



RTGWG                                                        Z. Han, Ed.
Internet-Draft                                                   R. Pang
Intended status: Standards Track                                  Y. Yue
Expires: 1 September 2026                                   China Unicom
                                                                 J. Dong
                                                     Huawei Technologies
                                                                 Z. Ruan
                                                            China Unicom
                                                                Q. Xiong
                                                         ZTE Corporation
                                                        28 February 2026


Use cases and Requirement for Flow Control Collaboration Across DCNs and
                                  WAN
                draft-han-rtgwg-codeployment-pfc-fgfc-02

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.  This document
   describes the use cases and requirements for the collaboration of
   flow control mechanisms across DCNs and WANs.

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 1 September 2026.




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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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction and Background {#intro and backg}  . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Collaboration deployment scenarios of PFC and fine-grained Flow
           Control . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Interworking between PFC and fine-Grained Flow Control  . . .   5
     4.1.  PFC to fine-grained flow control  . . . . . . . . . . . .   5
     4.2.  Fine-grained flow control to PFC  . . . . . . . . . . . .   6
   5.  Requirement of collaboration deployment . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction and Background {#intro and backg}

   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.  To mitigate these
   issues in [I-D.dong-fantel-problem-statement], the fine - grained
   flow control is required for WANs.




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   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, backpressure bandwidth, flow
   identifier etc.).  These capabilities collectively contribute to
   achieving efficient and refined flow control in WANs
   [I-D.han-rtgwg-fine-grained-backpressure].

   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.  Collaboration deployment scenarios 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 over WAN

     Figure 1: Codeployment of PFC and fine-grained flow control





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   As shown in Figure 1, there are two data centers, A and B.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.

   All or some of the WAN nodes R1-R5 support fine-grained flow control
   [I-D.han-rtgwg-fine-grained-backpressure] to mitigate PFC
   backpressure issues, enabling flow/tenant-level congestion handling
   with granular parameters for precise and intelligent backpressure.
   These nodes also support HQoS (Hierarchical Quality of Service)
   queuing mechanisms and slicing.

   Edge nodes R1 and R5 support both PFC and fine-grained flow
   control[I-D.han-rtgwg-fine-grained-backpressure] , 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.

   Based on factors such as distance, number of users, network topology
   and node capabilities in WAN, the interworking and collaboration
   scenarios of PFC and fine-grained flow control can be classified into
   the following two categories.

   1) Single-hop direct interconnection without intermediate node
   participation

   In this scenario, Data Center A and Data Center B are directly
   connected through edge node R1 and R5 with a single hop, without any
   intermediate devices in between, or intermediate nodes (R2, R3, R4)
   are legacy network devices that whithout flow control capability.  In
   this case, intermediate nodes do not participate in the flow control
   process, and a tunnel is established between R1 and R5 to transmit
   fine-grained flow control packet.

   2) Multi-hop interconnection scenario with intermediate node
   participation

   In this scenario, Data Center A and Data Center B are connected by
   WAN via nodes R1 -> R2 -> R3 -> R4 -> R5.Intermediate nodes R2, R3,
   and R4 (or a subset of them) support fine-grained flow control
   capabilities.  These intermediate nodes actively participate in the
   flow control process.  The WAN nodes can adopt either hop-by-hop
   backpressure or cross-hop Backpressure mechanisms
   [I-D.ruan-spring-priority-flow-control-sid] to handle congestion.



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4.  Interworking between PFC and fine-Grained Flow Control

4.1.  PFC to fine-grained flow control

   tenant traffic
 |------------>
+--------------+
| Slice ID = 1 |
+--------------+                                            Congestion Occurs
      |                                                           |
      |                                                           |
      v                                                           v
    ---->  +--- +  -2/0/0  1/0/0-   +----+  -2/0/0  3/0/0-   +----------+
           | Rx |  -------------->  | R5 |  -------------->  |   Data   |
           |    |                   |    |                   | center B |
           +----+                   +----+                   +----------+
                    <- - - - - - - -|        <- - - - - - - -|
               fine-grained flow control     PFC backpressure
                  backpressure packet              frame
                          ^
                          |
                          |
                   +--------------+
                   | Slice ID = 1 |
                   +--------------+
                   +--------------+
                   | Slice ID = N |
                   +--------------+

              Figure 2: PFC to fine-grained flow control

   Edge node R5 responds to the PFC frame sent by the data center and
   transmits fine - grained flow control packet
   [I-D.han-rtgwg-fine-grained-backpressure] to the WAN.  The process
   follows these steps:

   1) When congestion occurs at the incoming port 3/0/0 of data center
   B.

   2) The data center B sends a PFC backpressure frame to the 2/0/0 port
   of edge node R5.  The PFC frame carries the queue priority of the
   traffic to be backpressured, which is af1.

   3) Edge node R5 needs to support responding to the PFC frame and
   buffers the traffic with the priority af1 through the 2/0/0 physical
   port.





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   4) The 1/0/0 port of edge device R5 has multiple slices.  When the
   buffer queue corresponding to the 2/0/0 port of edge device R5
   reaches the buffer threshold.

