Network Working Group                                            J. Haas
Internet-Draft                                    Juniper Networks, Inc.
Intended status: Standards Track                                   A. Fu
Expires: May 4, 2020                                      Bloomberg L.P.
                                                        November 1, 2019

                   BFD Encapsulated in Large Packets


   The Bidirectional Forwarding Detection (BFD) protocol is commonly
   used to verify connectivity between two systems.  BFD packets are
   typically very small.  It is desirable in some circumstances to know
   that not only is the path between two systems reachable, but also
   that it is capable of carrying a payload of a particular size.  This
   document discusses thoughts on how to implement such a mechanism
   using BFD in Asynchronous mode.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   be interpreted as described in [RFC2119] only when they appear in all
   upper case.  They may also appear in lower or mixed case as English
   words, without normative meaning.

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

   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 4, 2020.

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Copyright Notice

   Copyright (c) 2019 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
   ( 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 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.  BFD Encapsulated in Large Packets . . . . . . . . . . . . . .   3
   3.  Implementation and Deployment Considerations  . . . . . . . .   3
     3.1.  Implementations that do not support Large BFD Packets . .   3
     3.2.  Selecting MTU size to be detected . . . . . . . . . . . .   4
     3.3.  Detecting MTU mismatches  . . . . . . . . . . . . . . . .   4
     3.4.  Equal Cost Multiple Paths (ECMP) or other Load Balancing
           Considerations  . . . . . . . . . . . . . . . . . . . . .   5
     3.5.  S-BFD . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Related Features . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The Bidirectional Forwarding Detection (BFD) [RFC5880] protocol is
   commonly used to verify connectivity between two systems.  However,
   some applications may require that the Path MTU [RFC1191] between
   those two systems meets a certain minimum criteria.  When the Path
   MTU decreases below the minimum threshold, those applications may
   wish to consider the path unusable.

   BFD may be encapsulated in a number of transport protocols.  An
   example of this is single-hop BFD [RFC5881].  In that case, the link
   MTU configuration is typically enough to guarantee communication
   between the two systems for that size MTU.  BFD Echo mode

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   (Section 6.4 of [RFC5880]) is sufficient to permit verification of
   the Path MTU of such directly connected systems.  Previous proposals
   ([I-D.haas-xiao-bfd-echo-path-mtu]) have been made for testing Path
   MTU for such directly connected systems.  However, in the case of
   multi-hop BFD [RFC5883], this guarantee does not hold.

   The encapsulation of BFD in multi-hop sessions is a simple UDP
   packet.  The BFD elements of procedure (Section 6.8.6 of [RFC5880])
   covers validating the BFD payload.  However, the specification is
   silent on the length of the encapsulation that is carrying the BFD
   PDU.  While it is most common that the transport protocol payload
   (i.e.  UDP) length is the exact size of the BFD PDU, this is not
   required by the elements of procedure.  This leads to the possibility
   that the transport protocol length may be larger than the contained

2.  BFD Encapsulated in Large Packets

   Support for BFD between two systems is typically configured, even if
   the actual session may be dynamically created by a client protocol.
   A new BFD variable is defined in this document:

       The BFD transport protocol payload size is increased to this
       value.  The contents of this additional payload MUST be zero.
       The minimum size of this variable MUST NOT be smaller than
       permitted by the element of BFD procedure; 24 or 26 - see
       Section 6.8.6 of [RFC5880].

   The Don't Fragment bit (Section 2.3 of [RFC0791]) of the IP payload,
   when using IPv4 encapsulation, MUST be set.

3.  Implementation and Deployment Considerations

3.1.  Implementations that do not support Large BFD Packets

   While this document proposes no change to the BFD protocol,
   implementations may not permit arbitrarily padded transport PDUs to
   carry BFD packets.  While Section 6 of [RFC5880] warns against
   excessive pedantry, implementations may not work with this mechanism
   without additional support.

   [RFC5880], section 6.8.6, discusses the procedures for receiving BFD
   Control packets.  When an implementation is incapable of processing
   Large BFD Packets, it could manifest in one of two possible ways:

   o  A receiving BFD implementation is incapable of accepting Large BFD
      Packets.  This is identical to the packet being discarded.

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   o  A receiving BFD implementation is capable of accepting Large BFD
      Packets, but the Control packet is improperly rejected during
      validation procedures.  This is identical to the packet being

   In each of these cases, the BFD state machine would behave as if it
   were not receiving Control packets and the implementation would
   follow normal BFD procedures with regards to not having received
   Control packets.

