]> ZTE Corporation

xiong.quan@zte.com.cn

ZTE Corporation

zhu.xiangyang@zte.com.cn

rtgwg This document proposes Precise Priority-based Flow Control (PPFC) mechanism implemented at the network gateway, it designed to efficiently manage congestion in scenarios where multiple flows converge toward a common destination. By enabling fast, per-flow congestion control at the network gateway, PPFC allows incoming traffic destined for a congested bottleneck to be paused or rate-limited before entering the network. This approach decouples the congestion feedback loop from the end-to-end data path, significantly reducing reaction time and improving the effectiveness of congestion management.

Introduction In modern network deployments such as data centers and wide-area interconnections, multiple source clients frequently communicate through a shared ingress gateway. When substantial traffic is directed to a common server, these flows often converge onto a single bottleneck link within the network. Traditional end-to-end congestion control mechanisms��such as BBR and ECN��rely on feedback that must traverse the full return path to senders, introducing at least one round-trip time (RTT) of delay before rate adjustments take effect. This delay is particularly problematic in high-throughput, low-latency environments where transient congestion can severely impact performance. The PPFC (Precise Priority-based Flow Control) as per can address this limitation by enabling network-assisted congestion control near the sources clients. The key idea is to instantaneously pause or throttle sources at the ingress point (e.g., a gateway), allowing the network to quickly recover from congestion. This document proposes an enhancement to existing congestion control by introducing a PPFC mechanism at the gateway. By deploying PPFC at a gateway that aggregates traffic from multiple sources destined for a common destination, the gateway can prevent new flows from exacerbating existing congestion. It will significantly mitigate the congested link when the traffic transfers to the same destination.

Conventions Used in This Document

Abbreviations PPFC: Precise Priority-based Flow Control RTT: Round-Trip Time TCP: Transfer Control Protocol RDMA: Remote Direct Memory Access Round-Trip Time QUIC: Quick UDP Internet Connections ECN: Explicit Congestion Notification PFC: Priority-based Flow Control RoCEv2: RDMA over Converged Ethernet version 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 when, and only when, they appear in all capitals, as shown here.

Precise Priority-based Flow Control with Gateway Upon receiving the congestion in PPFC mechanism, the gateway can record the states and apply immediate congestion control to any subsequent traffic for that server. Furthermore, the gateway can perform source flow control, when new traffic arrives, it may reject or pause new flows to prevent worsening existing congestion. The following figure illustrates this interaction: |Gateway|<--->|Node X |<--->|Node Y |<--->|Server | +--------+ +----->+-------+ +-------+ +-------+ +-------+ 4-Traffic B| ****+ 2-Congestion +--------+ | * Occurs |Client B+<-----+ * 5-PPFC Fast Feedback (e.g.Server, Pause, 10us) +--------+<********* Figure 1: PPFC Mechanism at Gateway ]]> The example at gateway is shown in Figure 1 and the steps are as follows. Step 1: Client A sends traffic to the Server via the Gateway, Node X, and Node Y. Step 2: Congestion occurs at the interface of Node Y towards the Server. Step 3: Node Y generates an PPFC Notification containing the destination (Server) and a pause duration (10us), sending it to the Node X and the Node X transits it to the the Gateway. Step 4: Client B attempts to send traffic to the same Server. Step 5: The Gateway, having recorded the congested state for the Server, applies source flow control to Client B's new traffic. This may involve holding packets or sending an PPFC Feedback message with pause duration (e.g. 10us). which may apply the format as per to Client B, preventing its traffic from entering the network and aggravating the congestion at Node Y.

Security Considerations To be discussed in future versions of this document.

IANA Considerations This document does not currently require any IANA actions.

Contributors The following people have substantially contributed to this document:

Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. ZTE Corporation ZTE Corporation This document specifies the notification mechanism for Precise Priority-based Flow Control (PPFC), defining the message formats and network actions for communicating congestion information among network nodes. The PPFC notification enables rapid congestion signaling, allowing per-flow flow control for traffic from source.