public PacketType(PacketIP ip, PacketIcmp icmp)
 {
     _ip   = ip;
     _icmp = icmp;
 }
 public PacketType(PacketIP ip, PacketTcp tcp)
 {
     _ip  = ip;
     _tcp = tcp;
 }
 public PacketType(PacketIP ip, PacketUdp udp)
 {
     _ip  = ip;
     _udp = udp;
 }
 public PacketType(PacketIP ip)
 {
     _ip = ip;
 }