BFD Working Group                                               W. Cheng
Internet-Draft                                                   R. Wang
Updates: 5880 (if approved)                                 China Mobile
Intended status: Standards Track                             X. Min, Ed.
Expires: 28 July 12 August 2022                                        ZTE Corp.
                                                               R. Rahman
                                                              Individual
                                                           R. Boddireddy
                                                        Juniper Networks
                                                         24 January
                                                         8 February 2022

                         Unaffiliated BFD Echo
                  draft-ietf-bfd-unaffiliated-echo-03
                  draft-ietf-bfd-unaffiliated-echo-04

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.

   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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   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 28 July 12 August 2022.

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   document authors.  All rights reserved.

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Table of Contents

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

1.  Introduction

   To minimize the impact of device/link faults on services and improve
   network availability, a network device must 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.

   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.

   The latter scenario is referred to as Unaffiliated BFD Echo in this
   document.

   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.  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.

   *

      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.

   *

      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...

   *

      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...

3.  Unaffiliated BFD Echo Procedures

   Device A                                  Device B

   BFD Enabled                               BFD Echo packets loopback
   +--------+        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 BFD Echo packets, and after
   receiving the 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 BFD Echo packet or parse any BFD protocol field within the
   BFD Echo packet.

   To rapidly detect any IP forwarding faults between device A and
   device B, a BFD Echo session MUST be created at device A, and the 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 echoed
   from the remote system, and AdminDown state is ruled out because
   AdminDown effectively means removal of BFD Echo session.  In this
   case, although BFD Echo packets are transmitted with destination UDP
   port 3785 as defined in [RFC5881], the BFD Echo packets sent by
   device A are BFD Control packets too, the looped BFD Echo packets
   back from device B would drive BFD state change at device A,
   substituting the BFD Control packets sent from the BFD peer.  Also
   note that when device A receives looped BFD Control packets, the
   validation procedures of [RFC5880] are used.

   Once a BFD Echo session is created at device A, it starts sending BFD
   Echo packets, which MUST include BFD Echo session demultiplexing
   fields, such as BFD "Your Discriminator" defined in [RFC5880] (BFD
   "My Discriminator" can be set to 0 to avoid confusion), except for
   BFD "Your Discriminator", device A can also use IP source address or
   UDP source port to demultiplex BFD Echo session, or there is only one
   BFD Echo session running at device A.  Device A would send BFD Echo
   packets with IP destination address destined for itself, such as the
   IP address of interface 1 of device A.  All BFD Echo packets for the
   session MUST be sent with a Time to Live (TTL) or Hop Limit value of
   255.

   Within the BFD Echo packet, the "Desired Min TX Interval" and
   "Required Min RX Interval" defined in [RFC5880] may be populated with
   one second, which however has no real application and would be
   ignored by the receiver.

   Considering that the BFD peer device B wouldn't advertise "Required
   Min Echo RX Interval" as defined in [RFC5880], the transmission
   interval for sending BFD Echo packets MUST be provisioned at device
   A, how to make sure the BFD peer device B is willing and able to loop
   back BFD Echo packets sent with the provisioned transmission interval
   is outside the scope of this document.  Similar to what's specified
   in [RFC5880], the BFD Echo session begins with the periodic, slow
   transmission of BFD Echo packets, the slow transmission rate SHOULD
   be no less then one second per packet, until the session is Up, after
   that the provisioned transmission interval is applied, and reverting
   back to the slow rate once the session goes Down.  Considering that
   the BFD peer wouldn't advertise "Detect Mult" as defined in

   [RFC5880], the "Detect Mult" for calculating the Detection Time MUST
   be provisioned at device A, the Detection Time at device A is equal
   to the provisioned "Detect Mult" multiplied by the provisioned
   transmission 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.  The 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 BFD Echo packets.

5.  Security Considerations

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

   Note that the Unaffiliated BFD Echo prevents the 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 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 BFD Echo packet.

   In order to mitigate the potential reflector attack by the remote
   attackers, or infinite loop of the BFD Echo packets, it's RECOMMENDED
   to put two requirements on the device looping 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.

6.  IANA Considerations

   This document has no IANA action requested.

7.  Acknowledgements

   The authors would like to acknowledge Ketan Talaulikar, Greg Mirsky
   and Santosh Pallagatti for their careful review and very helpful
   comments.

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

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>.

   [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://www.ietf.org/archive/id/draft-wang-bfd-one-arm-
              use-case-00.txt>.

   [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>.

   [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

   Xiao Min (editor)
   ZTE Corp.
   Nanjing
   China

   Email: xiao.min2@zte.com.cn

   Reshad Rahman
   Individual
   Kanata
   Canada

   Email: reshad@yahoo.com
   Raj Chetan Boddireddy
   Juniper Networks

   Email: rchetan@juniper.net