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[133.18.180.162]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-1a75d76814bsm13556801c88.10.2026.10.29.03.21.35 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Thu, 29 Oct 2026 03:21:40 -0700 (PDT) Content-Type: text/plain; charset=us-ascii X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3901.100.1.1.12\)) Subject: Re: [PATCH net 2/2] Revert "net: advertise TCP MSS from the configured MTU, not the learned PMTU" From: Esko Mobius In-Reply-To: <20261029101859.6893f1a0@pumpkin> Date: Thu, 29 Oct 2026 18:21:22 +0800 Cc: netdev@vger.kernel.org, Jakub Kicinski , Paolo Abeni , "David S. Miller" , Simon Horman , David Ahern , Ido Schimmel , Jiayuan Chen , Shuah Khan , Thorsten Leemhuis , regressions@lists.linux.dev, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Eric Dumazet Content-Transfer-Encoding: 7bit Message-Id: <22077B76-12B0-4DD6-813E-A3968003575F@gmail.com> References: <20261026-advmss-revert-v1-0-272fa4b7ecb6@gmail.com> <20261026-advmss-revert-v1-2-272fa4b7ecb6@gmail.com> <20261029101859.6893f1a0@pumpkin> To: David Laight X-Mailer: Apple Mail (2.3901.100.1.1.12) 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