LSR                                                             L. Gong
Internet Draft                                             China Mobile
Intended status: Standards Track                                 C. Lin
Expires: April 20, 2025                            New H3C Technologies
                                                       October 17, 2024


                        Flexible Algorithms Exclude Node
                 draft-gong-lsr-flex-algo-exclude-node-00


Abstract

   Flexible Algorithms provide mechanisms for creating constraint-based
   paths in IGP. This document introduces a routing constraint based on
   Node Admin-Tags, allowing for easy exclusion of device nodes from
   path computation.

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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.





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   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents


   1. Introduction...................................................2
   2. Use Case ......................................................2
   3. Solution.......................................................3
      3.1. Requirements Language.....................................4
   4. Flexible Algorithm Definition constraints......................4
      4.1. IS-IS Flex-Algorithm Exclude Admin-Tag sub-TLV............4
      4.2. OSPF Exclude Admin-Tag sub-TLV............................4
      4.3. Calculation of Flex-Algorithm paths.......................5
   5. Backward Compatibility.........................................5
   6. Security Considerations........................................5
   7. IANA Considerations............................................6
      7.1. IS-IS Exclude Admin-Tag sub-TLV...........................6
      7.2. OSPF Exclude Admin-Tag sub-TLV............................6
   8. References.....................................................6
      8.1. Normative References......................................6
      8.2. Informational References..................................6
   Authors' Addresses................................................7

1. Introduction

   [RFC9350] describes a method to define constraints based on link
   attributes, allowing path calculation in Flex-Algo using these
   constraints. However, in some cases, administrators may want to
   simply exclude entire nodes from the path. In such scenarios,
   defining constraints based on links is not very convenient. This
   document defines a method to directly and entirely exclude nodes
   based on certain characteristics.

2. Use Case

   As shown in the Figure 1, there are two ABRs in the network. The
   primary path for upstream traffic on R1 is R1->R2->ABR1->PE. For the
   backup path calculation, we want it to pass through ABR2. Therefore,
   device ABR1 needs to be excluded during the backup path calculation.


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            +--+      +--+  +----+
        ----+R1+------+R2+--+ABR1+---+
            +--+      ++++  +-+-++    \
             |\       /  \   /  |      \
             |  \    /    \ /   |      ++-+
             |   \  /      /    |      |PE|(LoopBack: FA 129,SID1)
             |    /       / \   |      ++-+           FA 130,SID2
             |   / \     /   \  |      /
             | /     \  /     \ |     /
            +--+     ++++   +--+++   /
            |R3+-----+R4+---+ABR2+--+
            +--+     +--+   +----+
              Figure 1 LFA


3. Solution

   In the above use case, one solution is to specify an algorithm for
   calculating the backup path. The main path can be an SR route within
   algorithm 128, and the backup path is specified using a separate
   algorithm, with constraints to calculate backup path. During backup
   path calculation, set the constraint to exclude ABR1. This
   constraint can be implemented by excluding the node tags advertised
   by ABR1. The specific steps are:

   1) ABR1 advertises a node tag as Tag1.
   2) PE device associates two specific SIDs in different FlexAlgo with
      the same loopback address Public FlexAlgo 128: SID1 as primary,
      advertised under algorithm 129, and SID2 as backup, advertised
      under algorithm 130. Separate constraints are specified for each
      algorithm. In algorithm 129, the constraint is set to exclude the
      adminTag specified by A. Thus, when device R1 calculates the route
      to the PE device's loopback address, it evaluates the constraints
      in both the primary and backup algorithms.
   3) The administrator specifies Algorithm 129 for backup path
      calculation, with the constraint to exclude Node Tag1.
   4) Algorithm 129 is used to compute the backup path, directly
      excluding nodes with Tag1, thereby excluding ABR1, resulting in
      the backup path R1->R4->ABR2.

   [RFC7917] defines how the IS-IS protocol advertises node
   administrative tags, while [RFC7777] covers the OSPF protocol. Node
   administrative tags are primarily used to express and apply locally
   defined network policies, offering significant operational
   capabilities. This document describes how to define new constraints

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   in Flex-Algo utilizing node administrative tags to enable flexible
   network planning.



3.1. 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.


4. Flexible Algorithm Definition constraints

   To exclude specified nodes during Flex-Algo path computation, this
   document defines the "Flex-Algorithm Exclude Admin-Tag (FAEAT)
   Constraint" within the context of Flexible Algorithms. If a node
   advertises an Admin-Tag value that needs to be excluded, that node
   is removed from the Flex-Algo topology.

