Stateful-TCP: Bypassing TCP Slow-Start Using Cached Per-Destination Path Bandwidth

This document specifies Stateful-TCP, an experimental sender-side mechanism that accelerates the startup of TCP connections by reusing path-bandwidth information estimated from earlier connections to the same destination. When a usable estimate is available, the sender bypasses Slow-Start and instead enters a paced startup phase whose initial congestion window and pacing rate are derived from the cached estimate. When no usable estimate is available, the sender falls back to standard Slow-Start. Stateful-TCP is sender-only and does not require any change to the TCP receiver or to the wire format. It is orthogonal to the congestion control algorithm in use after the first round-trip time and may therefore be combined with existing TCP variants such as CUBIC. This document also specifies a Gap-Compensated Bandwidth Estimation (GCBE) procedure used to produce the cached per- destination estimate. Stateful-TCP extends the framework of TCP Control Block Interdependence (RFC 9040) by sharing additional per-destination state across connections. The mechanism is published as Experimental to enable independent implementation, evaluation, and review.