3.2.  Selecting MTU size to be detected

   Since the consideration is path MTU, BFD sessions using this feature
   only need to use a bfd.PaddedPduSize appropriate to exercise the path
   MTU for the desired application.  This may be significantly smaller
   than the system's link MTU; e.g. desired path MTU is 1500 bytes while
   the interface MTU that BFD with large packets is running on is 9000

   In the case multiple BFD clients desire to test the same BFD
   endpoints using different bfd.PaddedPduSize parameters,
   implementations should select the largest bfd.PaddedPduSize parameter
   from the configured sessions.  This is similar to how implementations
   of BFD select the most aggressive timing parameters for multiple
   sessions to the same endpoint.

3.3.  Detecting MTU mismatches

   The accepted MTU for an interface is impacted by packet encapsulation
   considerations at a given layer; e.g. layer 2, layer 3, tunnel, etc.
   A common misconfiguration of interface parameters is inconsistent
   MTU.  In the presence of inconsistent MTU, it is possible for
   applications to have unidirectional connectivity.

   When it is necessary for an application using BFD with Large Packets
   to test the bi-directional Path MTU, it is necessary to configure the
   bfd.PaddedPduSize parameter on both sides of an interface.  E.g., if
   the desire is to verify a 1500 byte MTU in both directions on an
   Ethernet or point to point link, each side of the BFD session must
   have bfd.PaddedPduSize set to 1500.  In the absence of such
   consistent configuration, BFD with Large Packets may correctly
   determine unidirectional connectivity at the tested MTU, but bi-
   directional MTU may not be properly validated.

   It should be noted that some interfaces may intentionally have
   different MTUs.  Setting the bfd.PaddedPduSize appropriately for each
   side of the interface supports such scenarios.

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3.4.  Equal Cost Multiple Paths (ECMP) or other Load Balancing

   Various mechanisms are utilized to increase throughput between two
   endpoints at various network layers.  Such features include Link
   Aggregate Groups (LAGs) or ECMP forwarding.  Such mechanisms balance
   traffic across multiple physical links while hiding the details of
   that balacing from the higher networking layers.  The details of that
   balancing are highly implementation specific.

   In the presence of such load balancing mechanisms, it is possible to
   have member links that are not properly forwarding traffic.  In such
   circumstances, this will result in dropped traffic when traffic is
   chosen to be load balanced across those member links.

   Such load balancing mechanisms may not permit all link members to be
   properly tested by BFD.  This is because the BFD Control packets may
   be forwarded only along links that are up.  BFD on LAG, [RFC7130],
   was developed to help cover one such scenario.  However, for testing
   forwarding over multiple hops, there is no such specified general
   purpose BFD mechanism for exercising all links in an ECMP.  This may
   result in a BFD session being in the Up state while some traffic may
   be dropped or otherwise negatively impacted along some component

   Some BFD implementations utilize their internal understanding of the
   component links and their resultant forwarding to exercise BFD in
   such a way to better test the ECMP members and to tie the BFD session
   state to the health of that ECMP.  Due to the implementation specific
   load balancing, it is not possible to standardize such additional
   mechanisms for BFD.

   Mis-configuration of some member MTUs may lead to Load Balancing that
   may have an inconsistent Path MTU depending on how the traffic is
   balanced.  While the intent of BFD with Large Packets is to verify
   path MTU, it is subject to the same considerations above.

3.5.  S-BFD

   This mechanism also can be applied to other forms of BFD, including
   S-BFD [RFC7880].

4.  Security Considerations

   This document does not change the underlying security considerations
   of the BFD protocol or its encapsulations.

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5.  IANA Considerations

   This document introduces no additional considerations to IANA.

6.  Acknowledgments

   The authors would like to thank Les Ginsberg, Mahesh Jethandani,
   Robert Raszuk, and Ketan Talaulikar, for their valuable feedback on
   this proposal.

7.  References

7.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981, <https://www.rfc-

   [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-

   [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, <https://www.rfc-

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
              June 2010, <>.

   [RFC7130]  Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed.,
              Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional
              Forwarding Detection (BFD) on Link Aggregation Group (LAG)
              Interfaces", RFC 7130, DOI 10.17487/RFC7130, February
              2014, <>.

   [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,

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7.2.  Informative References

              Haas, J. and M. Xiao, "Application of the BFD Echo
              function for Path MTU Verification or Detection", draft-
              haas-xiao-bfd-echo-path-mtu-01 (work in progress), July

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990, <https://www.rfc-

   [RFC3719]  Parker, J., Ed., "Recommendations for Interoperable
              Networks using Intermediate System to Intermediate System
              (IS-IS)", RFC 3719, DOI 10.17487/RFC3719, February 2004,

Appendix A.  Related Features

   IS-IS [RFC3719] supports a Padding feature for its hellos.  This
   provides the ability to detect inconsistent link MTUs.

Authors' Addresses

   Jeffrey Haas
   Juniper Networks, Inc.
   1133 Innovation Way
   Sunnyvale, CA  94089


   Albert Fu
   Bloomberg L.P.


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