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

> On 18. Sep 2025, at 20:22, Michael Tuexen <Michael.Tuexen@lurchi.franken.de> wrote:
> 
>> On 18. Sep 2025, at 18:35, Tilnel <deng1991816@gmail.com> wrote:
>> 
>> On Thu, Sep 18, 2025 at 6:25 PM Michael Tuexen
>> <michael.tuexen@lurchi.franken.de> 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.
>> 
>> 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.
> Hi Tilnel,
> 
> OK, now I understand your scenario. Let me test it and come back to you.
> Give me a day or two.
I can confirm the behavior you observed. It is a bug, I will fix it.

Thanks for reporting,

Best regards
Michael
> 
> Best regards
> Michael
>> Best regards
>> Tilnel
> 
>