Re: [PATCH packetdrill] mptcp: dss: validate tcp_rto_max_ms on DATA_FIN retransmissions

Hi Kalpan,

(Note: no need to add Mat, Paolo and myself in cc: only the MPTCP ML is
enough)

On 05/08/2026 08:47, Kalpan Jani wrote:
> The kernel patch "mptcp: honour configured min/max RTO in retransmit
> paths" makes the MPTCP-level DATA_FIN retransmission backoff follow
> the tcp_rto_min_us / tcp_rto_max_ms sysctls instead of the hard-coded
> TCP_RTO_MIN / TCP_RTO_MAX constants.
> 
> Validate it in dss_fin_retrans_established.pkt: set tcp_rto_max_ms to
> its minimum (1000ms). With the default 200ms rto_min, the backoff
> shift is then capped at ilog2(1000 / 200) = 2, so the retransmission
> intervals stop doubling at 200ms << 2 = 800ms. Add two more expected
> DATA_FIN retransmissions at that capped interval.
> 
> Without the kernel change, the backoff keeps doubling and the 5th
> retransmission arrives after ~1.6s instead of ~800ms, making the test
> fail.
> 
> Link: https://lore.kernel.org/all/[email protected]/
> Signed-off-by: Kalpan Jani <[email protected]>
> ---
> Notes:
> - This depends on the kernel patch linked above: the test fails on
>   kernels without it (5th DATA_FIN retransmission at ~1.6s instead
>   of ~800ms).
> - Validated with the mptcp-upstream-virtme-docker environment:
>   passes on a patched kernel (ipv4/ipv6/ipv4-mapped-v6), fails
>   without the patch as described.
Thank you, it looks good to me!

Do you mind opening a PR instead, please?

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

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