下面是一个简单流量控制程序片断：

#define MAX_SEQ  1
#define MAX_PKT  1024
typedefine enum {frame,arrival,chksum_err,timeout} event_type;
typedefine enum {data,ack,nak} frame_kind;
typedefine struct {unsigned char data[MAX_PKT];} packet;
typedefine struct {
   frame_kind kind;
   unsigned int seq;
   unsigned int ack;
   packet info;
}frame;
void protocol(void)
{
   unsigned int next_frame_to_send;
   unsigned int frame_expected;
   frame s,r;
   packet bufffer;
   event_type event;
   
   next_frame_to_send=0;
   frame_expected=0;
   from_network_layer(&buffer);
   s.info=buffer;
   s.seq=next_frame_to_send;
   s.ack=1-frame_expected;
   to_physical_layer(&s);
   start_timer(s.seq);
   
   while (1){
       wait_for_event(&event);
       if (event==frame_arrival) {
          from_physical_layer(&r);
          if (r.seq==frame_expected) {
             to_network_layer(&r.info)
             inc(frame_expect);
          }
          if (r.ack==next_frame_to_send) {
             from_network_layer(&buffer);
             inc(next_frame_to_send);
          }
       }
       s.info=buffer;
       s.seq=next_frame_to_send;
       s.ack=1-frame_expected;
       to_physical_layer(&s);
       start_timer(s.seq);
   }
}