Date: Thu, 18 Sep 2025 12:24:58 +0200 From: Michael Tuexen <michael.tuexen@lurchi.franken.de> To: Tilnel <deng1991816@gmail.com> Cc: freebsd-net@freebsd.org Subject: Re: Two different places between TCP socket behavior and RFC documents Message-ID: <38DCEDDE-7BAB-4A1D-ACB4-6B2E8FCEB6CE@lurchi.franken.de> In-Reply-To: <CADvKEf-vkJ-eKpwe_-x-z0pUTyx2sZRE3v7%2BZRV7cP_pq7h__w@mail.gmail.com> References: <CADvKEf-vkJ-eKpwe_-x-z0pUTyx2sZRE3v7%2BZRV7cP_pq7h__w@mail.gmail.com>
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> On 18. Sep 2025, at 10:50, Tilnel <deng1991816@gmail.com> wrote: >=20 > Hi, >=20 > I found two behaviors different with RFC recommendations in FreeBSD = 14.3 TCP > socket. >=20 > 1. Failure to RST on close with data pending > According to RFC2525 section 2.17, RST should be sent when close() on = socket > with pending data to read in receive buffer. > According to RFC1122: A host MAY implement a "half-duplex" TCP close = sequence, > ... cannot continue to read data ... If such a host issues a = CLOSE > call while received data is still pending in TCP, or if new = data is > received after CLOSE is called, its TCP SHOULD send a RST to = show that > data was lost. I agree that FreeBSD is inconsistent here. It reacts different if data = is received after the reading end is closed or before. I don't know why it is that way, but it is there for a long time... I plan to fix this. > It's not the case with FreeBSD TCP socket. Here is TCPDUMP output, > showing close() > on socket with pending data emit FIN instead of RST. > A > B: Flags [S], seq 2636678338, win 65535, length 0 > B > A: Flags [S.], seq 1969223298, ack 2636678339, win 65535, length = 0 > A > B: Flags [.], ack 1, win 1277, length 0 > A > B: Flags [P.], seq 1:6, ack 1, win 1277, length 5 > B > A: Flags [.], ack 6, win 1277, length 0 > B > A: Flags [F.], seq 1, ack 6, win 1277, length 0 > A > B: Flags [.], ack 2, win 1277, length 0 > All close()/shutdown(SHUT_RDWR)/shutdown(SHUT_RD) and both SO_LINGER = on or off > give the same trace. While on Linux the same execution gives this: > A > B: Flags [S], seq 2879877684, win 65495, length 0 > B > A: Flags [S.], seq 1538598692, ack 2879877685, win 65483, length = 0 > A > B: Flags [.], ack 1, win 512, length 0 > A > B: Flags [P.], seq 1:6, ack 1, win 512, length 5 > B > A: Flags [.], ack 6, win 512, length 0 > B > A: Flags [R.], seq 1, ack 6, win 512, length 0 >=20 > 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 > Does anyone know why these two violations exist? Did FreeBSD choose = not to > comply with the RFC for a specific reason, or is it simply an = implementation > error? I overall intention is to be RFC compliant... Best regards Michael >=20 > Thanks. >=20
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