Formal description techniques (FDTs) provide formal and abstract ways to specify what protocols have to do and what the features of the protocols need. Protocol verification is a process for detecting logical errors, such as deadlock, unspecified receptions, and channel overflow errors, in communication protocols. Estelle is a FDT defined by the International Organization for Standardization (ISO) for protocol specifications. The authors present an incremental protocol verification technique and an Estelle interpreter which accepts Estelle protocol specifications for incremental protocol verification.>