Network Working Group                                            J. Haas
Internet-Draft                                    Juniper Networks, Inc.
Intended status: Standards Track                                   A. Fu
Expires: February 27, 2020                                Bloomberg L.P.
                                                         August 26, 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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 27, 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
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   ( in effect on the date of
   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.  BFD Encapsulated in Large Packets . . . . . . . . . . . . . .   3
   3.  Implementation and Deployment Considerations  . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Related Features . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

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
   (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.

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   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

   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.  Additional changes to the base BFD
   protocol may be required to permit negotiation of this functionality
   and the padding value.

   It is also worthy of note that even if an implementation can function
   with larger transport PDUs, that additional packet size may have
   impact on BFD scaling.  Such systems may support a lower transmission
   interval (bfd.DesiredMinTxInterval) when operating in large packet
   mode.  This interval may depend on the size of the transport PDU.

   Given the impact on scaling larger PDU sizes may have on BFD
   implementations, operators should consider applying it only in
   situations where there is appropriate concern for path MTU.  An
   example of this is commercial WAN services.

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   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

   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.

5.  IANA Considerations

   This document introduces no additional considerations to IANA.

6.  References

6.1.  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, <>.

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   [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,

6.2.  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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