Note (2026): This post was written in 2015 for Linux kernel 2.4.x/2.6.x and is preserved here purely for historical interest. Several things have changed significantly since then — tcp_fack and route.flush have been removed from modern kernels entirely, the recommended window sizes are far too conservative for today’s connections (modern kernels auto-tune well beyond 512KB by default), and ECN is now widely supported and should generally be enabled, not disabled. If you’re trying to tune a modern Linux system for broadband, please look for a guide targeting the current kernel. Don’t copy-paste the sysctl.conf block below onto anything running kernel 4.x or later.