SEARCH -- a New Slow Start Algorithm for TCP and QUIC

TCP slow start is designed to ramp up to the network congestion point quickly, doubling the congestion window each round-trip time until the congestion point is reached, whereupon TCP exits the slow start phase. Unfortunately, the default Linux TCP slow start implementation -- TCP Cubic with HyStart [HYSTART] -- can cause premature exit from slow start, especially over wireless links, degrading link utilization. However, without HyStart, TCP exits slow start too late, causing unnecessary packet loss. To improve TCP slow start performance, this document proposes using the Slow start Exit At Right CHokepoint (SEARCH) algorithm [KCL24] where the TCP sender determines the congestion point based on acknowledged deliveries -- specifically, the sender computes the delivered bytes compared to the sent bytes, smoothed to account for link latency variation and normalized to accommodate link capacities, and initiates exits slow start if the delivered bytes are lower than expected. We implemented SEARCH in Linux, FreeBSD, and QUIC and evaluated it over WiFi, 4G/ LTE, and low earth orbit (LEO) and geosynchronous (GEO) satellite links. Analysis of the results show that SEARCH reliably exits from slow start after the congestion point is reached but before inducing packet loss.