RFC 9355

Internet Engineering Task Force (IETF)                K. Talaulikar, Ed.
Request for Comments: 9355                                     P. Psenak
Updates: 2328                                        Cisco Systems, Inc.
Category: Standards Track                                          A. Fu
ISSN: 2070-1721                                                Bloomberg
                                                               M. Rajesh
                                                        Juniper Networks
                                                           February 2023

       OSPF Bidirectional Forwarding Detection (BFD) Strict-Mode


   This document specifies the extensions to OSPF that enable an OSPF
   router to signal the requirement for a Bidirectional Forwarding
   Detection (BFD) session prior to adjacency formation.  Link-Local
   Signaling (LLS) is used to advertise the requirement for strict-mode
   BFD session establishment for an OSPF adjacency.  If both OSPF
   neighbors advertise BFD strict-mode, adjacency formation will be
   blocked until a BFD session has been successfully established.

   This document updates RFC 2328 by augmenting the OSPF neighbor state
   machine with a check for BFD session up before progression from Init
   to 2-Way state when operating in OSPF BFD strict-mode.

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

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  LLS B-Bit Flag
   3.  Local Interface IPv4 Address TLV
   4.  Procedures
     4.1.  OSPFv3 IPv4 AF Specifics
     4.2.  Graceful Restart Considerations
   5.  Operations and Management Considerations
   6.  Backward Compatibility
   7.  IANA Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References

   Authors' Addresses

1.  Introduction

   Bidirectional Forwarding Detection (BFD) [RFC5880] enables routers to
   monitor data plane connectivity and to detect faults in the
   bidirectional path between them.  BFD is leveraged by routing
   protocols like OSPFv2 [RFC2328] and OSPFv3 [RFC5340] to detect
   connectivity failures for established adjacencies faster than the
   OSPF Hello dead timer detection and to trigger rerouting of traffic
   around the failure.  The use of BFD for monitoring routing protocol
   adjacencies is described in [RFC5882].

   When BFD monitoring is enabled for OSPF adjacencies by the network
   operator, the BFD session is bootstrapped based on the neighbor
   address information discovered by the exchange of OSPF Hello packets.
   Faults in the bidirectional forwarding detected via BFD then result
   in the OSPF adjacency being brought down.  A degraded or poor-quality
   link may result in intermittent packet drops.  In such scenarios,
   implementations prior to the extensions specified in this document
   may still get an OSPF adjacency established over such a link;
   however, given the more aggressive monitoring intervals supported by
   BFD, a BFD session may not get established and/or may flap.  The
   traffic forwarded over such a link would experience packet drops, and
   the failure of the BFD session establishment will not enable fast
   routing convergence.  OSPF adjacency flaps may occur over such links
   when OSPF brings up the adjacency only for it to be brought down
   again by BFD.

   To avoid the routing churn associated with these scenarios, it would
   be beneficial not to allow OSPF to establish an adjacency until a BFD
   session is successfully established and has stabilized.  However,
   this would preclude the OSPF operation in an environment where not
   all OSPF routers support BFD and have it enabled on the link.  A
   solution is to block OSPF adjacency establishment until a BFD session
   is established as long as both neighbors advertise such a
   requirement.  Such a mode of OSPF BFD usage is referred to as
   "strict-mode".  Strict-mode introduces signaling support in OSPF to
   achieve the blocking of adjacency formation until BFD session
   establishment occurs, as described in Section 4.1 of [RFC5882].

   This document specifies the OSPF protocol extensions using Link-Local
   Signaling (LLS) [RFC5613] for a router to indicate to its neighbor
   the willingness to require BFD strict-mode for OSPF adjacency
   establishment (refer to Section 2).  It also introduces an extension
   to OSPFv3 LLS of the interface IPv4 address (refer to Section 3) to
   be used for the BFD session setup when OSPFv3 is used for an IPv4
   Address Family (AF) instance.

   This document updates [RFC2328] by augmenting the OSPF neighbor state
   machine with a check for BFD session up before progression from Init
   to 2-Way state when operating in OSPF BFD strict-mode.

   The extensions and procedures for OSPF BFD strict-mode also apply for
   adjacency over virtual links using BFD multi-hop [RFC5883]

   A similar functionality for IS-IS is specified in [RFC6213].

