/* 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) {                        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);	/* only the ack field is use */        }  }}