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

Hi Matt,

Thank you for the review!

> Do you mind opening a PR instead, please?

Sure, here it is:

https://github.com/multipath-tcp/packetdrill/pull/204

Also noted for the cc list, I will only use the MPTCP ML next time.

Cheers,
Kalpan Jani


From: Matthieu Baerts <[email protected]>
To: "Kalpan Jani"<[email protected]>, <[email protected]>
Cc: <[email protected]>, <[email protected]>, <[email protected]>
Date: Wed, 05 Aug 2026 15:00:19 +0530
Subject: 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.
 > 
 >