BFD Working Group                                               W. Cheng
Internet-Draft                                                   R. Wang
Updates: 5880 (if approved)                                 China Mobile
Intended status: Standards Track                                  X. Min
Expires: March 13, 2021                                           A. Liu
                                                               ZTE Corp.
                                                               R. Rahman
                                                           Cisco Systems
                                                           R. Boddireddy
                                                        Juniper Networks
                                                       September 9, 2020

                     Unaffiliated BFD Echo Function


   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 BFD echo
   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
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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 March 13, 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

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   ( 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Unaffiliated BFD Echo Behavior  . . . . . . . . . . . . . . .   3
   3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   To minimize the impact of device 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 path between adjacent forwarding engines.  The faults
   can be interface, data link, and even forwarding engine faults.  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, also supports echo function to reduce the device
   requirement for BFD.  When the echo function is activated, the local
   system sends a BFD echo packet and the remote system loops back the
   packet through the forwarding path.  If several consecutive echo
   packets are not received, the session is declared to be Down.

   When using BFD echo function, it is not clear whether the devices
   using echo function need to support the full BFD procotol, including
   maintaining the state machine of BFD session as described in
   [RFC5880] and [RFC5881].  According to different understanding, there
   are two typical scenarios as below:

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      1.  Full BFD procotol capability with affiliated echo function:
      this scenario requires both the local device and the neighboring
      device to support BFD protocol.

      2.  Only BFD echo function without full BFD procotol capability:
      this scenario requires only the local device to support sending
      BFD packets.

   The two typical scenarios are both reasonable and useful, and the
   latter is referred to as unaffiliated BFD echo function in this

   Unaffiliated BFD echo function described in this document reuses the
   BFD echo function as described in [RFC5880] and [RFC5881], but
   independent of BFD asynchronous mode, that means it doesn't need BFD
   protocol capability of state machine, but only BFD echo function to a
   deployed device supporting BFD detection.  When using unaffiliated
   BFD echo function, just the local device works on BFD protocol and
   the BFD peer doesn't, which only loopback the received BFD echo
   packets as usual data packets without enabling BFD protocol.

   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.

2.  Unaffiliated BFD Echo Behavior

   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 capablity, 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.
   A BFD echo session can be established at the device that supports
   BFD, and the device will send the BFD echo packets with the IP
   address destined for itself, whereas the other peer device just
   loopback the received BFD echo packets.

   After receiving a BFD echo packet, the device that does not support
   BFD protocol immediately loops back the packet by normal IP
   forwarding, implementing quick link failure detection.  As shown in
   Figure 1, device A supports BFD, whereas device B does not support
   BFD.  To rapidly detect any faults with the IP link between device A
   and device B, a BFD echo session can be provisioned and created at
   device A, and device A starts sending BFD echo packets, which should
   include a BFD echo session demultiplexing field, such as BFD

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   discriminator defined in [RFC5880].  After receiving the BFD echo
   packets sent from device A, device B immediately loops back them,
   this allows device A to rapidly detect a connectivity loss to device

   Device A                                   Device B
                      BFD echo session
   BFD Enabled                                BFD Echo packets loopback
   +--------+                                 +---------+
   |   A    |---------------------------------|   B     |
   |        |Inf 1                       Inf 1|         |
   BFD is supported.                 BFD is not supported.

            Figure 1: Unaffiliated BFD Echo deployment scenario

3.  Discussion

   Unaffiliated BFD echo function is reasonable and useful.  Firstly,
   unaffiliated BFD echo can use BFD protocol capability in the local
   BFD-supported device, while using IP forwarding capability in the
   peer non-BFD-supported 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.

4.  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.

5.  IANA Considerations

   This document has no IANA action requested.

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6.  Acknowledgements


7.  References

7.1.  Normative References

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              DOI 10.17487/RFC5881, June 2010,

7.2.  Informative References

              Broadband Forum, "BBF Technical Report - Subscriber
              Sessions Issue 1", 2013, <https://www.broadband-

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <>.

   [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,

Authors' Addresses

   Weiqiang Cheng
   China Mobile


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   Ruixue Wang
   China Mobile


   Xiao Min
   ZTE Corp.


   Aihua Liu
   ZTE Corp.


   Reshad Rahman
   Cisco Systems


   Raj Chetan Boddireddy
   Juniper Networks


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