



Link State Routing                                    K. Talaulikar, Ed.
Internet-Draft                                                 P. Psenak
Updates: 2328 (if approved)                          Cisco Systems, Inc.
Intended status: Standards Track                                   A. Fu
Expires: 9 April 2023                                          Bloomberg
                                                               M. Rajesh
                                                        Juniper Networks
                                                          6 October 2022


                          OSPF BFD Strict-Mode
                 draft-ietf-lsr-ospf-bfd-strict-mode-10

Abstract

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

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 9 April 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  LLS B-bit Flag  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Local Interface IPv4 Address TLV  . . . . . . . . . . . . . .   4
   4.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  OSPFv3 IPv4 Address-Family Specifics  . . . . . . . . . .   6
     4.2.  Graceful Restart Considerations . . . . . . . . . . . . .   7
   5.  Operations & Management Considerations  . . . . . . . . . . .   7
   6.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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 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, in
   implementations prior to the extensions specified in this document,
   an OSPF adjacency may still get established over such a link but
   given the more aggressive monitoring intervals supported by BFD, a



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   BFD session may not get established and/or may flap over it.  The
   traffic that gets forwarded over such a link would experience packet
   drops and the failure of the BFD session establishment would not
   enable fast routing convergence.  OSPF adjacency flaps may occur over
   such links as 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 to not 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 both 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".  It introduces the signaling support in OSPF to
   achieve the blocking of adjacency formation until BFD session
   establishment 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
   for OSPFv3 Link-Local Signalling (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 Two-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]
   procedures.

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

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.







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2.  LLS B-bit Flag

   This document defines the B-bit in the LLS Type 1 Extended Options
   and Flags field.  This bit is defined for the LLS block included in
   Hello and Database Description (DD) packets and 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 TLV 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                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: 21

      Length: 4 octets

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

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
   first with that neighbor before bringing up the OSPF adjacency as
   described further in this section.





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   This document updates the OSPF neighbor state machine as described in
   [RFC2328].  Specifically, the operations related to the Init state
   are modified as 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 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
      Hello packet LLS Extended Options and Flags.

   Whenever the neighbor state transitions to Down state, the removal of
   the BFD session associated with that neighbor is requested by OSPF
   and subsequent BFD session establishment is similarly requested by
   OSPF upon transitioning into Init state.  This may result in the
   deletion and creation of the BFD session 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 its Hello LLS options.  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
   state 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 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



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   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 its subsequent Hello LLS
   options.  Disabling OSPF BFD strict-mode has no effect on BFD
   operations and would not result in bringing down of any established
   BFD sessions.  Disabling BFD would result in the BFD session being
   brought down due to Admin reason [RFC5882] and hence 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 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 non-DR 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 Address-Family Specifics

   Multiple AF support 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 AF, it is required that BFD is used to monitor and verify
   IPv4 data plane connectivity between the routers on the link and,
   hence, the BFD session is setup 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 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 of 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 of their Hello packets
   whenever the B-bit is also set in the LLS Options and Flags field.  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 IPv4 AF OSPFv3 instances.



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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
   GR aspects discussed in section 3.3 of [RFC5882] also apply with OSPF
   BFD strict-mode.  Additionally, in OSPF BFD strict-mode, since the
   OSPF adjacency formation is delayed until the BFD session
   establishment, 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 & 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.
   This in turn may cause routing churn.  The use of OSPF BFD strict-
   mode along with mechanisms such as hold-down (a delay in the initial
   OSPF adjacency bringup following BFD session establishment) and/or
   dampening (a delay in the OSPF adjacency bringup 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.

   [I-D.ietf-ospf-yang] specifies the base OSPF YANG model.  The
   required configuration and operational elements for this feature are
   expected to be introduce as augmentation to this base OSPF YANG
   model.

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



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   which results in the router not advertising the B-bit and BFD
   operation being performed in the same way as prior to this
   specification.

   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 of 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 of 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 implementations 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.

   IANA is requested to make permanent the following values that have
   been assigned via early allocation:

   o In the "LLS Type 1 Extended Options and Flags" registry, the B-bit
   is assigned the bit position 0x00000010

   o In the "Link Local Signaling TLV Identifiers (LLS Types)" registry,
   the Type 21 is 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
   failure to detect 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.









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9.  Acknowledgements

   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
   Hardaker.

   The authors would like to acknowledge Dylan van Oudheusden for
   highlighting the problems in using OSPF BFD strict-mode for BFD
   session for IPv4 AF instance with OSPFv3 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.

10.  References

10.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC5613]  Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
              Yeung, "OSPF Link-Local Signaling", RFC 5613,
              DOI 10.17487/RFC5613, August 2009,
              <https://www.rfc-editor.org/info/rfc5613>.

   [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,
              <https://www.rfc-editor.org/info/rfc5838>.

   [RFC5882]  Katz, D. and D. Ward, "Generic Application of
              Bidirectional Forwarding Detection (BFD)", RFC 5882,
              DOI 10.17487/RFC5882, June 2010,
              <https://www.rfc-editor.org/info/rfc5882>.





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   [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>.

10.2.  Informative References

   [I-D.ietf-ospf-yang]
              Yeung, D., Qu, Y., Zhang, J., Chen, I., and A. Lindem,
              "YANG Data Model for OSPF Protocol", Work in Progress,
              Internet-Draft, draft-ietf-ospf-yang-29, 17 October 2019,
              <https://www.ietf.org/archive/id/draft-ietf-ospf-yang-
              29.txt>.

   [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,
              <https://www.rfc-editor.org/info/rfc5880>.

   [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,
              <https://www.rfc-editor.org/info/rfc6213>.

   [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,
              <https://www.rfc-editor.org/info/rfc7474>.

Authors' Addresses

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










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


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


   Rajesh M
   Juniper Networks
   India
   Email: mrajesh@juniper.net
































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