[PATCH mptcp-next 0/3] mptcp: fallback to TCP after 3 MPC drop + cache

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

The SYN + MPTCP_CAPABLE packets could be explicitly dropped by firewall
somewhere in the network, e.g. with:

  iptables -t filter -A FORWARD -p tcp --tcp-option 30 -j DROP

The idea of this series is to fallback to TCP after 3 SYN+MPC drop
(patch 2). If the connection succeeds after the fallback, it very likely
means a blackhole has been detected. In this case (patch 3), MPTCP can
be disabled for a certain period of time, 1h by default. If after this
period, MPTCP is still blocked, the period is doubled.

This should help applications which want to use MPTCP by default on the
client side if available.

This series has been validated by a new packetdrill test:

  https://github.com/multipath-tcp/packetdrill/pull/156

Some questions:

- Should we let the user changes the number of retransmissions (2)
  before falling back to TCP? For TFO, the data are never retransmitted
  in a SYN. Maybe that's different here? A sysctl knob could always be
  added later on.
- Should we globally disable all MPTCP connections if any retransmission
  after the fallback is a success instead of only the first one? I guess
  we reduce risks of accidents by only looking at the following
  retransmission after the fallback.
- Is one hour a good time for the fallback?

For later:

- The restriction could be done per oif (sk_dst_get(ssk)->dev), but we
  would need to store it somehow, or per MPTCP entrypoint. Or let the PM
  calling mptcp_active_enable() when a new endpoint is added.
- Other cases could trigger mptcp_active_disable(): e.g. some fallbacks
  or corruptions in the middle of the connections.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Matthieu Baerts (NGI0) (3):
      mptcp: export mptcp_subflow_early_fallback()
      mptcp: fallback to TCP after SYN+MPC drops
      mptcp: disable active MPTCP in case of blackhole

 Documentation/networking/mptcp-sysctl.rst |  11 +++
 include/net/mptcp.h                       |   4 +
 net/ipv4/tcp_timer.c                      |   1 +
 net/mptcp/ctrl.c                          | 133 ++++++++++++++++++++++++++++++
 net/mptcp/mib.c                           |   3 +
 net/mptcp/mib.h                           |   3 +
 net/mptcp/protocol.c                      |  18 ++--
 net/mptcp/protocol.h                      |  16 +++-
 net/mptcp/subflow.c                       |   4 +
 9 files changed, 182 insertions(+), 11 deletions(-)
---
base-commit: 44106bc7908a7c9e461983032567a918e1d62b91
change-id: 20240822-mptcp-fallback-x-mpc-491bb35a8e66

Best regards,
-- 
Matthieu Baerts (NGI0) <matttbe@kernel.org>