Flexible Candidate Path Selection of SR Policy
draft-liu-spring-sr-policy-flexible-path-selection-10
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Yisong Liu , Changwang Lin , Shuping Peng , Ran Chen , Gyan Mishra , Yuanxiang Qiu | ||
| Last updated | 2025-06-30 | ||
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draft-liu-spring-sr-policy-flexible-path-selection-10
SPRING Working Group Y. Liu
Internet Draft China Mobile
Intended status: Standards Track C. Lin
Expires: 30 December 2025 New H3C Technologies
S. Peng
Huawei Technologies
R. Chen
ZTE Corporation
G. Mishra
Verizon Inc.
Y. Qiu
New H3C Technologies
30 June 2025
Flexible Candidate Path Selection of SR Policy
draft-liu-spring-sr-policy-flexible-path-selection-10
Abstract
This document proposes a flexible SR policy candidate path selection
method. Based on the real-time resource usage and forwarding quality
of candidate paths, the head node can perform dynamic path switching
among multiple candidate paths in the SR policy.
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
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reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on 30 December 2025.
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/license-info) in effect on the date of
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publication of this document. Please review these documents
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Table of Contents
1. Introduction...................................................2
2. Terminology....................................................3
3. Background of requirements.....................................3
4. Flexible Candidate Path Selection Method.......................5
4.1. Threshold Parameters of Candidate Paths...................6
4.2. Rules for Setting eligibility attribute...................8
4.3. Flexible Candidate Path Selection Process.................8
5. Usecases of Flexible Candidate Path Selection..................9
5.1. Select the Best Path Based on End-to-End Delay............9
5.2. Select the Best Path Based on Available Bandwidth........10
5.3. Select the Best Path Based on Actual Bandwidth...........11
6. IANA Considerations...........................................11
7. Security Considerations.......................................11
8. References....................................................12
8.1. Normative References.....................................12
8.2. Informative References...................................12
9. Acknowledgments...............................................14
Authors' Addresses...............................................14
1. Introduction
Segment routing (SR) [RFC8402] is a source routing paradigm that
explicitly indicates the forwarding path for packets at the ingress
node. The ingress node steers packets into a specific path according
to the Segment Routing Policy (SR Policy) as defined in [RFC9256].
An SR Policy may have multiple candidate paths that are provisioned
or signaled [I-D.ietf-idr-sr-policy-safi] [RFC8664] from one of more
sources. The tie-breaking rules defined in [RFC9256] result in
determination of a single "active path" in a formal definition.
Refer to [RFC9256] only the active candidate path MUST be used for
forwarding traffic that is being steered onto that policy except for
certain scenarios such as fast reroute where a backup candidate path
may be used. A candidate path can be represented as a segment list
or a set of segment lists. If a set of segment lists is associated
with the active path of the policy, then the steering is per flow
and weighted-ECMP (W-ECMP) based according to the relative weight of
each valid segment list.
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According to the criteria for the validity of candidate paths
described in Section 5 of [RFC9256], if there is a valid segment
list in the active candidate path, the active candidate path is
still valid. When some segment lists of the active candidate path
are invalid, the active candidate path may still be valid, but it
may not continue to meet the actual forwarding requirements.
[I-D.karboubi-spring-sr-policy-eligibility] introduces a concept of
an eligibility attribute at the candidate path level, not only at
the time of the computation but also through topology and network
changes to ensure that user intentions are preserved while carrying
service traffic.
This document proposes a method for setting the eligibility
attribute, which can influence the selection of candidate paths. For
specific preference rules, refer to [I-D.karboubi-spring-sr-policy-
eligibility].
Based on real-time resource usage and forwarding quality of
candidate paths, the head node can dynamically adjust
the eligibility attribute value, enabling it to perform dynamic path
switching among multiple candidate paths within the SR policy.
