Does TCP_FASTOPEN actually work?

[Pieter de Goeje]

I'm trying to test this new feature, but I have trouble getting the 
client to actually send a cached cookie. It keeps requesting new ones 
and as a consequence it never sends data in the initial SYN packet. 
Tcpdump shows that the server correctly replies to a cookie request with 
a cookie.

Or am I misunderstanding how it should work and is the cookie cache 
per-process instead of system wide?

I'm using the test programs from 
https://people.freebsd.org/~pkelsey/tfo-tools/ for this purpose.

Server and client run on r335760 or later, with no changes to 
net.inet.tcp.fastopen except that server_enable was set to 1.

Related issue is that TCP_FASTOPEN is currently undocumented in tcp(4).

- Pieter