   5) According to the port, tenant traffic, and slice mapping
   relationship, the 1/0/0 port of edge device R5 sends a fine - grained
   flow control backpressure packet to the network node Rx.  Rx is the
   upstream network node with fine-grained flow control capability, it
   can be the intermediate node or the edge node R1 in WAN for different
   deployment scenarios mentioned in clause 3.  The packet carries the
   tenant traffic information to be backpressured, with the queue
   priority af1, sliceID, and pause time, etc.

   6) Based on the congestion handling situation, if the RX node fails
   to resolve the congestion:

   * For multi-hop scenario mentioned in clause 3 , the RX node sends fine-grained flow control packets to upstream WAN nodes as needed;

   * For the single-hop scenario mentioned in clause 3, where RX is edge node R1, the RX node sends PFC frames to the DCN as needed.

4.2.  Fine-grained flow control to PFC

                                  +--------------+
                                  | Slice ID = 1 |
                                  +--------------+
                                          |         Congestion Occurs
                                          |              |
                                          v              |
                                    tenant traffic       v
                |--------------------------------------------->
   +----------+  -3/0/0  2/0/0-   +----+  -1/0/0-     +----+
   |   Data   |  -------------->  | R1 |  --------->  | Rx |
   | Center A |                   |    |              |    |
   +----------+                   +----+              +----+
                  <- - - - - - - -|         <- - - - -|
               PFC backpressure frame   fine-grained flow
                                       control backpressure
                                               ^
                                               |
                                               |
                                        +--------------+
                                        | Slice ID = 1 |
                                        +--------------+

                 Figure 3: fine-grained flow control to PFC






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   Edge node R1 responds to fine - grained flow control
   packet[I-D.han-rtgwg-fine-grained-backpressure] from WAN, then sends
   PFC frame to the data center.  The process follows these steps:

   1) When congestion occurs in the traffic of queue af1 with sliceID =
   1 at the egress port of network node R2.

   2) Network node Rx sends a fine - grained flow control backpressure
   packet to edge node R1.  This packet carries the tenant traffic
   information to be backpressured, with the queue priority af1, sliceID
   = 1, and the pause timed, etc.

   3) Edge node R1 performs traffic control and buffers the tenant
   traffic with priority af1 and sliceID = 1.

   4) When the buffer queue corresponding to port 1/0/0 of edge device
   R1 reaches the buffer threshold, port 2/0/0 of edge node R1 sends
   backpressure to the data center according to the standard PFC packet.

   5) Data center A performs standard PFC backpressure and stops all
   traffic with priority af1 destined for port 3/0/0.

5.  Requirement of collaboration deployment

   Requirement 1: Edge node needs support the coordination and
   bidirectional translation between the fine-grained flow control
   mechanism in the WAN and the PFC mechanism in the DCN, enabling
   seamless end-to-end flow control across WAN and DCN domains.

   Requirement 2: Edge node needs to respond to PFC frames from the DCN.
   It includes the following capabilities:

   1) Learn task flow-to-port mappings to identify affected traffic;

   2) Configure appropriate buffer thresholds;

   3) Generate and send fine-grained flow control messages to WAN nodes
   with granular parameters.

   Requirement 3: Edge nodes needs to respond to fine-grained flow
   control messages from the WAN.  It includes the following
   capabilities:

   1) Use established flow-to-port mappings to determine target DCN
   ports;

   2) Configure appropriate buffer thresholds;




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   3) Generate and send standard PFC frames to corresponding DCN ports.

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>.

   [I-D.dong-fantel-problem-statement]
              Dong, J., McBride, M., Clad, F., Zhang, Z. J., Zhu, Y.,
              Xu, X., Zhuang, R., Pang, R., Lu, H., Liu, Y., Contreras,
              L. M., Mehmet, D., and R. Rahman, "Fast Network
              Notifications Problem Statement", Work in Progress,
              Internet-Draft, draft-dong-fantel-problem-statement-05, 2
              February 2026, <https://datatracker.ietf.org/doc/html/
              draft-dong-fantel-problem-statement-05>.

   [I-D.han-rtgwg-fine-grained-backpressure]
              Zhengxin, H., Pang, R., Ruan, Z., and X. Yi, "Fine-Grained
              Flow Control Backpressure Mechanism for Wide Area
              Networks", Work in Progress, Internet-Draft, draft-han-
              rtgwg-fine-grained-backpressure-00, 20 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-han-rtgwg-
              fine-grained-backpressure-00>.

   [I-D.ruan-spring-priority-flow-control-sid]
              Ruan, Z., Liu, Y., Han, M., Han, Z., and Y. Liu, "SRv6
              behavior extention for Flow Control in WAN", Work in
              Progress, Internet-Draft, draft-ruan-spring-priority-flow-
              control-sid-03, 27 February 2026,
              <https://datatracker.ietf.org/doc/html/draft-ruan-spring-
              priority-flow-control-sid-03>.

Authors' Addresses





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   Zhengxin Han (editor)
   China Unicom
   Beijing
   China
   Email: hanzx21@chinaunicom.cn


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


   Yi Yue
   China Unicom
   Beijing
   China
   Email: yuey80@chinaunicom.cn


   Jie Dong
   Huawei Technologies
   Email: jie.dong@huawei.com


   Zheng Ruan
   China Unicom
   Email: ruanz6@chinaunicom.cn


   Quan Xiong
   ZTE Corporation
   Email: xiong.quan@zte.com.cn

















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