LSR Working Group P. Psenak Internet-Draft J. Horn Intended status: Standards Track Cisco Systems Expires: 22 January 2026 A. Dhamija Arrcus 21 July 2025 IGP Flexible Algorithms Reverse Affinity Constraint draft-ietf-lsr-igp-flex-algo-reverse-affinity-08 Abstract An IGP Flexible Algorithm (Flex-Algorithm) enables the computation of constraint-based paths within an IGP domain, allowing operators to influence path selection according to administrative policies. This document defines an extension to Flex-Algorithm that allows the inclusion or exclusion of links from path computation based on Administrative Groups (also known as link affinities) associated with the reverse direction of the path under computation. This extension enhances the path selection capabilities of Flex- Algorithm by enabling reverse-affinity-based constraints, which are particularly useful for scenarios where path symmetry or directional link attributes are operationally significant. 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." This Internet-Draft will expire on 22 January 2026. Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. Psenak, et al. Expires 22 January 2026 [Page 1] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Use Case Example . . . . . . . . . . . . . . . . . . . . . . 3 4. Deployment Consideration . . . . . . . . . . . . . . . . . . 4 5. IS-IS Flexible Algorithm Exclude Reverse Admin Group Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 4 6. IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 6 8. OSPF Flexible Algorithm Exclude Reverse Admin Group Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 6 9. OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 7 10. OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 7 11. Calculation of Flexible Algorithm Paths . . . . . . . . . . . 8 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 12.1. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . 8 12.2. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry . . . . . . . . . . . . . . . . . . . . . . . . 9 12.3. IGP Flex-Algo Path Computation Rules Registry . . . . . 10 13. Security Considerations . . . . . . . . . . . . . . . . . . . 14 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 15. Normative References . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 1. Introduction IGP protocols historically computed the best paths over the network solely based on the IGP metric assigned to the links. An IGP Flex- Algorithm as specified in [RFC9350] allows IGPs to compute constraint-based paths. Several mechanisms to include or exclude the link during the Flex-Algorithm path calculation have been defined already: Psenak, et al. Expires 22 January 2026 [Page 2] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 - link inclusion or exclusion based on the presence of a specific Admin Group(s) - [RFC9350] - link exclusion based on the presence of a specific Shared Risk Link Group (SRLG) - [RFC9350] - link exclusion based on minimum bandwidth - [I-D.ietf-lsr-flex-algo-bw-con] - link exclusion based on maximum delay - [I-D.ietf-lsr-flex-algo-bw-con] This document extends IGP Flex-Algorithm with additional constraints for inclusion or exclusion of links in the path based on Admin Groups associated with the reverse direction of the SPF path computation. The term OSPF in this document is used to cover both OSPFv2 and OSPFv3 protocols. 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. Use Case Example The Flexible Algorithm definition (FAD) can specify Extended Administrative Groups that are used by the operator to include or exclude links during the Flex-Algorithm path computation [RFC9350]. These link Extended Administrative Groups are checked in the path direction of the SPF computation, e.g., in the direction from the root vertex toward verticies of increasing distance. In certain scenarios, it is beneficial to evaluate the Extended Administrative Groups associated with the reverse direction of a link, rather than solely those in the direction of path computation. Consider a point-to-point link represented as a pair of directed edges between two nodes, A and B. When computing a path from A to B, issues such as input errors on the link, detectable only at the receiving node B, may be operationally significant. An operator might monitor metrics like CRC errors or other input-related faults at node B and apply thresholds over a defined observation period. If such a threshold is exceeded, node B may locally assign specific Extended Administrative Groups to the link in the direction from B to A. Psenak, et al. Expires 22 January 2026 [Page 3] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 To accommodate this operational intent, the Flex-Algorithm can be extended to inspect the Extended Administrative Groups of the reverse-direction edge (from B to A) when evaluating the forward- direction edge (from A to B) during path computation. This enables the exclusion of links from the computed topology based on conditions detected at the far end of the link, improving network reliability and policy control. 4. Deployment Consideration Threshold based setting of the link Extended Administrative Groups must be done with care to avoid excessive flooding and constant Flex- Algorithm path re-calculation. One possible approach is to use two different thresholds when setting and un-setting the Extended Administrative Group on the link. For example, when setting the Extended Administrative Group on the link based on the percentage of certain incoming errors, higher threshold is used for setting and lower threshold is used for un-setting the Extended Administrative Group on the link. Many implementations provide a throttling mechanism that limits the rate at which the LSP in case of ISIS, or LSA in case of OSPFv2 and OSPFv3, is updated at the originator. Such mechanism is typically not specific to any particular link attribute, but rather considers any change in the LSP or LSA. Usage of such throttling mechanism can also be used to avoid frequent changes in the setting of the Extended Administrative Group on a link to affect the stability of the receivers. 