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  LLS B-Bit Flag

   This document defines the B-bit in the LLS Type 1 Extended Options
   and Flags.  This bit is defined for the LLS block that is included in
   Hello and Database Description (DD) packets.  The B-bit indicates
   that BFD is enabled on the link and that the router requests OSPF BFD
   strict-mode.  Section 7 describes the position of the B-bit.

   A router MUST include the LLS block with the B-bit set in the LLS
   Type 1 Extended Options and Flags in its Hello and DD packets when
   OSPF BFD strict-mode is enabled on the link.

3.  Local Interface IPv4 Address TLV

   The Local Interface IPv4 Address TLV is an LLS TLV defined for OSPFv3
   IPv4 AF instance [RFC5838] protocol operation as described in
   Section 4.1.

   It has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |              Type             |             Length            |
   |                 Local Interface IPv4 Address                  |


   Type:  21

   Length:  4 octets

   Local Interface IPv4 Address:  The primary IPv4 address of the local

4.  Procedures

   A router supporting OSPF BFD strict-mode advertises this capability
   through its Hello packets as described in Section 2.  When a router
   supporting OSPF BFD strict-mode discovers a new neighbor router that
   also supports OSPF BFD strict-mode, it will establish a BFD session
   with that neighbor first before bringing up the OSPF adjacency as
   described further in this section.

   This document updates the OSPF neighbor state machine as described in
   [RFC2328].  Specifically, the operations related to the Init state
   are modified as described below when OSPF BFD strict-mode is used:

   Init (without OSPF BFD strict-mode):
      In this state, a Hello packet has recently been received from the
      neighbor.  However, bidirectional communication has not yet been
      established with the neighbor (i.e., the router itself did not
      appear in the neighbor's Hello packet).  All neighbors in this
      state (or higher) are listed in the Hello packets sent from the
      associated interface.

   Init (with OSPF BFD strict-mode):
      In this state, a Hello packet has recently been received from the
      neighbor.  However, bidirectional communication has not yet been
      established with the neighbor (i.e., the router itself did not
      appear in the neighbor's Hello packet).  BFD session establishment
      with the neighbor is requested if it's not already completed
      (e.g., in the event of transition from 2-Way state).  Neighbors in
      Init state or higher will be listed in Hello packets associated
      with the interface if they either have a corresponding BFD session
      established or have not advertised OSPF BFD strict-mode in the LLS
      Type 1 Extended Options and Flags advertised in the Hello packet.

   When the neighbor state transitions to Down state, the removal of the
   BFD session associated with that neighbor is requested by OSPF;
   subsequent BFD session establishment is similarly requested by OSPF
   upon transitioning into Init state.  This may result in BFD session
   deletion and creation, respectively, when OSPF is the only client
   interested in the BFD session with the neighbor address.

   An implementation MUST NOT wait for BFD session establishment in Init
   state unless OSPF BFD strict-mode is enabled by the operator on the
   interface and the specific neighbor indicates OSPF BFD strict-mode
   capability via the LLS Type 1 Extended Options and Flags advertised
   in the Hello packet.  When BFD is enabled, but OSPF BFD strict-mode
   has not been signaled by both neighbors, an implementation SHOULD
   start BFD session establishment only in 2-Way or greater state.  This
   makes it possible for an OSPF router to support BFD operation in both
   strict-mode and normal mode across different interfaces or even
   across different neighbors on the same multi-access interface.

   Once the OSPF state machine has moved beyond the Init state, any
   change in the B-bit advertised in subsequent Hello packets MUST NOT
   result in any trigger in either the OSPF adjacency or the BFD session
   management (i.e., the B-bit is considered only when in Init state).
   Disabling BFD (or OSPF BFD strict-mode) on an OSPF interface would
   result in it not setting the B-bit in the LLS Type 1 Extended Options
   and Flags advertised in subsequent Hello packets.  Disabling OSPF BFD
   strict-mode has no effect on BFD operations and would not result in
   the bringing down of any established BFD sessions.  Disabling BFD
   would result in the BFD session being brought down due to AdminDown
   State (described in Section 3.2 of [RFC5882]); hence, it would not
   bring down the OSPF adjacency.

