Internet Engineering Task Force (IETF)                      C. Pignataro
Request for Comments: 7881                                       D. Ward
Category: Standards Track                                          Cisco
ISSN: 2070-1721                                                 N. Akiya
                                                     Big Switch Networks
                                                               July 2016


          Seamless Bidirectional Forwarding Detection (S-BFD)
                        for IPv4, IPv6, and MPLS

Abstract

   This document defines procedures for using Seamless Bidirectional
   Forwarding Detection (S-BFD) in IPv4, IPv6, and MPLS environments.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7881.

Copyright Notice

   Copyright (c) 2016 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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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. S-BFD UDP Port ..................................................2
   3. S-BFD Echo UDP Port .............................................3
   4. S-BFD Control Packet Demultiplexing .............................3
   5. Initiator Procedures ............................................3
      5.1. Details of S-BFD Control Packets Sent by SBFDInitiator .....4
           5.1.1. Target versus Remote Entity (S-BFD Discriminator) ...4
   6. Responder Procedures ............................................5
      6.1. Details of S-BFD Control Packets Sent by SBFDReflector .....5
   7. Security Considerations .........................................6
   8. IANA Considerations .............................................6
   9. References ......................................................7
      9.1. Normative References .......................................7
      9.2. Informative References .....................................7
   Acknowledgements ...................................................8
   Contributors .......................................................8
   Authors' Addresses .................................................8

1.  Introduction

   Seamless Bidirectional Forwarding Detection (S-BFD) [RFC7880] defines
   a generalized mechanism to allow network nodes to seamlessly perform
   continuity checks to remote entities.  This document defines
   necessary procedures for using S-BFD in IPv4, IPv6, and MPLS
   environments.

   The reader is expected to be familiar with the IP [RFC791] [RFC2460],
   BFD [RFC5880], MPLS BFD [RFC5884], and S-BFD [RFC7880] terms and
   protocol constructs.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  S-BFD UDP Port

   A new UDP port is defined for use by S-BFD in IPv4, IPv6, and MPLS
   environments: 7784.

   In S-BFD Control packets from the SBFDInitiator to the SBFDReflector,
   the SBFDReflector session MUST listen for incoming S-BFD Control
   packets on port 7784.  SBFDInitiator sessions MUST transmit S-BFD
   Control packets with destination port 7784.  The source port of the
   S-BFD Control packets transmitted by SBFDInitiator sessions can be
   any port, with one exception: it MUST NOT be 7784.  The same UDP

   source port number MUST be used for all S-BFD Control packets
   associated with a particular SBFDInitiator session.  The source port
   number is unique among all SBFDInitiator sessions on the system.

   In S-BFD Control packets from the SBFDReflector to the SBFDInitiator,
   the SBFDInitiator session MUST listen for reflected S-BFD Control
   packets at its source port.

3.  S-BFD Echo UDP Port

   The BFD Echo port defined by [RFC5881], port 3785, is used for the
   S-BFD Echo function in IPv4, IPv6, and MPLS environments.
   SBFDInitiator sessions MUST transmit S-BFD Echo packets with
   destination port 3785.  The setting of the UDP source port [RFC5881]
   and the procedures [RFC7880] for the S-BFD Echo function are outside
   the scope of this document.

4.  S-BFD Control Packet Demultiplexing

   S-BFD Control packet demultiplexing follows the procedure specified
   in Section 7.1 of [RFC7880].  A received S-BFD Control packet MUST be
   demultiplexed with the destination UDP port field.

   This procedure for an S-BFD packet is executed on both the initiator
   and the reflector.  If the port is 7784 (i.e., an S-BFD packet for
   the SBFDReflector), then the packet MUST be looked up to locate a
   corresponding SBFDReflector session based on the value from the
   Your Discriminator field in the table describing S-BFD
   Discriminators.  If the port is not 7784 but the packet is
   demultiplexed to be for an SBFDInitiator, then the packet MUST be
   looked up to locate a corresponding SBFDInitiator session based on
   the value from the Your Discriminator field in the table describing
   BFD Discriminators.  In that case, the destination IP address of the
   packet SHOULD be validated to be for itself.  If the packet
   demultiplexes to a classical BFD session, then the procedures from
   [RFC5880] apply.

5.  Initiator Procedures

   S-BFD Control packets are transmitted with an IP header, UDP header,
   and BFD Control packet ([RFC5880]).  When S-BFD Control packets are
   explicitly label switched (i.e., not IP routed and forwarded over a
   Label Switched Path (LSP), but explicitly sent on a specific LSP),
   the former is prepended with a label stack.  Note that this document
   does not make a distinction between a single-hop S-BFD scenario and a
   multi-hop S-BFD scenario; both scenarios are supported.

   The necessary values in the BFD control headers are described in
   [RFC7880].  Section 5.1 describes necessary values in the MPLS
   header, IP header, and UDP header when an SBFDInitiator on the
   initiator is sending S-BFD Control packets.

5.1.  Details of S-BFD Control Packets Sent by SBFDInitiator

   o  Specifications common to both IP-routed S-BFD Control packets and
      explicitly label-switched S-BFD Control packets:

      *  The Source IP Address field of the IP header MUST be set to a
         local IP address that is expected to be routable by the target
         (i.e., not an IPv6 link-local address when the target is
         multiple hops away).

      *  The UDP destination port MUST be set to a well-known UDP
         destination port assigned for S-BFD, i.e., 7784.

      *  The UDP source port MUST NOT be set to 7784.

   o  Specifications for IP-routed S-BFD Control packets:

      *  The Destination IP Address field of the IP header MUST be set
         to an IP address of the target.

