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

[Esko Mobius]

On Thu, 29 Oct 2026, David Laight wrote:
> If it isn't a silly question, how to connections in the other direction
> work?
>
> I'd have thought they'd need that ICMP message to get through.

Not silly at all. The frag-needed always goes back to whoever sent the
oversized packet, and only the client side filters inbound ICMP.

Server -> client (downloads): the server's full-size segment hits the
1456 hop, the router there sends frag-needed back to the server, which
accepts it, learns the PMTU and resends smaller segments. The server's
send side was never changed by 2640e6419594 (tcp_current_mss() still
uses the learned PMTU), so downloads always worked.

Client -> server (uploads): the client's full-size segment hits the same
hop, the frag-needed goes back towards the client (via the relay's
conntrack), and the client-side firewall drops it. The client never
learns to send smaller segments, so the upload stalls. Here the only
thing that kept segments small was the smaller MSS the server
advertised in its SYN-ACK, which is what the reverted commit removed.

Thanks,
Yongjian