MQTT-256 defines the basic mechanism for metadata but was originally designed to only show the metadata used on PUBLISH for message format. This leaves a lot of more general metadata items yet to be defined.
- What is the format of metadata on control packets other than PUBLISH?
- What are the rules for assigning IDs and the code to skip over metadata fields.
- One of the original concepts of metadata was that it should be possible for receivers of metadata to easily skip over metadata they did not want to process. This is especially important for small clients. Thus we should specify the minimum that a receiver must do to validate the received metadata.
- We should define the responsibility of the sender of metadata to only send valid metadata. This does imply that when a client or server passes on metadata it received, it needs to validate it first.