RTGWG Y. Liu Internet-Draft China Mobile Intended status: Standards Track Y. Liu Expires: 3 September 2026 ZTE Corporation 2 March 2026 Proactive Flow Control Point Detection in WAN draft-liu-rtgwg-wan-flowctrl-detect-01 Abstract [I-D.ietf-rtgwg-net-notif-ps] establishes usecases and requirements for fast notification delivery for network events (e.g., failure, congestion or state change) in modern network applications. This document proposes a proactive detection mechanism that enables the flow control notification message generating downstream node to be aware of the nearest upstream node capable of processing the message . The mechanism operates in the data plane, uses probe packets that follow the same path as data traffic, and maintains state only where needed. While proposed for flow control, the same mechanism may support other fast notification use cases that require distributed recipient node discovery. 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 September 2026. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Liu & Liu Expires 3 September 2026 [Page 1] Internet-Draft Flow Control Detection March 2026 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 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Flow Control in WAN . . . . . . . . . . . . . . . . . . . . . 4 4. Proactive Detection Mechanism Requirements . . . . . . . . . 4 5. Proactive Control Point Detection Method . . . . . . . . . . 5 6. Proactive Control Point Detection Packet Format . . . . . . . 5 7. Processing Procedures . . . . . . . . . . . . . . . . . . . . 6 8. Considerations for Other Usecases . . . . . . . . . . . . . . 7 9. Security Considerations . . . . . . . . . . . . . . . . . . . 7 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 11.1. Normative References . . . . . . . . . . . . . . . . . . 8 11.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction [I-D.ietf-rtgwg-net-notif-ps] establishes requirements for sub- millisecond to millisecond fast notification delivery for network events (e.g., failure, congestion or state change) in modern network applications. An example where this fast network notification problem arises is flow control in wide-area networks (WANs). With the growth of intelligent computing services, scenarios such as disaggregated computing and real-time inference require the lossless transmission of large volumes of bursty traffic over WANs. To achieve lossless data transmission over WAN, there're some recent works aiming to deploy flow control mechanisms in WAN to avoid packet loss in case of congestion, e.g, [I-D.ruan-spring-priority-flow-control-sid] discusses how to deploy PFC(Priority-based Flow Control, [IEEE802.1Qbb]) in WAN based on SRv6 data plane, and [I-D.liu-rtgwg-srv6-cc] proposes the method of precise/fine-grained flow control to achieve flow control at the network slice [RFC9543] level. Liu & Liu Expires 3 September 2026 [Page 2] Internet-Draft Flow Control Detection March 2026 As in [I-D.ietf-rtgwg-net-notif-ps], the fast network notification recipient nodes include adjacent routers/switches, non-adjacent routers/switches, ingress nodes, end hosts, and controllers. To deploy flow control mechanism in WAN, the node facing congestion needs to generate a flow control message and sends it to the upstream point which is able to perform the flow control action (e.g, stop sending the corresponding traffic or reducing the sending rate). In this case, the recipient node may be the adjacent router/switch, non- adjacent router/switch or the ingress node. To control the sending of the flow control message, one of the following mechanisms may be used: * Multicast. Although standard PFC propagates congestion information via Ethernet multicast frames, multicast-based mechanism is not preferred in WAN since it cannot accurately reach upstream nodes, potentially leading to incorrect flow suppression and impacting unrelated services. * Centralized configuration. The controller, with the awareness of all the node and the path information in the network, can configure the information of the upstream flow control point on each node that may generate the flow control message. But this methods will bring extra burden for the controller in large scale networks. * Distributed decision. The congested node decides the upstream node itself. The difficulty is that the congested node needs to be aware of the necessary information to make the proper decision. Based on the above considerations, this document proposes a proactive detection mechanism for the distributed decision mechanism, letting the congested node to know precisely which upstream point should the flow control message be sent to. Besides flow control in WAN, this mechanism potentially can support other types of network fast notification use cases if the distributed decision mechanism is required. The detailed flow control mechanism itself on the recipient node is out of the scope of this