\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
+specified in RFC~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}
+\enlargethispage{2\baselineskip}
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.
+\code{localpart@domain/resource}, where \code{domain} 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 at the same time, the
+\code{resource} part is a string chosen by the user to distinguish those
+clients. Only the part \code{localpart@domain} (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]
+\begin{wrapfigure}{r}{0.5\textwidth}
+ \tikzstyle{iconlabel}=[text width=3cm, align=center, font=\footnotesize]
+ \tikzstyle{label}=[font=\footnotesize]
+ \begin{tikzpicture}[node distance=0pt,scale=1.5,>=stealth,thick]
+ \def\nodelist{
+ juliet/{(-1,-1)}/XMPP client \code{juliet@example.net}/below/computer,
+ examplenet/{(-1,1)}/XMPP server \code{example.net}/above/server,
+ imexampleorg/{(1,1)}/XMPP server \code{im.example.org}/above/server,
+ romeo/{(1,-1)}/XMPP client \code{romeo@im.example.org}/below/computer%
+ }
+ \foreach \name/\pos/\text/\tpos/\icon in \nodelist {
+ \node (\name) at \pos { \includegraphics[width=1cm]{icon-\icon.pdf} };
+ \node[\tpos=of \name,iconlabel] (\name text) { \text };
+ }
+ \draw[<->,dashed] (juliet) -- node[anchor=east,label]{s2c} (examplenet);
+ \draw[<->] (examplenet) -- node[anchor=south,label]{s2s} (imexampleorg);
+ \draw[<->,dashed] (imexampleorg) -- node[anchor=west,label]{s2c} (romeo);
+ \end{tikzpicture}
\centering
- \includegraphics[width=0.4\textwidth]{fig-xmpp-architecture-mock.jpg}
- \caption{XMPP architecture}
+ \caption{XMPP architecture, showing server-to-server (s2s) and
+ server-to-client (s2c) connections}
\label{fig:xmpparch}
-%\end{figure}
\end{wrapfigure}
The original architecture underlying XMPP strongly leans on the established
design of Internet Mail, and an example is depicted in Fig.~\ref{fig:xmpparch}.
The distributed network is formed by \term{XMPP servers} on one hand, which make
-up the always-on backbone of the network used for routing message, and manage
-user accounts and statuses. On the other hand, \term{XMPP clients} represent a
-single logged-in user and are the interface for communication with other users.
+up the always-on backbone of the network used for message routing, and which
+manage user accounts and statuses. On the other hand, \term{XMPP clients}
+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
All communication over XMPP is based on XML. To minimize communication overhead,
only fragments of XML, called \term{stanzas}, are sent between hosts. A stanza
-is always well-formed as a whole; it consist of a root element, which also
-includes routing attributes (\code{to} and \code{from}), and its optional child
-elements.
+is always well-formed as a whole; it consists of a root element, which in most
+cases also includes routing attributes (\code{to} and \code{from}), and its
+optional child elements.
On top of that, living connections between hosts are represented by \term{XML
streams}. The client initiates a connection by sending an XML declaration
an opening \code{<stream>} tag. The client then performs SASL authentication and
binds its stream to a resource for proper addressing. If this process succeeded,
both client and server can send an unlimited number of stanzas, until the
-connection is closed by one side by sending an closing \code{</stream>} tag. The
-other side then has the chance to send all outstanding stanzas and likewise
+connection is closed by one side by sending a closing \code{</stream>} tag. The
+other side then has the chance to send all outstanding stanzas and then likewise
closes its stream. If both streams are closed, the underlying TCP connection is
terminated.
-\todo[Example stream]
+\subsubsection{Publish/Subscribe and Presence}
-\subsubsection{Multi-User Chats}
+Typically, a user wants to chat with a more or less fixed set of other users,
+whose JIDs she needs to know, so she needs some kind of ``address book'' that
+remembers the JIDs for her. In XMPP, this address book is called
+\term{roster}, and it also shows the users' willingness to chat (``presence'').
+In order to see their chat status (which can be one of ``online'', ``offline'',
+and several ``away'' or ``do not disturb'' states), a user needs to subscribe to
+the other user's status. The mechanism behind this is called
+\term{Publish-Subscribe} and is specified in XEP-0060~\cite{xep0060}. It can
+be used to notify interested users about changes in personal information, and
+implements the well-known Observer pattern.
-\cite{xep-0045}
+A user publishes information by creating a \term{node} on the XMPP server, which
+acts as a handle for the data. Interested users can then query the server for
+nodes, and request subscription to them. When the owner of the node confirms the
+subscription request, subscribers get notified whenever the owner updates the
+respective node.
