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Mon, 22 Sep 2025 19:25:18 -0700 (PDT) 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 References: <38DCEDDE-7BAB-4A1D-ACB4-6B2E8FCEB6CE@lurchi.franken.de> <20250919003104.5857ecd7@nuclight.lan> In-Reply-To: <20250919003104.5857ecd7@nuclight.lan> From: Tilnel Date: Tue, 23 Sep 2025 10:25:07 +0800 X-Gm-Features: AS18NWCqL2t1rEfL2ESk_5FQfI2cHSnOVm69jAQQnUNNXrPyw_bebX72FfeHds0 Message-ID: Subject: Re: Two different places between TCP socket behavior and RFC documents To: Vadim Goncharov Cc: Michael Tuexen , freebsd-net@freebsd.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Spamd-Bar: --- X-Spamd-Result: default: False [-4.00 / 15.00]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_MEDIUM(-1.00)[-1.000]; NEURAL_HAM_SHORT(-1.00)[-1.000]; DMARC_POLICY_ALLOW(-0.50)[gmail.com,none]; R_SPF_ALLOW(-0.20)[+ip6:2a00:1450:4000::/36]; R_DKIM_ALLOW(-0.20)[gmail.com:s=20230601]; MIME_GOOD(-0.10)[text/plain]; FREEMAIL_TO(0.00)[gmail.com]; RCVD_TLS_LAST(0.00)[]; FROM_HAS_DN(0.00)[]; ARC_NA(0.00)[]; TO_DN_SOME(0.00)[]; MIME_TRACE(0.00)[0:+]; DWL_DNSWL_NONE(0.00)[gmail.com:dkim]; FREEMAIL_FROM(0.00)[gmail.com]; DKIM_TRACE(0.00)[gmail.com:+]; MISSING_XM_UA(0.00)[]; RCVD_IN_DNSWL_NONE(0.00)[2a00:1450:4864:20::32b:from]; PREVIOUSLY_DELIVERED(0.00)[freebsd-net@freebsd.org]; TO_MATCH_ENVRCPT_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; RCPT_COUNT_THREE(0.00)[3]; MID_RHS_MATCH_FROMTLD(0.00)[]; ASN(0.00)[asn:15169, ipnet:2a00:1450::/32, country:US]; MLMMJ_DEST(0.00)[freebsd-net@freebsd.org]; RCVD_COUNT_ONE(0.00)[1]; FREEMAIL_ENVFROM(0.00)[gmail.com] X-Rspamd-Queue-Id: 4cW3jg02bbz47k2 On Fri, Sep 19, 2025 at 5:31=E2=80=AFAM Vadim Goncharov wrote: > > On Fri, 19 Sep 2025 00:35:18 +0800 > Tilnel wrote: > > > 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, whic= h 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 w= ait 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 devia= te > > from the RFC guidance and may lead to unnecessary connection resets in = edge > > cases. Best regards > > Tilnel > > Did you check it with about ~2 G out of window? That is, your examples ab= ove > were about ~200 M different sequence numbers, so that RST could be ignore= d. > Hi. I just checked it with ACK segment carrying different SEQ, including ~2G ou= t of the window. 1. expect seq=3D2, actual seq=3D0x80000001 2. expect seq=3D2, actual seq=3D0x80000002 3. expect seq=3D2, actual seq=3D0x80000003 4. expect seq=3D2, actual seq=3D0x90000000 5. expect seq=3D2000001 (0x1e8481), actual seq=3D1 6. expect seq=3D1, actual seq=3D2000001 (0x1e8481) All of them, under FreeBSD, give a RST reply. Here is the tcpdump of the fi= rst case. A > B: Flags [S], seq 1, win 8192, length 0 0x0000: 4500 0028 0000 4000 4006 0000 7f00 0001 0x0010: 7f00 0001 22b9 22b8 0000 0001 0000 0000 0x0020: 5002 2000 4c6e 0000 B > A: Flags [S.], seq 1643153760, ack 2, win 65535, options [mss 16344], length 0 0x0000: 4500 002c 0000 4000 4006 0000 7f00 0001 0x0010: 7f00 0001 22b8 22b9 61f0 8960 0000 0002 0x0020: 6012 ffff fe20 0000 0204 3fd8 A > B: Flags [.], seq 2147483648, ack 1, win 8192, length 0 0x0000: 4500 0028 0000 4000 4006 0000 7f00 0001 0x0010: 7f00 0001 22b9 22b8 8000 0001 61f0 8961 0x0020: 5010 2000 e10d 0000 B > A: Flags [R], seq 1643153761, win 0, length 0 0x0000: 4500 0028 0000 4000 4006 0000 7f00 0001 0x0010: 7f00 0001 22b8 22b9 61f0 8961 0000 0000 0x0020: 5004 0000 fe1c 0000 Best regards Tilnel > -- > WBR, @nuclight