Network Working Group Y. Wang
Internet-Draft G. Xu
Intended status: Standards Track X. Geng
Expires: 4 September 2025 J. Dong
Huawei
P. Psenak
Cisco Systems
3 March 2025
IGP Flexible Algorithm with Link Loss
draft-wang-lsr-flex-algo-link-loss-04
Abstract
This document proposes extensions to the IGP Flexible Algorithms
framework defined in [RFC9350]. It introduces a mechanism to exclude
links exceeding a specified packet loss rate threshold during path
computation. The solution leverages existing link loss measurements
advertised via IS-IS [RFC8570] and OSPF [RFC7471], 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 4 September 2025.
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Copyright Notice
Copyright (c) 2025 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/
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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. 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 for Flexible Algorithm Definition
Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. OSPF Sub-Sub-TLVs for Flexible Algorithm Definition
Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. 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. It is a critical metric for network performance evaluation.
High 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.
[I-D.ietf-lsr-flex-algo-bw-con] defines a family of generic metrics
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(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 link loss, as the path cost should be defined as
the maximum or minimum value among all links in the path.
To address this issue, two solutions are considered. First, new
operators, such as a maximum value operator, can be defined to
function 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
during 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max 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 loss 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 loss should be
derived from a form of averaging, such as an exponential weighted
average, of the collected loss values. The advertised loss can be
normalized to prevent the dissemination of non-significant changes
in loss metrics.
3. Flapping suppression: If frequent changes in the IGP-
advertised loss are detected, a timer can be implemented to delay
the update process, thereby stabilizing the routing computations.
5. IANA Considerations
5.1. IS-IS Sub-Sub-TLVs for 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 for Flexible Algorithm Definition Sub-TLV
Type: 252(TBA)
Description: OSPF Exclude Maximum Link Loss Sub-TLV
Reference: This document Section 2.2
6. References
6.1. 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>.
[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>.
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[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>.
6.2. Informative 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, <https://datatracker.ietf.org/doc/html/draft-ietf-
lsr-flex-algo-bw-con-22>.
[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>.
Authors' Addresses
Yifan Wang
Huawei
Huawei Bld., No. 156 Beiqing Rd.
Beijing
100095
China
Email: wangyifan82@huawei.com
Guoqi Xu
Huawei
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Email: xuguoqi@huawei.com
Xuesong Geng
Huawei
Email: gengxuesong@huawei.com
Jie Dong
Huawei
Email: jie.dong@huawei.com
Peter Psenak
Cisco Systems
Email: ppsenak@cisco.com
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