Re: [PATCH mptcp-next v4] mptcp: honour configured min/max RTO in retransmit paths

Hi Kalpan,

On 05/08/2026 08:39, Kalpan Jani wrote:
> 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.
Now in our tree:

New patches for t/upstream:
- 3dcc47c54bb6: mptcp: honour configured min/max RTO in retransmit paths
- Results: 570d3b306aec..0715f5ba093a (export)

Tests are now in progress:

- export:
https://github.com/multipath-tcp/mptcp_net-next/commit/810cb9f0a83e758a5e310b4dc7248b5d7cec76bc/checks

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.