Fine-Grained Flow Control Backpressure Mechanism for Wide Area Networks
draft-han-rtgwg-fine-grained-backpressure-01
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Han Zhengxin , Zheng Ruan , Ran Pang , Yi Yue , Jie Dong , Quan Xiong | ||
| Last updated | 2026-03-02 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
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draft-han-rtgwg-fine-grained-backpressure-01
RTGWG Z. Han, Ed.
Internet-Draft Z. Ruan
Intended status: Standards Track R. Pang
Expires: 3 September 2026 Y. Yue
China Unicom
J. Dong
Huawei Technologies
Q. Xiong
ZTE Corporation
2 March 2026
Fine-Grained Flow Control Backpressure Mechanism for Wide Area Networks
draft-han-rtgwg-fine-grained-backpressure-01
Abstract
This document specifies a fine-grained flow control backpressure
mechanism for Wide Area Networks (WANs). Leveraging data-plane
congestion detection and notification, it enables millisecond-level
congestion response. The mechanism enhances Layer 2 PFC by extending
network protocols (e.g., ICMPv6) for congestion backpressure
messaging in WANs, and leverages network slicing isolation to provide
fine-grained flow control at tenant or task granularity. It
addresses the limitations of traditional flow control mechanisms in
WAN environments through fast and precise backpressure, and supports
multi-hop propagation of congestion notifications along the
forwarding path.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 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
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
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Table of Contents
1. Introduction and Background . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Fine-grained flow control mechanism in WAN . . . . . . . . . 3
3.1. Network slicing & Elastic bandwidth . . . . . . . . . . . 4
3.2. Dual-watermark congestion detection . . . . . . . . . . . 5
3.3. Backpressure message generation and propagation . . . . . 5
4. Congestion backpressure message formats . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction and Background
With the rapid development of High-Performance Computing (HPC),
remote healthcare, multimedia content production, and AI-Generated
Content (AIGC)applications, the volume, velocity, and variety of data
are growing exponentially. This poses higher requirements for the
efficiency and reliability of massive data transmission across Wide
Area Networks (WANs). WANs are characterized by large scale, complex
topology, long round-trip times, diverse service types, continuously
increasing load, and frequent high-intensity burst traffic, making
them prone to congestion.
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Traditional congestion control mechanisms
[I-D.dong-fantel-problem-statement] like Priority Flow Control(PFC)
and Explicit Congestion Notification (ECN) face limitations in WAN
environments. PFC provides coarse-grained port-based flow control
that can lead to congestion spreading, head-of-line blocking, and
deadlocks. ECN requires end-host participation with slow and
inaccurate responses, making it unsuitable for long-distance
transmission in WANs.
This document proposes a fine-grained flow control backpressure
mechanism implemented in the data plane, achieving millisecond-level
response times. The controller is involved only in the pre-
deployment phase for path planning and static parameters
configuration. The mechanism extends network protocols (e.g.,
ICMPv6) and leverages network slicing isolation to provide precise
congestion backpressure at tenant or task granularity. It also
supports multi-hop congestion notification along the traffic path.
2. Terminology
* Fine-grained flow control(fgfc): An enhanced PFC mechanism that
enables precise flow control at tenant or other granular levels,
limits flow control to specified paths and slices, and provides
intelligent congestion backpressure to prevent network congestion
[I-D.han-rtgwg-wan-lossless-terms]
* Congestion Backpressure Message: Network protocol message carrying
congestion information and flow control policies, which can be
implemented using ICMP, UDP, or other suitable protocols.
* Dual-Watermark Mechanism: Congestion detection using Xoff
(trigger) and Xon (release) thresholds.
* Elastic bandwidth: The bandwidth is dynamically adjusted according
to the network conditions to improve network bandwidth utilization
and network transmission
efficiency[I-D.han-rtgwg-wan-lossless-terms].
* SRv6: IPv6 Segment Routing as defined in [RFC8754].
* SRH: Segment Routing Header as defined in [RFC8754].
3. Fine-grained flow control mechanism in WAN
The fine-grained flow control(fgfc) backpressure mechanism operates
entirely in the data plane to achieve millisecond-level congestion
response, avoiding buffer overflow caused by control-plane latency.
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The controller is involved only in the pre-deployment phase. Based
on the network topology and service requirements, controller pre-
calculates the forwarding path and distributes static information
(such as address information, slice configurations, watermark
thresholds, and backpressure message formats) to the routing tables
of data-plane devices. It does not participate in real-time
congestion handling. The address information includes node
addresses, port addresses, and SRv6 behavior
addresses[I-D.ruan-spring-priority-flow-control-sid].
Congestion Notification Congestion Occurs
|
<-----------------------------------|
v
+----------+ +----------+ +----------+
| | | | | |
|Enterprise|\ /| R2 |----| R3 |\
+----------+ \ +----------+ / +----------+ +----------+ \ +----------+ +----------+
\ | |/ \ | | | |
| R1 | | R6 |-----| IDC |
/ | |\ / | | | |
+----------+ / +----------+ \ +----------+ +----------+ / +----------+ +----------+
| |/ \| R4 |----| R5 |/
| IDC + | | | |
+----------+ +----------+ +----------+
|<-----------End-to-End Slice-------->|
|<-------------- Fine-grained Flow Control --------------->|
Figure 1: Fine-grained flow control mechanism in WAN
3.1. Network slicing & Elastic bandwidth
End-to-end network slices are established between WAN nodes R1 and
R6, with each slice assigned to a single tenant and supporting 1 to 8
PFC queues. Elastic bandwidth is enabled via the configuration of a
Committed Information Rate (CIR) and Peak Information Rate (PIR),
which optimizes bandwidth resource utilization and improves
transmission efficiency.
