Handling incorrect messages

[ericthegeek]

Originally Posted by eldoMS

2. What error to give with a wrong message class value -> format error?

This is a bit of a gray area. It's depends on how you interpret table 6-7.

Technically, NACK is only allowed for GET_COMMAND_RESPONSE and SET_COMMAND_RESPONSE. It doesn't address whether you can NACK an unknown command class. Scott is correct that if you do decide to NACK it, NR_UNSUPPORTED_COMMAND_CLASS is the correct NACK reason code to use.

One quirk of the standard: Per table 6-7 you can never NACK a discovery request. That means if you get an unknown PID with a discovery command class, all you can do is drop it since NACK is not allowed. I'm not sure this was the intended purpose, but that's how it's written. (The original motivation was to ensure that Discover Unique Branch, Mute, and Unmute were always handled immediately).

Originally Posted by eldoMS

3. When the packet data length mismatches with data length -> format error?

I disagree with Scott on this one. A responder needs to be very strict about enforcing the length fields. If the lengths don't match, the packet is almost certainly corrupt, and you should treat it like a checksum error and ignore the packet.

There are three lengths in RDM:
1: The "Message Length" Field in the packet.
2: The PDL field in the packet (should be exactly "Message Length" minus 24).
3: The number of bytes you actually receive with your UART (should be "Message Length" plus 2).

If all 3 don't match, then some part of the packet has been corrupted in transit. It's common to see this happen. The checksum used in RDM is relatively weak. Because it's an additive checksum, if a 0x00 gets added or dropped from the packet, the checksum won't change. It also can't detect many two-bit errors.

That means if a communication error occurs (RF noise, loose connection, UART Error), or a splitter/hub drops one or more bytes, the checksum can't reliably detect it. The only way you have to know if this has happened is to verify that the lengths all match.