/* Protocol 4 (sliding window) is bidirectional and is more robust than protocol 3. */

#define MAX_SEQ 1	/* must be 1 for protocol 4 */
typedef enum {frame_arrival, cksum_err, timeout} event_type;
#include "protocol.h"

void protocol4 (void)
{
  seq_nr next_frame_to_send;	/* 0 or 1 only */
  seq_nr frame_expected;	/* 0 or 1 only */
  frame r, s;	/* scratch variables */
  packet buffer;	/* current packet being sent */
  event_type event;

  next_frame_to_send = 0;	/* next frame on the outbound stream */
  frame_expected = 0;	/* number of frame arriving frame expected */
  from_network_layer(&buffer);	/* fetch a packet from the network layer */
  s.info = buffer;	/* prepare to send the initial frame */
  s.seq = next_frame_to_send;	/* insert sequence number into frame */
  s.ack = 1 - frame_expected;	/* piggybacked ack */
  to_physical_layer(&s);	/* transmit the frame */
  start_timer(s.seq);	/* start the timer running */

  while (true) {
        wait_for_event(&event);	/* could be: frame_arrival, cksum_err, timeout */
        if (event == frame_arrival) { /* a frame has arrived undamaged. */
                from_physical_layer(&r);	/* go get it */

                if (r.seq == frame_expected) {
                        /* Handle inbound frame stream. */
                        to_network_layer(&r.info);	/* pass packet to network layer */
                        inc(frame_expected);	/* invert sequence number expected next */
                }

                if (r.ack == next_frame_to_send) { /* handle outbound frame stream. */
                        from_network_layer(&buffer);	/* fetch new packet from network layer */
                        inc(next_frame_to_send);	/* invert sender's sequence number */
                }
        }
             
        s.info = buffer;	/* construct outbound frame */
        s.seq = next_frame_to_send;	/* insert sequence number into it */
        s.ack = 1 - frame_expected;	/* seq number of last received frame */
        to_physical_layer(&s);	/* transmit a frame */
        start_timer(s.seq);	/* start the timer running */
  }
}