The data, synchronization, cancel request, and cancel reply packet types are used to construct the protocols for handling MPI point-to-point transfers. A message with length of up to IMPI_Pk_maxlen bytes is categorized as a short message. It fits in a single data packet. Longer messages are split into several data packets. The IMPI_PK_DATASYNC packet type notifies the receiver that the sender is expecting a synchronization ACK for the message. Otherwise, the IMPI_PK_DATA packet type is used.