Re: Two different places between TCP socket behavior and RFC documents

On Fri, Sep 19, 2025 at 5:31 AM Vadim Goncharov <vadimnuclight@gmail.com> wrote:
>
> On Fri, 19 Sep 2025 00:35:18 +0800
> Tilnel <deng1991816@gmail.com> 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: <CTL=SYN><SEQ=x> to Socket B, which is in
> > the TCP_LISTEN state.
> > 2. Socket B transitions to TCP_SYN_RECV and responds with
> >    <CTL=SYN,ACK><SEQ=y><ACK=x+1>.
> > 3. Instead of sending the expected <CTL=ACK><SEQ=x+1><ACK=y+1> to complete
> > the three-way handshake, Socket A incorrectly sends
> > <CTL=ACK><SEQ=x><ACK=y+1>. According to the RFC, the appropriate response to
> > such a malformed ACK should be an empty ACK segment:
> > <CTL=ACK><SEQ=y+1><ACK=x+1>. After that, Socket B should either wait for a
> > valid ACK or retransmit the SYN-ACK if necessary. However, in FreeBSD’s
> > current implementation, a RST segment is sent instead: <CTL=RST><SEQ=y+1>,
> > which aborts the connection prematurely. This behavior appears to deviate
> > 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 above
> were about ~200 M different sequence numbers, so that RST could be ignored.
>

Hi.
I just checked it with ACK segment carrying different SEQ, including ~2G out of
the window.
1. expect seq=2, actual seq=0x80000001
2. expect seq=2, actual seq=0x80000002
3. expect seq=2, actual seq=0x80000003
4. expect seq=2, actual seq=0x90000000
5. expect seq=2000001 (0x1e8481), actual seq=1
6. expect seq=1, actual seq=2000001 (0x1e8481)
All of them, under FreeBSD, give a RST reply. Here is the tcpdump of the first
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