When sending a unicast message, time to time I'm getting an error code: 0xd4 ( Unicast frame does not have a route available but it is buffered for automatic resend.),
However I do not see any resend done by the ZigBee stack, as if I resend the message it usaly go through - and in between I got a NEW_ROUTE_DISCOVER event)
So it looks like 0xd4 express the fact that something has been implemented on the Zigbee stack level to avoid doing what is expected when receiving 0xd1 as specify in the documentation:
Note: If a message is unicast to a destination for which a route has not already been established, the message will not be sent and a route discovery will be performed instead. If this is the case, the unicast function will return ZPS_NWK_ENUM_ROUTE_ERROR. The application must then wait for the stack event ZPS_EVENT_NWK_ROUTE_DISCOVERY_CONFIRM (success or failure) before attempting to re-send the message by calling the same unicast function again.
Reference: JN-UG-3113 Revision 1.5