CATS H. Fu Internet-Draft ZTE Corporation Intended status: Standards Track X. Yi Expires: 30 August 2026 China Unicom B. Pang Beijing Jiaotong University D. Yuan W. Duan C. Miao ZTE Corporation 26 February 2026 Hybrid Forwarding Mechanism for Computing-Aware Traffic Steering (CATS) draft-fu-cats-hybrid-fwd-01 Abstract This document specifies a hybrid forwarding mechanism for Computing- Aware Traffic Steering (CATS). The mechanism integrates the CATS forwarding table with the IP forwarding table to optimize forwarding table capacity utilization. By customizing the forwarding model based on service identifiers, it accommodates both experience- sensitive and non-experience-sensitive services. Additionally, it supports flow-granularity load balancing, enhancing the utilization of computing and networking resources while ensuring differentiated service requirements are satisfied. 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 30 August 2026. Fu, et al. Expires 30 August 2026 [Page 1] Internet-Draft Hybrid Forwarding for CATS February 2026 Copyright Notice 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 Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4 5. Hybrid Forwarding Model . . . . . . . . . . . . . . . . . . . 5 5.1. Table Management and Working Mechanism . . . . . . . . . 6 5.2. Considerations for Hybrid Forwarding . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 9.1. Normative References . . . . . . . . . . . . . . . . . . 11 9.2. Informative References . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 1. Introduction Computing-Aware Traffic Steering (CATS) [I-D.ldbc-cats-framework] targets efficient routing at the network edge, directing traffic between service clients and providers. It relies on real-time computing and network status data for informed decisions. CATS operates as an overlay system, choosing optimal service instances for requests. The CATS framework does not assume any specific data plane or control plane solutions. As the integration of networking and computing deepens, users are placing increasingly higher demands on both the experience of network services and the efficiency of resource utilization. This is particularly evident for services with low latency requirements, such as AR/VR, and services with explicit requirements for computing resources. Traditional traffic steering mechanisms are not capable of meeting these evolving demands. Relevant studies have Fu, et al. Expires 30 August 2026 [Page 2] Internet-Draft Hybrid Forwarding for CATS February 2026 demonstrated that traffic steering that takes into account computing resource conditions can significantly improve service performance and user experience [I-D.ietf-cats-usecases-requirements]. To address these challenges, the Computing-Aware Traffic Steering (CATS) framework has been proposed in [I-D.ldbc-cats-framework]. The primary goal of the CATS framework is to efficiently steer traffic at the edge device of the network instructed by traffic steering decisions referring to both computing and network conditions. The CATS framework is designed as an overlay mechanism for selecting the optimal service instances, without relying on specific data-plane or control-plane solutions. This document specifies a hybrid forwarding mechanism for CATS with the following key features: * Integration of Forwarding Tables: The CATS forwarding table is integrated with the IP forwarding table to achieve shared and efficient utilization of the forwarding table capacity. * Customized Forwarding Model: The forwarding model is customized based on service identifiers to meet the demands of both experience-sensitive and non-experience-sensitive services. * Flow-Level Load Balancing: The mechanism supports flow-granularity load balancing, enhancing the utilization of computing and networking resources while meeting diverse requirements for differentiated services. This hybrid forwarding mechanism provides a flexible and efficient solution for network vendors and service providers, facilitating the deployment of computing-aware traffic steering. 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 uses terms defined in [I-D.ldbc-cats-framework], [I-D.lbdd-cats-dp-sr], and [I-D.fu-cats-flow-lb]. This document defines the following additional terms: Fu, et al. Expires 30 August 2026 [Page 3] Internet-Draft Hybrid Forwarding for CATS February 2026 CATS-IND: An indicator in the forwarding table to identify CATS service packets. MODE-IND: An indicator to differentiate service requirements (e.g., experience-sensitive vs. non-sensitive services). Flow Affinity Table: A data plane table maintaining flow-to-next-hop bindings to ensure consistent forwarding for packets belonging to the same flow. CATS Routing Table: A control plane data structure containing network paths, service instances, and associated load sharing ratios for a given service identifier. CATS Forwarding Table: A data plane construct derived from the CATS Routing Table, optimized for packet forwarding decisions. 