RFC 9438 · PROPOSED STANDARD · 2023

CUBIC for Fast and Long-Distance Networks

Overview

RFC 9438, “CUBIC for Fast and Long-Distance Networks”, is a Proposed Standard document published in August 2023 by L. Xu, S. Ha, I. Rhee, V. Goel, L. Eggert. It updates RFC 5681. It obsoletes RFC 8312. The canonical text is published by the RFC Editor.

Abstract

CUBIC is a standard TCP congestion control algorithm that uses a cubic function instead of a linear congestion window increase function to improve scalability and stability over fast and long-distance networks. CUBIC has been adopted as the default TCP congestion control algorithm by the Linux, Windows, and Apple stacks.

This document updates the specification of CUBIC to include algorithmic improvements based on these implementations and recent academic work. Based on the extensive deployment experience with CUBIC, this document also moves the specification to the Standards Track and obsoletes RFC 8312. This document also updates RFC 5681, to allow for CUBIC's occasionally more aggressive sending behavior.

Abstract as published in the RFC, via rfc-editor.org.

What “Proposed Standard” means

An entry-level standards-track specification: stable, peer-reviewed and a solid basis for implementation, though it may still evolve before becoming an Internet Standard.

Read this RFC

The canonical text of RFC 9438 is hosted at rfc-editor.org. Available in HTML,TXT,PDF,XML.

Relationships to other RFCs
This RFC obsoletes
RFC 8312
This RFC updates
RFC 5681
Other RFCs from 2023

Who Is Online

In total there are 72 users online: 0 registered, 66 guests and 6 bots.

Most users ever online was 1,226 on 13 Jun 2026, 3:56 am.

Bots: AhrefsBot Applebot Facebook Other Bot SemrushBot Sogou

Users active in the past 15 minutes. Total registered members: 354