If UNICAST receives a high number of messages and sends a high number of messages concurrently, there will be a lot of retransmissions. Reason is contention on 'connections', with up- and down messages accessing it concurrently. Both up- and down- messages impede each other and thus messages are not received in time, causing retransmissions (ACKs to be resent).

SOLUTION:
#1 Reduce contention and turn 'connections' from HashMap into ConcurrentHashMap
#2 Break 'connections' into a send-table and receive-table: sends and acks access the send-table, receives the receive-table