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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=cmWkw7W7; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8FA13606A323 for ; Thu, 22 Oct 2026 13:30:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=cmWkw7W7 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9DB411F000C2; Thu, 22 Oct 2026 13:30:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792675831; bh=7nZ+PTvIIYY32MbOGiiGjY3A9u7uFw/+tvgMhQC7g20=; h=Date:Subject:To:References:From:Cc:In-Reply-To; b=cmWkw7W7u+fHGMnFzJ7sb6AruZMIJ/Z87BtvRHe8zuw5zhc0+WgpdtCTwMHblmmir qfuV90gJ+Xj6DirQk7zXXzCOeoRvUjvk5nu0TSVDcAHSFOPMZd7fIvgR9N1FfnZCNW 3ICcDHfTINUK8njR+Vod1X86uknfIUFljOyZNG2awS8pseMU6TsCHm2a6dS2zYrizF 6FlJdABssJi4RmzkWlg7QFwBF7YgjzZNRj86aHnuKXWJlpp21mciEy9I9KlZvWQVqF yj/dn9KGBPyUPok9SOv3t7RDnspZCsx5I/Yqn3qR4RQRWbYjtkHoqTyfeGHFVdeJyJ lFf6KuGSeXOtA== Message-ID: Date: Thu, 22 Oct 2026 15:30:28 +0200 X-Mailing-List: mptcp@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Beta Subject: Re: [RFC PATCH mptcp-next v3 2/3] mptcp: pm: kernel: retry subflows closed after an error Content-Language: fr To: Anil Kaushik References: <20261014143349.768138-1-anilkaushikwireless@gmail.com> <20261014143349.768138-3-anilkaushikwireless@gmail.com> <7c7308c5-c150-41a1-9d84-9981acfb5962@kernel.org> <4180b794-8877-4516-9400-afb0894e8f49@kernel.org> <07243418-dc96-43c6-beb6-ff220234bdeb@kernel.org> From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 22/10/2026 13:04, Anil Kaushik wrote: > On Thu, Oct 22, 2026 at 3:58 PM Matthieu Baerts wrote: >> On 22/10/2026 11:24, Anil Kaushik wrote: >>> On Thu, Oct 22, 2026 at 1:02 PM Matthieu Baerts wrote: >>>> On 21/10/2026 20:20, Anil Kaushik wrote: >>>>> On Wed, Oct 21, 2026 at 11:18 PM Matthieu Baerts wrote: >>>>>> On 16/10/2026 10:43, Matthieu Baerts wrote: >>>>>>> On 16/10/2026 07:49, Anil Kaushik wrote: (...) >>>>>>>>> modify icsk_syn_retries instead [...] WDYT? >>>>>>>> >>>>>>>> Good idea, I will do that: one new subflow per failure, with its SYN >>>>>>>> retries set from the new netlink limit, 0 = disabled. It also removes >>>>>>>> the socket reuse, the retries counter and the 60s check. >>>>>>>> >>>>>>>> One point to think though, If ICMP unreachable stops the SYN at once. >>>>>>>> I would not retry >>>>>>>> again then. Is that OK? >>>>>>> >>>>>>> Good point. In your specific case, what do you observe? >>>>> >>>>> In my tests, the fiber outage is a silent drop: the SYNs just time >>>>> out, and the subflow comes back once the path is up again. >>>> >>>> Interesting. Out of curiosity, how long are these outages typically? >>>> Less than ~20 minutes I suppose? >>>> >>> It varies. Short ones are a few minutes, but when the fiber is cut, the >>> repair can take several hours. So a longer default would help. >> >> When the "fiber is cut", do you still have an IP address or is the >> connection still up? >> >> To me, if there is an issue in the network, we generally have two cases: >> either we are closed to an end (client / server) and the host should be >> disconnected from that network, or it is happening in the "middle" of >> the network where BGP will reroute the traffic. >> In case of disconnection, I would guess the IP is lost and the >> corresponding MPTCP endpoint is removed. The userspace will try to >> reconnect, then re-add the MPTCP endpoint, and the PM can restart the >> subflows. >> >> But that doesn't seem to be the case on your side :) >> Do you know why? >> > Yes. MPTCP runs on a gateway in front of the fiber and Starlink > routers, connected to each of them over Ethernet. The fiber cut is > between the fiber router and the ISP, so the gateway keeps its link and > its IP address on that side, nothing removes the MPTCP endpoint, and > the packets on that path are just dropped. There will be mix of > deployments like this. > > Any thoughts? So when you have an issue between the fiber router and the ISP, the fiber router keeps its connection up to the ISP, and no ICMP network unreachable are sent? In such setup, and if the fiber router gets a notification somehow in case of issue, I would find it better to forward this notification to the gateway to disable the MPTCP endpoint. An alternative is to have a simple service monitoring the links, and removing/re-adding endpoints: a simple "echo" service in TCP (or ICMP echo) with a server somewhere, one per link. With that, you have more control and can eventually ack quicker. Yet, the series you are suggesting here still makes sense because that would help people, or in case of issues not detected by the network manager. Cheers, Matt -- Sponsored by the NGI0 Core fund.