From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=209.85.128.43 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793269147; b=vCRDMHUh6Vn2Ek+gj/bbb475kP+5n7jxItwu6n52VmywaIemxnl35zRziDDaXITEimnA kfx7X0dH4Wzjygq50K/Tl2uniDFDGRe8ysfunQ7wZmnGsDW9JkfIr30mHda2MDuPy0rED hRf8uMqHri+FFhUTLfj1ELEqQpHOXZOZXYRwFPnwPLqXURjnkcfPwcx6wyOaVBQH6d0hJ Lf6jtE6w2VkDiMN5/ebkf8nVlq1ujaF3Z7SKdqg82Gs9RqGONF5nU2xyeHpsAJoy51Kar 8efAd0smTIgbFEpfFDM7aO1WLHcrzN5W1wtyTyKZaG3VtXLig333PlRmPlRSXu5mPTg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793269147; h=DMARC-Filter:Received:DKIM-Signature:X-Google-DKIM-Signature: X-Forwarded-Encrypted:X-Gm-Message-State:X-Gm-Gg:X-Received:Received: Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References:X-Mailer: MIME-Version:Content-Type:Content-Transfer-Encoding; bh=Vc0r3JbZbwBvUVapNGhL09kDmaTErdcaYY3i2MzSoa4=; b=DC1i/5rmIKlBwoPf7BOuNXDqGtMamSOj0Dxus9sL5ypwF4Ci+oaqMnOjMIqhTRz5rmMJ zmCa8vEwi3RkzF2yjelrkFWAwG0Tv7X4FqgPQUeVLwd3asVjvcu+ODNtEoauFMEGNGz09 Hwhj02e4tK700N1pnuLMjlxh0KIP8Xiq+Rb8bEfiy+yRp4jz0Solh8jjLyjUP0hLog+7N vUOLOryZnU/7Gq/4vMum0cOWXPziBABrMrsGx6QOaw7OqTs8onQX8wCzphqJ02l7G8w81 BUk2c6hMCQK55f89FTN2akCzVefbjigqmKE4yurfR14cEF//OOmAnyQlRMR5SryYSNA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256 header.s=20260707 header.b=zH9z4kbb; arc=none smtp.remote-ip=209.85.128.43 Received: from mail-wm1-f43.google.com (mail-wm1-f43.google.com [209.85.128.43]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CBB77606A321 for ; Thu, 29 Oct 2026 10:19:06 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org CBB77606A321 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256 header.s=20260707 header.b=zH9z4kbb Received: by mail-wm1-f43.google.com with SMTP id 5b1f17b1804b1-4a140e7405dso20161155e9.3 for ; Thu, 29 Oct 2026 03:19:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20260707; t=1793269145; x=1793873945; darn=vger.kernel.org; h=content-transfer-encoding:content-type:mime-version:references :in-reply-to:message-id:subject:cc:to:from:date:from:to:cc:subject :date:message-id:reply-to:content-type; bh=Vc0r3JbZbwBvUVapNGhL09kDmaTErdcaYY3i2MzSoa4=; b=zH9z4kbbMg0lkkb/HqgCLaFR9vONpMS7UnrW2ezQXaMot4yuVlvx8b5AsRhSxwcAbi Q4V4plM0s/t0NDmjNPggc0m3ZhTaJqh16rAqpGi9MHj+RiOCp8gednbCJjh+al/OBh3J k7dr5f93QI3uvgi5N70JgAPgUqm5BbmDSTg4+7OuWMyvYgfA37I5zYTduwmKcfRgttzJ kjcY4e0V4EKRJbYh1+4WRLNTrxvUqyF0Wh6n+2CKqJa9D29js7cu4+6Z8dPMKadEGaoD kMgHq+0nvRzb+a+PbaN9D/ZrUsU0nRFfB6YPncLGLwQk91Qn1/Z4TSQbNgEhroKs+9x9 ox+g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1793269145; x=1793873945; h=content-transfer-encoding:content-type:mime-version:references :in-reply-to:message-id:subject:cc:to:from:date:x-gm-gg :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=Vc0r3JbZbwBvUVapNGhL09kDmaTErdcaYY3i2MzSoa4=; b=HyXG9ZHrjY0+76GCqR6NGU8uCRddOboc/4lu719CTi3ZOtONVbr9Vej/ZPolL85YIU Gj2rU0BrgpMgQhPgj2di7uunazOuMYcATwx7nzCJtV7i/COlGFkKHS0HwCTt+lKooAs2 D2Tp3AUE+xXLVDy3DG3lSdvG1VsBSFjI//EG/V0dKO58s63c57Ip1Tx/Hg+pCFaTA3Y2 553aEmRe1mvREdJbWV2aNb7U7U/1qWzAoNZYa3PRwW70EqI00lrsQBhQU+p0dqGM662b f43nABurJOFzKKxtDyfX0AhCIFuxDE04as4VebP/by+JXxt9crW9a9aGthBYKYRpEaCZ YBag== X-Forwarded-Encrypted: i=1; AKwUvBwkHYzbYixmf12rH2U5ReIEZ0dAKR1+BLyeXP1Vt59qx41MRY8iVO5gy+PkGgTc/wa+TmUFaO5pr87/54PWZRw=@vger.kernel.org