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 433FD3A0EB3 for ; Thu, 13 Aug 2026 17:42:50 +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=1786642971; cv=none; b=iLyojJ9ei7GcTk0uy5EgDp9ZDrl8rymYAFn9rd01y4WOBFo/PSz0Z0ptt58V5u0Z5WTrgVTwesqjT0PHIun1Sn1T0SMXBw4nqKfTjuB5G4kD5TKJY9/rPB88BQK/IHDpBP/GPaj/U2aLfl/gySJBbfSD1BBmhoFFtiQChNYp1b4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786642971; c=relaxed/simple; bh=CHcilhNjuaMcjdKomSjRcEIvJJ3qwhcxwr5TsbHPpEY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rcvgIImCfr3uX6Cv9X7TwDOVSZQpaub8aH42Gp6a8yPG8P1zC8J+0DV/va2bZE71YE53grNYqm1ShKD4jyUd6m8WxvPHyuGpojQTRNKbQiXvP8z1ai/SaU4h8z2Y42VnLbtPYY4AZ2R514hcbcUYkZ9Uzi6vUQZk8XcEFCme0gY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JioHiYsc; 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="JioHiYsc" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 227821F000E9; Thu, 13 Aug 2026 17:42:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786642970; bh=A/oTGEjWDQTkgvIkvJJj2W5bv5b/WCvIA/hHPCkgv10=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=JioHiYscDfqzJGH3olSWB0cn4UL/VXDdd9ldN1Xq7XPcZ5yRsaThPViD0/eKhJDMo 978EqqkijsB/Mt8PuHEBEZFwRCiB6mbMO0VUshfUEb1WozW5c3uOjQ7O3JjebaU+Kj tUSY1IimM8HhYuFHEg/UosX9r5G5tC/mxez/0iQDsCfR/4x/J5/jr5o3KOref3wye9 0pnUUQdfv9NG1+W1gKX7YINxghR8E/U3c17g4WwWyxPlyhJ8DWUst8N6YnF+/SkcnL GFtSLlfWCtmOTuFe5b7Lv487nfzG7q78WTV68STPcSXkezZlbWSbddpuvgpNZgnClr Z6iiK4IN1m0GQ== Message-ID: Date: Thu, 13 Aug 2026 19:42:47 +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 export v3 2/4] mptcp: reject joins after fallback in mptcp_is_fully_established Content-Language: fr To: gang.yan@linux.dev, Paolo Abeni , Chenguang Zhao , mptcp@lists.linux.dev Cc: Chenguang Zhao References: <20260812054653.43799-1-chenguang.zhao@linux.dev> <20260812054653.43799-3-chenguang.zhao@linux.dev> <1d7c56b9-00c1-4550-a403-c3d704259267@redhat.com> <41078a8c-00c7-4cb1-8e57-237b4ea6c2f0@kernel.org> <564cb027be5e7972fc8f6f976707628ee32f38aa@linux.dev> From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit Hi Gang, On 13/08/2026 11:32, gang.yan@linux.dev wrote: > August 12, 2026 at 11:32 PM, "Matthieu Baerts" wrote: > > >> >> Note: Chenguang is looking at a bug where, when an MP_FAIL is received >> before being in fully_established, the kernel accept going to fully >> established, and creating new subflows, then reject them once done. >> >> Same when receiving a 4th packet without MPTCP options. >> >> I think the MP_FAIL should do a fallback if possible, but it is probably >> also required to check the fallback status or something similar when >> receiving an MPJ. > > Hi, Matt > > Sorry, I'm not very familiar with the details of this patch. > > No offense intended, I just have a question I'd like to ask based on my > understanding and the RFC documentation: Should MP_FAIL trigger a fallback? > > For the multiple‑subflow case, if one subflow encounters an MP_FAIL, it should > be closed with a RST, and the data will be retransmitted over the other healthy > subflows. This should not cause the entire MPTCP connection to fallback to regular > TCP, right? > RFC8684's contents: > ''' > The receiver of this option MUST discard all data following the data sequence number > specified. Failed data MUST NOT be DATA_ACKed and so will be retransmitted on other subflows. > ''' Correct, in this case it shouldn't fallback. > For the single‑subflow case, according to the RFC, MP_FAIL should subsequently > lead to a fallback under the infinite mapping (INFINITEMAP) situation, shouldn't it? > RFC 8684's contents: > ''' > A special case is when there is a single subflow and it fails with a checksum error. > If it is known that all unacknowledged data in flight is contiguous (which will usually > be the case with a single subflow), an infinite mapping can be applied to the subflow > without the need to close it first, essentially turning off all further MPTCP signaling. > ''' Correct. Please note that when I said "the MP_FAIL should [cause] a fallback if possible", I'm referring to what is done in mptcp_try_fallback() → a fallback is only possible in the single subflow situation, without reordering at the MPTCP level. In addition, in the case we are trying to fix here, the MP_FAIL is received before switching to fully established: the RFC doesn't explain this particular case, but it sounds like there is no need to send the infinite mapping, and a fallback can be done directly *I think*. The thing is this situation shouldn't happen: we are not supposed to receive an MP_FAIL at that stage, no data has been exchanged at the MPTCP level, so let's do the minimal -- i.e. just a fallback -- to avoid issues and reduce the complexity, no? Cheers, Matt -- Sponsored by the NGI0 Core fund.