LwIP raw api下使用tcp keep alive

// The following code is implemented in tcp_connected call back

... xxx_connected(..., struct tcp_pcb* tpcb, ...)

{

    tpcb->so_options |= SOF_KEEPALIVE;

    tpcb->keep_idle = 5000;      // ms

    tpcb->keep_intvl = 5000;     // ms

    tpcb->keep_cnt = 2;          // report error after 2 KA without response

}