Вот такой вот коммит обнаружился:
Remove TCP FastOpen support.
We never enabled it by default, and have no plans to, so we should just remove it. QUIC also makes it less useful, and TLS 1.2 0-RTT session restore means it potentially mutates state.
It also leaks also leaks data between profiles, including (but not limited to) incognito profiles, since it relies on global system state.
Интересно, что при этом TFO по-прежнему используется в Firefox, который на текущий момент пожалуй является единственным браузером, поддерживающим эту технологию. Вот так, например, выглядит статистика TCP Fast Open в MacOS:
% sudo netstat -s|fgrep -i tfo netstat: sysctl: net.inet.ip.input_perf_data: No such file or directory 0 time a TFO-cookie has been announced 0 SYN with data and a valid TFO-cookie have been received 0 SYN with TFO-cookie-request received 0 time an invalid TFO-cookie has been received 152 times we requested a TFO-cookie 50 times the peer announced a TFO-cookie 57 times we combined SYN with data and a TFO-cookie 1 time a connection-attempt with TFO fell back to regular TCP 0 time a TFO-connection blackhole'd 10 times a TFO-cookie we sent was wrong 110 times did not received a TFO-cookie we asked for 0 time TFO got disabled due to heuristicsn 0 time TFO got blackholed in the sending direction
— Основной вклад в не-0’ые счетчики был внесен именно запуском Firefox.