The only ones I believe might be used are 0x02 and 0x03, but those were only proposed and shouldn't actually be used. But when these "invalid" messages appear, what I would do is assume the payload is correct, read it but then drop it from further distribution. I'd also keep a counter for the occurence - if it happens too often within a short timespan, I'd consider the other client to be faulty and drop the connection.