From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 3A1C4480DCD for ; Wed, 19 Aug 2026 16:06:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787155617; cv=none; b=PcwqjO9bmn+acqBVXTqmxEPBFw8x5kPimHPdI+EhJn0QJ2W5ghFXyH7pvad1SMgRsCcMRL4zx9aZeLwooqyn7lb6I0MNq55NTD29loaBaLDCaF/TAtOpGAqx/QvhMYG3mzIIGcbdwLB94InhY2nik01O2SEWl1cDkoWDjFVPraI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787155617; c=relaxed/simple; bh=PKxzqKmnGi+MXtj0o7ndnz+h/V+Tcld+JysvEIeiCek=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=fZ3waVQnDilI1TnkJ1jDJDP8WUB/8pHScC7zuQms9p1rk2Eey3MIhpXRW9ji1O8nBv3PNFPd1LC92rvF27P/Er/Had+V41DARe3dFuyGAyYhqzPUBYJySrOgKo5I4ZbvkPnI+6BDr7agQOT9vZj2IuBkp+obUm6e3W0VwNJYMZQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SfzlACgM; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SfzlACgM" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 772DD1F000E9; Wed, 19 Aug 2026 16:06:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787155615; bh=FQKuV/0fj7wsBu2AIFN0JhIYGOC+geZxZA6/edM2GMA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=SfzlACgMFlfDhKB9DNpkMfNspxXVJ4Kcuvt6NVDHOSGQsw4NzN3WMLVdwCXP8eefl pJ6WzxrsDUfLkecdC7zCXcSBw2ad9jgLPttiGr7qEhWaBgX+a8mNlpCJ7d+HnYRg6f 5WNj+bzwC72Bh8iggsZYZDH8+NevDvoSSkf356kIuYd3ogkB6+4Qq70y6A8/m10l2J 2HBfOrxvK/1wAmBFqMnyd0dD6J4W4Hul1dbyrvsERHZv8vgBsJ61uOdcR9MtyGrLuP vv3F4tarvS/OpsW7GU314TShRTR3NpxGo6BCHOJDz5Rv4TdkTU0zTKvjGmopvRegJL upX0atjMHvsKg== Message-ID: <2ab65503-c388-43f8-90f2-ef676205043b@kernel.org> Date: Wed, 19 Aug 2026 18:06:51 +0200 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 mptcp-next RFC v4 2/2] mptcp: send MP_FAIL and infinite mapping on the same ACK Content-Language: fr To: Chenguang Zhao , mptcp@lists.linux.dev Cc: Chenguang Zhao References: <20260818091417.2200986-1-chenguang.zhao@linux.dev> <20260818091417.2200986-3-chenguang.zhao@linux.dev> <68791a9e-9310-4368-854e-d379ce0b416c@kernel.org> <691b982a-7825-490c-b516-ac23b601c64a@linux.dev> From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit Hi Chenguang, On 19/08/2026 13:16, Chenguang Zhao wrote: > > 在 2026/8/19 00:19, Matthieu Baerts 写道: >> Hi Chenguang, >> >> On 18/08/2026 11:14, Chenguang Zhao wrote: >>> From: Chenguang Zhao >>> >>> On a single subflow, accepting MP_FAIL must leave MPTCP mode at once >>> (RFC8684 §3.7). Emit the infinite mapping on the MP_FAIL reply ACK, >>> then call mptcp_try_fallback() immediately after. >> I think Sashiko is right about the issue of lacking option space to have >> both the MP_FAIL and the infinite mapping sent together. > Yes, Sashiko is right, I did not consider all the aspects. >> Please also note this from RFC 8684 §3.7.11 [1]: >> >>> This infinite mapping will be a DSS option (Section 3.3) on the first >>> new packet, containing a Data Sequence Mapping that acts retroactively >> The "on the first new packet" seems to suggest the infinite mapping >> shouldn't be attached to the MP_FAIL. >> >> [1] https://datatracker.ietf.org/doc/html/rfc8684#section-3.7-11 >> >> >> Maybe a fallback for the single subflow without csum is enough? Or maybe >> nothing needs to be done? Or maybe this fallback is just an >> optimisation, and can be done in net-next (and both commits can be >> combined)? WDYT? > > Hi Matthieu > > I am getting a bit confused, so let me summarize my current understanding to verify if I got this right. > In the existing kernel code, fallback will be triggered upon receiving MP_FAIL. However, the FALLBACK_DONE > flag is currently set inside mptcp_update_infinite_map(), which only gets invoked when transmitting a data packet. > This creates a timing window between receiving MP_FAIL and sending the first subsequent packet, resulting in this minor bug. > Our goal is to close this timing window: set the FALLBACK_DONE flag immediately after MP_FAIL is received. > Besides, add a __mptcp_check_fallback() check within mptcp_can_accept_new_subflow() to reject MP_JOIN requests ahead of time. > > Is my understanding correct? I will send out a v5 RFC based on the above ideas. I'm not sure that it can work: if there is a fallback, no MPTCP options are supposed to be sent after that point. That's why the fallback should be done after having sent the infinite mapping. I didn't check in the details: *maybe* the fully_established bit could be reset earlier in some cases, but it feels like it can only be done when a fallback can be done, so never had extra subflows or reinjection, and no checksum. But then again, in this case, we are not supposed to receive an MP_FAIL. Then why bother and not simply fallback, and that's it, no? Cheers, Matt -- Sponsored by the NGI0 Core fund.