TCP has dozens of tunable parameters: windows, timeouts, congestion
control algorithms, keepalive. You already know the basics, what a
tcp-handshake is and which tcp-states a connection goes through;
here you turn exactly these parameters. Most of them live in
/proc/sys/net/ipv4/tcp_* and are read and written through sysctl.
When a serious service hits a ceiling, these are usually what gets
tuned.
This lesson is a tour of the main parameters, switching the congestion control algorithm (CUBIC vs BBR), and reading a live session through cmd-ss (the internal TCP counters).