DDSIRTP21 — Computation of KeyHash is unspecified

Summary:
The specification does not describe how the KeyHash is computed from the Key.

This implicates that key hashes in messages coming from different RTPS implementations can never be interpreted, because one implementation may utilize a different key hash algorithm then the other.

I would prefer that the hash algorithm becomes part of the specification. RTPS implementations can choose to implement the prescribed algorithm or simply use a zero-valued key.

Resolution:
The UDP PIM should mandate that the KeyHash is computed either as the serialized key or else as an MD5 digest, depending on whether the serialize key for the type can exceed the 128 that are used for the KeyHash.

The UDP PIM should mandate that the KeyHash is computed either as the serialized key or else as an MD5 digest, depending on whether the serialize key for the type can exceed the 128 that are used for the KeyHash.

The resolution of this issue depends on the presence of new section 9.6.3.3 titled "Key Hash" added as part of the resolution of issue. 12504. If issue 12504 is resolved in a way that does not add this section, then the resolution proposed here would not be valid.