Network Working Group                                           L. Gong
Internet Draft                                                 W. Cheng
Intended status: Standards Track                           China Mobile
Expires: March 1, 2024                                           C. Lin
                                                                M. Chen
                                                   New H3C Technologies
                                                                R. Chen
                                                        ZTE Corporation
                                                               Y. Liang
                                              Ruijie Networks Co., Ltd.
                                                      September 1, 2023



           Advertising Exclusive Links for Flex-Algorithm in IGP
              draft-gong-lsr-exclusive-link-for-flex-algo-06


Abstract

   This document proposes the method to advertise exclusive links for
   Flex-Algorithm in IGP.

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
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   This Internet-Draft will expire on March 1 2024.






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Copyright Notice

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   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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   warranty as described in the Simplified BSD License.

Table of Contents


   1. Introduction ................................................ 2
      1.1. Requirements Language .................................. 3
   2. Problem Statement ........................................... 3
   3. Solution A: Maximum Link Metric ............................. 4
      3.1. Advertising Maximum Link Metric in IS-IS ............... 4
      3.2. Advertising Maximum Link Metric in OSPF ................ 4
      3.3. Considerations for Flex-Algorithm Using IGP Metric ..... 4
   4. Solution B: Unreachable Link Flag ........................... 5
      4.1. Advertising Unreachable Link Flag in IS-IS ............. 5
      4.2. Advertising Unreachable Link Flag in OSPF .............. 6
   5. Backward Compatibility ...................................... 7
   6. Security Considerations ..................................... 7
   7. IANA Considerations ......................................... 8
   8. References .................................................. 8
      8.1. Normative References ................................... 8
      8.2. Informative References ................................. 8
   9. Acknowledgments ............................................. 9
   Authors' Addresses ............................................ 10

1. Introduction

   Flexible Algorithm (Flex-Algorithm) allows IGP to compute
   constraint-based paths. [I-D.ietf-lsr-flex-algo] specifies the usage
   of Flex-Algorithm in Segment Routing (SR) data planes - SR MPLS and
   SRv6. [I-D.ietf-lsr-ip-flexalgo] extends the Flex-Algorithm for
   native IPv4 and IPv6 data planes.

   In some scenarios, exclusive links may be deployed for Flex-
   Algorithm, but not for best-effort service. However, these links


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   cannot be pruned in normal SPF calculation, and unexpected flows may
   be steered into these links.

   This document proposes the method to advertise exclusive links for
   Flex-Algorithm in IGP.

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. Problem Statement

   Flex-Algorithm allows IGP to compute the best paths along the
   constrained topology.

   A network topology is shown in Figure 1. Node A, B, C and D have an
   extra link between each other. These links have EAG attribute of
   "red" color.

   Flex-Algorithm 128 are enable on Node A, B, C and D, with metric
   type of IGP cost and EAG rule of including "red". The topology used
   by Flex-Algorithm 128 is shown in Figure 2.

   Flex-Algorithm 128 are used to transmit particular flows, such as
   network slice. The 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 exclusive links for
   Flex-Algorithm cannot be pruned in normal SPF calculation.












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   A======C------E
   ||     ||     |
   ||     ||     |
   ||     ||     |
   B======D------F

   Figure 1

   A------C
   |      |
   |      |
   |      |
   B------D

   Figure 2

3. Solution A: Maximum Link Metric

3.1. Advertising Maximum Link Metric in IS-IS

   As specified in [RFC5305], if a link is advertised with the maximum
   link metric (2^24 - 1), this link MUST NOT be considered during the
   normal SPF computation in IS-IS.

   The exclusive links for Flex-Algorithm may be advertised with the
   maximum link metric, so that they will be the pruned in normal SPF
   computation.

3.2. Advertising Maximum Link Metric in OSPF

   In OSPF protocol, if a link is advertised with the maximum link
   metric (2^16 - 1), it may be still reachable. [RFC1247] specifies
   that, if the cost of the link is (2^16 - 1), 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.

   If an exclusive link for Flex-Algorithm is advertised with the
   maximum link metric, OSPF routers will prefer alternate paths in the
   network, rather than the path through that link. However, if there
   is no alternate path, the path through the exclusive link will still
   be used.

3.3. Considerations for Flex-Algorithm Using IGP Metric

   If the associated Flex-Algorithm needs to use IGP Metric in path
   calculation, a user defined metric type (128-255) may be assigned to
   substitute IGP Metric, and the Generic Metric sub-TLV may be

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   advertised to carry the metric value, as specified in [I-D.ietf-lsr-
   flex-algo-bw-con].

