A PaScal extenSion for formal Specification of computer networkprotocolS. ProtocolS are deScribed by moduleS which are communicating NFAS. ModuleS are arranged in a dynamic hierarchy and communicate at named interaction pointS. EStPC CompileS EStelle into C. petdingo TranSlateS EStelle into C++ Adopted by ITU-T. ISO 9074 (1989). ["The Formal DeScription Technique EStelle", M. Diaz et al edS, N-H 1989].