DDSIRTP21 — Clarify usage of KeyHash

Usage of keyHash is currently underspecified. It is not clear whether its presence in the message is mandatory and how it should be processed.
Resolution:
Expanded descriptions of the intended usage of keyHash parameter are added.
Revised Text:
Insert new Section 8.7.8

8.7.8 "Key Hash"

The Key Hash provides a hint for the key that uniquely identifies the data-object that is being changed within the set of objects that have been registered by the RTPS Writer.

Nominally the key is part of the serialized data of a data submessage. Using the key hash benefits implementations providing a faster alternative than deserializing the full key from the received data-object.

When the key hash is not received by a DataReader, it should be computed from the data itself. If there is no data in the submessage, then a default zero-valued key hash should be used by the DataReader

If there is a KeyHash, if present, shall be computed as described in Section 9.6.3.3

Expanded descriptions of the intended usage of keyHash parameter are added