



Network Working Group                                            Y. Wang
Internet-Draft                                                   J. Dong
Intended status: Standards Track                                 X. Geng
Expires: 3 September 2026                                         Huawei
                                                               P. Psenak
                                                           Cisco Systems
                                                            2 March 2026


              IGP Flexible Algorithm with Link Packet Loss
                 draft-wang-lsr-flex-algo-link-loss-05

Abstract

   This document proposes extensions to the IGP Flexible Algorithm.  It
   introduces a mechanism to exclude links exceeding a specified packet
   loss rate threshold during path computation.  The solution leverages
   existing link packet loss advertisement via IS-IS and OSPF, and
   defines new constraints for Flex-Algorithm path calculation.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD 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.

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 3 September 2026.







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Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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.  Exclude Maximum Link Loss Sub-TLV . . . . . . . . . . . . . .   4
     2.1.  IS-IS Exclude Maximum Link Loss Sub-TLV . . . . . . . . .   4
     2.2.  OSPF Exclude Maximum Link Loss Sub-TLV  . . . . . . . . .   5
   3.  Calculation of Flexible Algorithm Paths . . . . . . . . . . .   6
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  IS-IS Sub-Sub-TLVs in Flexible Algorithm Definition
           Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  OSPF Sub-Sub-TLVs in Flexible Algorithm Definition
           Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Link packet loss rate (hereafter "link loss") refers to the
   percentage of data packets that are lost during transmission over a
   network link.  It is a critical metric for network performance
   evaluation.  High packet loss rates directly impact service quality,
   congestion management, and operational efficiency.  To maintain
   optimal forwarding paths, it is essential to avoid links with
   excessive packet loss during IGP path computation.

   The IGP Flexible Algorithms enable IGPs to compute constraint-based
   paths [RFC9350].  Current path computation methods focus on
   determining the minimum cost of the path from the source to the
   destination.  Flex-Algorithm already supports path computation based
   on IGP cost, minimum link delay, and traffic-engineering metrics.



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   [RFC9843] defines a family of generic metrics (e.g., bandwidth-based
   metric type) and bandwidth-related constraints to enable path
   computation based on bandwidth.  However, current flexible algorithm
   definitions lack native support for path computation based on packet
   loss, as the cost of packet loss is defined as the maximum or minimum
   packet loss value among all links along the path.

   To address this issue, two solutions are considered.  First, a new
   operator, such as a maximum value operator, can be defined as a step
   function.  Specifically, when the link loss exceeds a threshold, the
   link cost is set to the maximum value.  Second, new Flexible
   Algorithm Definition (FAD) constraints can be defined to exclude
   links that do not meet the link loss requirements from path
   calculation.  The second method is specifically demonstrated in this
   document, and the general ideas are as follows:

      1.  The link loss is used as a link constraint for path
      computation.  That is, links with a loss rate exceeding the
      specified value are excluded.

      2.  Metric-type remains unchanged: IGP, te, and delay.

   This document proposes the method to exclude links exceeding a
   specified packet loss rate by defining:

      a) A new Flexible Algorithm Definition (FAD) constraint to exclude
      links exceeding a configured maximum loss threshold (Section 2).

      b) Operational procedures for integrating loss constraints into
      Flex-Algorithm path computations (Section 3).

      c) Mechanisms to stabilize routing during loss metric fluctuations
      (Section 4).

   The solution reuses existing link loss advertisements defined in
   [RFC8570] for IS-IS and [RFC7471] for OSPF, ensuring backward
   compatibility with deployed networks.  The link packet loss rate can
   be measured using methods such as TWAMP [RFC5357] and STAMP
   [RFC8762].  However, these measurement techniques are beyond the
   scope of this document.  It is important to ensure that link-loss
   measurements are consistent throughout the IGP routing domain.










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2.  Exclude Maximum Link Loss Sub-TLV

   A new sub-TLV, the Exclude Maximum Link Loss 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 their routing updates, as specified in Sections 5, 6, and 7
   of [RFC9350].

   The Exclude Maximum Link Loss Sub-TLV is introduced to define the
   maximum allowable link loss value.  When this Sub-TLV is carried
   within the FAD TLV, all network links with packet loss rates
   exceeding the specified maximum value are excluded from the Flex-
   Algorithm path computation.

2.1.  IS-IS Exclude Maximum Link Loss Sub-TLV

   The IS-IS Flex-Algorithm Exclude Maximum Link Loss Sub-TLV (FAEML) is
   defined as a sub-TLV of the IS-IS FAD Sub-TLV.  The format follows
   standard TLV structure:

    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     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Maximum Link Loss                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: 252(TBA by IANA)

      Length: 3 octets

      Max Link Loss:  24-bit unsigned integer representing the maximum
      allowable loss percentage. Encoded with a resolution of 0.000003%
      per unit, providing a maximum expressible value of 50.331642%
      (0xFFFFFF * 0.000003). Values exceeding this cap MUST be advertised
      as 0xFFFFFF.

