From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4FC15223710 for ; Tue, 16 Dec 2025 16:38:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765903138; cv=none; b=Ycnn4NjRC9nnb6TXnD+El9eTVTAvhIDQph4qbfIQn9RwmZRBt36R4Q3A+vPRU666CgwL6tEGt+JyYMivRSrbU4ncnMvZUSV+KpRPjYW17IAsHS23VV/62uXA2D4gadZXJHD26XcszDchLNpQV9/nyb4LP0vUgun/i+coGk7Qmq4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765903138; c=relaxed/simple; bh=cJJRloXo6bgMJtvfpD2g+KNlM53nsktSimWrvkgmrJY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Fp1nCO39bh5oineCCKxPLNhQrM1LXHTHZuWaNFD5oBgrleN5CikgZyCECEa6uQLvjTBlgS3dLgpSjRn8OE4rVKx0VXcXSoBbleOeJO9g6I86H3h3AojVPEhv3hrt57FXaxtMBmffeANvmBOiZuReXPpnUpFf/HEc7PWierYXOiw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EhHYxAZe; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="EhHYxAZe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E8E83C4CEF1; Tue, 16 Dec 2025 16:38:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1765903137; bh=cJJRloXo6bgMJtvfpD2g+KNlM53nsktSimWrvkgmrJY=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=EhHYxAZeezEXTSz9PpLpCODud1glrNlgrAK9t8YjcsbPQyASNx6b286KXccWEh4Mg uWeb6soEbGDISZt9Ed3nDzBrvyqMH5V6C67V50nvPnNxkk4I2eCeq39GZCEMfT67HX p8ZW7AYo65fpsgRxZcjeOM1evinNYyZya212Nt74z5Y11EJnA72xOEL+61LrAXljzm yGKKi91FnU4yCqrwD/693oQhVvHseqUPIlTPj6fUrbCIgl5NTmwyW1ufmdE6z0OM4k ffRwbVnPMwZCJItMKCHK/hCrQmqFaljeNXvP1+MpC47wfrnZSBpmoRfGNb6LPgPeH1 Wag38Y+jfQ+ag== Message-ID: <49d1fc83-5805-43ac-893b-c1913e84cdaf@kernel.org> Date: Tue, 16 Dec 2025 17:38:54 +0100 Precedence: bulk X-Mailing-List: mptcp@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 mptcp-next 5/6] mptcp: better mptcp-level RTT estimator Content-Language: en-GB, fr-BE To: Paolo Abeni , Mat Martineau Cc: mptcp@lists.linux.dev References: <6ffc28b03d35f1b5698ff9ed6b22bd3e82fc81be.1763625391.git.pabeni@redhat.com> From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Paolo, Mat, On 20/11/2025 09:39, Paolo Abeni wrote: > The current MPTCP-level RTT estimator has several issues. On high speed > links, the MPTCP-level receive buffer auto-tuning happens with a frequency > well above the TCP-level's one. That in turn can cause excessive/unneeded > receive buffer increase. > > On such links, the initial rtt_us value is considerably higher > than the actual delay, and the current mptcp_rcv_space_adjust() updates > msk->rcvq_space.rtt_us with a period equal to the such field previous > value. If the initial rtt_us is 40ms, its first update will happen after > 40ms, even if the subflows see actual RTT orders of magnitude lower. > > Additionally: > - setting the msk rtt to the maximum among all the subflows RTTs makes DRS > constantly overshooting the rcvbuf size when a subflow has considerable > higher latency than the other(s). > > - during unidirectional bulk transfers with multiple active subflows, the > TCP-level RTT estimator occasionally sees considerably higher value than > the real link delay, i.e. when the packet scheduler reacts to an incoming > ack on given subflow pushing data on a different subflow. > > - currently inactive but still open subflows (i.e. switched to backup mode) > are always considered when computing the msk-level rtt. > > Address the all the issues above with a more accurate RTT estimation > strategy: the MPTCP-level RTT is set to the minimum of all the subflows > actually feeding data into the MPTCP receive buffer, using a small sliding > window. > > While at it, also use EWMA to compute the msk-level scaling_ratio, to that > mptcp can avoid traversing the subflow list is mptcp_rcv_space_adjust(). > > Use some care to avoid updating msk and ssk level fields too often. FYI, it looks like the recent "simult_flows" instabilities are due to this patch, see this ticket for more details: https://github.com/multipath-tcp/mptcp_net-next/issues/607 Good that there is no hurry to fix this, it is only in our tree for the moment. I could revert it, but anyway, it looks like there are other bugs in net, e.g. it is not possible to connect to it at the beginning via vsock. Cheers, Matt -- Sponsored by the NGI0 Core fund.