document. Liu & Liu Expires 3 September 2026 [Page 3] Internet-Draft Flow Control Detection March 2026 2. Terminology 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. Flow Control in WAN +----------+ +----------+ | Data | | Data | | center A | | center B | +----------+ +----------+ | Congestion ^ | | | v v | +----+ --> +----+ --> +----+ --> +----+ --> +----+ | R1 | | R2 | | R3 | | R4 | | R5 | +----+ +----+ +----+ +----+ +----+ Figure 1: Flow Control in WAN As shown in Figure 1, data center A and data center B are connected via a path(R1-R2-R3-R4-R5) in WAN. R1,R2,R4 and R5 are able to perform the function of flow control, and R3 is a legacy device which doesn't support any flow control technology. When congestion occurs at R4, R4 generates a flow control message (e.g, the PFC pause frame defined in [IEEE802.1Qbb], the congestion notification message defined in [I-D.liu-rtgwg-srv6-cc]) to the nearest upstream stream node that is able to perform flow control, i.e, node R2. R2 receives the notification and performs the flow control based on the content of the notification message and local capacity. If R2 cannot handle the congestion, a flow control message is forwarded further upstream to R1. 4. Proactive Detection Mechanism Requirements For the distributed decision mechanism, what is needed is a lightweight, data-plane method that allows each notification message generating node to be aware of the identity of the nearest upstream point capable of handling a specific class of fast notifications. This discovery should be: Liu & Liu Expires 3 September 2026 [Page 4] Internet-Draft Flow Control Detection March 2026 * Proactive: information is available before the event occurs, so that the notification can be sent immediately when needed. * Same-path: follows the same path as the data traffic, ensuring that the discovered node is topologically relevant. * State-efficient: only the nodes that may generate notifications need to maintain the discovered information. 5. Proactive Control Point Detection Method The basic concept of the proactive control point detection method in this document is to send a control point detection packet along the packet forwarding path. When receiving the control detection packet, the node that is capable of flow control updates the packet with its own information(e.g, the interface address or the corresponding SRv6 ajacency SID of the interface), so the detection packet will always carry the information of the nearest upstream node that's capable of flow control and the node receiving the detection packet would store this information and use it as the destination of the flow control message when congestion occurs. 6. Proactive Control Point Detection Packet Format A new Hop-by-Hop option (Section 4.3 of [RFC8200]) type is defined in this document to carry the fields above for control point detection. Its format is shown in Figure 2. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Type | Upstream Type | Context Type | Reversed | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Upstream Point Info (4 or 16 octets) ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Context (optional) ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Control Point Detection Option * Option Type: TBA for the control point detection option. Liu & Liu Expires 3 September 2026 [Page 5] Internet-Draft Flow Control Detection March 2026 * Opt Data Len: 8-bit unsigned integer indicates the length of the option Data field of this option, in octets. * Message Type: 8-bit identifier of the type of the message. The type of Flow Control Detection message is TBD1. * Upstream Type: indicates the type of the upstream point that is able perform corresponding control for the traffic. When set to 1, the upstream Point Info carries a 128-bit IPv6 interface address, when set to 2, the upstream Point Info carries a 128-bit SRv6 adj-SID. * Upstream Point Info: Variable-length field carrying the corresponding upstream point information based on the value of the Upstream Type. * Context Type: The Context Type field is an 8-bit bitmap that specifies which contexts are included in the Context field of the packet. Each bit in this field corresponds to a specific context. When a bit is set to 1, it indicates the presence of the corresponding context. * Context: Optional, Context-Type-specific information. The packet fields defined above can be carried in-band or out-band as long as the packet is forwarded along the same path of the normal traffic flow. 7. Processing Procedures As in Figure 1, the traffic of network slice 1 is forwarded along an SRv6 path, the corresponding SID list is , whereas SID-R12 is the adjacency SID of R1 for the adjacency between R1 and R2, SID-R23 and SID-R45 are also the corresponding adj-SID on R2 and R4. And precise flow control is enabled in the network to control the congestion at the network slice level. R1,R2,R4 and R5 are able to perform flow control