X-Gm-Message-State: AFq9FYLF/HWcUhMXSVyok/QQcOUIg5/e8pZ8wndLJ2VZJB2q23sif5TV mUTTbJJtmTs5Fl5GXRN8BenFdjxU61kuauxmfjAqIhJ2qbp1FqlCu8LJ X-Gm-Gg: AYBFou3NJT7zg/pSRL16ZA6ONuD6h7kLW2OH0vmAie6R2IP2uWzofUrFSi+sBGns+6C 5b2YtHTb6aOrvS6x7iouPGdeVvqOiso450Pa7bysRNEK96VrTn9jIUTSBJ4D5mDySYNrznA/ENI a0OP640I56P+VoRI2/c3eap5ZNwvrB8HrqCOk1DKIZ0bZRaEFGM7GiV///z85B8CM/m9GPsgd9a nodI/uDhQAbSUswYBfCdAprPMMRL8pb1XvFIg77quUIqrHc5Mdcjp00w12VchusoIaCw87OtKsY e4nvvHF2L/dDSOyoqceQOxt6X3nc9FCDxnS6C1Q0f23C+gzHWAKxYbrKo6i5xysiQ/+Tb4AcQJn pZOR9hKRZcPaAvOEbKV63sbx3T5sfH/+9o9bJtng9QfQ2xJA0Wdl+OeWyzNrmelvBrhYlVIrhXu KtK188Nd1TkLOrRpDjE1p8b7r/E9UnU75azVoeaTupRGqdrqLYWzMi79nD5GMYO/fyNDcAT3ytD ul8ebLa7nzkKAkyJ92EBQ1ckpKcOatu0zp7eHSjhJAZrFPwrIGUo14lQi6pKrC7msGo1zV5Iv7L feVQ8yyQPAgW9yodCSxlT6z3vjk4GlAwY649XsQYXxJm3L8yW3KBBdFXN5D7xUMLjg== X-Received: by 2002:a05:600c:6088:b0:4a1:742e:3 with SMTP id 5b1f17b1804b1-4a9677ca203mr118445565e9.27.1793269145060; Thu, 29 Oct 2026 03:19:05 -0700 (PDT) Received: from pumpkin (82-69-66-36.dsl.in-addr.zen.co.uk. [82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e3ecd354asm31946023f8f.43.2026.10.29.03.19.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Oct 2026 03:19:04 -0700 (PDT) Date: Thu, 29 Oct 2026 10:18:59 +0000 From: David Laight To: Esko Mobius via B4 Relay Cc: echoowall@gmail.com, 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 Subject: Re: [PATCH net 2/2] Revert "net: advertise TCP MSS from the configured MTU, not the learned PMTU" Message-ID: <20261029101859.6893f1a0@pumpkin> In-Reply-To: <20261026-advmss-revert-v1-2-272fa4b7ecb6@gmail.com> References: <20261026-advmss-revert-v1-0-272fa4b7ecb6@gmail.com> <20261026-advmss-revert-v1-2-272fa4b7ecb6@gmail.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) X-Mailing-List: linux-kselftest@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=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 26 Oct 2026 15:23:09 +0800 Esko Mobius via B4 Relay wrote: > From: Esko Mobius > > This reverts commit 2640e6419594 ("net: advertise TCP MSS from the > configured MTU, not the learned PMTU"). > > That commit stopped the learned PMTU from lowering the MSS we advertise > in SYN and SYN-ACK. For symmetric paths it argued that "the peer runs > its own PMTU discovery and usually already knows the real path MTU". > That only holds if ICMP "fragmentation needed" can reach the peer. When > the peer sits behind a firewall that drops inbound ICMP, its PMTU > discovery is a black hole (RFC 2923, section 2.1), and the smaller MSS > we advertised was the only thing keeping its full-size segments off the > narrow hop. > > Inbound ICMP filtering is very common in enterprise and carrier > networks, and it is pervasive in mainland China. We hit this in > production after the commit was backported to 6.12.111: TCP connections > that reach a server through a relay with a smaller-MTU hop complete the > handshake, then stall as soon as the client sends full-size segments. > Clients on networks that let ICMP through, and connections that bypass > the relay, are unaffected, which made it hard to trace back to the > kernel. 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. David