-\subsubsection{Publish/Subscribe, Presence and the Roster}
+All communication takes place between the client and the server over \code{<iq>}
+(``information query'') stanzas.
-\cite{xep-0060}
+\subsubsection{Multi-User Chats}
+Besides one-to-one messaging, XMPP also allows users to create multi-user chat
+rooms, which is specified in XEP-0045~\cite{xep0045}. Each chat room is given a
+unique JID on the server managing the room to which the users send their
+messages to. Each incoming message is then dispatched to all users which have
+joined the room.
-\cite{rfc6121}
+To join a room, the user sends a \code{<presence>} stanza to the room JID, where
+the resource part of the room JID specifies the desired nick name.
\subsubsection{XMPP Serverless Messaging}\label{sec:xsm}
-\todo
To overcome the need for a central server and authentication, XMPP Serverless
-Messaging~\cite{xep-0174} allows XMPP clients on a network to build a
+Messaging~\cite{xep0174} allows XMPP clients on a network to build a
peer-to-peer mesh network and chat directly with each other. This feature was
first introduced by Apple as part of their \term{Bonjour} project, and nowadays
it is also available in many other XMPP clients.
-With XMPP Serverless Messaging, XMPP clients simply open a port on the host, and
-then rely on mDNS and DNS-SD (see Section~\ref{sec:dns})
-to publish instance names in the domain \code{\_presence.\_tcp.local}. For
-example, if Juliet uses her machine (named \code{capulet}) with serverless
-messaging, her client would publish the following four mDNS records:
+%\begin{wrapfigure}{R}{0.4\textwidth}
+ %\tikzstyle{iconlabel}=[text width=2cm, align=center, font=\footnotesize]
+ %\tikzstyle{label}=[font=\footnotesize]
+ %\begin{tikzpicture}[node distance=0pt,scale=1.2,>=stealth,thick]
+ %\def\nodelist{
+ %juliet/{(-1,-1)}/\code{juliet@\ balcony.local}/below/computer,
+ %tybalt/{(-1,1)}/\code{tybalt@\ montague.local}/above/computer,
+ %mercutio/{(1,1)}/\code{mercutio@\ capulet.local}/above/computer,
+ %romeo/{(1,-1)}/\code{romeo@\ romeo.local}/below/computer%
+ %}
+ %\foreach \name/\pos/\text/\tpos/\icon in \nodelist {
+ %\node (\name) at \pos { \includegraphics[width=1cm]{icon-\icon.pdf} };
+ %\node[\tpos=of \name,iconlabel] (\name text) { \text };
+ %}
+ %\draw[<->,dashed] (juliet) -- (tybalt);
+ %\draw[<->,dashed] (juliet) -- (romeo);
+ %\draw[<->,dashed] (juliet) -- (mercutio);
+ %\draw[<->,dashed] (romeo) -- (mercutio);
+ %\draw[<->,dashed] (romeo) -- (tybalt);
+ %\draw[<->,dashed] (mercutio) -- (tybalt);
+ %\end{tikzpicture}
+ %\centering
+ %\caption{XMPP architecture with Serverless Messaging}
+ %\label{fig:xmpparch2}
+%\end{wrapfigure}
+
+With XMPP Serverless Messaging, XMPP clients simply open a port on their host,
+and then rely on mDNS and DNS-SD (see Section~\ref{sec:dns}) to publish instance
+names in the domain \code{\_presence.\_tcp.local}. For example, if Juliet uses
+her machine (named \code{capulet}) with serverless messaging, her client would
+publish the following four mDNS records:
\begin{itemize}
\item an A record \code{capulet.local}, specifying her IP address,
\item an SRV record \code{juliet@capulet.\_presence.\_tcp.local}, specifying
- the port on which her XMPP client listens, and refering to
+ the port on which her XMPP client listens, and referring to
\code{capulet.local} as the host name
\item a PTR record \code{\_presence.\_tcp.local} for service discovery,
pointing to \code{juliet@capulet.\_presence.\_tcp.local}
\item and a TXT record \code{juliet@capulet.\_presence.\_tcp.local} specifying
- more information about her (e.~g. her online status, contact data, etc.)
+ more information about her (e.~g. her online status, contact data, etc.) in
+ standardized key-value pairs.
\end{itemize}
When other clients in the same network enumerate the available services by
over the corresponding TXT record in the mDNS. To go offline, a client
announces the deletion of its mDNS records.
-\pages{1}
-
% vim: set ft=tex et ts=2 sw=2 :
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