iptables nf_conntrack_tcp_timeout_established
http://www.lognormal.com/blog/2012/09/27/linux-tcpip-tuning/
http://www.artiflo.net/2008/12/nf_conntrack-table-full-dropping-packet/
https://wiki.khnet.info/index.php/Conntrack_tuning
tcp window scaling
advanced
https://access.redhat.com/solutions/30453
https://vincent.bernat.im/fr/blog/2014-tcp-time-wait-state-linux
https://blogs.dropbox.com/tech/2017/09/optimizing-web-servers-for-high-throughput-and-low-latency/