DDSIRTP21 — Some submessage kinds are logically redundant and not extensible

The submessages required to send Keyed and Unkeyed data are mapped in the PSM as two different submessages (DATA and NOKEY_DATA). These submessages have almost identical content so there is no reason to use two submessages for this. Furthermore having two different submessages at the PSM level and exposing the KeyHash as part of the submessage introduces problems with regards to the implementation and the extensibility of the protocol. For example, future versions of the protocol may want to support keyhashes that are either bigger or smaller than the 16 Bytes used today, and do that in an inter-operable manner without breaking backwards compatibility. They may also want to omit sending a key hash when the size of the key itself is small. As the key is always included as part of the data, sending the KeyHash for small keys is inefficient at best.
This could be resolved by combining the two PSM submessages and moving the KeyHash into the InlineQos.

Resolution:
Combine the DATA and NOKEY_DATA submessages. Also combine the DATA_FRAG and NOKEY_DATA_FRAG. Include additional fields in these submessages to enable extensibility for future submessage-specific options.

Combine the DATA and NOKEY_DATA submessages. Also combine the DATA_FRAG and NOKEY_DATA_FRAG. Include additional fields in these submessages to enable extensibility for future submessage-specific options