  Replica field theory for the Ising spin glass in zero magnetic field is
studied around the upper critical dimension d=6. A scaling theory of the spin
glass phase, based on Parisi's ultrametrically organised order parameter, is
proposed. We argue that this infinite step replica symmetry broken (RSB) phase
is nonperturbative in the sense that amplitudes of scaling forms cannot be
expanded in term of the coupling constant w^2. Infrared divergent integrals
inevitably appear when we try to compute amplitudes perturbatively,
nevertheless the \epsilon-expansion of critical exponents seems to be
well-behaved. The origin of these problems can be traced back to the unusual
behaviour of the free propagator having two mass scales, the smaller one being
proportional to the perturbation parameter w^2 and providing a natural infrared
cutoff. Keeping the free propagator unexpanded makes it possible to avoid
producing infrared divergent integrals. The role of Ward-identities and the
problem of the lower critical dimension are also discussed.
