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mptcp: honour configured min/max RTO in retransmit paths

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The MPTCP-level retransmit timers (DATA_FIN retransmissions and the
fallback timeout) used the hard-coded TCP_RTO_MIN / TCP_RTO_MAX
constants, ignoring the tcp_rto_min_us and tcp_rto_max_ms sysctls.

Make them follow the sysctls instead: seed icsk_rto_min / icsk_rto_max
on the MPTCP socket from the per-netns sysctls in __mptcp_init_sock()
-- the msk does not go through tcp_init_sock(), so these fields would
otherwise stay zero -- and read them directly where the constants were
used:

- mptcp_set_datafin_timeout(): both the backoff cap computation and
  the resulting timer_ival. The two sysctls are validated
  independently, so rto_min > rto_max is a valid configuration; keep
  a max_t() guard so ilog2() is never called with 0.

- __mptcp_set_timeout(): the fallback when no subflow timeout is
  available.

The icsk fields are read directly instead of using the
tcp_rto_min()/tcp_rto_max() helpers: the MPTCP socket does not perform
routing lookups in these paths, so the rto_min route metric checked by
tcp_rto_min() can never apply here. The TCP_RTO_MIN_US /
TCP_RTO_MAX_MS socket options are not supported by MPTCP setsockopt()
either; this can be revisited if they get supported on MPTCP sockets.

The remaining uses of TCP_RTO_MAX in net/mptcp/ctrl.c (default
add_addr_timeout) and net/mptcp/subflow.c (MP_FAIL timeout) are
intentionally left unchanged: they use the constant as a default
duration, not as an RTO bound on a retransmit timer.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/618
Signed-off-by: Kalpan Jani <kalpan.jani@mpiricsoftware.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260812-net-next-mptcp-misc-feat-7-3-v1-4-1905a818f6cb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Kalpan Jani 2026-08-12 16:55:39 +02:00 • committed by Jakub Kicinski
commit ea4eb2adb0
1 changed files with 18 additions and 4 deletions
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