CATS Fallback Decision Framework
draft-pang-cats-fallback-decision-framework-00

CATS                                                        R. Pang, Ed.
Internet-Draft                                               M. Han, Ed.
Intended status: Standards Track                            China Unicom
Expires: 7 January 2027                                    T. Huang, Ed.
                                                                CNIC CAS
                                                           N. Zhang, Ed.
                                                            China Unicom
                                                             6 July 2026

                    CATS Fallback Decision Framework
             draft-pang-cats-fallback-decision-framework-00

Abstract

   This document describes the framework and considerations for fallback
   decision-making in Computing-Aware Traffic Steering (CATS).  While
   existing OAM frameworks provide multi-dimensional telemetry
   collection capabilities, how the CATS Path Selector (C-PS) should
   react to partial or transient failures remains an open issue.  This
   document highlights the problems of misjudgment due to correlated
   failures, steering flapping, and non-deterministic fallback
   behaviors.  It outlines the high-level considerations for logical
   decoupling of status dimensions and stable state transition
   mechanisms to ensure carrier-grade reliability.  Specific protocol
   extensions and algorithm implementations will be explored in future
   revisions based on working group discussions.

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 7 January 2027.

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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   4.  High-Level Considerations for the Framework . . . . . . . . .   3
     4.1.  Logical Decoupling of Dimensions  . . . . . . . . . . . .   4
     4.2.  Graceful Degradation  . . . . . . . . . . . . . . . . . .   4
     4.3.  State Stability Mechanisms  . . . . . . . . . . . . . . .   4
     4.4.  Deterministic Fallback Behavior . . . . . . . . . . . . .   4
   5.  CATS Fallback Decision Framework  . . . . . . . . . . . . . .   4
   6.  Next Steps  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   As described in [I-D.ietf-cats-framework], Computing-Aware Traffic
   Steering (CATS) optimizes service instance selection by incorporating
   dynamic states of computing and network resources.  Existing OAM
   frameworks, such as [I-D.fu-cats-oam-fw], define mechanisms to
   collect these multi-dimensional metrics.

   However, when the C-PS receives telemetry indicating a failure (e.g.,
   metric timeout or path unreachable), the decision logic governing how
   to fallback is not defined.  Simply removing the instance or
   immediately switching to an alternative often leads to service
   disruption, especially in edge computing scenarios where stateful
   sessions are common.

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   This document initiates the discussion on the CATS Fallback Decision
   Framework.  It identifies key challenges in existing approaches and
   proposes high-level considerations for improving the robustness of
   the C-PS.  The specific protocol extensions and detailed state
   machine algorithms are outside the scope of this initial version and
   will be addressed in subsequent revisions.

2.  Conventions and Definitions

   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.  Abbreviations and definitions used in this
   document:

3.  Problem Statement

   In the absence of a well-defined fallback decision framework, several
   critical issues persist in CATS deployments:

   *  Correlated Failure Misjudgment: When the telemetry reporting
      channel (e.g., for computing metrics) experiences a transient
      network flap, the C-PS may incorrectly mark a reachable service
      instance as unavailable.  This conflation of channel failure and
      instance failure causes unnecessary traffic black-holing.

   *  Steering Flapping: High-frequency volatility of computing metrics
      or intermittent network issues can cause the C-PS to oscillate
      between normal and fallback states.  This instability breaks
      stateful sessions (e.g., online gaming, video conferencing) and
      degrades user experience.

   *  Non-deterministic Fallback: In multi-device or Anycast
      deployments, if fallback algorithms are not consistent across
      nodes, the same client request may be steered to different
      instances during a failure, breaking session consistency.

   *  Binary State Limitation: Existing models often use a binary (up/
      down) status.  When a partial failure occurs (some metrics are
      stale but the instance is reachable), the C-PS abruptly abandons
      policy steering, failing to utilize the remaining valid resources.

4.  High-Level Considerations for the Framework

   To address the problems described in Section 2, the CATS Fallback
   Decision Framework SHOULD consider the following principles in its
   future definition:

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4.1.  Logical Decoupling of Dimensions

   The framework SHOULD allow the C-PS to logically decouple the
   evaluation of metric availability (from Instance OAM) and network
   reachability (from Path/Link OAM).  A failure in one dimension SHOULD
   NOT automatically result in the immediate withdrawal of an instance
   if other dimensions indicate normal operation.  This helps mitigate
   misjudgments caused by isolated channel failures.