[RFC2386] provides valuable background on QoS-based routing, details
some issues and requirements associated with QoS-based routing, and
proposes a framework for employing QoS-based routing within the
Internet. This document describes an SR Policy mechanism where the
path state is switched based on the resource status of the traversed
path. However, it does not address the challenges related to dynamic
distributed scheduling or resource reservation along intermediate
paths. The document specifies the capability to switch to
alternative paths within a strategy when the current path fails to
satisfy designated link quality criteria, such as bandwidth, delay,
or packet loss. In instances where a controller issues an SR Policy
encompassing multiple paths, should a path's link quality not meet
the established requirements, a switch to a backup path for
forwarding is executed.
2. Terminology
The definitions of the basic terms are identical to those found in
Segment Routing Policy Architecture [RFC9256].
3. Background of requirements
When some segment lists of the active candidate path are invalid,
according to [RFC9256], if there is a valid segment list in the
active candidate path, the active candidate path is still valid. But
the paths of remaining segment lists may not meet the SR policy
forwarding performance requirements, such as insufficient path
bandwidth. Even if there are other candidate paths with lower
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preference that can meet the forwarding performance requirements in
the SR policy, the traffic will continue to be forwarded along the
original active candidate path.
As an example, consider the following SR Policy to illustrate the
issues present in the current candidate path selection process in
detail.
SR Policy POL1
Candidate Path CP1
Preference 200
Segment List 1 <SID11...SID1i>, Weight 1
Segment List 2 <SID21...SID2j>, Weight 1
Segment List 3 <SID31...SID3k>, Weight 1
Candidate Path CP2
Preference 100
Segment List 4 <SID41...SID4i>, Weight 1
Segment List 5 <SID51...SID5j>, Weight 1
Segment List 6 <SID61...SID6k>, Weight 1
There are two static candidate paths CP1 and CP2 in SR policy POL1.
CP1 has a higher preference. Both candidate paths are composed of
three static segment lists with the same weight. The path indicated
by each segment list can carry traffic of 100Mbps bandwidth. When
all Segment Lists in CP1 are valid, the effective bandwidth of the
candidate path is 300Mbps.
The bandwidth of the actual traffic forwarded by the SR policy is
between 100Mbps and 150Mbps. Because the traffic forwarded on the
candidate path will share the load on the three segment list paths
according to the weight value. Therefore, normally, the candidate
path can meet the forwarding requirements. The traffic is forwarded
on the three segment lists of the high preference candidate paths of
the SR policy.
When the segment list 1 and 2 in the high-preference candidate path
CP1 are invalid, according to the candidate path validity criteria
described in [RFC9256] Section 5, because the segment list 3 in CP1
is still valid, the active candidate path CP1 is still valid. All
traffic of SR policy POL1 will continue to be forwarded through the
path of CP1. However, because segment list 3 can only forward
100Mbps traffic, over-bandwidth traffic will be discarded.
Of course, when the Segment List path fault is detected, the network
device can report the detected fault information to the controller.
The controller optimizes the forwarding path after receiving the
message. However, this interaction process is relatively long, and
it is difficult to meet the requirement for fast switching.
When the quality of high-preference candidate paths deteriorates,
due to issues such as insufficient available bandwidth, increased
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end-to-end transmission delay, or segment lists that fail to meet
service requirements, the same need arises. The goal is to switch
traffic to other candidate paths within the SR policy that better
satisfy the forwarding quality requirements.
To address this issue, this document proposes a new candidate path
selection rule that defines resource thresholds and forwarding
quality requirements for candidate paths.
If a candidate path does not satisfy the forwarding quality
requirements, its eligibility attribute MUST be set to false. During
the active CP selection process, the head-end SHALL use this
eligibility attribute as an additional mandatory criterion, in
conjunction with the rules defined in [RFC9256], Section 2.9. When a
CP's eligibility attribute is false, it indicates that the path
cannot forward traffic and therefore MUST NOT be considered for
active CP selection.
