Recent modern operating system technology enables protocol processing in user space using in-kernel packet filter and user-level protocol processing library for flexibility without sacrificing the performance of traditional kernelized protocol processing. This technology can be adapted to build a highly preemptable protocol processing mechanism for distributed real-time environment. In this paper, we discuss the structural difference of various operating systems from the protocol processing point of view, and propose an extended mechanism of packet filter and user-level protocol processing library for real-time communication. Using this mechanism, priority of the client can be handed off to the server without priority inversion problem while protocol processing.