% $Id: failure_results.tex,v 1.5 2005/04/08 21:21:45 gokhale Exp $

\subsection{Fault Tolerent Behavior of EASM}
\label{sec:ft_results}

In this section we study how random node failures affect the working
of EASM. Packet delivery fraction for differnt values of node failure
is plotted in Figure~\ref{fig:fail_pdf}. Note that the
replication uses a system parameter $n$, indicating the {\em
depth} of replication. Delivery fraction reduces gradually as more
nodes are failed in the network.

However, as $n$ is increased the success ratio increases.  Even if
some rendezvous points fail during the simulation, the replication
allows choosing an alternate rendezvous point thereby successfully
forwarding the packet to its destination.  As failure rate is
increased, we expect that packets take alternate routes to their
destination thereby increasing the delays.  This is 
corroborated by the results in Figure~\ref{fig:fail_avg_delay}. It
shows the variation of average delay when rate of node failure is
increased.  For example when a forwarder node fails for an
interceptor, it chooses an alternate forwarder to send the packets to
which geographically may not be the closest choice thus adding to the
overall delay.  Since more and more packets have to take alternate
routes to their destination nodes the routing load increases. 
% Amogh, so is this good or bad

Replication adds to the routing load only marginally, as shown in
Figure~\ref{fig:fail_routing_load}.  Thus EASM is able to handle high
failure rates and route packets to their respective destinations with
marginal overheads. 

\begin{figure*}[!ht]
\begin{center}
\subfloat[Packet Delivery Fraction]
{\includegraphics[width=2in]{eps/failure_pdf.ps}
\label{fig:fail_pdf}}
\subfloat[Average Delay]
{\includegraphics[width=2in]{eps/failure_avg_delay.ps}
\label{fig:fail_avg_delay}}
\subfloat[Routing Load]
{\includegraphics[width=2in]{eps/failure_routing_load.ps}
\label{fig:fail_routing_load}}
\bfcaption{Performance in Presence of Failures}
\end{center}
\end{figure*}

