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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=Eo+tl64L; 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 11DBD606A31D for ; Thu, 22 Oct 2026 15:50:54 +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=Eo+tl64L Received: by smtp.kernel.org (Postfix) with ESMTPSA id 317241F01825; Thu, 22 Oct 2026 15:50:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792684254; bh=HUd5m39kGzzCln+TjdiGkkhbcxtb48mI5rxpseWprHA=; h=Date:Subject:To:References:From:Cc:In-Reply-To; b=Eo+tl64L5Qbtl1EBOizdwfHZA73vrLaj7lfnfaiVBbZ6PB7Zylwz6a7x5PgEiRUmt EN/+y60exUgK39hDAuN9G8GNfJqouT76joyV88DG5hgo+wUpiP6uWtJcZ/Kd8AoWm2 0YIsEcA0O73hoEMX75vbSZykPs1oyp/0EIrxl3rsK5ciE1endC9f0M5aTO+oTivbp9 427bolAJStfat3z0li6/QC+zezZ7le5eW3ru7HMyyiW7wjy04WaCdsDwqrrBSu1sNy WOf2L1gGvxiObEiAKI+3w/1cNUCIp4O9k6a1hCTgli/VFt1tbsS/52AohVX2tA5AXs sJbCY4Mx6hIkQ== Message-ID: <2ce6ae1c-e023-411d-85df-350367ae30ef@kernel.org> Date: Thu, 22 Oct 2026 17:50:52 +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 Cc: mptcp@lists.linux.dev Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 22/10/2026 16:31, Anil Kaushik wrote: > Hi Matt, > > Sorry my earlier response had html content. > > Resending my response again. Couple of more points too. > > On Thu, Oct 22, 2026 at 7:00 PM Matthieu Baerts wrote: >> >> 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. >> > > Yes. The router still has its default route, so it does not send ICMP > unreachable, it just drops the packets. And when its WAN link really goes > down, it has no way to tell the gateway. It could eventually turn off the link to the gateway. > Agreed on the link monitoring. A small probe per link, removing and > re-adding the endpoint, is a good idea and I will send a separate RFC. > Let me know what you think? That's something that should be done in userspace, no? >> 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. >> > I will send v8 with the default set to the maximum, 127. Thank you, I will try to have a look. Cheers, Matt -- Sponsored by the NGI0 Core fund.