The question of extensibility of D-mode to more advanced functionality has been
raised multiple times. Latest from Lars Eggert:

> >Section 5.7.1., paragraph 1:
> >>    A key attribute of GIST is that it is flexible in its
> >ability to use
> >>    existing transport and security protocols.  Different transport
> >>    protocols may have performance attributes appropriate
> to different
> >>    environments; different security protocols may fit
> >appropriately with
> >>    different authentication infrastructures.
> >
> >  All protocols defined in the subsections of this section are for  
> > C-mode - what about D-mode?

D mode only uses UDP+nothing. No flexibility is intended; if 
you wanted to negotiate another protocol, one would do so with the C-mode
mechanisms. It is probably worth saying this explicitly.