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Mon, 26 Oct 2026 07:23:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792999390; bh=gL4Y4LLW2G3VJwIBXj9jDelLMgccnmv4YV0UaBDP1+Q=; h=From:Subject:Date:To:Cc:Reply-To; b=ZwcsoOIh97NogTnRDp/9R0PVea18/TOaCQ1mmcvhCLQEz+Set1y5naNcfnetzY0jF y2O88qwWb6ZtWhx39wpCFqI1ctDOMbNqCAQiiq2lWFrnKpWu/uecwIQNMrjgowtft8 wSOtpPw3A/azXVma2cMgC/NvNKdEZStzyi0RqNZPxU9b5z9C79YGSHRtGT0I147Z5C Ceg7l3CvxGWAcEQhlyOL/pJKM5bFkElnp0aD9JEMEgOou2H7+3r7z6vRkgZHecE62S Uian5GSLXSNESgDEjl97s4joBQQNkMbx9neHJ8MUG/1oRvyUU0xiOw2IaNUU3qcrjm MyBvGB2sVijTQ== Received: from nexus.subspace.kernel.org (localhost [IPv6:::1]) by lkml.kernel.org (Postfix) with ESMTP id C477244C43; Mon, 26 Oct 2026 07:23:09 +0000 (UTC) From: Esko Mobius via B4 Relay Subject: [PATCH net 0/2] Revert advertising the TCP MSS from the configured MTU Date: Mon, 26 Oct 2026 15:23:07 +0800 Message-Id: <20261026-advmss-revert-v1-0-272fa4b7ecb6@gmail.com> 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 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-B4-Tracking: v=1; b=H4sIANv/3moC/6tWKk4tykwtVrJSqFYqSi3LLM7MzwNyDHUUlJIzE vPSU3UzU4B8JSMDIzNDINZNTCnLLS7WBapNLSrRNbUwME1JNTQ3NrNMUwLqKShKTcusAJsXrZS XWqIUCxEsLk3KSk0uAZmkVFsLAFqhlOV2AAAA X-Change-ID: 20261026-advmss-revert-5805de17369f To: netdev@vger.kernel.org Cc: 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, Esko Mobius , Eric Dumazet X-Mailer: b4 0.16.0 X-Developer-Signature: v=1; a=ed25519-sha256; t=1792999388; l=1884; i=echoowall@gmail.com; s=20261026; h=from:subject:message-id; bh=XaR0Gk8SmZ1BSXypb/zmzgZFcyeFs/xIK7D652IwkuE=; b=1UtGo0a2fgm7Z14XWVa6/EaFpZ4ZKjTgedpVnVQW7PmVrgnzJb5IAT2ykAQfhMa27+gN/Y4Su 9CVc7UOGdAYCpkkdt3jSZAgu/96ThC6LY37mBdYYOdPI1tHLgZrpP84 X-Developer-Key: i=echoowall@gmail.com; a=ed25519; pk=9XP6JhqjjPsHCF6UZZZTeqTRnXIa4+ufggnxXtTJ+no= X-Endpoint-Received: by B4 Relay for echoowall@gmail.com/20261026 with auth_id=1323 X-Original-From: Esko Mobius Reply-To: echoowall@gmail.com Commit 2640e6419594 ("net: advertise TCP MSS from the configured MTU, not the learned PMTU") breaks TCP for clients behind networks that filter inbound ICMP, when the server reaches them through a hop with a smaller MTU: the handshake completes, then the connection stalls as soon as the client sends full-size segments. This was reported in [1] and confirmed on 7.3-rc7; reverting the commit on top of 7.3-rc7 fixes it. Details and numbers are in patch 2. The asymmetric (DSR-style) case that commit targets can still be served by the existing per-route advmss metric, which overrides the default after the revert; patch 2 includes a test of that as well. If you would rather keep the new behaviour as an explicit opt-in some other way, I am happy to test or rework this. Patch 1 removes the packetdrill tests that assert the reverted behaviour, patch 2 is the revert itself. [1] https://lore.kernel.org/regressions/4F0FAEB6-A13B-47F3-BFF1-C706686C6F64@gmail.com/ Signed-off-by: Esko Mobius --- Esko Mobius (2): Revert "selftests: net: packetdrill: add tests for advertised MSS with PMTU exceptions" Revert "net: advertise TCP MSS from the configured MTU, not the learned PMTU" include/net/ip.h | 25 -------- include/net/ip6_route.h | 37 ------------ net/ipv4/route.c | 4 +- net/ipv6/route.c | 2 +- .../net/packetdrill/tcp_advmss_pmtu_ipv4.pkt | 67 ---------------------- .../net/packetdrill/tcp_advmss_pmtu_ipv6.pkt | 67 ---------------------- 6 files changed, 3 insertions(+), 199 deletions(-) --- base-commit: 7076f684d5132e6a6145e1da2fa74771ecd052e5 change-id: 20261026-advmss-revert-5805de17369f Best regards, -- Esko Mobius