at the network slice level, and R3 is a legacy device which doesn't support any flow control technology. To detect the flow control point along the path of slice 1, R1 sends a flow control detection packet in band with the traffic of slice 1, since R1 is capable of flow control, R1 puts SID-R12 into the Upstream Point Info. Liu & Liu Expires 3 September 2026 [Page 6] Internet-Draft Flow Control Detection March 2026 When R2 receives the packet, it stores the mapping between slice 1 and SID-R12, and updates the Upstream Point Info with its own information, i.e, SID-R23. Since R3 doesn't recognize the flow control detection packet, it just forwards the packet based on the SID-list and slice-ID of the packet. When R4 receives the packet, it stores the mapping between slice 1 and SID-R23, and updates the Upstream Point Info with its own information, i.e, SID-R45. When R5 receives the packet, it stores the mapping between slice 1 and SID-R45, and since R5 is the endpoint, it stops processing the packet further. When congestion occurs at R4 in slice 1, R4 would generate a flow control message for slice 1, and based on the local information, R4 finds the information of upstream flow control point, i.e, SID-R23, and uses it as the destination of the flow control message. When R2 receives the flow control message with DA set as local adj- SID SID-R23, R2 perform the flow control for slice 1 on the port related with SID-R23 based on the context of the flow control message. If R2 is not able to control the congestion and generates a flow control message further, R2 would send the message with DA set to SID-R12 based on the local information. 8. Considerations for Other Usecases The proactive control point detection mechanism described in this document is not limited to flow control in WAN. It provides a generic recipient discovery primitive for fast network notification scenarios for the distributed decision method, especially when intended recipient node is a non-adjacent node on the forwarding path. Future version of this document will discuss more about other fast network notification usecases. 9. Security Considerations The security considerations with IPv6 Hop-by-Hop Options header are described in [RFC8200],, [RFC7045][RFC9098], [RFC9099], [RFC9673]. This document introduces a new IPv6 Hop-by-Hop option which is either processed in the fast path or ignored by network nodes, thus it does not introduce additional security issues. Liu & Liu Expires 3 September 2026 [Page 7] Internet-Draft Flow Control Detection March 2026 10. IANA Considerations This document requests IANA to assign a new option type from "Destination Options and Hop-by-Hop Options" registry [IANA-HBH]. Hex Value Binary Value Description Reference act chg rest -------------------------------------------------------- TBA 00 0 tba Control Point [this document] Detection Option 11. References 11.1. Normative References [IANA-HBH] IANA, "Internet Protocol Version 6 (IPv6) Parameters", . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing of IPv6 Extension Headers", RFC 7045, DOI 10.17487/RFC7045, December 2013, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017, . [RFC9098] Gont, F., Hilliard, N., Doering, G., Kumari, W., Huston, G., and W. Liu, "Operational Implications of IPv6 Packets with Extension Headers", RFC 9098, DOI 10.17487/RFC9098, September 2021, . [RFC9099] Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey, "Operational Security Considerations for IPv6 Networks", RFC 9099, DOI 10.17487/RFC9099, August 2021, . Liu & Liu Expires 3 September 2026 [Page 8] Internet-Draft Flow Control Detection March 2026 [RFC9673] Hinden, R. and G. Fairhurst, "IPv6 Hop-by-Hop Options Processing Procedures", RFC 9673, DOI 10.17487/RFC9673, October 2024, . 11.2. Informative References [I-D.ietf-rtgwg-net-notif-ps] 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-ietf-rtgwg-net-notif-ps-00, 11 February 2026, . [I-D.liu-rtgwg-srv6-cc] Liu, Y., Yao, J., Lin, C., and X. Min, "Congestion Control Based on SRv6 Path", Work in Progress, Internet-Draft, draft-liu-rtgwg-srv6-cc-01, 27 February 2026, . [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, . [IEEE802.1Qbb] IEEE, "IEEE Standard for Local and metropolitan area networks--Media Access Control (MAC) Bridges and Virtual Bridged Local Area Networks--Amendment 17: Priority-based Flow Control", DOI 10.1109/IEEESTD.2011.6032693, IEEE Std 802.1Qbb-2011, September 2011, . [RFC9543] Farrel, A., Ed., Drake, J., Ed., Rokui, R., Homma, S., Makhijani, K., Contreras, L., and J. Tantsura, "A Framework for Network Slices in Networks Built from IETF Technologies", RFC 9543, DOI 10.17487/RFC9543, March 2024, . Authors' Addresses Yisong Liu China Mobile Liu & Liu Expires 3 September 2026 [Page 9] Internet-Draft Flow Control Detection March 2026 Email: liuyisong@chinamobile.com Yao Liu ZTE Corporation Email: liu.yao71@zte.com.cn Liu & Liu Expires 3 September 2026 [Page 10]