Link State Routing K. Talaulikar Internet-Draft P. Psenak Updates: 2328 (if approved) Cisco Systems, Inc. Intended status: Standards Track A. Fu Expires: 25 March 2023 Bloomberg M. Rajesh Juniper Networks 21 September 2022 OSPF BFD Strict-Mode draft-ietf-lsr-ospf-bfd-strict-mode-08 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 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 25 March 2023. Copyright Notice Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved. Talaulikar, et al. Expires 25 March 2023 [Page 1] Internet-Draft OSPF BFD Strict-Mode September 2022 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 . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 10.1. Normative References . . . . . . . . . . . . . . . . . . 9 10.2. Informative References . . . . . . . . . . . . . . . . . 9 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 Talaulikar, et al. Expires 25 March 2023 [Page 2] Internet-Draft OSPF BFD Strict-Mode September 2022 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 adjaceny 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 establishement 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. Talaulikar, et al. Expires 25 March 2023 [Page 3] Internet-Draft OSPF BFD Strict-Mode September 2022 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. 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. 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: Talaulikar, et al. Expires 25 March 2023 [Page 4] Internet-Draft OSPF BFD Strict-Mode September 2022 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 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 Talaulikar, et al. Expires 25 March 2023 [Page 5] Internet-Draft OSPF BFD Strict-Mode September 2022 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 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. Talaulikar, et al. Expires 25 March 2023 [Page 6] Internet-Draft OSPF BFD Strict-Mode September 2022 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. 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 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 specification. Talaulikar, et al. Expires 25 March 2023 [Page 7] Internet-Draft OSPF BFD Strict-Mode September 2022 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] SHOULD also be used to protect the contents of the LLS block. 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. Talaulikar, et al. Expires 25 March 2023 [Page 8] Internet-Draft OSPF BFD Strict-Mode September 2022 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, . [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, . 10.2. Informative References Talaulikar, et al. Expires 25 March 2023 [Page 9] Internet-Draft OSPF BFD Strict-Mode September 2022 [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, . [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, . [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, . Authors' Addresses Ketan Talaulikar Cisco Systems, Inc. India Email: ketant.ietf@gmail.com 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 Talaulikar, et al. Expires 25 March 2023 [Page 10] Internet-Draft OSPF BFD Strict-Mode September 2022 Email: mrajesh@juniper.net Talaulikar, et al. Expires 25 March 2023 [Page 11]