Advertising Infinity Links in OSPF
draft-ietf-lsr-ospf-ls-link-infinity-01

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

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   This Internet-Draft will expire on April 20, 2025.

Copyright Notice

   Copyright (c) 2024 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

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   publication of this document. Please review these documents
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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.

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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.

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   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

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   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.

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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":

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   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, <https://www.rfc-
             editor.org/info/rfc2328>.

   [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, <https://www.rfc-editor.org/info/rfc7684>.

   [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, <http://www.rfc-editor.org/info/rfc7770>.

   [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, <https://www.rfc-editor.org/info/rfc8362>.

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,
             <https://www.rfc-editor.org/info/rfc5340>.

   [RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D.
             McPherson, "OSPF Stub Router Advertisement", RFC 6987, DOI
             10.17487/RFC6987, September 2013, <http://www.rfc-
             editor.org/info/rfc6987>.

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   [RFC7308] Osborne, E., "Extended Administrative Groups in MPLS
             Traffic Engineering (MPLS-TE)", RFC 7308, DOI
             10.17487/RFC7308, July 2014, <https://www.rfc-
             editor.org/info/rfc7308>.

   [RFC8770] Patel, K., Pillay-Esnault, P., Bhardwaj, M., and S.
             Bayraktar, "Host Router Support for OSPFv2", RFC 7308, DOI
             10.17487/RFC8770, April 2020,  <https://www.rfc-
             editor.org/info/rfc8770>.

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

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   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

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