LSR Working Group R. Gandhi, Ed. Internet-Draft Cisco Systems, Inc. Intended status: Standards Track 18 July 2026 Expires: 19 January 2027 IGP Flexible Algorithm Utilizing Link Bit Error Rate Metric draft-gandhi-lsr-flex-algo-link-ber-00 Abstract Networks may experience transmission bit errors due to various factors, such as poor fiber quality. This document defines extensions to the IGP Flexible Algorithm to utilize the link Bit Error Rate (BER) metric as a link constraint. It defines a mechanism to exclude links whose BER metric exceeds a configured threshold during Flex-Algorithm path computation. The mechanism utilizes BER metric advertisement defined for IS-IS and OSPF in draft-gandhi-lsr- ber. 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 19 January 2027. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Gandhi Expires 19 January 2027 [Page 1] Internet-Draft Flexible-Algorithm with Link BER July 2026 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. Conventions Used in This Document . . . . . . . . . . . . . . 3 2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Exclude Maximum Link BER Sub-TLV . . . . . . . . . . . . . . 4 4.1. IS-IS Exclude Maximum Link BER Sub-TLV . . . . . . . . . 4 4.2. OSPF Exclude Maximum Link BER Sub-TLV . . . . . . . . . . 5 5. Calculation of Flexible Algorithm Paths . . . . . . . . . . . 6 6. Operational Considerations . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . 6 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 8.1. IS-IS Sub-Sub-TLVs in Flexible Algorithm Definition Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 6 8.2. OSPF Sub-Sub-TLVs in Flexible Algorithm Definition Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 7 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 9.2. Informative References . . . . . . . . . . . . . . . . . 8 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction Link Bit Error Rate (hereafter "link BER metric") refers to the percentage of transmitted bits that are received in error over a network link. It is a key metric for network performance evaluation. A high BER metric can impact service quality and forwarding efficiency. To maintain optimal forwarding paths, it is desirable to avoid links with excessive BER metric values during IGP path computation. Gandhi Expires 19 January 2027 [Page 2] Internet-Draft Flexible-Algorithm with Link BER July 2026 The IGP Flexible Algorithms enable IGPs to compute constraint-based paths [RFC9350]. Current path computation methods focus on determining the minimum path cost from source to destination. Flex- Algorithm already supports path computation based on IGP cost, minimum link delay, and traffic-engineering metrics. [RFC9843] defines additional generic metric types and constraints. [I-D.gandhi-lsr-ber] defines BER metric advertisement extensions for IS-IS and OSPF. This document specifies a BER-metric-based exclusion constraint for Flex-Algorithm path computation. 2. Conventions Used in This Document 2.1. 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. 2.2. Abbreviations BER: Bit Error Rate. EMA: Exponential Moving Average. FA-EMB: Flex-Algorithm Exclude Maximum Link BER. FAD: Flexible Algorithm Definition. OSPF: Open Shortest Path First. TLV: Type-Length-Value. 3. Overview This document defines a new Flexible Algorithm Definition (FAD) that exclude links exceeding a configured maximum BER threshold. Existing metric-type (for example, IGP, TE, and delay) remains unchanged. [I-D.gandhi-lsr-ber] defines extensions for advertising Packets with Bit Errors (PER) metric for links. Utilizing the link PER metric for Flex-Algorithm path computation will be covered in the future revision of this document. Gandhi Expires 19 January 2027 [Page 3] Internet-Draft Flexible-Algorithm with Link BER July 2026 4. Exclude Maximum Link BER Sub-TLV A new sub-sub-TLV, the "Exclude Maximum Link BER Sub-TLV", is defined as part of the FAD TLV. To ensure loop-free forwarding, all routers participating in a Flex-Algorithm MUST agree on the FAD definition. Selected nodes within the IGP domain MUST advertise FADs by including them in routing updates, as specified in Sections 5, 6, and 7 of [RFC9350]. The "Exclude Maximum Link BER Sub-TLV" defines the maximum allowable BER metric value for links in the Flex-Algorithm topology. When this sub-sub-TLV is present, links whose advertised BER metric exceeds the configured maximum BER metric value are excluded from path computation. 4.1. IS-IS Exclude Maximum Link BER Sub-TLV The IS-IS FA-EMB Sub-TLV is defined as a sub-sub-TLV of the IS-IS FAD 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 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Maximum Link BER | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: IS-IS FA-EMB Sub-TLV Type: TBA1 (to be assigned by IANA) Length: 3 octets Max Link BER: 24-bit unsigned integer representing the maximum allowable BER percentage. Encoded with a resolution of 0.000003% per unit, providing a maximum expressible value of 50.331642% (0xFFFFFF * 0.000003). Values exceeding this limit MUST be advertised as 0xFFFF FF. The FA-EMB sub-sub-TLV MUST appear at most once in the IS-IS FAD Sub- TLV. If it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the receiving node. The maximum link BER metric advertised in the FA-EMB sub-sub-TLV MUST be compared with the BER metric advertised for the link as defined in [I-D.gandhi-lsr-ber]. Gandhi Expires 19 January 2027 [Page 4] Internet-Draft Flexible-Algorithm with Link BER July 2026 If the advertised link BER metric exceeds the maximum BER metric in FA-EMB, the link MUST be excluded from the Flex-Algorithm topology. If a link does not advertise the BER metric but the FAD contains the FA-EMB sub-sub-TLV, the link MUST NOT be excluded from the Flex- Algorithm topology. 