Flow-Level Load Balancing of Computing-Aware Traffic Steering (CATS)
draft-fu-cats-flow-lb-03
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Huakai.Fu , Daniel Huang , Wei Duan , Bin Tan | ||
| Last updated | 2026-02-26 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
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| Stream | Stream state | (No stream defined) | |
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draft-fu-cats-flow-lb-03
CATS H. Fu
Internet-Draft D.H. Huang
Intended status: Standards Track W. Duan
Expires: 30 August 2026 B. Tan
ZTE Corporation
26 February 2026
Flow-Level Load Balancing of Computing-Aware Traffic Steering (CATS)
draft-fu-cats-flow-lb-03
Abstract
This document specifies a flow-level load balancing mechanism for
Computing-Aware Traffic Steering (CATS) that reduces control plane
overhead and improves resource utilization through data plane
autonomous flow distribution.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 30 August 2026.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
5. Architectural Model . . . . . . . . . . . . . . . . . . . . . 3
6. Control plane Operation . . . . . . . . . . . . . . . . . . . 5
7. Data Plane Operation . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Computing-Aware Traffic Steering (CATS) [I-D.ldbc-cats-framework]
directs traffic between service clients and providers based on real-
time computing and network status. While CATS operates as an overlay
system for selecting optimal service instances, the framework does
not assume specific data plane or control plane solutions.
This document defines a flow-level load balancing mechanism
addressing two operational challenges: control plane scalability
limitations caused by reactive path computation, and resource
utilization imbalances resulting from coarse-grained status
reporting. The mechanism enables the control plane to pre-compute
multiple viable forwarding alternatives while allowing the data plane
to autonomously distribute traffic across these alternatives using
flow-based affinity.
2. 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.
3. Terminology
This document makes use of the terms defined in
[I-D.ldbc-cats-framework].
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* CATS Routing Table: Control plane structure containing service
identifiers, network paths, service instances, and load sharing
weights.
* CATS Forwarding Table: Data plane structure derived from CATS
Routing Table via UCMP-to-ECMP expansion.
* Flow Affinity Table: Data plane table maintaining flow-to-
forwarding-entry bindings.
4. Problem Statement
Current CATS implementations rely on periodic or threshold-triggered
resource status reports to optimize service instance and path
selection. This approach creates uneven computing resource
utilization when status updates lag behind actual load changes,
potentially directing multiple requests to already overloaded
instances. Additionally, frequent metric fluctuations trigger
repeated control plane path recalculation and policy updates,
creating scalability constraints that incremental calculation alone
cannot resolve.
5. Architectural Model
The Flow-Level Load Balancing of Computing-Aware Traffic Steering is
constructed based on the framework established in the CATS
architecture [I-D.ldbc-cats-framework](Figure 1 for a visual
representation).
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+-----+ +------+ +------+
+------+| +------+ | +------+ |
|client|+ |client|-+ |client|-+
+---+--+ +---+--+ +---+--+
| | |
| +----------------+ | +-----+----------+
+-+ C-TC#1 +-+ +-----+ C-TC#2 |
|----------------| | |----------------|
| |C-PS#1 | +------+ |CATS-Forwarder 4|
......| +----------|....|C-PS#2|..| |...
: |CATS-Forwarder 2| | | | | .
: +----------------+ +------+ +----------------+ :
: :
: +-------+ :
: Underlay | C-NMA | :
: Infrastructure +-------+ :
: :
: :
: +----------------+ +----------------+ :
: |CATS-Forwarder 1| +-------+ |CATS-Forwarder 3| :
:.| |..|C-SMA#1|.... | |....:
+---------+------+ +-------+ +----------------+
| | | C-SMA#2 |
| | +-------+--------+
| | |
| | |
+------------+ +------------+
+------------+ | +------------+ |
| Service | | | Service | |
| Contact | | | Contact | |
| Instance |-+ | Instance |-+
+------------+ +------------+
service site 1 service site 2
Figure 1: CATS-Functional-Components
The mechanism operates through three functional entities: the Path
Selector (C-PS) situated in the control plane collecting metrics via
Metric Agents (C-SMA and C-NMA) and computing forwarding
alternatives; the Forwarder operating in the data plane performing
flow identification, affinity maintenance and packet forwarding; and
the Metric Agents reporting service instance and network status to
enable path computation.
