  We discuss a dispersion relation in the photon mass and show how (in
principle) model-independent constraints on the parton distribution functions
of the photon, notably a momentum sumrule, can be obtained. We present two sets
of parametrizations, SaS~1 and~2, corresponding to two rather extreme
realizations of the non-perturbative part. Inclusive electron scattering off a
real photon is found to be insufficient to constrain the non-perturbative
components. The additional sensitivity provided by the photon virtuality is
outlined. Previous approaches to model the non-perturbative input distributions
are commented upon.
