IGP Flexible Algorithm Utilizing Link Bit Error Rate Metric
draft-gandhi-lsr-flex-algo-link-ber-00

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

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   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.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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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.

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   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.

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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].

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   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.

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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:

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    +=======+========================================+===============+
    | 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,
              <https://datatracker.ietf.org/doc/html/draft-gandhi-lsr-
              ber-02>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9350]  Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
              and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
              DOI 10.17487/RFC9350, February 2023,
              <https://www.rfc-editor.org/info/rfc9350>.

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   [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,
              <https://www.rfc-editor.org/info/rfc9479>.

   [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,
              <https://www.rfc-editor.org/info/rfc9492>.

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,
              <https://www.rfc-editor.org/info/rfc7471>.

   [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, <https://www.rfc-editor.org/info/rfc8570>.

   [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, <https://www.rfc-editor.org/info/rfc9843>.

Acknowledgments

   TBA.

Author's Address

   Rakesh Gandhi (editor)
   Cisco Systems, Inc.
   Canada
   Email: rgandhi@cisco.com

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