The Path Selector maintains the CATS Routing Table containing for
each CS-ID a set of Forwarding Alternatives comprising network path
identifiers, service instance identifiers, and load sharing weights
representing desired traffic distribution proportions. The Forwarder
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generates the CATS Forwarding Table through UCMP-to-ECMP expansion,
creating a uniform lookup structure where each Forwarding Alternative
appears in proportion to its weight, and maintains the Flow Affinity
Table binding flow identifiers to specific forwarding entries.
6. Control plane Operation
The Path Selector identifies for each CS-ID the Service Instance Set
containing all Forwarding Alternatives satisfying SLA requirements,
computing load sharing weights based on service instance metrics,
network path characteristics and policy objectives. The resulting
CATS Routing Table is translated into a CATS Forwarding Table and
distributed to Forwarders..
Figure 2 shows an example of a representation of multi-next-hop CATS
routing table designed for a specific CS-ID1.
+-------+-------+--------------------------------------------------+
| | | NEXT HOP |
|VRF-ID |PREFIX +-----------------+-----------+--------------------+
| | |SR-Policy |Service SID| Load Sharing Ratio |
+-------+-------+-----------------+-----------+--------------------+
|100 |CS-ID1 |SR-Policy1(2ms) |END.DX-1 | 20% |
| | +-----------------+-----------+--------------------+
| | |SR-Policy1(2ms) |END.DX-2 | 30% |
| | +-----------------+-----------+--------------------+
| | |SR-Policy2(1.5ms)|END.DX-3 | 30% |
| | +-----------------+-----------+--------------------+
| | |SR-Policy2(1.5ms)|END.DX-4 | 20% |
+-------+-------+-----------------+-----------+--------------------+
Figure 2: An example of CATS routing table
Figure 3 shows an example of the CATS forwarding table following the
changes.
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+-------+-------+-----------------+-----------+--------+
|VRF-ID |PREFIX |SR-Policy |Service SID| offset |
+-------+-------+-----------------+-----------+--------+
|100 |CS-ID1 |SR-Policy1(2ms) |END.DX-1 | 0 |
| | +-----------------+-----------+--------+
| | |SR-Policy1(2ms) |END.DX-1 | 1 |
| | +-----------------+-----------+--------+
| | |SR-Policy1(2ms) |END.DX-2 | 2 |
| | +-----------------+-----------+--------+
| | |SR-Policy1(2ms) |END.DX-2 | 3 |
| | +-----------------+-----------+--------+
| | |SR-Policy1(2ms) |END.DX-2 | 4 |
| | +-----------------+-----------+--------+
| | |SR-Policy2(1.5ms)|END.DX-3 | 5 |
| | +-----------------+-----------+--------+
| | |SR-Policy2(1.5ms)|END.DX-3 | 6 |
| | +-----------------+-----------+--------+
| | |SR-Policy2(1.5ms)|END.DX-3 | 7 |
| | +-----------------+-----------+--------+
| | |SR-Policy2(1.5ms)|END.DX-4 | 8 |
| | +-----------------+-----------+--------+
| | |SR-Policy2(1.5ms)|END.DX-4 | 9 |
+-------+-------+-----------------+-----------+--------+
Figure 3: An example of CATS forwarding table
To minimize control plane load, the Path Selector regenerates the
CATS Routing Table only when metric changes cross predefined
thresholds rather than reacting to every fluctuation, optionally
employing hysteresis mechanisms to prevent rapid oscillation between
configurations.
7. Data Plane Operation
Upon receiving a packet the Forwarder extracts the CS-ID and computes
the flow identifier, typically derived from the five-tuple. The
Forwarder queries the Flow Affinity Table and if an existing binding
is found forwards the packet using the bound entry. For unbound
flows the Forwarder computes a hash over the flow identifier, selects
the CATS Forwarding Table entry at the resulting index, creates a
Flow Affinity Entry binding the flow to this entry, and forwards the
packet.
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The CATS Forwarding Table remains stable during flow affinity
establishment ensuring consistent forwarding for all packets of a
flow. Table updates received from the Path Selector apply only to
new flows while existing bindings remain valid until flow termination
or timeout.