5. IS-IS Flexible Algorithm Exclude Reverse Admin Group Sub-TLV The IS-IS Flexible Algorithm Exclude Reverse Admin Group (FAERAG) Sub-TLV is used to advertise the exclude rule that is used during the Flex-Algorithm path calculation as specified in Section 11. The IS-IS FAERAG Sub-TLV is a Sub-TLV of the IS-IS FAD Sub-TLV. It has the following format: Psenak, et al. Expires 22 January 2026 [Page 4] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Admin Group | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where: Type (1 octet): 10 Length (1 octet): Variable, dependent on the size of the Extended Admin Group. The length MUST be a multiple of 4 octets. If the length is not a multiple of 4 octets, the entire Sub-TLV MUST be ignored by the receiver. Extended Administrative Group: Extended Administrative Group as defined in [RFC7308]. The IS-IS FAERAG Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub- TLV MUST be ignored by the receiver. The IS-IS FAERAG Sub-TLV MUST NOT appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once in such set, the IS-IS FAERAG Sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used and any other occurrences MUST be ignored by the receiver. 6. IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV The IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV is used to advertise the include-any rule that is used during the Flex-Algorithm path calculation as specified in Section 11. The format of the IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV is identical to the format of the FAERAG Sub-TLV in Section 5. IANA has assigned Type value of 11 for the IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV. The IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver. Psenak, et al. Expires 22 January 2026 [Page 5] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 The IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV MUST NOT appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once in such set, the IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used and any other occurrences MUST be ignored by the receiver. 7. IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV The IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV is used to advertise the include-all rule that is used during the Flex-Algorithm path calculation as specified in Section 11. The format of the IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV is identical to the format of the FAERAG Sub-TLV in Section 5. IANA has assigned Type value of 12 for the IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV. The IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiver. The IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV MUST NOT appear more than once in the set of FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it appears more than once in such set, the IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV in the first occurrence in the lowest numbered LSP from a given IS MUST be used and any other occurrences MUST be ignored by the receiver. 8. OSPF Flexible Algorithm Exclude Reverse Admin Group Sub-TLV The OSPF Flexible Algorithm Exclude Reverse Admin Group (FAERAG) Sub- TLV is used to advertise the exclude rule that is used during the Flex-Algorithm path calculation as specified in Section 11. The OSPF FAERAG Sub-TLV is a Sub-TLV of the OSPF FAD TLV. It has the following format: Psenak, et al. Expires 22 January 2026 [Page 6] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Admin Group | +- -+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where: Type (2 octets): 10 Length (2 octets): Variable, dependent on the size of the Extended Admin Group. The length MUST be a multiple of 4 octets. If the length is not a multiple of 4 octets, the entire Sub-TLV MUST be ignored by the receiver. Extended Administrative Group: Extended Administrative Group as defined in [RFC7308]. The OSPF FAERAG Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver. 9. OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV The OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV is used to advertise the include-any rule that is used during the Flex-Algorithm path calculation as specified in Section 11. The format of the OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV is identical to the format of the OSPF FAERAG Sub-TLV in Section 8. IANA has assigned Type value of 11 for the OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV. The OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver. 10. OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV The OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV is used to advertise the include-all rule that is used during the Flex-Algorithm path calculation as specified in Section 11. Psenak, et al. Expires 22 January 2026 [Page 7] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 The format of the OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV is identical to the format of the OSPF FAERAG Sub-TLV in Section 8. IANA has assigned Type value of 12 for OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV. The OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV MUST NOT appear more than once in an OSPF FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored by the receiver. 