4. Flexible Candidate Path Selection Method
As described in [RFC9256], the candidate path selection process
operates primarily on the candidate path Preference. A candidate
path is selected when it is valid and it has the highest Preference
value among all the valid candidate paths of the SR Policy.
[I-D.karboubi-spring-sr-policy-eligibility] introduce a new
attribute at the candidate path level called eligibility. Only
candidate paths with eligibility as true must be considered as part
of the active candidate path selection defined in [RFC9256].
This document proposes using forwarding quality
requirements and resource requirements of candidate paths
as eligibility criteria for path selection.
A headend MAY be informed about the forwarding quality requirements
of a candidate path for an SR Policy <Color, Endpoint> through
various means, including configuration, PCEP, or BGP. The extensions
of BGP and PCEP are described in
[I-D.liu-idr-bgp-sr-policy-cp-threshold] and
[I-D.liu-pce-sr-policy-cp-threshold].
When a candidate path fails to meet forwarding quality requirements,
its eligibility attribute should be set to false, thereby excluding
it from active candidate path selection.
For candidate paths containing multiple segment lists:
- If a segment list fails to meet forwarding quality requirements,
it should be excluded from forwarding operations.
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- When all segment lists under a candidate path fail to meet
forwarding quality requirements, the path's eligibility attribute
should be set to false, subsequently disqualifying it from active
candidate path selection.
4.1. Threshold Parameters of Candidate Paths
The threshold parameters of candidate paths can include but are not
limited to the following:
* Jitter
* Latency
* Packet loss
Delay, jitter, and packet loss are thresholds at the segment list
level.
When the jitter, delay, or packet loss of a valid segment list
does not meet the specified threshold requirements, the segment
list will be deemed invalid and will no longer participate in
load sharing traffic.
* Available bandwidth
The bandwidth threshold is the threshold at the candidate path
level.
CP available bandwidth = CP preset bandwidth * (Sum of Segment
List weights in Up state / Sum of all Segment List weights)
* Actual bandwidth
The actual bandwidth refers to the sum of the actual available
remaining bandwidth of each valid segment list in the candidate
path.
Due to the different congestion conditions of each node on the
forwarding path, the actual bandwidth that can forward service
packets may differ from the preset bandwidth. By utilizing some
measurement mechanisms, the actual minimum available bandwidth
and actual minimum remaining bandwidth of all nodes along the
path can be obtained. The specific measurement mechanism is not
within the scope of this document.
* Precision Availability Metrics (PAM)
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Consider a candidate path of SR policy as a Service Level
Objective (SLO), based on the Precision Availability Metrics
(PAM) defined in [RFC9544], determine whether the candidate path
meets the forwarding requirements.
If both segment list-level thresholds (such as latency, jitter, or
packet loss) and candidate path-level thresholds (such as available
bandwidth) are specified, then when one or more segment lists in the
candidate path fail to meet the segment list-level thresholds, it
indicates that these segment lists cannot provide forwarding
capabilities that meet the SLA requirements. These segment lists
will be marked as unavailable and will no longer participate in
packet forwarding. After excluding these segment lists, it should be
verified whether the candidate path still meets the forwarding
quality requirements. If it does, traffic can continue to be
forwarded along that candidate path.
For example, two threshold parameters, delay and available
bandwidth, are specified simultaneously for the candidate path with
multiple segment lists. When the delay of a segment list exceeds the
threshold, the following processing is performed:
1) Remove the segment list from the forwarding path first.
2) Calculate the current available bandwidth of CP based on the
weight ratio of the remaining effective segment lists and the
bandwidth of CP.
3) Check whether the current available bandwidth of CP still meets
the bandwidth threshold requirements.
* If the available bandwidth still meets the requirements, the
candidate path still meets the forwarding quality
requirements, and the traffic is still forwarded along this
candidate path.
* Otherwise, set the eligibility attribute of this CP to false.
The system should then consider switching service traffic to
another active candidate path with better forwarding quality.
If the candidate path does not specify any threshold parameters,
select the primary candidate path according to the selection method
defined in [RFC9256].