4.1. IS-IS Flex-Algorithm Exclude Admin-Tag sub-TLV

   IS-IS Flex-Algorithm Exclude Admin-Tag sub-TLV (FAEAT) is a sub-TLV
   of the IS-IS FAD sub-TLV[RFC9350].  It has the following format.



       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     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Admin Tag                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      where:
          Type: TBD1
          Length: 3 octets
          Admin Tag:  Node Admin Tag

                        Figure 6: IS-IS FAEAT Sub-TLV

4.2. OSPF Exclude Admin-Tag sub-TLV

   The OSPF Flex-Algorithm Exclude Admin-Tag sub-TLV (FAEAT) sub-TLV is
   a sub-TLV of the OSPF FAD TLV [RFC9350].  It has the following
   format.

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       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                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Admin Tag                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      where:
          Type: TBD2
          Length:  4 octets
          Admin Tag:  Node Admin Tag

                           Figure 7: OSPF FAEAT Sub-TLV



4.3. Calculation of Flex-Algorithm paths

   A new additional rules are added to the existing rules in the Flex-
   Algorithm calculations specified in sec 13 of [RFC9350].

   6. Check if any exclude FAEAT rule is part of the Flex-Algorithm
       definition. If such a rule exists and the originating node of the
       link has advertised an Admin Tag, verify if the node's Admin Tag
       matches the one specified in the FAEAT rule. If they match, the
       node must be removed from the Flex-Algorithm computation.



5. Backward Compatibility

   This extension brings no new backward-compatibility issues.  This
   document defines new FAD constraints.  As described in [RFC9350],
   any node that does not understand sub-TLVs in a FAD TLV, stops
   participation in the corresponding Flex-Algorithm.  The new
   extensions can be deployed among the nodes that are upgraded to
   understand the new extensions without affecting the nodes that are
   not upgraded.

6. Security Considerations

   TBD.







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7. IANA Considerations

7.1. IS-IS Exclude Admin-Tag sub-TLV

   A new IS-IS sub-TLV type is requested to be allocated from the "IS-
   IS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV" Registry
   to specify the use of admin-tag for path constraint selection.

    +-------------------+-------------------+
    | Value             | TBD1              |
    +-------------------+-------------------+
    | Description       | Exclude Admin Tag |
    +-------------------+-------------------+
    | Reference         | [This Document]   |
    +-------------------+-------------------+

7.2. OSPF Exclude Admin-Tag sub-TLV

   A new OSPF sub-TLV type is requested to be allocated from the "OSPF
   Flexible Algorithm Definition TLV Sub-TLVs" Registry to specify the
   use of admin-tag for path constraint selection.

    +-------------------+-------------------+
    | Value             | TBD2              |
    +-------------------+-------------------+
    | Description       | Exclude admin tag |
    +-------------------+-------------------+
    | Reference         | [This Document]   |
    +-------------------+-------------------+

8. References

8.1. Normative References

   TBD

8.2. Informational References

   [RFC7917] P. Sarkar, Individual Contributor, H. Gredler, RtBrick
             Inc., S. Hegde, Juniper Networks, Inc., S. Litkowski, B.
             Decraene, Orange, "Advertising Node Administrative Tags in
             IS-IS", RFC 7917, DOI 10.17487/RFC7917, July 2016,
             <https://www.rfc-editor.org/info/rfc7917>.






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   [RFC7777] S. Hegde, Juniper Networks, Inc., R. Shakir, Jive
             Communications, Inc., A. Smirnov, Cisco Systems, Inc., Z.
             Li, Huawei Technologies, B. Decraene, Orange, "Advertising
             Node Administrative Tags in OSPF", RFC 3602, DOI
             10.17487/RFC7777, March 2016, <https://www.rfc-
             editor.org/info/rfc7777>.

   [RFC9350] P. Psenak, Ed., Cisco Systems, Inc., S. Hegde, Juniper
             Networks, Inc., C. Filsfils, Cisco Systems, Inc., K.
             Talaulikar, Cisco Systems, Inc, A. Gulko, Edward Jones,
             "IGP Flexible Algorithm", RFC 9350, DOI 10.17487/RFC9350,
             March 2016, <https://www.rfc-editor.org/info/rfc9350>.





Authors' Addresses


   Liyan Gong
   China Mobile
   China

   Email: gongliyan@chinamobile.com



   Changwang Lin
   New H3C Technologies
   China
   Email: linchangwang.04414@h3c.com
















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