   When BFD is enabled on an interface over which we already have an
   existing OSPF adjacency, it would result in the router setting the
   B-bit in its subsequent Hello packets and the initiation of BFD
   session establishment to the neighbor.  If the adjacency is already
   up (i.e., in its terminal state of Full or 2-Way with routers that
   are not designated routers on a multi-access interface) with a
   neighbor that also supports OSPF BFD strict-mode, then an
   implementation SHOULD NOT bring this adjacency down into the Init
   state to avoid disruption to routing operations and instead use the
   OSPF BFD strict-mode wait only after a transition to Init state.
   However, if the adjacency is not up, then an implementation MAY bring
   such an adjacency down so it can use the OSPF BFD strict-mode for its
   adjacency establishment.

4.1.  OSPFv3 IPv4 AF Specifics

   Support for multiple AFs in OSPFv3 [RFC5838] requires the use of an
   IPv6 link-local address as the source address for Hello packets, even
   when forming adjacencies for IPv4 AF instances.  In most deployments
   of OSPFv3 IPv4 AFs, it is required that BFD is used to monitor and
   verify IPv4 data plane connectivity between the routers on the link;
   hence, the BFD session is set up using IPv4 neighbor addresses.  The
   IPv4 neighbor address on the interface is learned only later in the
   adjacency formation process when the neighbor's Link-LSA (Link State
   Advertisement) is received.  This results in the setup of the BFD
   IPv4 session either after the adjacency is established or later in
   the adjacency formation sequence.

   To operate in OSPF BFD strict-mode, it is necessary for an OSPF
   router to learn its neighbor's IPv4 link address during the Init
   state of adjacency formation (ideally, when it receives the first
   Hello).  The use of the Local Interface IPv4 Address TLV (as defined
   in Section 3) in the LLS block advertised in OSPFv3 Hello packets for
   IPv4 AF instances makes this possible.  Implementations that support
   OSPF BFD strict-mode for OSPFv3 IPv4 AF instances MUST include the
   Local Interface IPv4 Address TLV in the LLS block advertised in their
   Hello packets whenever the B-bit is also set in the LLS Type 1
   Extended Options and Flags.  A receiver MUST ignore the B-bit (i.e.,
   not operate in strict-mode for BFD) when the Local Interface IPv4
   Address TLV is not present in OSPFv3 Hello messages for OSPFv3 IPv4
   AF instances.

4.2.  Graceful Restart Considerations

   An implementation needs to handle scenarios where both graceful
   restart (GR) and the OSPF BFD strict-mode are deployed together.  The
   graceful restart aspects related to process restart scenarios
   discussed in Section 3.3 of [RFC5882] also apply with OSPF BFD
   strict-mode.  Additionally, since the OSPF adjacency formation is
   delayed until the BFD session establishment in OSPF BFD strict-mode,
   the resultant delay in adjacency formation may affect or break the
   GR-based recovery.  In such cases, it is RECOMMENDED that the GR
   timers are set such that they provide sufficient time to allow for
   normal BFD session establishment delays.

5.  Operations and Management Considerations

   An implementation SHOULD report the BFD session status along with the
   OSPF Init adjacency state when OSPF BFD strict-mode is enabled and
   support logging operations on neighbor state transitions that include
   the BFD events.  This allows an operator to detect scenarios where an
   OSPF adjacency may be stuck waiting for BFD session establishment.

   In network deployments with noisy or degraded links with intermittent
   packet loss, BFD sessions may flap, resulting in OSPF adjacency
   flaps.  In turn, this may cause routing churn.  The use of OSPF BFD
   strict-mode along with mechanisms such as hold-down (a delay in
   bringing up the initial OSPF adjacency following BFD session
   establishment) and/or dampening (a delay in bringing up the OSPF
   adjacency following failure detected by BFD) may help reduce the
   frequency of adjacency flaps and therefore reduce the associated
   routing churn.  The details of these mechanisms are outside the scope
   of this document.

   [RFC9129] specifies the base OSPF YANG module.  The required
   configuration and operational elements for this feature are expected
   to be introduced as augmentation to this base OSPF YANG module.

