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[133.18.180.162]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-cd8ccc46482sm5225375a12.11.2026.10.29.05.36.46 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Thu, 29 Oct 2026 05:36:51 -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: <20261029112045.5e7d4317@pumpkin> Date: Thu, 29 Oct 2026 20:36:33 +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: <40993B2C-6390-4BC6-B4AD-967518B0CBE9@gmail.com> References: <20261026-advmss-revert-v1-0-272fa4b7ecb6@gmail.com> <20261026-advmss-revert-v1-2-272fa4b7ecb6@gmail.com> <20261029101859.6893f1a0@pumpkin> <22077B76-12B0-4DD6-813E-A3968003575F@gmail.com> <20261029112045.5e7d4317@pumpkin> To: David Laight X-Mailer: Apple Mail (2.3901.100.1.1.12) On Thu, 29 Oct 2026, David Laight wrote: > But that relies on something having gone the other way first. > In general that won't be true. Agreed, that is the same point Jiayuan made. > It is almost worth implement something that retries with the shorter mtu > to detect the PPPoE mtu blackhole at the sending end. Linux already has this: net.ipv4.tcp_mtu_probing (RFC 4821). With it set to 1, once the retransmission timeouts reach tcp_retries1 the connection drops to tcp_base_mss and probes up from there. It is off by default, and the same goes for Windows (EnablePMTUBHDetect); macOS turns it on by default. > Or even just use the shorter mtu for long RTT connections. Both of these have to happen at the sending end, which in my case is the client (it is an upload), so the server can't help. The only lever the server has is the MSS it advertises in the SYN-ACK, and it has no RTT sample at that point. For now the MSS is clamped on the path. While reading this code I noticed there are no selftests for the black hole detection or for MTU probing. I'd like to add packetdrill tests for both once net-next reopens. Thanks, Yongjian