  We show how to prescribe the initial data of a characteristic problem
satisfying the costraints, the smallness, the regularity and the asymptotic
decay suitable to prove a global existence result. In this paper, the first of
two, we show in detail the construction of the initial data and give a sketch
of the existence result. This proof, which mimicks the analogous one for the
non characteristic problem in [Kl-Ni], will be the content of a subsequent
paper.
