The design and development process of communication protocols and real-time systems and particularly transport protocol mechanisms requires adequate methodology and efficient instrumental support. In this paper, an extensible and QoS-oriented development framework integrating design and simulation with UML (Unified Modeling Language), and implementation and evaluation with emulation is introduced. An early use of the proposed framework is illustrated with the design and development of simple transport mechanism. Categories and Subject Descriptors C.2.2 [Network Protocols]: [Transport mechanisms] General Terms Languages, Verification Keywords QoS, UML, design, simulation, emulation, transport protocol.