Credit-based flow control mechanisms, such as those used in the Transmission Control Protocol, allow flow control and error control procedures of data transfer protocols to operate independently. We create a Coloured Petri Net model of a class of data transfer protocols, which uses retransmissions and acknowledgements for error control and "credits" for flow control. This model is characterized by 3 parameters: the maximum sequence number, the maximum number of retransmissions and the maximum receiver buffer size. From the analysis results, we derive expressions in these parameters for the channel bounds and the number of terminal states. These expressions are verified for a range of values of the parameters.