When UNICAST3 or NAKACK2 are absent, messages can get lost in the following scenario:

• A unicast message M is sent to P
• The physical address for P is not in the logical address cache
• An async lookup is performed, but the message is dropped

This would not be an issue with retransmission present, as M will be resent, and the physical address for P should be present on the next retransmission. But, without retransmission, M is lost.

[1] https://issues.redhat.com/browse/JGRP-2681