Internet DRAFT - draft-gong-lsr-ospf-unreachable-link

draft-gong-lsr-ospf-unreachable-link



Network Working Group                                           L. Gong
Internet Draft                                                 W. Cheng
Intended status: Standards Track                           China Mobile
Expires: August 21, 2024                                         C. Lin
                                                   New H3C Technologies
                                                              A. Lindem
                                                    LabN Consulting LLC
                                                              R. Chen
                                                        ZTE Corporation
                                                      February 23, 2024


                   Advertising Unreachable Links in OSPF
                  draft-gong-lsr-ospf-unreachable-link-05


Abstract

   This document proposes the method to advertise links as unreachable
   in OSPF. In some scenarios, there are requirements to advertise
   unreachable links in OSPF for purposes other than building the
   normal Shortest Path Tree.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 21, 2024.







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Copyright Notice

   Copyright (c) 2023 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
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   warranty as described in the Simplified BSD License.

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 based on MaxLinkMetric................................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 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.


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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 the SPF computation, undesired flows of best-effort
   traffic service may utilize the 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". Flex-Algorithm allows an IGP to compute the

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   paths 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 is used to transmit particular flows, such as 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 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. 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: SPF Topology after Excluding Unreachable Links

3. Solution based on MaxLinkMetric

   This document specifies that if a link is advertised with the
   MaxLinkMetric (0xffff), it MUST NOT be considered during the normal
   SPF computation.

   In OSPF protocol, there are some inconsistencies when a link is
   advertised with the MaxLinkMetric (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 not treat links with the cost 0xffff as
   unreachable.



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   However, 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 Informational Capabilities TLV [RFC7770]
   including the following Router Informational 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 in the area MUST
   recalculate routes without considering MaxLinkMetric.

   MaxLinkMetric is applicable for the following TLVs/LSAs:

   o The Router-LSA [RFC2328]

   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.




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   Upon detecting the absence of that capability from any router in the
   same area, all routers MUST recalculate routes without considering
   MaxLinkMetric.

4.1. Stub Router Advertisement Backward Compatibility

   Stub Router Advertisement [RFC6987] also uses MaxLinkMetric (0xffff)
   to indicate a router-LSA link should not be used for transit
   traffic.

   When an OSPFv2 router supports [RFC6987] and the MaxLinkMetric
   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 MaxLinkMetric - 1 (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 MaxLinkMetric - 1 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 MaxLinkMetric capability SHOULD be configurable.

   In some networks, the operator may still want links with maximum
   metric 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 MaxLinkMetric
   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 MaxLinkMetric - 1
   (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
   -------------------------------------------------------
   TBA           MaxLinkMetric 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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