Interface method, system, and program product for facilitating layering of a data communications protocol over an active message layer protocol

A protocol interface is provided for an active message protocol of a computing environment and a client process employing the active message protocol. The protocol interface includes an interface to a header handler function associated with the client process. The interface to the header handler function has parameters to be passed by and a parameter to be returned to the active message protocol when processing a message received through the active message protocol. The parameters to be passed include current message state information and current message type information for the received message. These parameters facilitate message-specific decisions by the header handler function about processing data of the message by the active message protocol. The parameter to be returned to the active message protocol instructs the active message protocol how to process the received message other than just where to store the message.