TCP_FASTOPEN cookie is saved per what?

I’ve been reading about TCP_FASTOPEN in Linux and I can’t figure out per what is cookie saved.

If one program on my Linux server connects using tcp fast open, does other program that connects to same server uses previous ones cookie for fast open?

Or only sockets from same program can use each other cookies?

Answer

Few years ago the concept of TCP_FASTOPEN (TFO) was introduced as a solution to improve performance on TCP connections reducing one roundtrip of the handshake process. The first operating system that implements TFO is Linux and have been demonstrated good improvements when used in a common network.

The implementation in the Linux Kernels have been made by parts, being Linux Kernel 3.6.1 the first one into implement the client side requirements and then Linux Kernel 3.7 who implements the server side socket behavior.

When using TCP_FASTOPEN the behavior its a little different. You not longer need to use connect(2), instead you use sendto(2) and it also gives you the opportunity to let the Kernel buffer some initial outgoing data. For short, the call sendto(2) its like an implicit connect(2) and send/write(2) same time

For more information http://edsiper.linuxchile.cl/blog/2013/02/21/linux-tcp-fastopen-in-your-sockets/

EDIT

The Fast Open Cookie is designed to mitigate new security vulnerabilities in order to enable data exchange during a handshake. The cookie is a MAC tag generated by the server and is opaque to the client; the client simply caches the cookie and passes it back on subsequent SYN packets to open new connections. The server can expire the cookie at any time to enhance security. It doesn’t die when application close, as its stored in system memory