By default, there is no threshold parameter specified on the
candidate path.
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4.2. Rules for Setting eligibility attribute
When a candidate path's current forwarding quality meets specified
threshold requirements, its eligibility attribute MUST be set
to true, indicating this path is valid for:
* Traffic forwarding operations.
* Active candidate path selection (per [RFC9256] selection
methodology)
Conversely, when a candidate path fails to meet quality
requirements, its eligibility attribute MUST be set to false.
For candidate paths without defined threshold parameters:
* The eligibility attribute MUST default to true.
* Primary path selection follows [RFC9256] procedures.
When multiple eligible candidate paths coexist in an SR policy:
* Only paths with eligibility=true MAY participate in active
path selection.
* Detailed behavior specified in
[I-D.karboubi-spring-sr-policy-eligibility].
4.3. Flexible Candidate Path Selection Process
The process of selecting the best candidate path for SR policy
through the threshold parameter of the candidate path is as follows.
1) Configure the threshold parameters on the candidate path of the
head node through static manual configuration or controller
distribution.
2) The head node monitors whether the available resources and
forwarding quality of the SR policy candidate path exceed the
thresholds.
3) The forwarding quality of path can be obtained through active or
passive performance measurement methods, such as iOAM [RFC9378],
STAMP [I-D.ietf-spring-stamp-srpm], TWAMP [RFC5375], etc. The
real-time quality data can be calculated by the controller and
distributed to the head node, or calculated by the head node
according to the network measurement data. The measurement method
and quality data acquisition method are beyond the scope of this
document.
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4) According to the rules described in Section 4.2, when the
available resources are less than the threshold, or the
forwarding quality cannot meet the threshold requirements, select
a new active candidate path.
5) After the old active candidate path eliminates the fault or
improves the forwarding quality, whether to recover can be
specified by the configuration. If fault recovery is required,
start a wait timer for delay recovery. When the timer expires and
the old active candidate path still meets the threshold
requirements, the traffic will be switched to the old higher
preference candidate path.
For avoiding path switching frequently, both over-threshold
switching and fault recovery should be delayed. The interval of
delay waiting can be adjusted by configuration.
5. Usecases of Flexible Candidate Path Selection
The SR policy in Section 3 is still used to illustrate how the
flexible candidate path selection method switches candidate paths.
SR policy POL1 has two candidate paths CP1 and CP2. The Preference
of CP1 is 200, and the Preference of CP2 is 100. Both candidate
paths are composed of three segment lists with the same weight.
5.1. Select the Best Path Based on End-to-End Delay
The quality requirement for the services carried on the SR policy is
that the transmission delay must be less than 200ms. The bandwidth
of the actual traffic forwarded by the SR policy is between 100Mbps
and 150Mbps.
When the delay of Segment List 1 does not meet the requirements,
continue to check the available bandwidth of CP1. Due to segment
list 2 only having 100Mbps bandwidth, it cannot meet the actual
traffic forwarding requirements. CP1's eligibility attribute MUST be
set to false, triggering the selection of CP2 as POL1's new active
candidate path. The traffic forwarded by POL1 is switched to the
path of CP2 for forwarding.
SR Policy POL1
Candidate Path CP1
Preference 200
Delay threshold 200ms //Delay<=200ms
Segment List 1 <SID11...SID1i>, Weight 1 //100M, Delay>1s
Segment List 2 <SID21...SID2i>, Weight 1 //100M, Delay<100ms
Candidate Path CP2
Preference 100
Delay threshold 200ms //Delay<=200ms
Segment List 3 <SID31...SID3i>, Weight 1 //100M, Delay<100ms
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Segment List 4 <SID41...SID4i>, Weight 1 //100M, Delay<100ms
5.2. Select the Best Path Based on Available Bandwidth
The preset bandwidth for CP1 and CP2 is both 300Mbps. Each segment
list can carry a maximum of 100Mbps traffic. The quality requirement
for service traffic is that the available bandwidth of the
forwarding path must not be less than 150Mbps.
