ascender, descender, cap-height and x-height

Hi again www-font folks,

I'd like to know whether TrueType or OpenType fonts contain fields that
give reliable ascender, descender, cap-height and x-height info. If so,
what are the names of those fields, and which tables are they in?

If the fonts don't contain reliable info for those parameters, would it
be a good idea (for ascender) to compute the median of the ascents of
the glyphs for the letters with ascenders? (I.e. b, d, f, h, etc?)

When I say "reliable", I'm referring to the ability to use those fields
in any existing, reasonably popular font.

Thanks again,

Erik