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Versions: (draft-cw-bfd-unaffiliated-echo) 00 01

BFD Working Group                                               W. Cheng
Internet-Draft                                                   R. Wang
Updates: 5880 (if approved)                                 China Mobile
Intended status: Standards Track                                  X. Min
Expires: May 6, 2021                                           ZTE Corp.
                                                               R. Rahman
                                                           Cisco Systems
                                                           R. Boddireddy
                                                        Juniper Networks
                                                        November 2, 2020


                     Unaffiliated BFD Echo Function
                  draft-ietf-bfd-unaffiliated-echo-01

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
   function where the local system supports BFD but the neighboring
   system does not support BFD.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 6, 2021.

Copyright Notice

   Copyright (c) 2020 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



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   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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

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 interface, data link, and even forwarding
   engine.  It is a single, unified mechanism to monitor any media and
   protocol layers in real time.

   BFD defines Asynchronous mode to satisfy various deployment
   scenarios, and also supports 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:






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

      o Only BFD Echo function without full BFD protocol capability:
      this scenario requires only the local device to support sending
      and demultiplexing BFD Control packets.

   The two typical scenarios are both reasonable and useful, and the
   latter is referred to as Unaffiliated BFD Echo function in this
   document.

   Section 6.2.2 of [BBF-TR-146] describes one use case of the
   Unaffiliated BFD Echo function, and at least one more use case is
   known in the field BFD deployment.

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

2.  Updates to RFC 5880

   The Unaffiliated BFD Echo function described in this document reuses
   the BFD Echo function as described in [RFC5880] and [RFC5881], but
   does not require BFD asynchronous mode.  When using the Unaffiliated
   BFD Echo function, only the local system has the BFD protocol
   enabled, the remote system just loops back the received BFD Echo
   packets as regular data packets.

   With that said, this document updates [RFC5880] with respect to its
   descriptions on the BFD Echo function as follows.

   o [RFC5880] states in the 4th paragraph of Section 3.2:

      An adjunct to both modes is the Echo function.  When the Echo
      function is active, a stream of BFD Echo packets is transmitted in
      such a way as to have the other system loop them back through its
      forwarding path.  If a number of packets of the echoed data stream
      are not received, the session is declared to be down.  The Echo
      function may be used with either Asynchronous or Demand mode.
      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).

   * This paragraph is now updated to:

      An adjunct or complement to both modes is the Echo function.  When
      the Echo function is active, a stream of BFD Echo packets is



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      transmitted in such a way as to have the other system loop them
      back through its forwarding path.  If a number of packets of the
      echoed data stream are not received, the session is declared to be
      down.  The Echo function may be used with either Asynchronous or
      Demand mode.  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.

   o [RFC5880] states in the 3rd and 9th paragraphs of Section 6.1:

      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.

      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 two paragraphs are now updated to:

      When a system is running with Asynchronous 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 when
      the Echo function is active.

      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.

   o [RFC5880] states in the 2nd paragraph of Section 6.4:

      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).

   * This paragraph is now updated to:

      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




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      packets.  This can be controlled by manipulating the Required Min
      RX Interval field (see section 6.8.3).

   o [RFC5880] states in the 2nd paragraph of Section 6.8:

      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.

   * This paragraph is now updated to:

      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.

   o [RFC5880] states in the 7th paragraph of Section 6.8.3:

      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.

   * This paragraph is now updated to:

      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.

   o [RFC5880] states in the 1st and 2nd paragraphs of Section 6.8.9:

      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.

      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, except as follows:

         A 25% jitter MAY be applied to the rate of transmission, such
         that the actual interval MAY be between 75% and 100% of the



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         advertised value.  A single BFD Echo packet MAY be transmitted
         between normally scheduled Echo transmission intervals.

   * The two paragraphs are now updated to:

      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.

      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, except as follows:

         A 25% jitter MAY be applied to the rate of transmission, such
         that the actual interval MAY be between 75% and 100% of the
         advertised value.  A single BFD Echo packet MAY be transmitted
         between normally scheduled Echo transmission intervals.

3.  Unaffiliated BFD Echo Procedures

   As shown in Figure 1, device A supports BFD, whereas device B does
   not support BFD.  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 is RECOMMENDED to 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.  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.

   Once a BFD Echo session is created at device A, it starts sending BFD
   Echo packets, which SHOULD include a BFD Echo session demultiplexing
   field, such as BFD Your Discriminator defined in [RFC5880] (BFD My
   Discriminator can be set to 0 to avoid confusion), except that device
   A can 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.




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   Considering the BFD peer wouldn't advertise Required Min Echo RX
   Interval as defined in [RFC5880], the transmit interval for sending
   BFD Echo packets MUST be provisioned at device A, how to make sure
   the BFD peer is willing and able to loop back BFD Echo packets sent
   with the provisioned transmit interval is outside the scope of this
   document.  Considering 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 in device A
   is equal to the provisioned Detect Mult multiplied by the provisioned
   transmit interval.

   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.


   Device A                                   Device B
                      BFD Echo session
   BFD Enabled                                BFD Echo packets loopback
   +--------+                                 +---------+
   |   A    |---------------------------------|   B     |
   |        |Inf 1                       Inf 1|         |
   +--------+10.1.1.1/24           10.1.1.2/24+---------+
   BFD is supported.                 BFD is not supported.


            Figure 1: Unaffiliated BFD Echo deployment scenario

4.  Unaffilicated BFD Echo Applicability

   With the more and more application of BFD detection, there are some
   scenarios the BFD Echo function is deployed.  And due to the
   different capabilities of the devices deploying BFD Echo function,
   it's required to apply Unaffiliated BFD Echo to the devices that
   couldn't afford the overhead of the full BFD protocol capability,
   such as the servers running virtual machines or some Internet of
   Things (IoT) devices.  Unaffiliated BFD Echo can be used when two
   devices are connected and only one of them supports BFD protocol
   capability.

   Unaffiliated BFD Echo function is reasonable and useful.  Firstly,
   Unaffiliated BFD Echo can use BFD protocol capability at the local
   BFD-supported device, while using IP forwarding capability at the
   peer BFD-unsupported device, so Unaffiliated BFD Echo can support
   fast detecting and manage BFD sessions very effectively.  Secondly,
   it is scalable when using Unaffiliated BFD Echo to adapt to different
   capabilities of devices.



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5.  Security Considerations

   Unicast Reverse Path Forwarding (uRPF), as specified in [RFC3704] and
   [RFC8704], is a security feature that prevents the IP address
   spoofing attacks which is commonly used in DoS, DDoS. uRPF has two
   modes called strict mode and loose mode. uRPF strict mode means that
   the router will perform checks for all incoming packets on a certain
   interface: whether the router has a matching entry for the source IP
   in the routing table and whether the router uses the same interface
   to reach this source IP as where the router received this packet on.
   Note that the use of BFD Echo function would prevent the use of uRPF
   in strict mode.

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.

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

   [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>.







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   [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>.

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>.

   [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
   CN

   Email: chengweiqiang@chinamobile.com


   Ruixue Wang
   China Mobile
   Beijing
   CN

   Email: wangruixue@chinamobile.com


   Xiao Min
   ZTE Corp.
   Nanjing
   CN

   Email: xiao.min2@zte.com.cn






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   Reshad Rahman
   Cisco Systems
   Kanata
   CA

   Email: rrahman@cisco.com


   Raj Chetan Boddireddy
   Juniper Networks

   Email: rchetan@juniper.net







































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