HTTP persistent connection

https://en.wikipedia.org/wiki/HTTP_persistent_connection

https://developers.google.com/web/fundamentals/performance/http2/

https://http2.github.io/http2-spec/

https://tools.ietf.org/html/rfc7540

https://www.safaribooksonline.com/library/view/http-the-definitive/1565925092/ch04s05.html

https://tools.ietf.org/id/draft-thomson-hybi-http-timeout-01.html

http://stackoverflow.com/questions/1480329/what-exactly-does-a-persistent-connection-mean

https://www.safaribooksonline.com/library/view/http-the-definitive/1565925092/ch04s05.html

https://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html

https://www.w3.org/Protocols/HTTP/Issues/http-persist.html

https://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html

http://stackoverflow.com/questions/23581777/tcp-ip-long-term-connections