LSR Working Group L. Gong Internet Draft W. Cheng Updates: 6987, 8770 (if approved) China Mobile Intended status: Standards Track C. Lin Expires: April 20, 2025 New H3C Technologies A. Lindem LabN Consulting LLC R. Chen ZTE Corporation October 17, 2024 Advertising Infinity Links in OSPF draft-ietf-lsr-ospf-ls-link-infinity-01 Abstract In certain scenarios, it is necessary to advertise infinity links in OSPF, which should be explicitly excluded from the related SPF calculation. This document proposes the method to advertise infinity links in OSPF. 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/. 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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.....................................2 2. Use Case.......................................................3 2.1. Case 1: Traffic Engineering...............................3 2.2. Case 2: Flexible Algorithm................................3 3. Solution based on LSLinkInfinity...............................4 4. Backward Compatibility.........................................5 4.1. Stub Router Advertisement Backward Compatibility..........6 5. Management Considerations......................................6 6. Security Considerations........................................6 7. IANA Considerations............................................6 8. References.....................................................7 8.1. Normative References......................................7 8.2. Informative References....................................7 Contributors......................................................8 Authors' Addresses................................................8 1. Introduction In specific scenarios, there is a requirement to advertise infinity links in OSPF, which MUST NOT be considered during the standard SPF computation. For example, a link may be available for Traffic Engineering (TE) purposes but not suitable for hop-by-hop routing. Another example involves links with dedicated resources for network slicing included in a Flexible Algorithm (Flex-Algorithm) but excluded from the default topology. This document proposes a mechanism to advertise infinity links in OSPF. 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. Gong, et al. Expires April 20, 2025 [Page 2] Internet-Draft OSPF Infinity Link October 2024 2. Use Case 2.1. Case 1: Traffic Engineering A network topology is shown in Figure 1. There is a link available for Traffic Engineering between Node A and E. If this link is used for SPF calculations, unexpected flows of best-effort service will be steered onto that link. TE Link --------- / \ / \ A------C------E | | | | | | | | | B------D------F Figure 1: Network Topology 2.2. Case 2: Flexible Algorithm A network topology is shown in Figure 2. Nodes A, B, C, and D have an extra link between each other. These links have an Extended Administrative Group (EAG) [RFC7308] attribute specifying the "red" color. ****** A------C------E |* |* | |* |* | ******: "red" link |* |* | B------D------F ****** Figure 2: Network Topology Flex-Algorithm 128 is enabled on Nodes A, B, C, and D, with an EAG rule of including "red" and the Metric-Type is designed to be of a type other than the IGP metric. Flex-Algorithm allows IGP to compute the paths along the constrained topology. The topology used by Flex- Algorithm 128 is shown in Figure 3. Gong, et al. Expires April 20, 2025 [Page 3] Internet-Draft OSPF Infinity Link October 2024 A******C * * * * * * B******D Figure 3: Topology of Flex-Algorithm 128 Flex-Algorithm 128 is used to transmit particular flows, such as those for a network slice. The "red" links used by Flex-Algorithm 128 are sub-interfaces with dedicated queues for bandwidth guarantee. So, it is expected that only the particular flows are transmitted on these links using Flex-Algorithm 128. However, these links are also contained in the default topology used by normal SPF calculation, and unexpected flows of best-effort service may be steered onto these links. Therefore, it is a problem that the dedicated links for Flex-Algorithm are still reachable in base SPF calculation. If the IGP metric of all the "red" links are advertised as infinity, the base topology will be as Figure 4, excluding all the "red" links. This allows only the network slice traffic will be steered into the "red" links by Flex-Algorithm 128. A------C------E | | | | | | | | | B------D------F Figure 4: Base SPF Topology Excluding Infinity Links 3. Solution based on LSLinkInfinity This document specifies that if the IGP metric of a link is advertised as LSLinkInfinity (0xffff), it MUST NOT be considered during the related SPF computation. This applies to both Flex- Algorithm SPF and base SPF as long as the IGP metric is being used. In OSPF protocol, there are some inconsistencies when a link is advertised with the LSLinkInfinity (0xffff). [RFC1247] specified that, if the cost of the link is 0xffff, the link should not be used for data traffic. However, this was changed in [RFC1583] and subsequent OSPF versions to treat links with the cost 0xffff as reachable. Such inconsistency may lead to routing loops. For example, in the network shown as Figure 5, link D-F is advertised with Gong, et al. Expires April 20, 2025 [Page 4] Internet-Draft OSPF Infinity Link October 2024 LSLinkInfinity (65535/0xffff). Router A supports LSLinkInfinity, but router B does not. Router A sees link D-F as reachable, and the shortest path to F is A->B->D->F. Router B sees link D-F as unreachable, and the shortest path to F is B->A->C->E->F. As a result, A forwards the packets to B, but B returns them to A, which causes routing loops. 