Short messages are sent eagerly, relying on the packet protocol ACKs for flow control. The pk_len and pk_msglen fields have the same value. If the IMPI_PK_DATASYNC packet type is used, the destination process sends a synchronization ACK packet back to the source after it receives the message.
The pk_srqid field in the ACK packet must be set to the value of the pk_srqid field in the message packet. The sender must store the send request identifier in the outgoing packet. It receives it back in the ACK packet. This mechanism is used by the sender to locate the request that matches the ACK packet.
Short messages generated by MPI_SSEND and MPI_ISSEND are mapped onto the short-message protocol with the IMPI_PK_DATASYNC packet type. All other short messages are mapped onto this protocol with the IMPI_PK_DATA packet type.