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[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-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=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