



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,
              <https://datatracker.ietf.org/doc/html/draft-hs-rtgwg-wan-
              lossless-uc-01>.

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]