OSPF K. Patel Internet-Draft Arrcus Updates: 2328,6987 (if approved) P. Pillay-Esnault Intended status: Standards Track Futurewei Expires: January 9, 2020 M. Bhardwaj S. Bayraktar Cisco Systems July 8, 2019 Host Router Support for OSPFv2 draft-ietf-ospf-ospfv2-hbit-08 Abstract The OSPFv2 specifies an SPF algorithm that identifies transit vertices based on their adjacencies. Therefore, OSPFv2 does not have a mechanism to prevent traffic transiting a participating node if it is a transit vertex in the only existing or shortest path to the destination. The use of metrics to make the node undesirable can only help to repel traffic if an alternative better route exists. This document defines the Host-bit functionality to prevent other OSPFv2 routers from using the router for transit traffic in OSPFv2 routing domains. This document updates the Open Shortest Path First v2 specification (OSPFv2 rfc2328) by assigning a new bit (Host-bit) in the OSPF Router-LSA bit registry. In addition, if the Host-bit is set, the calculation of the shortest-path tree for an area, as described in OSPFv2, is modified by including a new check to verify that transit vertices have the Host-bit clear. In addition, this document updates OSPF Stub Router Advertisement (rfc6987) to advertise for type-2 External and NSSA LSAs with a high cost in order to repel traffic effectively. 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." Patel, et al. Expires January 9, 2020 [Page 1] Internet-Draft July 2019 This Internet-Draft will expire on January 9, 2020. Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Host-bit Support . . . . . . . . . . . . . . . . . . . . . . 3 4. SPF Modifications . . . . . . . . . . . . . . . . . . . . . . 5 5. Auto Discovery and Backward Compatibility . . . . . . . . . . 6 6. OSPF AS-External-LSAs/NSSA LSAs with Type 2 Metrics . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . 9 10.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction The OSPFv2 specifies an SPF algorithm that identifies transit vertices based on their adjacencies. Therefore, OSPFv2 does not have a mechanism to prevent traffic transiting a participating node if it is a transit vertex in the only existing or shortest path to the destination. The use of metrics to make the node undesirable can only help to repel traffic if an alternative better route exists. This functionality is particularly useful for a number of use cases: 1. To isolate a router to avoid blackhole scenarios when there is a reload and possible long reconvergence times. Patel, et al. Expires January 9, 2020 [Page 2] Internet-Draft July 2019 2. Closet Switches are usually not used for transit traffic but need to participate in the topology. 3. Overloaded routers could use such a capability to temporarily repel traffic until they stabilize. 4. BGP Route reflectors known as virtual Route Reflectors (vRRs), that are not in the forwarding path but are in central locations such as data centers. Such Route Reflectors typically are used for route distribution and are not capable of forwarding transit traffic. However, they need to learn the OSPF topology to perform spf computation for optimal routes and reachbility resolution for its clients [I-D.ietf-idr-bgp-optimal-route-reflection]. This document defines the Host-bit (H-Bit)functionality to prevent other OSPFv2 routers from using the router for transit traffic in OSPFv2 routing domains. This document updates the [RFC2328] by - assigning the Host-bit in the OSPFv2 Router Properties Registry - if the host-bit is set then the calculation of the shortest-path tree for an area, as described in section 16.1 of [RFC2328], is modified by including a new check to verify that transit vertices DO NOT have the host-bit set. 2. 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. 3. Host-bit Support This document defines a new router-LSA bit known as the Host Bit or the H-bit. An OSPFv2 router advertising a router-LSA with the H-bit set indicates to other OSPFv2 routers in the area supporting the functionality that it MUST NOT be used as a transit router (see section 4). If the host-bit is NOT set routers MUST act transit routers as described in [RFC2328] ensuring backward compatibility. Patel, et al. Expires January 9, 2020 [Page 3] Internet-Draft July 2019 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |H|0|0|N|W|V|E|B| 0 | # links | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | # TOS | metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TOS | 0 | TOS metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | Host Bit in router-LSA 