[lwip-users] Spurious retransmission of FIN, ACK and ACK

[Adrian Figueroa]

Hello!

 

I have implemented a simple TCP server on lwIP 1.4.1 running on chibiOS (a realtime OS) with the netconn api on an STM32F746.

 

It goes like this:

 

TCP server is opened and waits for connections:

 

    conn = netconn_new(NETCONN_TCP);

    LWIP_ERROR("http_server: invalid conn", (conn != NULL), chThdExit(MSG_RESET););

 

    netconn_bind(conn, NULL, SERVER_THREAD_PORT);

    netconn_listen(conn);

 

    err_t err;

    while (true) {

      err = netconn_accept(conn, &newconn);

      if (err != ERR_OK)

        continue;

      serve(newconn);

      netconn_delete(newconn);

    }

 

The serve function kicks in when a connection was accepted and receives some data from the client:

 

void serve(struct netconn * conn)

{

  struct netbuf *inbuf;

  uint8_t *buf;

  u16_t buflen;

  err_t err;

 

  /*

   * Read the data from the port, blocking if nothing yet there.

   * Assume that the request is in one netbuf.

   */

  err = netconn_recv(conn, &inbuf);

 

  if (err == ERR_OK) {

    netbuf_data(inbuf, (void **)&buf, &buflen);

    // evaluate the input!

  }

 

  /* Close the connection */

  netconn_close(conn);

 

  /*

   * Delete the buffer (netconn_recv gives us ownership,

   * so we have to make sure to deallocate the buffer)

   */

  netbuf_delete(inbuf);

}

 

My TCP client (it is a Matlab PC) connects, sends some data, and disconnects. The TCP server also closes the connection on its side and then deletes the netconn structure.

 

The problem is that I get spurious ACK and FIN, ACK in Wireshark. I do not get those, when I do not let Matlab close the TCP connection on client side. However, I have to, since I want to open it again. The spurious retransmission does not hurt my system but I wonder why it exists.

 

Is there something that is obviously wrong? Should I NOT close the connection on the TCP server side? Find a capture attached.

 

Thanks in advance,

Adrian

 

dump.pcapng
Description: dump.pcapng