#define DBRU_SIZE   5
struct dbru_st  // Unpacked DBRU structure (Upstream)
{
    unsigned int onu_id; 	// unique ID of ONU
    unsigned int counter; 	// TCONT ID within  that ONU
    unsigned int dbru; 		// Buffer Occupancy in Bytes, which is being requested
};

// Serialise Byte DBRU into packed character stream
void pack_dbru(struct dbru_st *xdbru, unsigned char res[]){
    res[0] = (xdbru->onu_id >> 4) & 0x3f;
    res[1] = ((xdbru->onu_id & 0x0f) << 4) | (xdbru->counter & 0x0f);
    res[2] = (xdbru->dbru >> 16) & 0xff;
    res[3] = (xdbru->dbru >> 8) & 0xff;
    res[4] = xdbru->dbru & 0xff;
}

struct dbru_st dbru_tx; unsigned char stream[DBRU_SIZE];
dbru_tx.onu_id=ont; dbru_tx.counter=tcont;
dbru_tx.dbru=n;
pack_dbru(&dbru_tx,stream);
sendto(sockfd, stream, DBRU_SIZE, 0, (struct sockaddr*)&serv_addr, slen);