When the network is idle, tenant traffic may preempt available idle
resources to scale up to the PIR. During network congestion, the
backpressure mechanism enforces a fallback to the CIR to ensure the
minimum guaranteed bandwidth for the tenant. CIR may optionally be
set to zero, in which case lossless transmission is ensured entirely
by the flow control mechanism.
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3.2. Dual-watermark congestion detection
Each node deploys a dual watermark (Xon/Xoff) monitoring mechanism
for the queue,which is used to detect congestion and avoid frequent
backpressure oscillation.
When the queue length exceeds Xon threshold, the node generates a
backpressure message to upstream nodes to reduce the sending
rate.When the queue length falls below Xoff threshold, the node may
send a release message (or stop sending backpressure) to resume
normal forwarding.The threshold can be configured according to the
queue buffer depth and business latency requirements.
The dual watermark mechanism avoids the frequent switching of
backpressure and cancellation caused by small fluctuations of the
queue depth, and ensures the stability of the flow control process.
3.3. Backpressure message generation and propagation
When the R3 node detects congestion (queue > Xon), it
triggers a fine-grained flow control backpressure message. The
message is sent upstream nodes that support fine-grained flow
control(fgfc).
Per-hop backpressure: If all intermediate nodes in WAN support fgfc,
the backpressure is applied hop-by-hop, gradually reducing the rate
at each node.
Cross-hop backpressure: If only some nodes support fgfc, the
backpressure message bypasses non-supporting nodes and is processed
only by capable nodes.
The time from congestion detection to backpressure message
transmission is controlled within a few milliseconds. Upon receiving
the message, the upstream node identifies the affected queue(s) and
performs rate limiting within nanoseconds or microseconds, achieving
fast congestion response,near-zero packet loss and high throughput.
The backpressure path can be carried in the SRv6 Segment Routing
Header (SRH) via encapsulation or insertion. The SRH path is
computed by the controller or locally by nodes based on the forward
path and pre-configured policies.
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4. Congestion backpressure message formats
The Fine-Grained Flow Control mechanism conveys congestion
information by extending network protocols. The backpressure message
can based on ICMPv6 notification messages, enabling data-plane
congestion notification without host involvement. The format is
shown in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flag | Priority | Reserve |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow-Control-Argument[0] | Flow-Control-Argument[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow-Control-Argument[2] | Flow-Control-Argument[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow-Control-Argument[4] | Flow-Control-Argument[5] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow-Control-Argument[6] | Flow-Control-Argument[7] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Backpressure Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Slice ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: ICMPv6-based Backpressure Message Format
* Type: 1 byte. ICMPv6 message type. A new Type value (170) is
suggested to be assigned by IANA to identify this message as a
congestion backpressure notification message.
* Code: 1 byte. Set to 0 for fgfc messages.
* Checksum: 2 bytes. Standard ICMPv6 checksum.
* Flag: 1 byte. Flags field. Currently unused and MUST be set to zero on transmission and ignored on receipt.
* Priority: 1 byte. Flow control priority. Each bit corresponds to one of eight queues. A bit set to 1 indicates that the
corresponding queue is subject to flow control (backpressure); a bit set to 0 indicates no backpressure.
* Reserve: 2 bytes. Reserved for future use. MUST be set to zero
on transmission and ignored on receipt.
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* Flow-Control-Argument[]: 16 bytes. Currently used to specify the
backpressure duration. It is interpreted as an array of eight
16-bit unsigned integers, each representing the pause time in
microseconds for the corresponding queue (matching the priority
queues ).
* Backpressure Bandwidth: 4 bytes. Specifies the bandwidth
throttling value (rate limit) to be applied.
* Slice ID: 4 bytes. Tenant identifier, used to identify the tenant
path or slice to which the backpressure applies.
This format allows queue-level pause and tenant-specific
backpressure, ensuring congestion does not spread across slices. The
message is processed entirely in the data plane without host
involvement.
5. Security Considerations
This document does not introduce any new security considerations.
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. Normative References
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
7.2. Informative References
[I-D.han-rtgwg-wan-lossless-terms]
Zhengxin, H., Pang, R., and T. He, "Terminology for
Implementing Lossless Techniques in Wide Area Networks",
Work in Progress, Internet-Draft, draft-han-rtgwg-wan-
lossless-terms-01, 29 June 2025,
<https://datatracker.ietf.org/doc/html/draft-han-rtgwg-
wan-lossless-terms-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
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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.ruan-spring-priority-flow-control-sid]
Ruan, Z., Liu, Y., Han, M., Zhengxin, H., and Ying, "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
Zhengxin Han (editor)
China Unicom
Beijing
China
Email: hanzx21@chinaunicom.cn
Zheng Ruan
China Unicom
Email: ruanz6@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
Quan Xiong
ZTE Corporation
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Email: xiong.quan@zte.com.cn
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