RTGWG Z. Han, Ed. Internet-Draft R. Pang Intended status: Standards Track Z. Ruan Expires: 3 January 2026 X. Yi China Unicom 2 July 2025 Usecase and requirement of deploying PFC and fine-grained flow control draft-han-rtgwg-codeployment-pfc-fgfc-00 Abstract The demand for lossless network transmission and the application of flow control mechanisms have expanded from DCNs (Data Center Networks) to WANs(Wide Area Networks). To mitigate PFC - related issues in WANs, the fine - grained flow control is proposed. This mechanism aims to achieve precise control at flow / tenant levels, limits flow control to specified paths and slices, and provides intelligent congestion backpressure. As current DCN already adopts PFC mechanisms, the fine-grained flow control in WANs needs to work with PFC in DCNs to achieve end-to-end flow control. 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 3 January 2026. 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 publication of this document. Han, et al. Expires 3 January 2026 [Page 1] Internet-Draft Req of PFC and fine-grained flow control July 2025 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 and Background . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Interworking deployment of PFC and fine-grained Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Procedure of end-to-end flow control . . . . . . . . . . . . 4 4.1. PFC to fine-grained flow control . . . . . . . . . . . . 4 4.2. Fine-grained flow control to PFC . . . . . . . . . . . . 4 5. Requirement of joint deployment . . . . . . . . . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . 4 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 8. Informative References . . . . . . . . . . . . . . . . . . . 4 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction and Background DCNs are typically characterized by a limited network scale, short path and predictable traffic patterns, so flow control mechanisms like PFC (Priority Flow Control) and ECN (Explicit Congestion Notification) operate effectively. With the growth of AI LLM distributed training and inference, lossless transmission of massive data between geographically separated data centers is required [I-D.hs-rtgwg-wan-lossless-uc], and the flow control mechanisms need to be extended from DCNs to WANs. Unlike DCNs, WANs are large-scale with complex topologies, long paths, and diverse traffic type. PFC based on port-level feedback ensures lossless transmission of RDMA protocol, by pausing/resuming specific priority queues to prevent congestion. When using it in the WANs, the backpressure from PFC will cause head-of-line blocking, deadlocks, and congestion spreading, which degrade network throughput [I-D.hs-rtgwg-wan-lossless-uc]. To mitigate these issues, the fine - grained flow control is required for WANs. Fine-grained flow control improves upon the coarse-grained port-based PFC mechanism. It enables precise control at the flow, tenant, or other granular levels, limits flow control to specified paths and slices, and provides intelligent congestion backpressure with granular parameters (pausing time, and buffer thresholds etc.). These capabilities collectively contribute to achieving efficient and refined flow control in WANs. Han, et al. Expires 3 January 2026 [Page 2] Internet-Draft Req of PFC and fine-grained flow control July 2025 This draft focuses on the scenarios where PFC is employed in DCNs and the fine-grained flow control is utilized in WANs. Usecase and requirements for the interworking deployment of PFC and fine-grained flow control mechanisms are described, achieving end-to-end flow control through coordination and policy mapping between DCNs and WANs. 2. Terminology PFC: Priority-based Flow Control DCN: Data Center Network WAN: Wide Area Network RDMA: Remote Direct Memory Access RoCE: RDMA over Converged Ethernet 3. Interworking deployment of PFC and fine-grained Flow Control +----------+ +----------+ -- | Data | | Data | ^ | center A | | center B | ^ | +----------+ +----------+ | | | | | |PFC | | PFC| | v v | v +----+ --> +----+ --> +----+ --> +----+ --> +----+ v -- | R1 | | R2 | | R3 | | R4 | | R5 | -- +----+ +----+ +----+ +----+ +----+ | | |-------------------------------------------------> | fine-grained flow control WAN Figure 1: Codeployment of PFC and fine-grained flow control As shown in Figure 1, there are two data centers, A and B, connected by WAN via nodes R1 -> R2 -> R3 -> R4 -> R5. The internal nodes of data center A and data center B employ the PFC mechanism. Because most DCN NICs today are optimized for legacy protocols (e.g., Ethernet, DCB) and lack SRv6 processing capabilities. This limitation prevents the direct extension for refined flow control. Hardware/firmware upgrades are needed to enable fine-grained flow control deployment. WAN nodes R1-R5 deploy fine-grained flow control to avoid PFC backpressure issues, enabling flow/tenant-level congestion handling with granular parameters for precise and intelligent backpressure. WAN nodes support HQOS (Hierarchical Quality of Service) queuing mechanisms and slicing. Han, et al. Expires 3 January 2026 [Page 3] Internet-Draft Req of PFC and fine-grained flow control July 2025 Edge nodes R1 and R5 support both PFC and fine-grained flow control, interworking DCN and WAN flow control mechanisms and ensuring seamless end-to-end flow control. The NNI ports of edge nodes R5 and R1 can establish multiple slices, each corresponding to a tenant and supporting 1-8 queues. 4. Procedure of end-to-end flow control 4.1. PFC to fine-grained flow control TBD 4.2. Fine-grained flow control to PFC TBD 5. Requirement of joint deployment TBD 6. Security Considerations This document does not introduce any new security considerations. 7. IANA Considerations This document has no IANA actions. 8. Informative References [I-D.hs-rtgwg-wan-lossless-uc] Zhengxin, H., He, T., Shi, H., and T. Zhou, "Use Cases and Requirements for Implementing Lossless Techniques in Wide Area Networks", Work in Progress, Internet-Draft, draft- hs-rtgwg-wan-lossless-uc-01, 2 July 2025, . Contributors Authors' Addresses Zhengxin Han (editor) China Unicom Beijing China Email: hanzx21@chinaunicom.cn Han, et al. Expires 3 January 2026 [Page 4] Internet-Draft Req of PFC and fine-grained flow control July 2025 Ran Pang China Unicom Beijing China Email: pangran@chinaunicom.cn Zheng Ruan China Unicom Beijing China Email: ruanz6@chinaunicom.cn Xinxin Yi China Unicom Beijing China Email: yixx3@chinaunicom.cn Han, et al. Expires 3 January 2026 [Page 5]