      *  The TTL / Hop Limit field of the IP header SHOULD be set
         to 255.

   o  Specifications for explicitly label-switched S-BFD Control
      packets:

      *  S-BFD Control packets MUST have the label stack that is
         expected to reach the target.

      *  The TTL field of the topmost label SHOULD be 255.

      *  The destination IP address MUST be chosen from the 127/8 range
         for IPv4 and from the 0:0:0:0:0:ffff:7f00:0/104 range for IPv6,
         as per [RFC5884].

      *  The TTL / Hop Limit field of the IP header MUST be set to 1.

5.1.1.  Target versus Remote Entity (S-BFD Discriminator)

   Typically, an S-BFD Control packet will have the Your Discriminator
   field corresponding to an S-BFD Discriminator of the remote entity
   located on the target network node defined by the destination IP
   address or the label stack.  It is, however, possible for an

   SBFDInitiator to carefully set the Your Discriminator and TTL fields
   to perform a continuity test in the direction towards a target, but
   destined to a transit network node and not to the target itself.

   Section 5.1 intentionally uses the word "target" instead of "remote
   entity" to accommodate this possible S-BFD usage through TTL expiry.
   This also requires that S-BFD Control packets not be dropped by the
   responder node due to TTL expiry.  Thus, implementations on the
   responder MUST allow received S-BFD Control packets taking a TTL
   expiry exception path to reach the corresponding SBFDReflector
   session.  This is an existing packet-processing exception practice
   for Operations, Administration, and Maintenance (OAM) packets, where
   the control plane further identifies the type of OAM by the protocol
   and port numbers.

6.  Responder Procedures

   S-BFD Control packets are IP routed back to the initiator and will
   have an IP header, UDP header, and BFD control header.  If an
   SBFDReflector receives an S-BFD Control packet with a UDP source port
   of 7784, the packet MUST be discarded.  Necessary values in the BFD
   control header are described in [RFC7880].  Section 6.1 describes
   necessary values in the IP header and UDP header when an
   SBFDReflector on the responder is sending S-BFD Control packets.

6.1.  Details of S-BFD Control Packets Sent by SBFDReflector

   o  The Destination IP Address field of the IP header MUST be copied
      from the Source IP Address field of the received S-BFD Control
      packet.

   o  The Source IP Address field of the IP header MUST be set to a
      local IP address that the initiator expects to be visible (i.e.,
      not an IPv6 link-local address when the initiator is multiple hops
      away).  The source IP address SHOULD be copied from the
      Destination IP Address field of the received S-BFD Control packet,
      except when it is from the 127/8 range for IPv4 or from the
      0:0:0:0:0:ffff:7f00:0/104 range for IPv6.

   o  The TTL / Hop Limit field of the IP header MUST be set to 255.

   o  The UDP destination port MUST be copied from the received UDP
      source port.

   o  The UDP source port MUST be copied from the received UDP
      destination port.

7.  Security Considerations

   Security considerations for S-BFD are discussed in [RFC7880].
   Additionally, implementing the following measures will strengthen
   security aspects of the mechanism described by this document:

   o  Implementations MUST provide filtering capability based on source
      IP addresses of received S-BFD Control packets; see [RFC2827].

   o  Implementations MUST NOT act on received S-BFD Control packets
      containing source Martian IP addresses (i.e., addresses that, by
      application of the current forwarding tables, would not have their
      return traffic routed back to the sender).

   o  Implementations MUST ensure that response S-BFD Control packets
      generated by the SBFDReflector and sent to the initiator have a
      reachable target (e.g., destination IP address).

8.  IANA Considerations

   A new port number value, 7784, was allocated from the "Service Name
   and Transport Protocol Port Number Registry".  The allocated registry
   entry is:

     Service Name (REQUIRED)
       s-bfd

     Transport Protocol(s) (REQUIRED)
       udp

     Assignee (REQUIRED)
       IESG <iesg@ietf.org>

     Contact (REQUIRED)
       IETF Chair <chair@ietf.org>

     Description (REQUIRED)
       Seamless Bidirectional Forwarding Detection (S-BFD)

     Reference (REQUIRED)
       RFC 7881

     Port Number (OPTIONAL)
       7784

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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <http://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,
              <http://www.rfc-editor.org/info/rfc5881>.

   [RFC7880]  Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
              Pallagatti, "Seamless Bidirectional Forwarding Detection
              (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
              <http://www.rfc-editor.org/info/rfc7880>.

9.2.  Informative References

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC791, September 1981,
              <http://www.rfc-editor.org/info/rfc791>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <http://www.rfc-editor.org/info/rfc2827>.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
              June 2010, <http://www.rfc-editor.org/info/rfc5884>.

Acknowledgements

   The authors would like to thank the BFD WG members for helping to
   shape the contents of this document.  In particular, significant
   contributions were made by the following people: Marc Binderberger,
   Jeffrey Haas, Santosh Pallagatti, Greg Mirsky, Sam Aldrin, Vengada
   Prasad Govindan, Mallik Mudigonda, and Srihari Raghavan.

Contributors

   The following are key contributors to this document:

      Tarek Saad, Cisco Systems, Inc.
      Siva Sivabalan, Cisco Systems, Inc.
      Nagendra Kumar, Cisco Systems, Inc.

Authors' Addresses

   Carlos Pignataro
   Cisco Systems, Inc.

   Email: cpignata@cisco.com


   Dave Ward
   Cisco Systems, Inc.

   Email: wardd@cisco.com


   Nobo Akiya
   Big Switch Networks

   Email: nobo.akiya.dev@gmail.com