The UNICAST protocol is used for unicast messages. It uses ACK. It is configured as a sub-element under the JGroups Config element. If the JGroups stack is configured with TCP transport protocol, UNICAST is not necessary because TCP itself guarantees FIFO delivery of unicast messages. Here is an example configuration for the UNICAST protocol.
<UNICAST timeout="100,200,400,800" down_thread="false" up_thread="false"/>
There is only one configurable attribute in the UNICAST element.
timeout specifies the retransmission timeout (in milliseconds). For instance, if the timeout is "100,200,400,800", the sender resends the message if it hasn't received an ACK after 100 ms the first time, and the second time it waits for 200 ms before resending, and so on.