Minor This issue was originally reported in MQTT-417, but doesn't seem to be addressed in WD14.
Lines 1860-1862
"If a Topic Alias mapping has been set at the receiver, a sender can send a PUBLISH packet that contains that Topic Alias and a zero length Topic Name. The receiver then treats the incoming PUBLISH as if it had contained the Topic Name of the Topic Alias."
How does the sender knows whether a topic alias mapping has been set at the receiver if the message is QoS 0?