Network Working Group L. Gong Internet Draft W. Cheng Intended status: Standards Track China Mobile Expires: October 20, 2023 C. Lin M. Chen New H3C Technologies R. Chen ZTE Corporation Y. Liang Ruijie Networks Co., Ltd. April 20, 2023 Advertising Unreachable Links in OSPF draft-gong-lsr-ospf-unreachable-link-01 Abstract This document proposes the method to advertise unreachable 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), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." 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Table of Contents 1. Introduction...................................................2 1.1. Requirements Language.....................................3 2. Use Case.......................................................3 2.1. Case 1: Traffic Engineering...............................3 2.2. Case 2: Flexible Algorithm................................3 3. Solution A: Maximum Link Metric................................4 4. Solution B: Unreachable Link Flag..............................6 5. Backward Compatibility.........................................7 6. Security Considerations........................................7 7. IANA Considerations............................................7 8. References.....................................................7 8.1. Normative References......................................7 8.2. Informative References....................................8 Authors' Addresses................................................9 1. Introduction In some scenarios, there are requirements to advertise unreachable links in OSPF for purposes other than building the normal Shortest Path Tree. One example is a link that is available for Traffic Engineering (TE), but not for hop-by-hop routing. Another example is that specific links with dedicated resources for network slicing are included in Flexible Algorithm (Flex-Algorithm), but should be excluded in the default topology. This document proposes the method to advertise unreachable links in OSPF. Gong, et al. Expires October 20, 2023 [Page 2] Internet-Draft OSPF Unreachable Link April 2023 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. 2. Use Case 2.1. Case 1: Traffic Engineering A network topology is shown in Figure 1. There is a link only available for Traffic Engineering between Node A and E. If that link is reachable in SPF computation, unexpected flows of best-effort service may be steered into it, which is not desired. 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. Node A, B, C and D have an extra link between each other. These links have EAG attribute of "red" color. ****** A------C------E |* |* | |* |* | ******: "red" link |* |* | B------D------F ****** Figure 2: Network Topology Flex-Algorithm 128 are enabled on Node A, B, C and D, with EAG rule of including "red". Flex-Algorithm allows IGP to compute the paths Gong, et al. Expires October 20, 2023 [Page 3] Internet-Draft OSPF Unreachable Link April 2023 along the constrained topology. The topology used by Flex-Algorithm 128 is shown in Figure 3. A******C * * * * * * B******D Figure 3: Topology of Flex-Algorithm 128 Flex-Algorithm 128 are used to transmit particular flows, such as 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 into these links. Therefore, it is a problem that the dedicated links for Flex-Algorithm are still reachable in normal SPF calculation. If all the "red" links are advertised as unreachable, the default topology used in normal SPF calculation will be as Figure 4. So that only the network slice traffics will be steered into the "red" links by Flex-Algorithm 128. A------C------E | | | | | | | | | B------D------F Figure 4: SPF Topology after Excluding Unreachable Links 3. Solution A: Maximum Link Metric In OSPF protocol, there are some inconsistencies when a link is advertised with the maximum link metric (0xffff). [RFC1247] specifies that, if the cost of the link is 0xffff, the link should not be used for data traffic. However, if a router performs an intra-area Dijkstra calculation as specified in [RFC1583] and higher, it do not treat links with maximum link metric as unreachable. [RFC6987] defines the MaxLinkMetric (0xffff) which indicates a router-LSA link to be unreachable, in order to support stub router advertisement. Gong, et al. Expires October 20, 2023 [Page 4] Internet-Draft OSPF Unreachable Link April 2023 About the backward compatibility, [RFC6987] states that "Note that this inconsistency will not lead to routing loops, because if there are some alternate paths in the network, both types of routers will agree on using them rather than the path through the stub router. If the path through the stub router is the only one, the routers of the first type will not use the stub router for transit (which is the desired behavior), while the routers of the second type will still use this path". However, if the MaxLinkMetric is used for general purposes, such inconsistency may lead to routing loops. For example, in the network shown as Figure 5, link D-F is advertised with MaxLinkMetric (65535/0xffff). Router A supports MaxLinkMetric, 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 MaxLinkMetric Causing Loops To improve backward compatibility, this document defines that all routers supporting MaxLinkMetric must advertise a Router Information (RI) LSA with a Router Functional Capabilities TLV [RFC7770] including the following Router Functional Capability Bit: Bit Capabilities TBD MaxLinkMetric support Upon detecting the presence of a reachable Router-LSA without a companion RI LSA that has the bit set, all routers MUST recalculate routes without considering MaxLinkMetric. In addition, this document extends MaxLinkMetric to be applicable for the following TLVs/LSAs as well: o The OSPFv2 Extended Link TLV of OSPFv2 Extended Link Opaque LSA [RFC7684] o The Router-Link TLV of OSPFv3 E-Router-LSA [RFC8362] Gong, et al. Expires October 20, 2023 [Page 5] Internet-Draft OSPF Unreachable Link April 2023 4. Solution B: Unreachable Link Flag A new OSPF Link Flags sub-TLV is defined in OSPF. The format is as the following: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o Type: TBD. o Length: Variable, dependent on the size of the Flags field. MUST be a multiple of 4 octets. o Flags: Following flags are currently defined. 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |U| ~ +-+-+-+-+-+-+-+-+ o U-Flag: Unreachable Link Flag. The associated link MUST be treated as unreachable during SPF calculation. The OSPF Link Flags sub-TLV is advertised in the TLVs/sub-TLVs below: o OSPFv2 Extended Link TLV of OSPFv2 Extended Link Opaque LSA [RFC7684] o Router-Link TLV of OSPFv3 E-Router-LSA [RFC8362] Due to the change of procedures in the SPF calculation, all routers in an area must support the changes specified in this section. To ensure that, if an area is provisioned to support Unreachable Link Flag, all routers supporting this capability must advertise a Router Information (RI) LSA with a Router Functional Capabilities TLV [RFC7770] that includes the following Router Functional Capability Bit: Gong, et al. Expires October 20, 2023 [Page 6] Internet-Draft OSPF Unreachable Link April 2023 Bit Capabilities TBD Unreachable Link Flag support Upon detecting the presence of a reachable Router-LSA without a companion RI LSA that has the bit set, all routers MUST recalculate routes without considering any Unreachable Link Flag. 5. Backward Compatibility Whether using solution A or solution B, all nodes in the same area must support that feature. To avoid topology inconsistence and achieve backward compatibility, routers MUST advertise the corresponding capability as described in Section 3 and Section 4. Upon detecting the absence of that capability from any router in the same area, all routers MUST recalculate routes without considering any unreachable link advertisement. 6. Security Considerations TBD 7. IANA Considerations TBD 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. [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. Gong, et al. Expires October 20, 2023 [Page 7] Internet-Draft OSPF Unreachable Link April 2023 [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. [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 October 20, 2023 [Page 8] Internet-Draft OSPF Unreachable Link April 2023 Authors' Addresses Liyan Gong China Mobile Email: gongliyan@chinamobile.com Weiqiang Cheng China Mobile Email: chengweiqiang@chinamobile.com Changwang Lin New H3C Technologies Email: linchangwang.04414@h3c.com Mengxiao Chen New H3C Technologies Email: chen.mengxiao@h3c.com Ran Chen ZTE Corporation Email: chen.ran@zte.com.cn Yanrong Liang Ruijie Networks Co., Ltd. Email: liangyanrong@ruijie.com.cn Gong, et al. Expires October 20, 2023 [Page 9]