/* Protocol 4 (sliding window) is bidirectional. */#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;		/* frame expected next */  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);	/* frame_arrival, cksum_err, or 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 seq number expected next */                }                if (r.ack == next_frame_to_send) {  /* handle outbound frame stream. */                        stop_timer(r.ack);	    /* turn the timer off */                        from_network_layer(&buffer);/* fetch new pkt 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 */  }}