In Modbus, data transactions are traditionally stateless, making them highly resistant to disruption from noise and yet requiring minimal recovery information to be maintained at either end.
Programming operations, on the other hand, expect a connection-oriented approach. This was achieved on the simpler variants by an exclusive ‘login’ token, and on the MODBUS Plus variant by explicit ‘Program Path’ capabilities which maintained a duplex association until explicitly broken down.
Modbus TCP handles both situations. A connection is easily recognized at the protocol level, and a single connection may carry multiple independent transactions.
In addition, TCP allows a very large number of concurrent connections, so in most cases it is the choice of the initiator whether to reconnect as required or re-use a long-lived connection.