SR Policy POL1
Candidate Path CP1
Preference 200
Preset bandwidth 300Mbps
Available bandwidth threshold 150Mbps
Segment List 1 <SID11...SID1i>, Weight 1
Segment List 2 <SID21...SID2j>, Weight 1
Segment List 3 <SID31...SID3k>, Weight 1
Candidate Path CP2
Preference 100
Preset bandwidth 300Mbps
Available bandwidth threshold 150Mbps
Segment List 4 <SID41...SID4i>, Weight 1
Segment List 5 <SID51...SID5j>, Weight 1
Segment List 6 <SID61...SID6k>, Weight 1
First, take the available bandwidth as the threshold parameter of
POL1. The threshold for configuring the available bandwidth is
150Mbps. When the available bandwidth of the candidate path is less
than 150Mbps, perform path switching.
Normally, the three segment lists of CP1 and CP2 are valid. The
available bandwidth of CP1 is 300Mbps, and the available bandwidth
can meet the threshold requirements. CP1's eligibility attribute
MUST be set to true, CP1 is selected as the active candidate path
according to the Preference.
If the paths indicated by Segment List 1 and 2 fail, Segment List 1
and 2 become invalid, and the available bandwidth of CP1 becomes
100Mbps. Because the available bandwidth of CP1 is lower than the
specified threshold, CP1 has failed to meet the forwarding quality
requirements. CP1's eligibility attribute MUST be set to false. Need
to reselect the active candidate path for POL1.
The three segment lists of the low-preference candidate path CP2 of
POL1 are valid, and the available bandwidth can meet the threshold
requirements. CP2's eligibility attribute MUST be set to true. CP2
is selected as the new active candidate path of POL1. The traffic
forwarded by POL1 will switch to the path of CP2 for forwarding.
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5.3. Select the Best Path Based on Actual Bandwidth
In scenarios involving the actual available bandwidth measurement
method for SRv6, as described in
[I-D.liu-ippm-srv6-bandwidth-measurement], the quality requirement
for the services carried on the SR policy mandates that the actual
available bandwidth of the forwarding path must exceed 80 Mbps. If
traffic congestion occurs on a node in Segment List 1, resulting in
a maximum forwarding capacity of only 50 Mbps for service traffic,
and if Segment List 2 is either in a down state or has exceeded the
delay threshold, Segment List 2 will not participate in load sharing
traffic.
When the aggregate available bandwidth of CP1 falls below 80 Mbps:
* CP1's eligibility attribute MUST be set to false.
* CP2's eligibility attribute MUST be set to true (provided it
meets forwarding requirements).
* CP2 SHALL become POL1's new active candidate path.
SR Policy POL1
Candidate Path CP1
Preference 200
Preset bandwidth 200Mbps
Actual available bandwidth threshold 80Mbps
Segment List 1 <SID11...SID1i>, Weight 1
(Actual available bandwidth is only 50Mbps.)
Segment List 2 <SID21...SID2j>, Weight 1
(In Down state, or the delay has exceeded the threshold.)
Candidate Path CP2
Preference 100
Preset bandwidth 300Mbps
Actual available bandwidth threshold 80Mbps
Segment List 3 <SID41...SID4i>, Weight 1 (100Mbps)
Segment List 4 <SID51...SID5j>, Weight 1 (100Mbps)
Segment List 5 <SID61...SID6k>, Weight 1 (100Mbps)
The traffic forwarded by POL1 will switch to the path of CP2 for
forwarding.
6. IANA Considerations
This document has no IANA actions.
7. Security Considerations
This document does not introduce any security considerations.
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8. References
8.1. Normative References
[I-D.ietf-idr-sr-policy-safi] Previdi, S., Filsfils, C., Talaulikar,
K., Mattes, P., and Jain, D., "Advertising Segment Routing
Policies in BGP", draft-ietf-idr-sr-policy-safi-13 (work
in progress), 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, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,July
2018, <https://www.rfc-editor.org/info/rfc8402>.