                    Figure 1: IS-IS FAEML Sub-TLV

   The FAEML sub-TLV MUST appear at most once in the 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 loss advertised in the FAEML Sub-TLV MUST be
   compared with the link loss advertised in Sub-Sub-TLV 36 [RFC8570] of
   ASLA Sub-TLV [RFC9479].  If the L-Flag is set in the ASLA sub-TLV,



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   the maximum link loss advertised in the FAEML sub-TLV MUST be
   compared with the link loss advertised by the sub-TLV 36 of the TLV
   22/222/23/223/141 [RFC5305] as defined in [RFC9479] Section 4.2.

   If the link loss exceeds the maximum link loss advertised in the
   FAEML sub-TLV, the link MUST be excluded from the Flex-Algorithm
   topology.  However, if a link does not advertise the link loss but
   the FAD contains the FAEML sub-TLV, the link MUST NOT be excluded
   from the Flex-Algorithm topology.

2.2.  OSPF Exclude Maximum Link Loss Sub-TLV

   The OSPF Flex-Algorithm Exclude Maximum Link Loss Sub-TLV (FAEML) is
   defined as a sub-TLV of the OSPF FAD Sub-TLV.  The format follows
   standard TLV structure:

    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           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Maximum Link Loss              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: 252(TBA)

      Length: 3 octets

      Max Link Loss:  24-bit unsigned integer representing the maximum
      allowable loss percentage. Encoded with a resolution of 0.000003%
      per unit, providing a maximum expressible value of 50.331642%
      (0xFFFFFF * 0.000003). Values exceeding this cap MUST be advertised
      as 0xFFFFFF.

                     Figure 2: OSPF FAEML Sub-TLV

   The FAEML sub-TLV MUST appear at most once in the FAD Sub-TLV.  If it
   appears more than once, the OSPF FAD Sub-TLV MUST be ignored by the
   receiving node.

   The maximum link loss advertised in the FAEML Sub-TLV MUST be
   compared with the link loss advertised in Sub-Sub-TLV 30 [RFC7471] of
   the ASLA Sub-TLV [RFC9492].  The ASLA Sub-TLV is advertised in
   Extended Link Opaque LSAs [RFC7684] for OSPFv2 and E-Router-LSAs
   [RFC8362] for OSPFv3.






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   If the link loss exceeds the maximum link loss advertised in the
   FAEML sub-TLV, the link MUST be excluded from the Flex-Algorithm
   topology.  However, if a link does not advertise the link loss but
   the FAD contains the FAEML sub-TLV, the link MUST NOT be excluded
   from the Flex-Algorithm topology.

3.  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 FAEML rule is part of the Flex-Algorithm
      definition.  If such exclude rule exists and the link has link
      loss advertised, check if the link satisfies the FAEML rule.  If
      not, the link MUST be pruned from the computation.

4.  Operational Considerations

   In certain scenarios, the link status may fluctuate between available
   and unavailable due to the link packet loss rate oscillating around
   the threshold value.  Consequently, Flex-Algorithm computation may be
   triggered repeatedly.  Several mechanisms are considered to address
   this issue:

      1.  Delayed collection: The IGP-advertised link packet loss rate
      can be calculated over a sufficiently long interval, such as 10
      minutes, to reduce the frequency of updates.

      2.  Averaging and normalization: The IGP-advertised link packet
      loss rate should be derived from a form of averaging, such as an
      exponential weighted average, of the collected loss values.  The
      advertised loss rate can be normalized to prevent the
      dissemination of non-significant changes in loss metrics.

      3.  Flapping suppression: If frequent changes in the IGP-
      advertised link packet loss rate are detected, a timer can be
      implemented to delay the update process, thereby stabilizing the
      routing computation.

5.  IANA Considerations

5.1.  IS-IS Sub-Sub-TLVs in Flexible Algorithm Definition Sub-TLV

   Type: 252(TBA)

   Description: IS-IS Exclude Maximum Link Loss Sub-TLV

   Reference: This document Section 2.1



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5.2.  OSPF Sub-Sub-TLVs in Flexible Algorithm Definition Sub-TLV

   Type: TBA

   Description: OSPF Exclude Maximum Link Loss Sub-TLV

   Reference: This document Section 2.2

6.  Acknowledgements

   The authors would like to thank Guoqi Xu for the review and valuable
   discussion of this document.

7.  References

7.1.  Normative References

   [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>.

   [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>.

   [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>.

   [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>.

   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
              F. Baker, "OSPFv3 Link State Advertisement (LSA)
              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
              2018, <https://www.rfc-editor.org/info/rfc8362>.

   [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>.

7.2.  Informative References

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
              <https://www.rfc-editor.org/info/rfc5357>.

   [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>.

   [RFC8762]  Mirsky, G., Jun, G., Nydell, H., and R. Foote, "Simple
              Two-Way Active Measurement Protocol", RFC 8762,
              DOI 10.17487/RFC8762, March 2020,
              <https://www.rfc-editor.org/info/rfc8762>.

   [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>.

Authors' Addresses

   Yifan Wang
   Huawei
   Huawei Bld., No. 156 Beiqing Rd.
   Beijing
   100095
   China
   Email: wangyifan82@huawei.com





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   Jie Dong
   Huawei
   Email: jie.dong@huawei.com


   Xuesong Geng
   Huawei
   Email: gengxuesong@huawei.com


   Peter Psenak
   Cisco Systems
   Email: ppsenak@cisco.com






































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