diff --git a/thesis/src/03.manipulate.tex b/thesis/src/03.manipulate.tex
index de409f0..289bcf6 100644
--- a/thesis/src/03.manipulate.tex
+++ b/thesis/src/03.manipulate.tex
@@ -61,10 +61,40 @@ Figure~\ref{fig:control-flow} illustrates the control flow for manipulating func
 
 \section{Defining a Protocol}\label{sec:defining-a-protocol}
 
-Lorem Ipsum.
+When using a socket to communicate with another process, a protocol definition is needed.
+This works defines a text-based protocol in which line breaks denote the end of a message.
+The following subsections describe the defined message types.
+
+\subsection{\textit{Init} Message (Client \textrightarrow{} Server)}\label{subsec:init-message}
+
+This message is the first message sent in a newly established connection.
+The client (\texttt{intercept.so}) uses it to identify the running program to the server (PID, path, \dots).
+
+\subsection{\textit{Call} Message (Client \textrightarrow{} Server)}\label{subsec:call-message}
+
+For each function call the client sends this message to the server and waits for a reply (\textit{Action} message).
+The contents of this message type correspond to the first line of an intercepted function call (see Section~\ref{sec:automated-testing-on-intercepted-function-calls}).
+
+\subsection{\textit{Action} Message (Server \textrightarrow{} Client)}\label{subsec:action-message}
+
+After receiving a \textit{Call} message from the client, the server decides what the client should do with this call.
+The server responds in one of four possible ways:
+
+\begin{description}
+  \item[\texttt{"ok"}] indicates that the function should be called normally.
+  \item[\texttt{"modify \textit{ARG...}"}] indicates that the arguments of the function should be changed according to the message before the call to the function.
+  \item[\texttt{"fail \textit{ERROR}"}] indicates that the function should not be called and instead should fail with the given error code.
+  \item[\texttt{"return \textit{VALUE}"}] indicates that the function should simply return the provided value without calling the real function.
+\end{description}
+
+\subsection{\textit{Return} Message (Client \textrightarrow{} Server)}\label{subsec:return-message}
+
+This message informs the server about the resulting return value.
+The server does not acknowledge this message.
+The contents of this message type correspond the second line of an intercepted function call (see Section~\ref{sec:automated-testing-on-intercepted-function-calls}).
 
 
-\section{Parsing Responses}\label{sec:parsing-responses}
+\section{Creating a Socket Server in Python}\label{sec:creating-a-socket-server-in-python}
 
 Lorem Ipsum.