40000 40000 Traffic: A->F A------C------E A sees link D-F as reachable | | A's shortest path: A->B->D->F 5| |5 B sees link D-F as unreachable | | B's shortest path: B->A->C->E->F B------D------F 5 65535 Figure 5: Inconsistency of LSLinkInfinity Causing Loops To improve backward compatibility, this document defines that all routers supporting LSLinkInfinity must advertise a Router Information (RI) LSA with a Router Informational Capabilities TLV [RFC7770] including the following Router Informational Capability Bit: Bit Capabilities TBD LSLinkInfinity support Upon detecting the change of a Router Information (RI) LSA, all routers in the area MUST recalculate routes. LSLinkInfinity is applicable for the following TLVs/LSAs: o The Router-LSA [RFC2328] and [RFC5340] o The OSPFv2 Extended Link TLV of OSPFv2 Extended Link Opaque LSA [RFC7684] o The Router-Link TLV of OSPFv3 E-Router-LSA [RFC8362] 4. Backward Compatibility To avoid topology inconsistency and achieve backward compatibility, routers MUST advertise the corresponding capability as described in Section 3. Upon detecting the change of a Router Information (RI) LSA, all routers in the area MUST perform SPF calculation again. Gong, et al. Expires April 20, 2025 [Page 5] Internet-Draft OSPF Infinity Link October 2024 4.1. Stub Router Advertisement Backward Compatibility Stub Router Advertisement [RFC6987] defines MaxLinkMetric (0xffff) to indicate a router-LSA link should not be used for transit traffic. This document updates [RFC6987] and [RFC8770]. When an OSPFv2 router supports LSLinkInfinity capability defined in this document, The MaxLinkMetric(0xffff) MUST be updated to MaxReachableLinkMetric(0xfffe). When an OSPFv2 router supports [RFC6987] and the LSLinkInfinity capability defined in this document, it MUST also support [RFC8770]. When announcing itself as a stub router, it MUST set the H-bit in the router-LSA and advertise all its non-stub links with a link cost of MaxReachableLinkMetric (0xfffe). Since MaxLinkMetric will not be used to indicate a link is unreachable unless all OSPFv2 routers support this specification as specified in section 3, all routers will also support the H-bit and the usage of MaxReachableLinkMetric to indicate a link should not be used for transit traffic. An OSPFv3 router can simply use the R-bit [RFC5340] for stub router advertisement. 5. Management Considerations Support of the LSLinkInfinity capability SHOULD be configurable. In some networks, the operator may still want links with maximum metric(0xffff) to be treated as reachable. For example, the auto- costing of links is used and there is a mix of low-speed and high- speed links. In such cases, the updated routers can disable the LSLinkInfinity capability and still treat links with maximum metric as reachable. It is also RECOMMENDED that implementations supporting this document and auto-costing limit the maximum cost to MaxReachableLinkMetric (0xfffe). 6. Security Considerations The document does not introduce any new security issues into the OSPF protocol. 7. IANA Considerations This document defines a new bit in the registry "OSPF Router Informational Capability Bits": Gong, et al. Expires April 20, 2025 [Page 6] Internet-Draft OSPF Infinity Link October 2024 Bit Number Capability Name Reference 0 TBA LSLinkInfinity support This document 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, April 1998, . [RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W., Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute Advertisement", RFC 7684, DOI 10.17487/RFC7684, November 2015, . [RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 7770, DOI 10.17487/RFC7770, February 2016, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, May 2017. [RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and F. Baker, "OSPFv3 Link State Advertisement (LSA) Extensibility", DOI 10.17487/RFC8362, RFC 8362, April 2018, . 8.2. Informative References [RFC1247] Moy, J., "OSPF Version 2", RFC 1247, July 1991. [RFC1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994. [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, . [RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D. McPherson, "OSPF Stub Router Advertisement", RFC 6987, DOI 10.17487/RFC6987, September 2013, . Gong, et al. Expires April 20, 2025 [Page 7] Internet-Draft OSPF Infinity Link October 2024 [RFC7308] Osborne, E., "Extended Administrative Groups in MPLS Traffic Engineering (MPLS-TE)", RFC 7308, DOI 10.17487/RFC7308, July 2014, . [RFC8770] Patel, K., Pillay-Esnault, P., Bhardwaj, M., and S. Bayraktar, "Host Router Support for OSPFv2", RFC 7308, DOI 10.17487/RFC8770, April 2020, . Contributors Mengxiao Chen New H3C Technologies China Email: chen.mengxiao@h3c.com Yanrong Liang Ruijie Networks Co., Ltd. China Email: liangyanrong@ruijie.com.cn Authors' Addresses Liyan Gong China Mobile China Email: gongliyan@chinamobile.com Weiqiang Cheng China Mobile China Email: chengweiqiang@chinamobile.com Changwang Lin New H3C Technologies China Email: linchangwang.04414@h3c.com Gong, et al. Expires April 20, 2025 [Page 8] Internet-Draft OSPF Infinity Link October 2024 Acee Lindem LabN Consulting LLC United States of America Email: acee.ietf@gmail.com Ran Chen ZTE Corporation China Email: chen.ran@zte.com.cn Gong, et al. Expires April 20, 2025 [Page 9]