The discrete event simulator OMNeT++, that is programmed in C++, shows a steady growing popularity. A shortcoming of it was the lack of a validated TCP implementation. In order to avoid a re-implementation of a fullfeatured TCP, including all potential implementation errors and costly validation tests, we integrated a TCP/IP stack of a real operating system into OMNeT++. In this paper we describe the difficulties of the integration process as well as the applied solutions. We also present some evaluation results that outline memory and CPU usage.