Large-scale scanning of TCP's initial window

Abstract

Improving web performance is fueling the debate of sizing TCP's initial congestion window (IW), which is a critical performance parameter especially for short-lived flows. This debate yielded several RFC updates to recommended IW sizes, e.g., an increase to IW10 in 2010. The current adoption of IW recommendations is, however, unknown. In this paper, we therefore conduct large-scale measurements covering the entire IPv4 space inferring the IW distribution size by probing HTTP and HTTPS servers. We present an HTTP and TLS scanning method implemented in ZMap, enabling quick estimations of IW sizes at Internet scale. For the first time since the standardization and implementation of IW 10, we shed light on the rugged landscape of IW configurations on the Internet.