enum opcodes {RRQ=1,WRQ=2,DATA=3,ACK=4,ERR=5};
int build_request(char *msg, unsigned short opcode, char *filename, char *mode);
int build_ack(char *msg, unsigned short block);
unsigned short get_shortval( char *p );
int get_opcode(char *msg);
int get_blocknum(char *msg);
void print_msg( char *msg, int len );
int wait_for_response(int sk,struct sockaddr_in *s,char *buff, int max);