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |H|0|0|N|W|V|E|B| +-+-+-+-+-+-+-+-+ Host Bit Bit H is the high-order bit of the OSPF as shown above. When set, an OSPFv2 router is a Host (non-transit) router and is incapable of forwarding transit traffic. An OSPFv2 router originating a router-LSA with the H-bit set MUST advertise all its router links with a link cost of MaxLinkMetric [RFC6987]. This is to increase the applicability of the H-bit to Patel, et al. Expires January 9, 2020 [Page 4] Internet-Draft July 2019 partial deployments where it is the responsibility of the operator to ensure that OSPFv2 routers not supporting the H-bit do not install routes causing routing loops. When the H-bit is set, an Area Border Router (ABR) MUST advertise the same H-bit setting in its self-originated router-LSAs for all attached areas. The consistency of the setting will prevent inter- area traffic transiting through the router by suppressing the suppressing advertisement of prefixes from other routers in the area in its summary LSAs. ONLY IPv4 prefixes associated with its local interfaces MAY be advertised in summary LSAs to provide reachability to end hosts attached behind a router with the H-bit set. When the H-bit is set cannot act as an AS Boundary Router (ASBR), as ASBR are transit routers to prefixes that are typically imported through redistribution of prefixes of other routing protocols. Therefore, non-local IPv4 prefixes, e.g., those exported from other routing protocols, MUST NOT be advertised in AS-external-LSAs for routers acting permanly as a host. However, in use cases such as an overloaded router or a router being gracefully isolated, these routers are only temporarily acting as host routers and therefore should continue to advertise their External LSAs but ensure they do not attract traffic. In addition to the procedure described above, temporary host routers advertising type 2-metric External LSAs MUST set the metrics to LSInfinity to repel traffic.(see Section 6 of this document). 4. SPF Modifications The SPF calculation described in section 16.1 [RFC2328] will be modified to ensure that the routers originating router-LSAs with the H-bit set will not be used for transit traffic. Step 2 is modified as follows: Patel, et al. Expires January 9, 2020 [Page 5] Internet-Draft July 2019 2) Call the vertex just added to the tree vertex V. Examine the LSA associated with vertex V. This is a lookup in the Area A's link state database based on the Vertex ID. If this is a router-LSA, and the H-bit of the router-LSA is set, and vertex V is not the root, then the router should not be used for transit and step (3) should be executed immediately. If this is a router-LSA, and bit V of the router-LSA (see Section A.4.2) is set, set Area A's TransitCapability to TRUE. In any case, each link described by the LSA gives the cost to an adjacent vertex. For each described link, (say it joins vertex V to vertex W): 5. Auto Discovery and Backward Compatibility To avoid the possibility of any routing loops due to partial deployment, this document defines a OSPF Router Information (RI) LSA [RFC7770] with and area flooding scope and a new bit assigned in the OSPF Router Informational Capability Bits Registry. Bit: Bit Capabilities 7 Host Router Support capability Auto Discovery via announcement of the Host Support Functional Capability ensures that the H-bit functionality and its associated SPF changes MUST only take effect if all the routers in a given OSPF area support this functionality. In normal operations, there is no guarantee that the RI LSA will reach all routers in an area in a timely manner which may result in rooting loops in partial deployments. For example, in a new router joins an area which previous had only H-bit capable routers with H-bit set then it may take some time for the RI to propagate to all routers. The following recommendations will mitigate transient routing loops: o Implementations are RECOMMENDED to provide a configuration parameter to manually override enforcement of the H-bit functionality in partial deployments where the topology guarantees Patel, et al. Expires January 9, 2020 [Page 6] Internet-Draft July 2019 that OSPFv2 routers not supporting the H-bit do not compute routes resulting in routing loops. o All routers, with the H-bit set, MUST advertise all of the router's non-local links with a metric equal to MaxLinkMetric in its LSAs in order to avoid OSPFv2 (unless last resort) routers not supporting the H-bit from attempting to use it for transit traffic. o All routers supporting H-Bit MUST check all the RI LSAs of nodes in the area before actively running the modified SPF to account for the H-bit in order to verify that all routers are in routing capability. If any router does not have the H-Bit support then all routers in the areas MUST run the normal SPF. o Any router not supporting the H-bit capability is detected (by examination of RI- LSA or RTR LSA in the area database) then all routers in the area MUST revert back to normal operations. 