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 AB3392D877B for ; Thu, 27 Nov 2025 18:13:55 +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=1764267235; cv=none; b=HMgPNf+dxN16QE2TURfOeVFQIwTqMW4MJx4BSl4+xRxe3PPZry7ia0BCVjH++epspJW12d7P3PY0R0xwN2IllSbsIyG4K/r0xQf1enSWc8j33WYKLMRborewZiQjZiTGngfLXG2tzFWavjTTtyZDUB1auvy5ahQRkxIC3SQHjls= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764267235; c=relaxed/simple; bh=W30WrMsV1Ytyt+nKzCFI1NOld66xFtvavZMvGf5Tb70=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=seIs6vUunFRnYx1FndvF7yWmUXXJBusm7t1CtPc3GSEcd60RNYrnVC6qE2fM4Y283tvNCSSeVO/rHD2cVRzXSaYjZFJFDVgC6BGxqP6JI+FlyLOkVFnDvZhShi1k/XQkiroC0Wjzp+Zx3iZyD+YlKIlFuHyKKY8lGWptuyRiMIQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=q00iDqbO; 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="q00iDqbO" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 69AACC4CEF8; Thu, 27 Nov 2025 18:13:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764267235; bh=W30WrMsV1Ytyt+nKzCFI1NOld66xFtvavZMvGf5Tb70=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=q00iDqbOwrHsPIXpfjM+gu5jYuB3farKdj58q88XaYJQ6h4Y+nVY+cO4dsO72KabJ Bd0Od4lnwlIABwFngbupXORiy2klZQCUTEKd8FFF7V0gVCm85SZ/U/oElJLijz7T0o Bu8G9k82Q1ohsdIKnLsb5IKzL1SbbftpRDq+LkaXRcE2JG66yZH2hfP03Bnm6HjNVB 5w72mGLbGVGAuh55gbYJR7oy728po2bMMJbU2T44XEUrvUTTroPr1+Bv0jTfcttqkS tP569N9665ZVTdjXKYfIMenC5/canDRCgPssAsc6jyG+6sR/h3fP20Jy/fSQY9Pq7i suhUYpNGwdqdA== Message-ID: <08ca5b33-8592-4060-9211-ecbef30cda0e@kernel.org> Date: Thu, 27 Nov 2025 19:13:52 +0100 Precedence: bulk X-Mailing-List: mptcp@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Beta Subject: Re: [PATCH v7 mptcp-next 5/6] mptcp: better mptcp-level RTT estimator Content-Language: en-GB, fr-BE To: Paolo Abeni , martineau@kernel.org 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. (...) > diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h > index ee0dbd6dbacf..b392d7855928 100644 > --- a/net/mptcp/protocol.h > +++ b/net/mptcp/protocol.h > @@ -269,6 +269,13 @@ struct mptcp_data_frag { > struct page *page; > }; > > +/* Arbitrary compromise between as low as possible to react timely to subflow > + * close event and as big as possible to avoid being fouled by biased large > + * samples due to peer sending data on a different subflow WRT to the incoming > + * ack. > + */ > +#define MPTCP_RTT_SAMPLES 5 > + > /* MPTCP connection sock */ > struct mptcp_sock { > /* inet_connection_sock must be the first member */ > @@ -340,11 +347,17 @@ struct mptcp_sock { > */ > struct mptcp_pm_data pm; > struct mptcp_sched_ops *sched; > + > + /* Most recent rtt_us observed by in use incoming subflows. */ > + struct { > + u32 samples[MPTCP_RTT_SAMPLES]; > + u32 next_sample; > + } rcv_rtt_est; I'm sorry to react only now, I didn't manage to follow this in details, but I have one question: why not using a smooth RTT [1]? Is it because the goal is to mix data from the active/recently used subflows and only to take the minimum, and not "combining" RTT from different subflows? [1] https://datatracker.ietf.org/doc/rfc6298/ srtt = old * (1-alpha) + new * alpha # alpha is 1/8 in RFC6298 Cheers, Matt -- Sponsored by the NGI0 Core fund.