[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>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
Hardwick, J., "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC8664,
DOI 10.17487/RFC8664, December 2019, <https://www.rfc-
editor.org/info/rfc8664>.
[RFC9256] Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", RFC
9256, DOI 10.17487/RFC9256, July 2022, <https://www.rfc-
editor.org/info/rfc9256>.
8.2. Informative References
[I-D.liu-idr-bgp-sr-policy-cp-threshold] Liu, Y., Lin, C., Qiu,
Y., " BGP Extension for Distributing CP Threshold
Constraints of SR Policy", draft-liu-idr-bgp-sr-policy-cp-
threshold-02 (work in progress), November 2024.
[I-D.liu-pce-sr-policy-cp-threshold] Liu, Y., Lin, C., Qiu, Y., "
PCEP Extension to Support Signaling Candidate Path
Threshold Constraints of SR Policy", draft-liu-pce-sr-
policy-cp-threshold-03 (work in progress), February 2025.
[I-D.liu-ippm-srv6-bandwidth-measurement] Liu, Y., Lin, C., Qiu, Y.,
Liu, Y., Liang, Y., " Measurement Method for Bandwidth of
SRv6 Forwarding Path", draft-liu-ippm-srv6-bandwidth-
measurement (work in progress), November 2024.
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[I-D.karboubi-spring-sr-policy-eligibility] Karboubi, A., Shah, H.,
Sivalaban, S., Stone, A. and Schmutz, C., "Eligibility
Concept in Segment Routing Policies", draft-karboubi-
spring-sr-policy-eligibility-02 (work in progress), June
2025.
[I-D.ietf-spring-stamp-srpm] Gandhi, R., Filsfils, C., Janssens, B.,
Chen, M., and R.F. Foote, "Performance Measurement Using
Simple Two-Way Active Measurement Protocol (STAMP) for
Segment Routing Networks", Work in Progress, Internet-
Draft, draft-ietf-spring-stamp-srpm-19, 20 June 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
stamp-srpm-19>.
[RFC2386] Crawley, E., Nair, R., Rajagopalan, B. and H. Sandick, "A
Framework for QoS-based Routing in the Internet", RFC
2386, August 1998.
[RFC5375] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., Babiarz,
J., "A Two-Way Active Measurement Protocol (TWAMP)", RFC
5375, DOI 10.17487/RFC5375, October 2008,
<https://www.rfc-editor.org/info/rfc5375>.
[RFC9378] Brockners, F., Bhandari, S., Bernier, D., Mizrahi, T., "In
Situ Operations, Administration, and Maintenance (IOAM)
Deployment", RFC 9378, DOI 10.17487/RFC9378, April 2023,
<https://www.rfc-editor.org/info/rfc9378>.
[RFC9544] Mirsky, G., Halpern, J., Min, X., Clemm, A., Strassner,
J., Francois, J., "Precision Availability Metrics for
Services Governed by Service Level Objectives (SLOs)", RFC
9544, DOI 10.17487/RFC9544, March 2024, <https://www.rfc-
editor.org/info/rfc9544>.
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9. Acknowledgments
The authors would like to thank the following for their valuable
contributions of this document:
TBD
Authors' Addresses
Yisong Liu
China Mobile
Beijing
China
Email: liuyisong@chinamobile.com
Changwang Lin
New H3C Technologies
Beijing
China
Email: linchangwang.04414@h3c.com
Shuping Peng
Huawei Technologies
Beijing
China
Email: pengshuping@huawei.com
Ran Chen
ZTE Corporation
Nanjing
China
Email: chen.ran@zte.com.cn
Gyan S. Mishra
Verizon Inc.
Email: gyan.s.mishra@verizon.com
Yuanxiang Qiu
New H3C Technologies
Beijing
China
Email: qiuyuanxiang@h3c.com
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