4.2. OSPF Exclude Maximum Link BER Sub-TLV The OSPF FA-EMB Sub-TLV is defined as a sub-sub-TLV of the OSPF FAD 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 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Maximum Link BER | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: OSPF FA-EMB Sub-TLV Type: TBA2 (to be assigned by IANA) Length: 3 octets Max Link BER: 24-bit unsigned integer representing the maximum allowable BER percentage. Encoded with a resolution of 0.000003% per unit, providing a maximum expressible value of 50.331642% (0xFFFFFF * 0.000003). Values exceeding this limit MUST be advertised as 0xFFFF FF. The FA-EMB sub-sub-TLV MUST appear at most once in the OSPF FAD Sub- TLV. If it appears more than once, the OSPF FAD Sub-TLV MUST be ignored by the receiving node. The maximum link BER metric advertised in the FA-EMB sub-sub-TLV MUST be compared with the BER metric advertised for the link as defined in [I-D.gandhi-lsr-ber]. If the advertised link BER metric exceeds the maximum BER metric in FA-EMB, the link MUST be excluded from the Flex-Algorithm topology. If a link does not advertise the BER metric but the FAD contains the FA-EMB sub-sub-TLV, the link MUST NOT be excluded from the Flex- Algorithm topology. Gandhi Expires 19 January 2027 [Page 5] Internet-Draft Flexible-Algorithm with Link BER July 2026 5. Calculation of Flexible Algorithm Paths The following rule is added to the topology pruning rules in Section 13 of [RFC9350]: 1. Check if any exclude FA-EMB rule is part of the Flex-Algorithm definition. If such exclude rule exists and the link has BER metric advertised, check whether the link satisfies the FA-EMB rule. If not, the link MUST be pruned from the computation. 6. Operational Considerations In some deployments, the BER metric may oscillate around the configured threshold, causing repeated topology updates and recomputations. The following mechanisms can improve stability: 1. Delayed collection: BER metric values advertised into IGP can be computed over a sufficiently long observation interval. 2. Averaging and normalization: BER metric values SHOULD be derived from averaged data (for example EMA) and MAY be normalized to avoid advertising insignificant changes. 3. Flapping suppression: Implementations MAY apply dampening or hold-down timers before advertising BER metric changes that repeatedly cross the exclusion threshold. 7. Security Considerations The security considerations in [RFC9350], [RFC8570], [RFC7471], [RFC9479], [RFC9492], and [I-D.gandhi-lsr-ber] apply. 8. IANA Considerations This document requests allocation of two new code points. 8.1. IS-IS Sub-Sub-TLVs in Flexible Algorithm Definition Sub-TLV IANA maintains the registry for IS-IS sub-sub-TLVs in the Flexible Algorithm Definition Sub-TLV. IANA is requested to allocate the following IS-IS sub-sub-TLV type: Gandhi Expires 19 January 2027 [Page 6] Internet-Draft Flexible-Algorithm with Link BER July 2026 +=======+========================================+===============+ | Value | Description | Reference | +=======+========================================+===============+ | TBA1 | IS-IS Exclude Maximum Link BER Sub-TLV | This document | +-------+----------------------------------------+---------------+ Table 1: IS-IS Sub-Sub-TLV Types 8.2. OSPF Sub-Sub-TLVs in Flexible Algorithm Definition Sub-TLV IANA maintains the registry for OSPF sub-sub-TLVs in the Flexible Algorithm Definition Sub-TLV. IANA is requested to allocate the following OSPF sub-sub-TLV type: +=======+=======================================+===============+ | Value | Description | Reference | +=======+=======================================+===============+ | TBA2 | OSPF Exclude Maximum Link BER Sub-TLV | This document | +-------+---------------------------------------+---------------+ Table 2: OSPF Sub-Sub-TLV Types 9. References 9.1. Normative References [I-D.gandhi-lsr-ber] Gandhi, R., "IS-IS, OSPF, and BGP-LS Extensions for Advertising Bit Error Rate Metric for Traffic Engineering", Work in Progress, Internet-Draft, draft- gandhi-lsr-ber-02, 18 July 2026, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . Gandhi Expires 19 January 2027 [Page 7] Internet-Draft Flexible-Algorithm with Link BER July 2026 [RFC9479] Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and J. Drake, "IS-IS Application-Specific Link Attributes", RFC 9479, DOI 10.17487/RFC9479, October 2023, . [RFC9492] Psenak, P., Ed., Ginsberg, L., Henderickx, W., Tantsura, J., and J. Drake, "OSPF Application-Specific Link Attributes", RFC 9492, DOI 10.17487/RFC9492, October 2023, . 9.2. Informative References [RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S. Previdi, "OSPF Traffic Engineering (TE) Metric Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015, . [RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March 2019, . [RFC9843] Hegde, S., Britto, W., Shetty, R., Decraene, B., Psenak, P., and T. Li, "IGP Flexible Algorithms: Bandwidth, Delay, Metrics, and Constraints", RFC 9843, DOI 10.17487/RFC9843, September 2025, . Acknowledgments TBA. Author's Address Rakesh Gandhi (editor) Cisco Systems, Inc. Canada Email: rgandhi@cisco.com Gandhi Expires 19 January 2027 [Page 8]