fig-3-12.c

计算机网络第四版的实现源代码

/* Protocol 3 (par) allows unidirectional data flow over an unreliable channel. */
#define MAX_SEQ 1		/* must be 1 for protocol 3 */
typedef enum  {frame_arrival, cksum_err, timeout} event_type;
#include "protocol.h"
void sender3(void)
{
  seq_nr next_frame_to_send;			/* seq number of next outgoing frame */
  frame s;					/* scratch variable */
  packet buffer;				/* buffer for an outbound packet */
  event_type event;
  next_frame_to_send = 0;			/* initialize outbound sequence numbers */
  from_network_layer(&buffer);			/* fetch first packet */
  while (true) {
        s.info = buffer;			/* construct a frame for transmission */
        s.seq = next_frame_to_send;		/* insert sequence number in frame */
        to_physical_layer(&s);			/* send it on its way */
        start_timer(s.seq);			/* if answer takes too long, time out */
        wait_for_event(&event);			/* frame_arrival, cksum_err, timeout */
        if (event == frame_arrival) {
                from_physical_layer(&s);	     /* get the acknowledgement */
                if (s.ack == next_frame_to_send) {
                        stop_timer(s.ack);	     /* turn the timer off */
                        from_network_layer(&buffer); /* get the next one to send */
                        inc(next_frame_to_send);     /* invert next_frame_to_send */
                }
        }
  }
}

void receiver3(void)
{
  seq_nr frame_expected;
  frame r, s;
  event_type event;
  frame_expected = 0;
  while (true) {
        wait_for_event(&event);			/* possibilities: frame_arrival, cksum_err */
        if (event == frame_arrival) {		/* a valid frame has arrived. */
                from_physical_layer(&r);	/* go get the newly arrived frame */
                if (r.seq == frame_expected) {	/* this is what we have been waiting for. */
                        to_network_layer(&r.info); /* pass the data to the network layer */
                        inc(frame_expected);	/* next time expect the other sequence nr */
                }
                s.ack = 1 - frame_expected;	/* tell which frame is being acked */
                to_physical_layer(&s);		/* send acknowledgement */
        }
  }
}