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[133.18.180.162]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-36a002a486csm19339403eec.22.2026.10.27.06.13.04 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Tue, 27 Oct 2026 06:13:09 -0700 (PDT) Content-Type: text/plain; charset=us-ascii X-Mailing-List: netdev@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: Date: Tue, 27 Oct 2026 21:12:52 +0800 Cc: Jiayuan Chen , netdev@vger.kernel.org, Jakub Kicinski , Paolo Abeni , "David S. Miller" , Simon Horman , David Ahern , Ido Schimmel , Shuah Khan , Thorsten Leemhuis , regressions@lists.linux.dev, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Content-Transfer-Encoding: 7bit Message-Id: References: <20261026-advmss-revert-v1-0-272fa4b7ecb6@gmail.com> <20261026-advmss-revert-v1-2-272fa4b7ecb6@gmail.com> To: Eric Dumazet X-Mailer: Apple Mail (2.3901.100.1.1.12) 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 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. Thanks, Esko