\TUchapter{Utilization OF MESSAGE PASSING INTERFACE}

\TUsection{Introduction to MPI Utilization for Attack Graph Generation}

\TUsection{Necessary Components}
\TUsubsection{Serialization}
In order to distribute workloads across nodes in a distributed system, various
types of data will need to be sent and received. Support and mechanisms vary based 
on the MPI implementation, but most fundamental data types such as integers, doubles,
characters, and Booleans are incorporated into the MPI implementation. While this does
simplify some of the messages that need to be sent and received in the MPI approaches of
attack graph generation, it does not cover the vast majority of them.

RAGE implements many custom classes and structs that are used throughout the generation process.
Qualities, topologies, network states, and exploits are a few such examples. Rather than breaking
each of these down into fundamental types manually, serialization functions are leveraged to handle
most of this. RAGE already incorporates Boost graph libraries for auxiliary support, so this work
extended this further to utilize the serialization libraries also provided by Boost. These
libraries also include support for serializing all STL classes, and many of the RAGE 
classes have members that make use of the STL classes. One additional advantage of the Boost 
library approach is that many of the RAGE class members are nested. For example, the NetworkState
class has a member vector of Quality classes. When serializing the NetworkState class, boost will
recursively serialize all members, including the custom class members, assuming they also have
serialization functions.

When using the serialization libraries, this work opted to use the intrusive route, where the
class instances are altered directly. This was preferable to the non-intrusive approach, since 
the class instances were able to be altered with relative ease, and many of the class instances 
did not expose enough information for the non-intrusive approach to be viable.  
\TUsubsection{Data Consistency}

\TUsection{Tasking Approach}
\TUsubsection{Introduction to the Tasking Approach}
The high-level overview of the compliance graph generation process can be broken down into six main tasks.
These tasks are described in Figure \ref{fig:tasks}. Prior works such as that seen by the 
authors of \cite{li_concurrency_2019}, \cite{9150145}, and \cite{7087377} work to parallelize the graph generation using
OpenMP, CUDA, and hyper-graph partitioning. This approach, however, utilizes Message Passing Interface (MPI)
to distribute the six identified tasks of RAGE to examine the effect on speedup, efficiency, and scalability for 
attack and compliance graph generation.

\begin{figure}[htp]
    \includegraphics[width=\linewidth]{"./Chapter5_img/horiz_task.drawio.png"}
    \vspace{.2truein} \centerline{}
        \caption{Task Overview of the Attack Graph Generation Process}
        \label{fig:tasks}
\end{figure}

\TUsubsection{Algorithm Design}
The design of the tasking approach is to leverage a pipeline structure with the six tasks and MPI nodes. Each stage of the pipeline will pass the necessary data to the next stage through various MPI messages, where the next stage's nodes will receive the data and execute their tasks. The pipeline is considered fully saturated when each task has a dedicated node. When there are less nodes than tasks, some nodes will processing multiple tasks. When there are more nodes than tasks, additional nodes will be assigned to Tasks 1 and 2. Timings were collected in the serial approach for various networks that displayed more time requirements for Tasks 1 and 2, with larger network sizes requiring vastly more time to be taken in Tasks 1 and 2. As a result, additional nodes are assigned to Tasks 1 and 2.
\TUsubsubsection{Communication Structure}
\TUsubsubsection{Task Zero}
\TUsubsubsection{Task One}
\TUsubsubsection{Task Two}
\TUsubsubsection{Task Three}
\TUsubsubsection{Task Four}
\TUsubsubsection{Task Five}

\TUsubsection{Performance Expectations}

\TUsection{Subgraphing Approach}
\TUsubsection{Introduction to the Subgraphing Approach}

\TUsubsection{Algorithm Design}
\TUsubsubsection{Communication Structure}
\TUsubsubsection{Worker Nodes}
\TUsubsubsection{Root Node}
\TUsubsubsection{Database Node}

\TUsubsection{Performance Expectations}