



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

   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
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   This Internet-Draft will expire on 30 August 2026.

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   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
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   provided without warranty as described in the Revised BSD License.



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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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