4.2.  Graceful Degradation

   Instead of a binary up/down model, the framework MAY support a multi-
   level state approach.  When partial failures occur, the C-PS SHOULD
   attempt to maintain policy steering within a valid candidate subset
   rather than immediately falling back to default routing.  This
   graceful degradation maximizes resource utilization.

4.3.  State Stability Mechanisms

   To prevent steering flapping, the framework SHOULD define mechanisms
   to absorb transient state fluctuations.  State transitions,
   particularly recovery transitions, SHOULD be stabilized to ensure
   that intermittent metric refreshes do not cause rapid oscillation of
   traffic.

4.4.  Deterministic Fallback Behavior

   When a complete fallback is unavoidable, the selection mechanism
   SHOULD be deterministic.  This means that given the same network
   state and client context, different C-PS nodes SHOULD select the same
   target instance to maintain session consistency.  The prioritization
   of deterministic fallback methods SHOULD be consistent across the
   network to maintain session continuity.
   

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5.  CATS Fallback Decision Framework

   Figure 1 shows the framework diagram of CATS fallback decision.
                            +----------------------+
                         |    Policy Engine     |
                         +----------+-----------+
                                    |
                                    | Policy decision / metadata feedback
                                    v
+--------------------+     +--------+-------------------------------+
| Service Metric     |     |          Network Edge Device           |
| Collection Agent   |---->|                                        |
+--------------------+     |  +-------------------+                 |
   Metric channel          |  | Path Selection    |                 |
   (logically independent) |  | Module            |                 |
                           |  +---------+---------+                 |
+--------------------+     |            |                           |
| Reachability       |---->|            | Decision result           |
| Detection Mechanism|     |            v                           |
+--------------------+     |  +-------------------+                 |
   Reachability channel    |  | Data-plane        |                 |
   (logically independent) |  | Forwarding Unit   |                 |
                           |  +---------+---------+                 |
                           +------------|---------------------------+
                                        |
                                        | User traffic steering
                                        v
                         +--------------+--------------+
                         | Compute Service Instance    |
                         +-----------------------------+

   *  The service metric collection agent reports service metrics to the
      path selection module through a logically independent metric
      channel.

   *  The reachability detection mechanism provides network-layer
      reachability status to the path selection module through a
      logically independent reachability channel.

   *  The path selection module makes path decisions and sends the
      decision result to the data-plane forwarding unit.

   *  The policy engine is deployed externally or as a sidecar.  It
      provides policy decisions to the path selection module, and
      receives decision metadata feedback from the path selection
      module.

   *  The data-plane forwarding unit steers user traffic to the selected
      compute service instance.

6.  Next Steps

   This document initiates the discussion on CATS fallback decision
   mechanisms.  Based on working group feedback, future revisions of
   this draft will define specific requirements, state machine models,
   and YANG data models to support the framework.

7.  Security Considerations

   The fallback decision mechanism relies on the integrity of OAM data.
   If an attacker injects fake stale metrics, it could trigger
   unnecessary fallbacks.  Implementations MUST ensure that OAM feeds
   used for decision-making are authenticated and integrity-protected.

8.  IANA Considerations

   TBD

9.  References

9.1.  Normative References

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   [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>.

9.2.  Informative References

   [I-D.ietf-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-ietf-
              cats-framework-24, 2 April 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cats-
              framework-24>.

   [I-D.fu-cats-oam-fw]
              Huakai.Fu, Xiong, Q., Du, Z., Liu, B., and Z. Li,
              "Computing-Aware Traffic Steering (CATS) Operations,
              Administration, and Maintenance (OAM) Framework", Work in
              Progress, Internet-Draft, draft-fu-cats-oam-fw-07, 18 June
              2026, <https://datatracker.ietf.org/doc/html/draft-fu-
              cats-oam-fw-07>.

Authors' Addresses

   Ran Pang (editor)
   China Unicom
   Beijing
   China
   Email: pangran@chinaunicom.cn

   Mengyao Han (editor)
   China Unicom
   Beijing
   China
   Email: hanmy12@chinaunicom.cn

   Tianyi Huang (editor)
   CNIC CAS
   Beijing
   China
   Email: tyhuang@cnic.cn

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   Naihan Zhang (editor)
   China Unicom
   Beijing
   China
   Email: zhangnh12@chinaunicom.cn

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