TCP Fast Open (TFO) is a relatively new TCP feature. The corresponding RFC7413 was published in 2014 and labeled “Experimental” [1]. The feature has been showing up in browsers and operating systems in the last couple of years. In particular, together with TLS 1.3, it can significantly decrease the time it takes to set up a connection.
Article Link: InfoSec Handlers Diary Blog - SANS Internet Storm Center