This paper presents a Unified Modeling Language profile for describing communications protocols. UML is a popular, standardized, general-purpose visual language, but the current version lacks formal action semantics, which is needed to define any complicated communications system. It is also difficult to generate an efficient protocol specific implementation from standard UML notation only.The authors developed a Graphical Protocol Description Language, a UML profile, to fulfill the needs of protocol engineering. UML stereotypes are used to add protocol-specific semantic information to class diagrams, enabling code generation for protocol implementations.GPDL contains graphical elements and a textual language that is used to describe actions in statechart transitions, called the Generic Action Extension Language. A system described with GPDL can be converted to an implementation for any protocol framework. As an example, the authors developed a chain of tools, which performs a translation from GPDL to SDL.