\subsection{XMPP}
\todo
\pages{3-4}

The \term{Extensible Messaging and Presence Protocol (XMPP)} is a distributed,
XML-based protocol for real-time communication. Its core functionalities are
specified in RFCs~6120~\cite{rfc6120} and RFC~6122~\cite{rfc6121}, while protocol
extensions are usually defined by the XMPP community in \term{XMPP Extension
Proposals (XEPs)}.

\subsubsection{Addressing}

Every user account in XMPP is addressed by a globally unique identifier, called
the \term{Jabber ID (JID)}~\cite{rfc6122}. It has the form
\code{localpart@domainpart/resource}, where \code{domainpart} is the DNS name of
an XMPP server, and \code{localpart} is the name of a user account on that
server. Since a user can be logged in from multiple clients, the \code{resource}
part is a string chosen by the user to distinguish those clients. Only the part
\code{localpart@domainpart} (the \term{bare JID}) is needed to identify a user,
the resource is only needed for routing between client and server.

\subsubsection{Architecture}
\begin{wrapfigure}{r}{0.4\textwidth}
%\begin{figure}[htop]
  \centering
  \includegraphics[width=0.4\textwidth]{xmpp-architecture-mock.jpg}
  \caption{XMPP architecture}
%\end{figure}
\end{wrapfigure}

The original architecture underlying XMPP strongly leans on the established
design of Internet Mail. The distributed network is formed by \term{XMPP
servers} on one hand, which make up the always available backbone of the network
and manage user accounts and statuses, and \term{XMPP clients} on the other
hand, which represent a single logged-in user and make up the interface for
communication with other users.

Every client communicates only with the server that manages the respective user
account which is configured in the client, as given in the user's JID. The
server then routes the messages to their recipients, using the JID to determine
the correct server for a message to be sent to. Finally, the receiving server
sends the message to a client where the receiving JID is logged in. If the user
is not logged in at the time the message is sent, the server can store it for
the user and deliver it on the next login.

\todo{use of DNS-SD?}

\subsubsection{Communication primitives}

\subsubsection{Multi-User Chats}

\cite{xep-0045}

\subsubsection{Publish/Subscribe and the Roster}

is this really needed?

\cite{xep-0060}

\subsubsection{Presence}


\cite{rfc6120}
\begin{itemize}
	\item architecture: client-server, use of DNS-SD
	\item addressing: JIDs, resources
	\item XML-based communication primitives, stanzas and streams
	\item presence
	\item publish/subscribe \cite{xep-0060}, roster
	\item multi-user chats \cite{xep-0045}
\end{itemize}



\subsubsection{XMPP Serverless Messaging}
\todo
\cite{xep-0174} \pages{1}

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