RFC 9743: Specifying New Congestion Control Algorithms | RFC Editor

RFC 5033 discusses the principles and guidelines for standardizing new congestion control algorithms. This document obsoletes RFC 5033 to reflect changes in the congestion control landscape by providing a framework for the development and assessment of congestion control mechanisms, promoting stability across diverse network paths. This document seeks to ensure that proposed congestion control algorithms operate efficiently and without harm when used in the global Internet. It emphasizes the need for comprehensive testing and validation to prevent adverse interactions with existing flows.¶
This memo documents an Internet Best Current Practice.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9743.¶
Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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This document provides guidelines for the IETF to use when evaluating a proposed congestion control algorithm that differs from the general congestion control principles outlined in [RFC2914]. The guidance is intended to be useful to authors proposing congestion control algorithms and for the IETF community when evaluating whether a proposal is appropriate for publication in the RFC Series and for deployment in the Internet.¶
This document obsoletes [RFC5033], which was published in 2007 as a Best Current Practice for evaluating proposed congestion control algorithms for publication in Experimental or Proposed Standard RFCs.¶
The IETF specifies standard Internet congestion control algorithms in the RFC Series. These congestion control algorithms can suffer performance challenges when used in differing environments (e.g., high-speed networks, cellular and Wi-Fi wireless technologies, and long-distance satellite links), and also when flows carry specific workloads (e.g., Voice over IP (VoIP), gaming, and videoconferencing).¶
When [RFC5033] was published, TCP [RFC9293] was the primary focus of IETF congestion control efforts, with proposals typically discussed within the Internet Congestion Control Research Group (ICCRG). Concurrently, the Datagram Congestion Control Protocol (DCCP) [RFC4340] was developed to define new congestion control algorithms for datagram traffic, while the Stream Control Transmission Protocol (SCTP) [RFC9260] reused TCP congestion control algorithms.¶
Since then, several changes have occurred. The range of protocols utilizing congestion control algorithms has expanded to include QUIC [RFC9000] and RTP Media Congestion Avoidance Techniques (RMCAT) (e.g., [RFC8836]). Additionally, some alternative congestion control algorithms have been tested and deployed at scale without full IETF review. There is increased interest in specialized use cases, such as data centers (e.g., [RFC8257]), and in supporting a variety of upper-layer protocols and applications, such as real-time protocols. Moreover, the community has gained significant experience with congestion indications beyond packet loss.¶
Multicast congestion control is a considerably less mature field of study and is not in the scope of this document. However, Section 4 of [RFC8085] provides additional guidelines for multicast and broadcast usage of UDP.¶
Congestion control algorithms have been developed outside of the IETF, including at least two that saw large scale deployment. These include CUBIC [HRX08] and Bottleneck Bandwidth and Round-trip propagation time (BBR) [BBR].¶
CUBIC was documented in a research publication in 2008 [HRX08], and was then adopted as the default congestion control algorithm for the TCP implementation in Linux. It was already used in a significant fraction of TCP connections over the Internet before being documented in an Internet-Draft in 2015, and published as an Informational RFC in 2017 as [RFC8312] and then as a Proposed Standard in 2023 [RFC9438].¶
At the time of writing, BBR is being developed as an internal research project by Google, with the first implementation contributed to Linux kernel 4.19 in 2016. BBR was described in an Internet-Draft in 2018 and was first presented in the IRTF Internet Congestion Control Research Group. It has since been regularly updated to document the evolving versions of the algorithm [BBR]. BBR is currently widely used for Google services using either TCP or QUIC and is also widely deployed outside of Google.¶
We cannot say whether the original authors of [RFC5033] expected that developers would be waiting for IETF review before widely deploying a new congestion control algorithm over the Internet, but the examples of CUBIC and BBR illustrate that deployment of new algorithms is not, in fact, gated by the publication of the algorithm as an RFC.¶
Nevertheless, a specification for a congestion control algorithm provides a number of advantages:¶
It can help implementers, operators, and other interested parties develop a shared understanding of how the algorithm works and how it is expected to behave in various scenarios and configurations.¶
It can help potential contributors understand the algorithm, which can make it easier for