  A full interpolation theory for Sobolev functions with smoothness between 0
and 1 and vanishing trace on a part of the boundary of an open set is
established. Geometric assumptions are of mostly measure theoretic nature and
reach beyond Lipschitz regular domains. Previous results were limited to
regular geometric configurations or Hilbertian Sobolev spaces. Sets with porous
boundary and their characteristic multipliers on smoothness spaces play a major
role in the arguments.
