1. A method of defining a common interactions protocol between two entities, the method comprising: the method performed by a communication server arranged for: converting syntactic specifications of multiple documents to be passed between the entities, into a skeleton semantic web ontology comprising a set of classes; deriving for each entity a respective set of constraints including semantic constraints established by the entity on aspects of the classes of the skeleton ontology; calculating the union of the two sets of constraints; determining, using a constraint resolver that comprises a description logic reasoner, whether the union is satisfiable, and: where the union is satisfiable, deriving from the intersection of the two sets of constraints a restricted document specification that is compatible with the constraint sets of both entities; and where the union is not satisfiable, indicating where any incompatibility lies.