  We derive iterative scaling algorithms of the Sinkhorn-Knopp (SK) type for
constrained optimal transport. The constraints are in the form of prior-imposed
zeroes in the transport plan. Based on classical Bregman arguments, we prove
asymptotic convergence of our algorithms to a unique optimal solution. New
insights obtained from the convergence proof are highlighted. An example from
electrical vehicle charging in a smart city context is outlined, in which the
prior zero-constraints prevent energy from being transported from some
providers to some vehicles.
