establishing, on a message sending device, a sender message model, the sender message model specifying a message format for interpreting application messages, the sender message model comprising one or more sender field specifications, each sender field specification specifying a message field for storing data in an application message, each sender field specification comprising sender field characteristics;

calculating, by the message sending device, a sender version number for the sender message model in dependence upon the sender field characteristics of one or more of the sender field specifications of the sender message model;

creating, by the message sending device, an application message according to the sender message model; and

transmitting, by the message sending device to a message receiving device, the application message and the sender version number for the sender message model.