1. A method of transferring image data between an image data source and image data sink, comprising the steps of: receiving a request at the image data source to push image data to the image data sink; writing transfer control information to a negotiation field by the image data source; modifying the negotiation field by the sink to change an item of transfer control information; reading the modified negotiation field by the source; and pushing the image data from the source to the sink in accordance with the transfer control information of the modified negotiation field.