[lwip-devel] One question about codes of tcp_input in tcp_in.c

[mud dog]

Hi all:
When reading the tcp codes of lwIP, I made a question.
After calling the tcp_process in tcp_input, we determine
the return status and flags of tcp_process, when err != ERR_ABRT 
and recv_flags' TF_RESET,TF_CLOSED is not set, call the tcp_output(pcb)
to send what? why do this?

    err = tcp_process(pcb);

    if (err != ERR_ABRT) {

      if (recv_flags & TF_RESET) {

       .................
      } else if (recv_flags & TF_CLOSED) {

       ...............
      } else {
          err = ERR_OK;
       ............
          if (err == ERR_OK) {

                 tcp_output(pcb); 
             ~~~~~~~~~~~~why this line? to send what?
          }
     }
  }

thanks