4. Problem Statement To effectively steer service request packets to appropriate forwarding paths and service instances, CATS faces several issues and challenges with existing technical solutions: Flow Affinity Mechanism Complexity: To ensure continuous access to the same computing resource during a service session, a flow affinity mechanism based on five-tuples or three-tuples is required. However, incorporating such lookups into the general routing process introduces additional processing overhead, which significantly increases packet forwarding latency and consumes chip resources. This additional processing burden can degrade the performance of latency-sensitive services. Control Plane Overhead: Computing services are characterized by ubiquity and state changes over time. If the control plane indiscriminately performs periodic or event-driven route computation and table updates for all managed computing service types, it will excessively consume CPU resources and increase the burden of system management and maintenance. Initial Packet Handling and Latency: Some solutions attempt to alleviate the control plane load by throttling and sending the first computing request packet to the control plane to generate a flow affinity table before the packet hits the flow affinity table. However, there is a risk of packet loss before the table entry takes effect, and the control plane processing of the packet can also lead to high latency for the first packet, which affects the quality of experience and service continuity. Fu, et al. Expires 30 August 2026 [Page 4] Internet-Draft Hybrid Forwarding for CATS February 2026 5. Hybrid Forwarding Model To address the aforementioned issues, this document specifies a hybrid forwarding mechanism. This solution features high flexibility, enabling customized service strategies tailored to different service characteristics: Experience-Sensitive Services: Utilize direct table-based forwarding to guarantee stringent QoS requirements and optimal user experience. Non-Sensitive Services: Employ packet-driven table updates to minimize resource overhead while maintaining forwarding efficiency. Key advantages of this mechanism include: (i) Reduced Control/Forwarding Plane Overhead: Mitigates update pressure through optimized table management. (ii) Decoupled Legacy Router Dependency: Enhances the flexibility of deployment in modern network environments. (iii) High Implementability and Scalability: Supports evolving service demands while maintaining backward compatibility. This approach provides an efficient and scalable solution for heterogeneous service delivery in next-generation networks. The Hybrid Forwarding mechanism operates within the framework defined in the CATS architecture [I-D.ldbc-cats-framework] (see Figure 1). Fu, et al. Expires 30 August 2026 [Page 5] Internet-Draft Hybrid Forwarding for CATS February 2026 +-----+ +------+ +------+ +------+| +------+ | +------+ | |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 5.1. Table Management and Working Mechanism Fu, et al. Expires 30 August 2026 [Page 6] Internet-Draft Hybrid Forwarding for CATS February 2026 CATS ROUTING TABLE -------------------------------------------------------------- NextHop VPN ID Prefix IP CATS-IND MODE-IND SR-Policy Service SID -------------------------------------------------------------- 1 CS-ID-1 1 1 SR-P-1 END-DX6-1 -------------------------------------------------------------- 1 CS-ID-1 1 0 SR-P-2 END-DX6-2 -------------------------------------------------------------- | v CATS FORWARDING TABLE -------------------------------------------------------------- NextHop VPN ID Prefix IP CATS-IND MODE-IND SR-Policy Service SID -------------------------------------------------------------- 1 CS-ID-1 1 1 SR-P-1 END-DX6-1 -------------------------------------------------------------- 1 CS-ID-1 1 0 NULL NULL -------------------------------------------------------------- Figure 2: CATS Table Management by the Control Plane Figure 2 presents a specific form of managing the CATS table entries on the control plane. The explanation for the entries in the table is as follows: CS-ID Usage: The CS-ID typically employs anycast IP addresses to identify services. As specified in [I-D.lbdd-cats-dp-sr] and [I-D.fu-cats-muti-dp-solution], the forwarding mechanism varies based on service instance connectivity: (1) when an egress gateway is connected to multiple service instances, traffic is forwarded to a specific instance via either a tunnel or a general interface (on the same CATS-Forwarder) using an END.DX4/6 Service SID; (2) when a single service instance is involved, an END.DT4/6 Service SID enables direct forwarding via anycast IP. Consequently, each Service SID maintains a strict one-to-one correspondence with its associated service instance. C-PS Deployment and Function: The C-PS component is deployed on the head node or a centralized computation-network controller. It collects and reports CS-ID, CIS-ID, and metric information related to service instances via C-SMA, as well as network-related capabilities and status information via C-NMA. Based on the collected information, the C-PS calculates the optimal network forwarding path and service instance, which is then referred to as the CATS routing table. The process of inserting or downloading these table entries t