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ModbusTCP is a connection oriented TCP protocol. The Acknowledgement TCP segment value includes the last received TCP segment value plus the number of bytes in the received ModbusTCP message. This way, the Master knows that the message was received intact.
If a Master's current TCP segment number is 100, and it sends a 14 byte ModbusTCP request, it expects to see a TCP Acknowledgement from that slave to be 114. If it is any other value, the slave did not receive the proper message. This should trigger an retransmission from the Master for a duplicate request.