#define?MAX_PKT?1024
#define?MAX_SEQ?1
typedef?unsigned?int?seq_nr;
typedef?struct{
???unsigned?char?data[MAX_PKT];
}packet;
typedef?enum{
???dateacknak
}frame_kind;
?typedef?struct{
?????frame_kind?kind;
?????seq_nr?seq;
?????seq_nr?ack;
?????packet?info;
?????int?nak;
?}frame;
typedef?enum{
?????frame_arrivalcksum_errtimeout
?}event_type;
?event_type?wait_for_event（）{
?????frame?rs;
	?if（s.nak==0&&s.seq==0&&s.ack==1）{
	?	return?frame_arrival;
	?}
?}
?void?protocol4（void）{
?	seq_nr?next_frame_to_send;
?	seq_nr?frame_exp