6.  Backward Compatibility

   An implementation MUST support OSPF adjacency formation and
   operations with a neighbor router that does not advertise the OSPF
   BFD strict-mode capability: both when that neighbor router does not
   support BFD and when it does support BFD but does not signal the OSPF
   BFD strict-mode as described in this document.  Implementations MAY
   provide a local configuration option to force BFD operation only in
   OSPF BFD strict-mode (i.e, adjacency will not come up unless BFD
   session is established).  In this case, an OSPF adjacency with a
   neighbor that does not support OSPF BFD strict-mode would not be
   established successfully.  Implementations MAY provide a local
   configuration option to enable BFD without the OSPF BFD strict-mode,
   which results in the router not advertising the B-bit and BFD
   operation being performed in the same way as prior to this

   The signaling specified in this document happens at a link-local
   level between routers on that link.  A router that does not support
   this specification would ignore the B-bit in the LLS block advertised
   in Hello packets from its neighbors and continue to establish BFD
   sessions (if enabled) without delaying the OSPF adjacency formation.
   Since a router that does not support this specification would not
   have set the B-bit in the LLS block advertised in its own Hello
   packets, its neighbor routers supporting this specification would not
   use OSPF BFD strict-mode with such OSPF routers.  As a result, the
   behavior would be the same as without this specification.  Therefore,
   there are no backward compatibility issues or implementation
   considerations beyond what is specified herein.

7.  IANA Considerations

   This specification makes the following updates under the "Open
   Shortest Path First (OSPF) Link Local Signaling (LLS) - Type/Length/
   Value Identifiers (TLV)" parameters.

   *  In the "LLS Type 1 Extended Options and Flags" registry, the B-bit
      has been assigned the bit position 0x00000010.

   *  In the "Link Local Signaling TLV Identifiers (LLS Types)"
      registry, the Type 21 has been assigned to the Local Interface
      IPv4 Address TLV.

8.  Security Considerations

   The security considerations for "OSPF Link-Local Signaling" [RFC5613]
   also apply to the extension described in this document.
   Inappropriate use of the B-bit in the LLS block of an OSPF Hello
   message could prevent an OSPF adjacency from forming or lead to the
   failure of detecting bidirectional forwarding failures.  If
   authentication is being used in the OSPF routing domain [RFC5709]
   [RFC7474], then the Cryptographic Authentication TLV [RFC5613] MUST
   also be used to protect the contents of the LLS block.

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,

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,

   [RFC5613]  Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
              Yeung, "OSPF Link-Local Signaling", RFC 5613,
              DOI 10.17487/RFC5613, August 2009,

   [RFC5838]  Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and
              R. Aggarwal, "Support of Address Families in OSPFv3",
              RFC 5838, DOI 10.17487/RFC5838, April 2010,

   [RFC5882]  Katz, D. and D. Ward, "Generic Application of
              Bidirectional Forwarding Detection (BFD)", RFC 5882,
              DOI 10.17487/RFC5882, June 2010,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [RFC5709]  Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, DOI 10.17487/RFC5709, October
              2009, <https://www.rfc-editor.org/info/rfc5709>.

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

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
              June 2010, <https://www.rfc-editor.org/info/rfc5883>.

   [RFC6213]  Hopps, C. and L. Ginsberg, "IS-IS BFD-Enabled TLV",
              RFC 6213, DOI 10.17487/RFC6213, April 2011,

   [RFC7474]  Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
              "Security Extension for OSPFv2 When Using Manual Key
              Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,

   [RFC9129]  Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem,
              "YANG Data Model for the OSPF Protocol", RFC 9129,
              DOI 10.17487/RFC9129, October 2022,


   The authors would like to acknowledge the review and inputs from Acee
   Lindem, Manish Gupta, Balaji Ganesh, Les Ginsberg, Robert Raszuk,
   Gyan Mishra, Muthu Arul Mozhi Perumal, Russ Housley, and Wes

   The authors would like to acknowledge Dylan van Oudheusden for
   highlighting the problems in using OSPF BFD strict-mode for BFD
   sessions for OSPFv3 IPv4 AF instances and Baalajee S for his
   suggestions on the approach to address it.

   The authors would like to thank John Scudder for his AD review and
   suggestions to improve the document.

Authors' Addresses

   Ketan Talaulikar (editor)
   Cisco Systems, Inc.
   Email: ketant.ietf@gmail.com

   Peter Psenak
   Cisco Systems, Inc.
   Apollo Business Center
   Mlynske nivy 43
   821 09 Bratislava
   Email: ppsenak@cisco.com

   Albert Fu
   United States of America
   Email: afu14@bloomberg.net

   Rajesh M
   Juniper Networks
   Email: mrajesh@juniper.net