\documentclass[landscape]{llncs}
\usepackage{epsfig}

\begin{document}

\begin{figure}[htc]
\begin{center}
% \leavevmode
% \mbox{\psfig{angle=0,figure=msg-flow2.1,height=3.5in}}
\leavevmode
\epsfbox{msg-flow2.1}
\end{center}
\end{figure}

% \begin{center}
% \huge{$N_{i-1}$ uses witnesses if $N_i$ is unreachable}
% \end{center}

\end{document}

\begin{figure}[htc]
\begin{center}
\leavevmode
\epsfbox{msg-flow2.1}
\caption{Message flow example}
\end{center}
\end{figure}