From nobody Thu Sep 18 18:22:34 2025 X-Original-To: freebsd-net@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 4cSPBd1Jvdz67pk4 for ; Thu, 18 Sep 2025 18:22:49 +0000 (UTC) (envelope-from michael.tuexen@lurchi.franken.de) Received: from drew.franken.de (mail-n.franken.de [193.175.24.27]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client CN "*.franken.de", Issuer "Certum Domain Validation CA SHA2" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 4cSPBc5qBDz4P7N for ; Thu, 18 Sep 2025 18:22:48 +0000 (UTC) (envelope-from michael.tuexen@lurchi.franken.de) Authentication-Results: mx1.freebsd.org; none Received: from smtpclient.apple (unknown [IPv6:2a02:8109:1101:be00:41c6:4036:5ab9:6ac6]) (Authenticated sender: lurchi) by drew.franken.de (Postfix) with ESMTPSA id 1739E721E283A; Thu, 18 Sep 2025 20:22:35 +0200 (CEST) Content-Type: text/plain; charset=utf-8 List-Id: Networking and TCP/IP with FreeBSD List-Archive: https://lists.freebsd.org/archives/freebsd-net List-Help: List-Post: List-Subscribe: List-Unsubscribe: Sender: owner-freebsd-net@FreeBSD.org Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3864.100.1.1.5\)) Subject: Re: Two different places between TCP socket behavior and RFC documents From: Michael Tuexen In-Reply-To: Date: Thu, 18 Sep 2025 20:22:34 +0200 Cc: freebsd-net@freebsd.org Content-Transfer-Encoding: quoted-printable Message-Id: References: <38DCEDDE-7BAB-4A1D-ACB4-6B2E8FCEB6CE@lurchi.franken.de> To: Tilnel X-Mailer: Apple Mail (2.3864.100.1.1.5) X-Spam-Status: No, score=-2.9 required=5.0 tests=ALL_TRUSTED,BAYES_00 autolearn=disabled version=3.4.1 X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on mail-n.franken.de X-Spamd-Bar: ---- X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[] X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-Rspamd-Queue-Id: 4cSPBc5qBDz4P7N > On 18. Sep 2025, at 18:35, Tilnel wrote: >=20 > On Thu, Sep 18, 2025 at 6:25=E2=80=AFPM Michael Tuexen > wrote: >>> 2. Sending RST to segment with old sequence SYN-RECEIVED instead of >>> acknowledgement >>> According to RFC793 page 69: If an incoming segment is not = acceptable, an >>> acknowledgement should be sent in reply. (here `should` is not = capitalized). >>> This should be applied to all states including and after = SYN-RECEIVED. But it's >>> not the case with FreeBSD TCP socket. I found this with manually = constructed TCP >>> segment: >>> A > B: Flags [S], seq 1, win 8192, length 0 >>> B > A: Flags [S.], seq 4054810353, ack 2, win 65535, length 0 >>> A > B: Flags [.], ack 1, win 8192, length 0 >>> B > A: Flags [R], seq 4054810354, win 0, length 0 >> I am not sure which scenario are you considering. Could you provide = SEG.SEQ >> for the this TCP segment? >>> Expected behavior is to send an empty ack: >>> A > B: Flags [S], seq 1, win 8192, length 0 >>> B > A: Flags [S.], seq 3620804602, ack 2, win 65495, length 0 >>> A > B: Flags [.], ack 1, win 8192, length 0 >>> B > A: Flags [.], ack 1, win 65495, length 0 >>> Which is the case with Linux. >=20 > I'd be happy to explain the scenario in more detail. > Consider the following TCP handshake sequence: > 1. Socket A sends a SYN segment: to Socket B, = which is in the > TCP_LISTEN state. > 2. Socket B transitions to TCP_SYN_RECV and responds with > . > 3. Instead of sending the expected = to complete the > three-way handshake, Socket A incorrectly sends = . > According to the RFC, the appropriate response to such a malformed ACK = should be > an empty ACK segment: . After that, = Socket B should > either wait for a valid ACK or retransmit the SYN-ACK if necessary. > However, in FreeBSD=E2=80=99s current implementation, a RST segment is = sent instead: > , which aborts the connection prematurely. > This behavior appears to deviate from the RFC guidance and may lead to > unnecessary connection resets in edge cases. Hi Tilnel, OK, now I understand your scenario. Let me test it and come back to you. Give me a day or two. Best regards Michael > Best regards > Tilnel