Normally just add an ID to the protocol. Each message starts at 1 and counts up. What layer 2 communication are you using? Ethernet, RS232, CAN, USB? TCP/IP does this for you. If you use UDP/IP then it's up to you. A typical protocol will have a header consisting of the senders ID, amount of bytes in the packet,Time stamp,receivers ID, packet counter, ect.. Followed by bytes of data and ending with a CRC checksum to guarantee that the data was not corrupted during transmission. Steve