I almost done the coding of MQTT-SN gateway and start debugging now on.
But, I have no idea how G/W handles WILLTOPICUPD and WILLMSGUPD.
I don't know G/W should keep Will topic and Will Message or not.
Those seem use less for G/W. MQTT protocol dose not have those messages.
if the client wants update those, it's enough to reconnect with updated will topic or will message.