This set of patches will bring "Fast Open" Option support to MPTCP.
The aim of Fast Open Mechanism is to eliminate one round trip
time from a TCP conversation by allowing data to be included as
part of the SYN segment that initiates the connection.

IETF RFC 8684: Appendix B.  TCP Fast Open and MPTCP.

[PATCH v3] includes "client-server" partial support for :
1. MPTCP cookie request from client.
2. MPTCP cookie offering from server.
3. MPTCP SYN+DATA+COOKIE from client.
4. subsequent write + read on the opened socket.

This patch is Work In Progress transitional draft. There was a pause
in code development that was unpaused recently. Now this code is based
on the top of mptcp-next branch. The option below will be modified in
future inelligently, depending on socket type (TCP||MPTCP):
*tcp_options ^= OPTION_TS
You also might notice some of commented pieces of the upstream code -
that (is probably not good) and was done to observe an
expected behavior of MPTCP Fast Open mechanism. Any comments how
to achive the same behavior of MPTCP_FO without commenting the related
parts of the code are welcome.

Signed-off-by: Dmytro SHYTYI <[email protected]>
Message-Id: <[email protected]>