CUBIC for Fast Long-Distance Networks
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I. Rhee, L. Xu, S. Ha, A. Zimmermann, L. Eggert, R. Scheffenegger · 2018-02
Abstract
RFC 8312 documents CUBIC, a TCP congestion control algorithm designed for high-bandwidth, long-delay networks. CUBIC uses a cubic function to compute the congestion window size, allowing faster window growth than the standard AIMD algorithm while remaining fair to legacy TCP flows.
Why This RFC Matters
CUBIC became the default TCP congestion control algorithm in Linux 2.6.19 (2006) and subsequently in most server operating systems, meaning it handles the majority of the world's internet traffic. Its key insight is that standard AIMD congestion control leaves bandwidth underutilized on high-bandwidth, high-latency paths (e.g., 10 Gbps transcontinental links) because the window grows too slowly after a loss event. CUBIC's cubic growth function reaches the previous window size quickly and then probes cautiously for more, achieving significantly higher throughput on these 'fat long-distance pipes' while remaining TCP-friendly. RFC 8312 documenting CUBIC as Informational reflects that it was already a de facto standard long before formal IETF publication.