Supporting the MPI_PROBE and MPI_IPROBE functions does not require special packet transfers. The protocol is purely local between the process and the agent. IMPI defines the conditions under which a message is considered to be available for the purpose of probing.
Packet transfers are considered atomic operations, independent of the medium's transfer mechanism. A message is considered available to the destination process after its first packet (its only packet for short messages) has been completely read by the agent, including the packet's user data segment. The total length of the message is available in the packet's pk_msglen field. Thus the status information needed by the probe calls is available.