What does the length_in_words field in capabilities/usage mean?

[cubanismo]

While reviewing the current prototype implementation, @mvicomoya pointed out the way I'd used length_in_words was more consistent with a field called extra_words or something, since it excludes the length of the header structure. Now I'm unclear which is preferable: Should length_in_words include the header structure or not? If not, should we change the name?

[mvicomoya]

Actually, would it make sense to have two fields "header_length" and "body_length"?

Note that if two drivers (or the library itself) use different versions of the allocator, and by any chance we modified the header in between, the current implementation would fail the comparison.

I think having two fields would help to keep backwards and forwards compatibility in this case.