tikzpicture draw inside node/frame

Question

I'm trying to reproduce a picture and I'm having a problem (since this is my very first experience with tikzpictures). The picture is the following:

My problem is of how to create the part that is inside the frame in the middle and more precisely how am I supposed to "frame" it like that (with the underlying part - description) and connect it later with the other nodes I have.

Sorry if the question is very naive. Thanks in advance for any help.

Answer 1

Here's a solution using the fit and calc libraries. The \pgflinewidth/2 is used to fix the width of the lower rectangle.

\documentclass[parskip]{scrartcl}
\usepackage[margin=15mm]{geometry}
\usepackage{tikz}
\usetikzlibrary{shapes,fit,arrows,calc}

\begin{document}

\begin{tikzpicture}
\node[ellipse,label=270:C++,align=left,draw] (a) at (0,0) {Simulation\\ Objects};
\node[ellipse,label=270:OT cl,align=left,draw] (b) at (4,0) {Simulation\\ Objects};
\draw[latex-latex] (a) -- (b);
\node (c) at ($(a.south west)+(-1,-1)$) {};
\node (d) at ($(b.north east)+(1,1)$) {};
\node[fit=(c)(d),draw] (e) {};
\draw ($(e.south west)+(\pgflinewidth/2,0)$) rectangle node {NS2 Shell Executable Command (ns)} ($(e.south east)+(-\pgflinewidth/2,-0.7)$);
\end{tikzpicture}

\end{document}