8. Security Considerations
The mechanism introduces potential denial of service vulnerabilities
through Flow Affinity Table exhaustion if an attacker generates
excessive new flows, mitigated by implementing binding creation rate
limits. Predictable flow identifiers could enable binding hijacking
requiring cryptographically robust hash functions. CATS Forwarding
Tables distributed from Path Selector to Forwarder may expose
topology information warranting protection during transmission.
9. Acknowledgements
To be added upon contributions, comments and suggestions.
10. IANA Considerations
TBA
11. References
11.1. 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>.
[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>.
[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>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
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[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
11.2. Informative References
[I-D.fu-cats-hybrid-fwd]
Huakai.Fu, Yi, X., Pang, B., Yuan, D., Duan, W., and C.
Miao, "Hybird Fordwarding of Computing-Aware Traffic
Steering (CATS)", Work in Progress, Internet-Draft, draft-
fu-cats-hybrid-fwd-00, 30 June 2025,
<https://datatracker.ietf.org/doc/html/draft-fu-cats-
hybrid-fwd-00>.
[I-D.fu-cats-muti-dp-solution]
Huakai.Fu, Liu, B., Li, Z., Huang, D., Yuan, D., Ma, L.,
and W. Duan, "Analysis for Multiple Data Plane Solutions
of Computing-Aware Traffic Steering", Work in Progress,
Internet-Draft, draft-fu-cats-muti-dp-solution-03, 19
August 2025, <https://datatracker.ietf.org/doc/html/draft-
fu-cats-muti-dp-solution-03>.
[I-D.huang-service-aware-network-framework]
Huang, D., Tan, B., and D. Yang, "Service Aware Network
Framework", Work in Progress, Internet-Draft, draft-huang-
service-aware-network-framework-01, 22 November 2022,
<https://datatracker.ietf.org/doc/html/draft-huang-
service-aware-network-framework-01>.
[I-D.ietf-cats-usecases-requirements]
Yao, K., Contreras, L. M., Shi, H., Zhang, S., and Q. An,
"Computing-Aware Traffic Steering (CATS) Problem
Statement, Use Cases, and Requirements", Work in Progress,
Internet-Draft, draft-ietf-cats-usecases-requirements-14,
2 February 2026, <https://datatracker.ietf.org/doc/html/
draft-ietf-cats-usecases-requirements-14>.
[I-D.lbdd-cats-dp-sr]
Li, C., Du, Z., Drake, J., and shangyuxiang, "Computing-
Aware Traffic Steering (CATS) Using Segment Routing", Work
in Progress, Internet-Draft, draft-lbdd-cats-dp-sr-06, 13
October 2025, <https://datatracker.ietf.org/doc/html/
draft-lbdd-cats-dp-sr-06>.
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[I-D.ldbc-cats-framework]
Li, C., Du, Z., Boucadair, M., Contreras, L. M., and J.
Drake, "A Framework for Computing-Aware Traffic Steering
(CATS)", Work in Progress, Internet-Draft, draft-ldbc-
cats-framework-06, 8 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ldbc-cats-
framework-06>.
[I-D.li-dyncast-architecture]
Li, Y., Iannone, L., Trossen, D., Liu, P., and C. Li,
"Dynamic-Anycast Architecture", Work in Progress,
Internet-Draft, draft-li-dyncast-architecture-08, 16
January 2023, <https://datatracker.ietf.org/doc/html/
draft-li-dyncast-architecture-08>.
[RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
"Architectural Considerations of IP Anycast", RFC 7094,
DOI 10.17487/RFC7094, January 2014,
<https://www.rfc-editor.org/info/rfc7094>.
Authors' Addresses
Huakai Fu
ZTE Corporation
Wuhan
China
Email: fu.huakai@zte.com.cn
Daniel Huang
ZTE Corporation
Nanjing
China
Email: huang.guangping@zte.com.cn
Wei Duan
ZTE Corporation
Nanjing
China
Email: duan.wei1@zte.com.cn
Bin Tan
ZTE Corporation
ShangHai
China
Email: tan.bin@zte.com.cn
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