This paper proposes a model describing resynchronization services in the data transfer phase of the Network layer. The model described the required properties in a very reduced form. It is functionally equivalent to a protocol model describing the protocol internal mechanisms. Assertions relative to the Network layer model can be conserved in the Transport layer model. These assertions can therefore be used for the validation of the Transport layer properties. This functional validation approach can be generalized to the characterization of any other hierarchical system. The joint development of specification and validation models will make it possible in the near future to make tools available for specification, modeling and validation in a rigorous and easy way.