Winsock - Retransmission Seq incremented - windows

Question

I get the following when communicating with a custom client.
With custom client i mean a housemade PCB with a FPGA which runs the Triple-Speed Ethernet Intel FPGA IP. It does not make a difference if a switch is between the PC and the PCB.
Workflow seen from the Server(Windows PC) where i detect this behaviour with wireshark:
Connect to client (Syn - Syn/Ack - Ack) Winsock2.connect
Sending data > MTU with Winsock2.WSASend (4092 Bytes on a 4088 Bytes MTU)
Packet gets "fragmented" into 2 packets - do not fragment bit is set
A Retransmission happens (because the client answered too slow?)
I am using delphi 10.4 and use the Winsock2 functions. Bevore each send i check with select if fdwrite is set if FD_Isset. Nagle is deactivated.
The "retransmission" does not happen everytime and i could not detect any sort of pattern when they occur. Except most of the time it is when the client needs more than 30ms to send his ACK.
When the "retransmission" happens it is not packet 1 or two whicht is re-send, but packet 1 with an offset of 60 which is the payload of packet 2. The sequence number of packet 1 is incremented by 60 too. Even the data is correct, it's correctly incremented by 60.
When I send 6000 Bytes i get the same behaviour with an incremented seq of 1968 which is correct too.
What is happening here?
Can i detect this with winsock2? Can i set the RTO with winsock? Why is the sequence number incremented and not packet 1, as it is, retransmitted?
Source Code of the send function:
function TZWinTCPSock.SendData (out ErrMsg : TAPILogStruct; SendOffset :
Cardinal = 0) : Boolean;
var
WSABuff : WSABUF;
res : Integer;
IPFlags : Cardinal;
t : Cardinal;
WSAErr : Cardinal;
begin
Result := FALSE;
WSAErr := WSAGetLastError;
try
if not CheckSockValid(ErrMsg) then // checks if fd_write is set
begin
exit(false);
end;
try
WSABuff.len := FMem.SendLength; // 4092 at this time Cardinal
WSABuff.buf := #FMem.SendData[SendOffset]; // 8192 Bytes reserved TArray<Byte>
IPFlags := 0;
res := WSASend(FSocket,#WSABuff,1,FMem.sentBytes,IPFlags,nil,nil);
if Res <> SOCKET_ERROR then
begin
if FMem.SendLength <> FMem.SentBytes then
begin
exit(false);
end
else
begin
Result := TRUE;
if WSAGetLastError <> WSAErr then // unexpected WSA error
begin
exit(FALSE);
end;
end;
end
else
begin
FLastWSAErr := WSAGetLastError;
if FLastWSAErr = WSAECONNRESET then
begin
Disconnect(ErrMsg);
exit(false);
end;
end;
except
on E : Exception do
begin
// Some error handling
end;
end;
finally
end;
end;
Edit1
The packets have the don't fragment Bit set.
I tried to detect this "retransmission" with the windows admin center, but I don't see anything popping up.

Answer 1

Got an answer on Microsoft Q&A.
Looks like it was a tail loss probe problem where the destination host is at fault, because the reply took too long and srtt timed out.