6. OSPF AS-External-LSAs/NSSA LSAs with Type 2 Metrics When calculating the path to an OSPF AS-External-LSA or NSSA-LSA with a Type-2 metric, the advertised Type-2 metric is taken as more significant than the OSPF intra-area or inter-area path. Hence, advertising the links with MaxLinkMetric as specified in [RFC6987] does not discourage transit traffic when calculating AS external or NSSA routes with Type-2 metrics. Consequently, OSPF routers implementing [RFC6987] and required to be the last resort transit then they MUST advertise a Type-2 metric of LSInfinity-1 for any self-originated type 2 AS-External-LSAs or NSSA- LSAs. However, in situations, the router needs to repel traffic and acts as a host router then, in addition of the host bit procedure described in this document they MUST advertise a Type-2 metric of LSInfinity for any self-originated type 2 AS-External-LSAs or NSSA- LSAs. 7. IANA Considerations This document requests the IANA to assign the 0x80 value to the Host- Bit (H-bit)in the OSPFv2 Router Properties Registry Value Description Reference 0x80 Host (H-bit) This Document Patel, et al. Expires January 9, 2020 [Page 7] Internet-Draft July 2019 This document requests the IANA to assign the Bit Number value of 7 to the Host Router Support Capability in the OSPF Router Informational Capability Bits Registry. [RFC7770] Bit Number Capability Name Reference 7 OSPF Host Router This Document 8. Security Considerations This document introduces the H-bit which is a capability that restricts the use of a router for transit except for its local destinations. This is a subset of the operations of a normal router and therefore should not introduce new security considerations beyond those already known in OSPF. The feature however does introduce the flooding of a capability information that allows discovery and verification that all routers in an area are capable before turning on the feature. In case. a rogue or buggy router advertise incorrectly its capability there are two possible cases: o The router does not have the capability but send H-Bit set in its LSAs: In this case, there is a possibility of a routing loop. However this is mitigated by the fact that this router should be avoided anyway. Moreover, the link metrics cost of this router should be MaxLinkMetric and will mitigate this situation. In any case a router advertising the H-bit capability without its links cost equal to MaxLinkMetric may be an indicator that this is a rogue router. o The router has the capability but sends the H-Bit clear in its LSAs: In this case, the router merely prevents support of other H-bit routers in the area and all the routers to run the modified SPF. The impact is also mitigated as other H-Bit routers in the area also advertise MaxLinkMetric cost so they will still be avoided unless they are the last resort path. 9. Acknowledgements The authors would like to acknowledge Hasmit Grover for discovery of the limitation in [RFC6987], Acee Lindem, Abhay Roy, David Ward, Burjiz Pithawala and Michael Barnes for their comments. 10. References Patel, et al. Expires January 9, 2020 [Page 8] Internet-Draft July 2019 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, . [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, DOI 10.17487/RFC8174, May 2017, . 10.2. Informative References [I-D.ietf-idr-bgp-optimal-route-reflection] Raszuk, R., Cassar, C., Aman, E., Decraene, B., and K. Wang, "BGP Optimal Route Reflection (BGP-ORR)", draft- ietf-idr-bgp-optimal-route-reflection-18 (work in progress), April 2019. [RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D. McPherson, "OSPF Stub Router Advertisement", RFC 6987, DOI 10.17487/RFC6987, September 2013, . Authors' Addresses Keyur Patel Arrcus Email: keyur@arrcus.com Padma Pillay-Esnault Futurewei 2330 Central Expressway Santa Clara, CA 95050 USA Email: padma.ietf@gmail.com Patel, et al. Expires January 9, 2020 [Page 9] Internet-Draft July 2019 Manish Bhardwaj Cisco Systems 170 W. Tasman Drive San Jose, CA 95134 USA Email: manbhard@cisco.com Serpil Bayraktar Cisco Systems 170 W. Tasman Drive San Jose, CA 95134 USA Email: serpil@cisco.com Patel, et al. Expires January 9, 2020 [Page 10]