A multiprocessor computer system comprises a sending processor node and a receiving processor node. The sending processor node is operable to send packets comprising part of a message to a receiver, and to send a message complete packet after all packets in the message are sent. The message complete packet includes an indicator of the number of packets in the message, and the message is recognized as complete in the receiver once the number of packets indicated in the message complete packet have been received for the message. The sender tracks acknowledgment from the receiver of receipt of the sent packets; and notifies the receiver when it has received all packets comprising a part of the message.