Scheduling of many different kinds of activities takes place in distributed real-time and multimedia systems. It includes scheduling of computations, window services, file system management, I/O services and communication protocol processing. In this paper, we investigate the problem of scheduling communication protocol processing in real-time systems. Communication protocol processing takes a relatively substantial amount of time and if not structured correctly, unpredictable priority inversion and undesirable timing behavior can result to applications communicating with other processors but are otherwise scheduled correctly. We describe the protocol processing architecture in the RT-Mach operating system, which allows the timing of protocol processing to be under strict application control. An added benefit is also obtained in the form of higher performance. This scheduling architecture is consistent with the other RT-Mach scheduling mechanisms including fixed priority scheduling and processor reservation. The benefits of this protocol architecture are demonstrated both under synthetic workloads and in a realistic distributed video-conferencing system we have implemented. End-to-end delays for both audio and video are as predicted even with other threads competing for the CPU and the network.