Re: [PATCH net 2/2] Revert "net: advertise TCP MSS from the configured MTU, not the learned PMTU"

[Jiayuan Chen]

On 10/27/26 9:12 PM, Esko Mobius wrote:
> On 10/27/26 1:45 PM, Jiayuan Chen wrote:
> > This precondition does all the work. The server only learns the PMTU
> > when it sends a full-size segment itself. [...]
> >
> > Please re-run the 6.12.107 case without seeding the PMTU.
> You are right. Without seeding, 6.12.107 stalls the same way:
>
>    kernel        PMTU seeded   SYN-ACK MSS   1 MB upload
>    6.12.107      no            1460          stalls
>    6.12.107      yes (ping)    1416          OK
>
> My reproducer took a shortcut there. What it should have modelled is
> our production setup, which is the masquerading relay you describe:
> the clients share the relay's address, so ordinary downloads by other
> clients keep the server's PMTU entry fresh. Modelling that directly,
> with no artificial seeding (cliA downloads 1 MB, then cliB, behind a
> firewall dropping frag-needed, uploads 1 MB through the same relay):
>
>    kernel        SYN-ACK MSS to cliB   cliB upload
>    6.12.107      1416                  OK
>    6.12.111      1460                  stalls


Thanks Esko for the testing, and Eric for the explanation. Agreed.


> So I agree it was never a designed mechanism, it is the side effect
> Eric describes. We have since added MSS clamping on our relays.
>
> If it helps, I can send a v2 that describes the masquerading case
> instead of the seeded one in the changelog. And I am happy to test
> the net-next follow-up on this setup.


No need for a v2 from my side, I just wanted to confirm the actual scenario.

I saw your report earlier but was swamped and could not get to it in 
time, sorry about that.


Reviewed-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>

> Thanks,
> Esko