Issue 269 MQTT-SN Feature: Topic Registration allows the protocol to continue where an invalid topic alias has been used.
The network is required to provided an in order lossless connection so the only possible causes of an invalid alias are an incorrect implementation or the sending of publications by the client before the connack has been received informing it of the servers maximum topic alias value.
Continuing the message flow risks data loss. In all cases where an invalid topic alias is received the receiver should disconnect with a 130 0x82 Protocol error disconnect code.