  General hypersurfaces of any causal character can be studied abstractly using
the hypersurface data formalism. In the null case, we write down all tangential
components of the ambient Ricci tensor in terms of the abstract data. Using
this formalism, we formulate and solve in a completely abstract way the
characteristic Cauchy problem of the Einstein vacuum field equations. The
initial data is detached from any spacetime notion, and it is fully
diffeomorphism and gauge covariant. The results of this paper put the
characteristic problem on a similar footing as the standard Cauchy problem in
General Relativity.
