other hand,
   specifies the contents of the Unaffiliated BFD Echo packet and what
   to do with them.

   Section 6.2.2 of [BBF-TR-146] describes one use case of the
   Unaffiliated BFD Echo.  Section 2 of [I-D.wang-bfd-one-arm-use-case]
   describes another use case of the Unaffiliated BFD Echo.

   This document describes the use of the Unaffiliated BFD Echo over
   IPv4 and IPv6 for single IP hop.

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

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   Device A                                         Device B

   BFD Enabled                                      BFD packets looped
   +--------+     Unaffiliated BFD Echo session     +--------+
   |   A    |---------------------------------------|   B    |
   |        |Interface 1                 Interface 1|        |
   +--------+                                       +--------+
   BFD is supported.                      BFD is not supported.

                  Figure 1: Unaffiliated BFD Echo diagram

   As shown in Figure 1, device A supports BFD, whereas device B 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.

   For unaffiliated echo, a Unaffiliated BFD Echo session is created on
   device A, and the Unaffiliated BFD Echo session MUST follow the BFD
   state machine defined in Section 6.2 of [RFC5880], except that the
   received state is not sent but looped back from the remote system.
   Unaffiliated BFD Echo does not use the AdminDown state.  BFD Control
   packets are transmitted and received as Unaffiliated BFD Echo packets
   using destination UDP port 3785, as defined in [RFC5881].  The
   procedures for BFD Async sessions are executed for the looped BFD
   Control packets as per [RFC5880], including validation and
   authentication.

   Once a Unaffiliated BFD Echo session is created on device A, it
   starts sending Unaffiliated BFD Echo packets.  Unaffiliated BFD Echo
   packets with zeroed "Your Discriminator" 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 Time to Live (TTL) or Hop
   Limit value of 255, and received with a TTL or Hop Limit value of
   254, otherwise the received packets MUST be dropped [RFC5082].

   Within the Unaffiliated BFD Echo packet, the "Desired Min TX
   Interval" and "Required Min RX Interval" defined in [RFC5880] SHOULD
   be populated with a certain value, which can avoid unset value being

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   a potential vector for disclosure of uninitialized memory.  A
   suggested value is 1 second (1,000,000 microseconds).  These values,
   however, MUST be ignored on receipt.  Furthermore, these values MUST
   NOT be used to calculate the Detection Time.

   The "Required Min Echo RX Interval" defined in [RFC5880] SHOULD be
   populated with a certain value.  A suggested value is 0.  This value
   MUST be ignored on receipt.  The transmission interval for
   Unaffiliated BFD Echo packets in the Up state MUST be provisioned on
   device A.  The Unaffiliated BFD Echo feature depends on device B
   performing IP forwarding (actually IP redirect) functionality.  While
   such functionality may normally be expected to be supported on a
   router, it may not be enabled on a host by default.  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 is Up.  After the session is
   Up, the provisioned transmission interval is used.  When the
   Unaffiliated BFD Echo session 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 be set to 0 initially, and then MUST be
      set to the same as My Discriminator looped back.

   *  Desired Min TX Interval SHOULD be set to a certain value.  A
      suggested value is 1 second (1,000,000 microseconds).

   *  Required Min RX Interval SHOULD be set to a certain value.  A
      suggested value is 1 second (1,000,000 microseconds).

   *  Required Min Echo RX Interval SHOULD be set to a certain value.  A
      suggested value is 0.

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   *  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.  When using the Unaffiliated
   BFD Echo, only the local system has the BFD protocol enabled; the
   remote system just loops back the received BFD Echo packets as
   regular data packets.

   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:

      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.

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      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, 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.  Demand mode MUST NOT be
      active if the session goes Down.

   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:

      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.

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      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.

      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...

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      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.  Unaffiliated BFD Echo Applicability

   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.

   Unaffiliated BFD Echo can be used when two devices are connected and
   only one of them supports the BFD protocol, and the other is capable
   of looping Unaffiliated BFD Echo packets.

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 Reverse Path Forwarding (URPF)
   [RFC3704] [RFC8704] in strict mode.

   As specified in Section 5 of [RFC5880], since BFD Echo packets may be
   spoofed, 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.

   In order to mitigate the potential reflector attack by the remote
   attackers, or infinite loop of the Unaffiliated BFD Echo packets,
   it's RECOMMENDED to put two requirements, also known as Generalized
   TTL Security Mechanism (GTSM) [RFC5082], on the device looping
   Unaffiliated BFD Echo packets, the first one is that a packet SHOULD
   NOT be looped unless it has a TTL or Hop Limit value of 255, and the
   second one is that a packet being looped MUST NOT reset the TTL or
   Hop Limit value to 255, and MUST use a TTL or Hop Limit value of 254.

   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 SHOULD be populated with a
   certain value, even if some of the fields are ignored on receipt.

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6.  IANA Considerations

   This document has no IANA action requested.

7.  Acknowledgements

   The authors would like to acknowledge Ketan Talaulikar, Greg Mirsky,
   Santosh Pallagatti, and Aijun Wang 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 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

   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>.

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   [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>.

   [I-D.wang-bfd-one-arm-use-case]
              Wang, R., Cheng, W., Zhao, Y., and A. Liu, "Using One-Arm
              BFD in Cloud Network", Work in Progress, Internet-Draft,
              draft-wang-bfd-one-arm-use-case-00, 18 November 2019,
              <https://datatracker.ietf.org/doc/html/draft-wang-bfd-one-
              arm-use-case-00>.

   [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>.

   [RFC8704]  Sriram, K., Montgomery, D., and J. Haas, "Enhanced
              Feasible-Path Unicast Reverse Path Forwarding", BCP 84,
              RFC 8704, DOI 10.17487/RFC8704, February 2020,
              <https://www.rfc-editor.org/info/rfc8704>.

Authors' Addresses

   Weiqiang Cheng
   China Mobile
   Beijing
   China
   Email: chengweiqiang@chinamobile.com

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

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   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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