4. Solution B: Unreachable Link Flag

4.1. Advertising Unreachable Link Flag in IS-IS

   A new ISIS Link Flags sub-TLV is defined in IS-IS. 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 ISIS Link Flags sub-TLV is advertised in the TLVs/sub-TLVs
   below:

   o TLV-22 (Extended IS reachability) [RFC5305]

   o TLV-222 (MT-ISN) [RFC5120]

   o TLV-23 (IS Neighbor Attribute) [RFC5311]

   o TLV-223 (MT IS Neighbor Attribute) [RFC5311]

   The ISIS Link Flags sub-TLV with U-Flag can be advertised for the
   exclusive links used by Flex-Algorithm, so that these links will be
   pruned during normal SPF calculation.


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   Due to the change of procedures in the SPF calculation, all routers
   in a level must support the changes specified in this section. To
   ensure that, if a level is provisioned to support Unreachable Link
   Flag, all routers supporting this capability must advertise an IS-IS
   Router Capability TLV-242 that includes the following Unreachable
   Link Flag Sub-TLV:

    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    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Upon detecting the presence of a reachable TLV-242 without an
   Unreachable Link Flag Sub-TLV, all routers MUST recalculate routes
   without considering any Unreachable Link Flag.

4.2. Advertising Unreachable Link Flag in OSPF

   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:


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   o OSPFv2 Extended Link TLV of OSPFv2 Extended Link Opaque LSA
      [RFC7684]

   o Router-Link TLV of OSPFv3 E-Router-LSA [RFC8362]

   The OSPF Link Flags sub-TLV with U-Flag can be advertised for the
   exclusive links used by Flex-Algorithm, so that these links will be
   pruned during normal SPF calculation.

   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:

   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

   An obvious benefit of solution A is that using maximum link metric
   is backward compatible. However, in OSPF, it may not work as well as
   in ISIS, since the links with maximum link metric are not always
   treated as unreachable by OSPF routers. Besides, additional
   mechanisms are required for the Flex-Algorithm using IGP Metric in
   path calculation.

   When using the Link Flags sub-TLV with U-Flag in solution B, all
   nodes in the same area or level must support this feature. To avoid
   topology inconsistence and achieve backward compatibility, routers
   supporting the Unreachable Link Flag MUST advertise that capability.
   Upon detecting the absence of that capability from any router in the
   same area or level, all routers MUST recalculate routes without
   considering any Unreachable Link Flag. The backward-compatibility
   procedures described in [RFC8042] should be followed to ensure loop-
   free routing.

6. Security Considerations

   TBD



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7. IANA Considerations

   Link Flags sub-TLV (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.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, May 2017

   [I-D.ietf-lsr-flex-algo] Psenak, P., Hegde, S., Filsfils, C.,
             Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm",
             draft-ietf-lsr-flex-algo-20 (work in progress), May 2022.

8.2. Informative References

   [RFC1247] Moy, J., "OSPF Version 2", RFC 1247, July 1991.

   [RFC1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994.

   [RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
             Topology (MT) Routing in Intermediate System to
             Intermediate Systems (IS-ISs)", RFC 5120, DOI
             10.17487/RFC5120, February 2008, <https://www.rfc-
             editor.org/info/rfc5120>.

   [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
             Engineering", RFC 5305, DOI 10.17487/RFC5305, October
             2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5311] McPherson, D., Ed., Ginsberg, L., Previdi, S., and M.
             Shand, "Simplified Extension of Link State PDU (LSP) Space
             for IS-IS", RFC 5311, DOI 10.17487/RFC5311, February 2009,
             <https://www.rfc-editor.org/info/rfc5311>.

   [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>.

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   [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>.

   [I-D.ietf-lsr-ip-flexalgo] Britto, W., Hegde, S., Kaneriya, P.,
             Shetty, R., Bonica, R., and P. Psenak, "IGP Flexible
             Algorithms (Flex- Algorithm) In IP Networks", draft-ietf-
             lsr-ip-flexalgo-06 (work in progress), May 2022.

   [I-D.ietf-lsr-flex-algo-bw-con] Hegde, S., J, W. B. A., Shetty, R.,
             Decraene, B., Psenak, P., and T. Li, "Flexible Algorithms:
             Bandwidth, Delay, Metrics and Constraints", draft-ietf-
             lsr-flex-algo-bw-con-03 (work in progress), July 2022.

9. Acknowledgments

   The authors would like to thank the following for their valuable
   contributions of this document:

   TBD



























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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












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