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[176.103.220.4]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48af502f491sm3154451f8f.12.2026.09.29.04.31.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 29 Sep 2026 04:31:53 -0700 (PDT) From: Stefano Brivio To: Aris Konstantoulas Subject: Re: [PATCH] tcp: Don't fast re-transmit if only our FIN is outstanding Message-ID: <20260929133152.680ca2ab@elisabeth> In-Reply-To: <20260928103120.233586-1-arist.kon@gmail.com> References: <20260928103120.233586-1-arist.kon@gmail.com> Organization: Red Hat X-Mailer: Claws Mail 4.2.0 (GTK 3.24.49; x86_64-pc-linux-gnu) MIME-Version: 1.0 Date: Tue, 29 Sep 2026 13:31:53 +0200 (CEST) X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: FiLmK7_A5XX5PO49yyAT3WWT5kyn2iCFDCHWUYxzs8o_1790681515 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-ID-Hash: RI763QWKLVNFA5MOKYK57EBVI35R6XB7 X-Message-ID-Hash: RI763QWKLVNFA5MOKYK57EBVI35R6XB7 X-MailFrom: sbrivio@redhat.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top, David Gibson , Aris Konstantoulas X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: On Mon, 28 Sep 2026 13:31:20 +0300 Aris Konstantoulas wrote: > [...] > > Don't consider a duplicate ACK as a fast re-transmit trigger if the > only outstanding sequence number is the FIN, and leave it to the timer. > Fast re-transmit of data is unaffected, with or without a FIN queued > after it. Thanks for the investigation and for the patch, I'll review it in a bit! Just a couple of quick (hopefully helpful) remarks for now: > [...] > > - Open question: I haven't established how a real flow first reaches > the state where the peer ACKs exactly up to, but not past, our FIN. If I recall correctly, a while ago, I managed to make this happen with a Linux kernel as peer, under memory pressure, with the receiver updating the remaining buffer estimation right after processing a large segment (the last data segment) that's assembled from smaller segments. You would need a slow userspace receiver as well (maybe reading exactly one byte after accepting the connection and then nothing else... something like that). In any case, I don't think it really matters to find a "natural" reproducer of this case, I wouldn't go crazy with it, your patch looks obviously correct to me anyway. > [...] This patch breaks the loop in either > case, but not that possible entry path. I'm happy to look into it > further, possibly as part of bug 125. Wow, yes, if you could also tackle bug #125, or even a part of it, that would be very warmly appreciated! -- Stefano