These patches focus on the Initiator and partially on Listener side.
The next options in userspace are available:
a) sendto(..., ..., ..., MSG_FASTOPEN, ..., ...);
b) setsockopt(..., SOL_TCP, TCP_FASTOPEN, ..., ...);

These patches implement Appendix-B of RFC8684 (MPTFO).

We would like to credit Paulo Abeni, Mat Martineau, Matthieu Baerts and
Benjamin Hesmans for advices and ideas that improved these patches.

Signed-off-by: Dmytro Shytyi <dmytro@shytyi.net>
---
v13 -> v14:
 - rebase on top of Paolo's patch: mptcp: factor out mptcp_connect().
 - add IPv6 only check for subflow_v6_send_synack().
 - new helper for sockopt (first subflow only) is used.
 - integrate selftests.
 - modify treq.
---
Dmytro Shytyi (5):
  mptcp: introduce MSG_FASTOPEN flag.
  mptcp: implement delayed seq generation for passive fastopen
  mptcp: add subflow_v(4,6)_send_synack()
  mptcp: add TCP_FASTOPEN sock option
  selftests: mptcp: mptfo Initiator/Listener

 net/mptcp/Makefile                            |   2 +-
 net/mptcp/fastopen.c                          |  97 ++++++++++++
 net/mptcp/options.c                           |   5 +
 net/mptcp/protocol.c                          |  11 +-
 net/mptcp/protocol.h                          |   9 ++
 net/mptcp/sockopt.c                           |   5 +-
 net/mptcp/subflow.c                           |  42 ++++++
 .../selftests/net/mptcp/mptcp_connect.c       | 140 +++++++++++++-----
 .../selftests/net/mptcp/mptcp_connect.sh      |  77 ++++++++++
 9 files changed, 346 insertions(+), 42 deletions(-)
 create mode 100644 net/mptcp/fastopen.c

-- 
2.34.1