11. Calculation of Flexible Algorithm Paths The following procedures augment the rules defined in Section 13 of [RFC9350] by introducing additional constraints based on Administrative Groups (AGs) associated with the reverse direction of a link. Check if any exclude reverse Admin Group rule is part of the Flex- Algorithm definition. If such exclude rule exists, check if any Admin Group that is part of the exclude rule is also set on the link in the reverse direction. If such Admin Group is set on the link in the reverse direction, the link MUST be pruned from the computation. Check if any include-any reverse Admin Group rule is part of the Flex-Algorithm definition. If such include-any rule exists, check if any Admin Group that is part of the include-any rule is also set on the link in the reverse direction. If no such Admin Group is set on the link in the reverse direction, the link MUST be pruned from the computation. Check if any include-all reverse Admin Group rule is part of the Flex-Algorithm definition. If such include-all rule exists, check if all Admin Groups that are part of the include-all rule are also set on the link in the reverse direction. If all such Admin Groups are not set on the link in the reverse direction, the link MUST be pruned from the computation. Please see rules 8, 9, and 10 in Section 12.3 for these added rules. 12. IANA Considerations 12.1. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV This document makes the following registration in the "IS-IS Sub-Sub- TLVs for Flexible Algorithm Definition Sub-TLV" registry under "IS-IS TLV Codepoints" registry group: Psenak, et al. Expires 22 January 2026 [Page 8] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 Type: 10 Description: Flexible Algorithm Exclude Reverse Admin Group MP-TLV Applicability: N Reference: This document (Section 5). Type: 11 Description: Flexible Algorithm Include-Any Reverse Admin Group MP-TLV Applicability: N Reference: This document (Section 6). Type: 12 Description: Flexible Algorithm Include-All Reverse Admin Group MP-TLV Applicability: N Reference: This document (Section 7). 12.2. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry This document makes the following registration in the "OSPF Flexible Algorithm Definition TLV Sub-TLVs" registry under "Open Shortest Path First (OSPF) Parameters" registry group: Type: 10 Description: Flexible Algorithm Exclude Reverse Admin Group Reference: This document (Section 8). Type: 11 Description: Flexible Algorithm Include-Any Reverse Admin Group Reference: This document (Section 9). Type: 12 Description: Flexible Algorithm Include-All Reverse Admin Group Reference: This document (Section 10). Psenak, et al. Expires 22 January 2026 [Page 9] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 12.3. IGP Flex-Algo Path Computation Rules Registry This document creates a new registry called "IGP Flex-Algorithm Path Computation Rules Registry" within the "Interior Gateway Protocol (IGP) Parameters" registry group. The registration procedure for the new registry is "Expert Review". The "IGP Flex-Algo Path Computation Rules" specifies the ordered set of rules that is used to prune links from the topology during the Flex-Algorithm path computation. When a new constraint is defined, the rule associated with that constraint MAY be inserted at any position. Backwards compatibility is guaranteed because nodes which don't support the new constraint will not participate in an algorithm where the FAD specifies a constraint they don't support. The relative ordering of existing rules MUST NOT be altered. Doing so has the potential to create backwards compatibility issues. Deletion of the rules MUST NOT be done. Given that the rules are only used conditionally based on the information carried in the winning FAD, deletion of the rule is not necessary. Merging or repeating of the rules MUST NOT be done. There is no upper bound on the number of rules that the registry supports. +==========+======================+=================================+ | Sequence | Description | Reference | | Number | | | +==========+======================+=================================+ | 1 | Check if any | Section 13 of [RFC9350] | | | exclude | | | | Administrative | | | | Group rule is part | | | | of the Flex- | | | | Algorithm | | | | Definition. If | | | | such exclude rule | | | | exists, check if | | | | any color that is | | | | part of the exclude | | | | rule is also set on | | | | the link. If such | | | | a color is set, the | | | | link MUST be pruned | | Psenak, et al. Expires 22 January 2026 [Page 10] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 | | from the | | | | computation | | +----------+----------------------+---------------------------------+ | 2 | Check if any | Section 13 of [RFC9350] | | | exclude SRLG rule | | | | is part of the | | | | Flex-Algorithm | | | | Definition. If | | | | such exclude rule | | | | exists, check if | | | | the link is part of | | | | any SRLG that is | | | | also part of the | | | | SRLG exclude rule. | | | | If the link is part | | | | of such SRLG, the | | | | link MUST be pruned | | | | from the | | | | computation. | | +----------+----------------------+---------------------------------+ | 3 | Check if any | Section 13 of [RFC9350] | | | include-any | | | | Administrative | | | | Group rule is part | | | | of the Flex- | | | | Algorithm | | | | Definition. If | | | | such include-any | | | | rule exists, check | | | | if any color that | | | | is part of the | | | | include-any rule is | | | | also set on the | | | | link. If no such | | | | color is set, the | | | | link MUST be pruned | | | | from the | | | | computation | | +----------+----------------------+---------------------------------+ | 4 | Check if any | Section 13 of [RFC9350] | | | include-all | | | | Administrative | | | | Group rule is part | | | | of the Flex- | | | | Algorithm | | | | Definition. If | | | | such include-all | | | | rule exists, check | | Psenak, et al. Expires 22 January 2026 [Page 11] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 | | if all colors that | | | | are part of the | | | | include-all rule | | | | are also set on the | | | | link. If all such | | | | colors are not set | | | | on the link, the | | | | link MUST be pruned | | | | from the | | | | computation. | | +----------+----------------------+---------------------------------+ | 5 | If the Flex- | Section 13 of [RFC9350] | | | Algorithm | | | | Definition uses | | | | something other | | | | than the IGP metric | | | | (Section 5 of | | | | [RFC9350]), and | | | | such metric is not | | | | advertised for the | | | | particular link in | | | | a topology for | | | | which the | | | | computation is | | | | done, such link | | | | MUST be pruned from | | | | the computation. A | | | | metric of value 0 | | | | MUST NOT be assumed | | | | in such a case. | | +----------+----------------------+---------------------------------+ | 6 | Check if any | Section 6 of | | | exclude FAEMB rule | [I-D.ietf-lsr-flex-algo-bw-con] | | | is part of the | | | | Flex-Algorithm | | | | definition. If | | | | such exclude rule | | | | exists and the link | | | | has Maximum Link | | | | Bandwidth | | | | advertised, check | | | | if the link | | | | bandwidth satisfies | | | | the FAEMB rule. If | | | | the link does not | | | | satisfy the FAEMB | | | | rule, the link MUST | | | | be pruned from the | | Psenak, et al. Expires 22 January 2026 [Page 12] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 | | Flex-Algorithm | | | | computation | | +----------+----------------------+---------------------------------+ | 7 | Check if any | Section 6 of | | | exclude FAEMD rule | [I-D.ietf-lsr-flex-algo-bw-con] | | | is part of the | | | | Flex-Algorithm | | | | definition. If | | | | such exclude rule | | | | exists and the link | | | | has Min | | | | Unidirectional link | | | | delay advertised, | | | | check if the link | | | | delay satisfies the | | | | FAEMD rule. If the | | | | link does not | | | | satisfy the FAEMD | | | | rule, the link MUST | | | | be pruned from the | | | | Flex-Algorithm | | | | computation. | | +----------+----------------------+---------------------------------+ | 8 | Check if any | This document, Section 11 | | | exclude reverse | | | | Admin Group rule is | | | | part of the Flex- | | | | Algorithm | | | | definition. If | | | | such exclude rule | | | | exists, check if | | | | any Admin Group | | | | that is part of the | | | | exclude rule is | | | | also set on the | | | | link in the reverse | | | | direction. If such | | | | Admin Group is set | | | | on the link in the | | | | reverse direction, | | | | the link MUST be | | | | pruned from the | | | | computation. | | +----------+----------------------+---------------------------------+ | 9 | Check if any | This document, Section 11 | | | include-any reverse | | | | Admin Group rule is | | | | part of the Flex- | | Psenak, et al. Expires 22 January 2026 [Page 13] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 | | Algorithm | | | | definition. If | | | | such include-any | | | | rule exists, check | | | | if any Admin Group | | | | that is part of the | | | | include-any rule is | | | | also set on the | | | | link in the reverse | | | | direction. If no | | | | such Admin Group is | | | | set on the link in | | | | the reverse | | | | direction, the link | | | | MUST be pruned from | | | | the computation | | +----------+----------------------+---------------------------------+ | 10 | Check if any | This document, Section 11 | | | include-all reverse | | | | Admin Group rule is | | | | part of the Flex- | | | | Algorithm | | | | definition. If | | | | such include-all | | | | rule exists, check | | | | if all Admin Groups | | | | that are part of | | | | the include-all | | | | rule are also set | | | | on the link in the | | | | reverse direction. | | | | If all such Admin | | | | Groups are not set | | | | on the link in the | | | | reverse direction, | | | | the link MUST be | | | | pruned from the | | | | computation. | | +----------+----------------------+---------------------------------+ Table 6: IGP Flex-algo Path Calculation Rules 13. Security Considerations This document inherits security considerations from [RFC9350]. Psenak, et al. Expires 22 January 2026 [Page 14] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 14. Acknowledgements TBD 15. Normative References [I-D.ietf-lsr-flex-algo-bw-con] Hegde, S., Britto, W., Shetty, R., Decraene, B., Psenak, P., and T. Li, "IGP Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints", Work in Progress, Internet- Draft, draft-ietf-lsr-flex-algo-bw-con-22, 13 February 2025, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7308] Osborne, E., "Extended Administrative Groups in MPLS Traffic Engineering (MPLS-TE)", RFC 7308, DOI 10.17487/RFC7308, July 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI 10.17487/RFC9350, February 2023, . Authors' Addresses Peter Psenak Cisco Systems Apollo Business Center Mlynske nivy 43 82109 Bratislava Slovakia Email: ppsenak@cisco.com Jakub Horn Cisco Systems Milpitas, CA 95035 United States of America Psenak, et al. Expires 22 January 2026 [Page 15] Internet-Draft Flex-Algorithm Reverse Affinity July 2025 Email: jakuhorn@cisco.com Amit Dhamija Arrcus Email: amitd@arrcus.com Psenak, et al. Expires 22 January 2026 [Page 16]