Abstract: A multiprocessor or clustered system with processing elements communicatively interconnected transmits and receives data in the form of message packets. Certain of the message packets are either responding to earlier requests with the data, or are requests that the data of the packet be written at the destination. Each message packet has an initial portion that includes information about the packet, including what the data is carrying (i.e., data in response to an earlier request, or data to be written), the source and ultimate destination of the message packet. This information prevents errant data being written at the destination by determining if the source has “permission” to send such data to the destination. When a message packet carrying data is received, processing of the packet is started in parallel with continued reception of the data it carries by using the information contained in the initial portion to check the permissions.