def _build_pkt(self, fields, ops):
        pkt_out = packet.Packet()
        pkt_ipv4 = pkt_out.get_protocol(ipv4.ipv4)
        pkt_icmp = pkt_out.get_protocol(icmp.icmp)

        def addIPv4(pkt_out, fields):
            pkt_out.add_protocol(ipv4.ipv4(dst=fields['dstip'],
                                version = 4,
                                header_length = 5,
                                tos = 0,
                                total_length = 0,
                                identification = fields['id'],
                                flags=0x02,
                                ttl = 63,
                                proto = fields['proto'],
                                csum = 0,
                                option = None,
                                src=fields['srcip']))
            return pkt_out

        def addARP(pkt_out,fields):
            pkt_out.add_protocol(arp.arp(opcode=arp.ARP_REPLY,
                                 src_mac=fields['srcmac'],
                                 src_ip=fields['srcip'],
                                 dst_mac=fields['dstmac'],
                                 dst_ip=fields['dstip']))
            return pkt_out

        pkt_out.add_protocol(ethernet.ethernet(ethertype=fields['ethtype'],
                                               dst=fields['dstmac'],
                                               src=fields['srcmac']))
        # Add if ARP
        if 'arp' in fields['ptype']:
            pkt_out.add_protocol(arp.arp(opcode=arp.ARP_REPLY,
                                 src_mac=fields['srcmac'],
                                 src_ip=fields['srcip'],
                                 dst_mac=fields['dstmac'],
                                 dst_ip=fields['dstip']))
        # Add if IPv4
        if 'ipv4' in fields['ptype']:
            pkt_out = addIPv4(pkt_out,fields)

        # Add if ICMP
        if 'icmp' in fields['ptype']:
            pkt_out = addIPv4(pkt_out,fields)

            pkt_out.add_protocol(icmp.icmp(type_=icmp.ICMP_ECHO_REPLY,
                                 code=icmp.ICMP_ECHO_REPLY_CODE,
                                 csum=0,
                                 data=None))
        # Add if UDP
        if 'udp' in fields['ptype']:
            #pkt_out = addARP(pkt_out,fields)
            pkt_out = addIPv4(pkt_out,fields)
            pkt_out.add_protocol(udp.udp(dst_port=fields['dstport'],
                                csum = 0,
                                total_length = 0,
                                src_port=fields['srcport']))

        # Add if TCP
        if 'tcp' in fields['ptype']:
            pkt_out = addIPv4(pkt_out,fields)
            pkt_out.add_protocol(tcp.tcp(dst_port=fields['dstport'],
				bits=fields['bits'],option=fields['opt'],
                                src_port=fields['srcport']))

        #Add covert channel information
        if fields['com'] != None:
            pkt_out.add_protocol(fields['com'])