This paper studies the semantics of protocol modules composition and interaction in configurable communication systems. We present a semantic model describing the x-kernel, Cactus and Appia --- three frameworks that are used for implementing modular systems. The model covers protocol graph, session and channel creation, and inter-module communication of events and messages. To build the model, we defined a source-code-validated specification of a large fragment of the programming interface provided by the frameworks; we developed an operational semantics describing the behaviour of the operations through state transitions, making explicit interactions between modules. Developing the model and a small example implementing a configurable multicast helped us to better understand the design choices in these frameworks. The work reported in this paper is our first step towards reasoning about systems composed from collections of modules.