RFC 9937 - Proportional Rate Reduction (PRR)

Basic Information

• RFC Number: 9937
• Title: Proportional Rate Reduction (PRR)
• Publication Date: December 2025
• Status: PROPOSED STANDARD
• Obsoletes: RFC 6937
• Authors:
  • M. Mathis
  • N. Cardwell (Google, Inc.)
  • Y. Cheng (Google, Inc.)
  • N. Dukkipati (Google, Inc.)

Abstract

This document specifies a Standards Track version of the Proportional Rate Reduction (PRR) algorithm that obsoletes the Experimental version described in RFC 6937. PRR regulates the amount of data sent by TCP or other transport protocols during fast recovery. PRR accurately regulates the actual flight size through recovery such that at the end of recovery it will be as close as possible to the slow start threshold (ssthresh), as determined by the congestion control algorithm.

Status of This Memo

This is an Internet Standards Track document.

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 Internet Standards 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/rfc9937\````.

Copyright Notice

Copyright (c) 2025 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 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. Conventions
• 3. Definitions
• 4. Changes Relative to RFC 6937
• 5. Relationships to Other Standards
• 6. Algorithm
  • 6.1. Initialization Steps
  • 6.2. Per-ACK Steps
  • 6.3. Per-Transmit Steps
  • 6.4. Completion Steps
• 7. Properties
• 8. Examples
• 9. Adapting PRR to Other Transport Protocols
• 10. Measurement Studies
• 11. Operational Considerations
  • 11.1. Incremental Deployment
  • 11.2. Fairness
  • 11.3. Protecting the Network Against Excessive Queuing and Packet Loss
• 12. IANA Considerations
• 13. Security Considerations
• 14. References
  • 14.1. Normative References
  • 14.2. Informative References
• Appendix A. Strong Packet Conservation Bound
• Acknowledgments
• Authors' Addresses

Reading Guide

This RFC document contains detailed specifications for TCP congestion control algorithms. Suggested reading order:

1. Quick Understanding: Read the Abstract and Section 1 Introduction
2. Core Algorithm: Focus on Section 6 Algorithm
3. Practical Guidance: Refer to Section 8 Examples and Section 11 Operational Considerations
4. In-depth Study: Read Appendix A for theoretical foundations

---

Note: Detailed content for each section is being translated and links will be added progressively.