1 \subsection{XMPP}
2 \pages{3-4}
4 The \term{Extensible Messaging and Presence Protocol (XMPP)} is a distributed,
5 XML-based protocol for real-time communication. Its core functionalities are
6 specified in RFCs~6120~\cite{rfc6120} and RFC~6122~\cite{rfc6121}, while protocol
7 extensions are usually defined by the XMPP community in \term{XMPP Extension
8 Proposals (XEPs)}.
12 Every user account in XMPP is addressed by a globally unique identifier, called
13 the \term{Jabber ID (JID)}~\cite{rfc6122}. It has the form
14 \code{localpart@domainpart/resource}, where \code{domainpart} is the DNS name of
15 an XMPP server, and \code{localpart} is the name of a user account on that
16 server. Since a user can be logged in from multiple clients, the \code{resource}
17 part is a string chosen by the user to distinguish those clients. Only the part
18 \code{localpart@domainpart} (the \term{bare JID}) is needed to identify a user,
19 the resource is only needed for routing between client and server.
21 \subsubsection{Architecture}
22 \begin{wrapfigure}{r}{0.5\textwidth}
23 \tikzstyle{iconlabel}=[text width=3cm, align=center, font=\footnotesize]
24 \begin{tikzpicture}[node distance=0pt,scale=1.5,>=stealth,thick]
25 \def\nodelist{
26 juliet/{(-1,-1)}/XMPP client \code{juliet@example.net}/below/computer,
27 examplenet/{(-1,1)}/XMPP server \code{example.net}/above/server,
28 imexampleorg/{(1,1)}/XMPP server \code{im.example.org}/above/server,
29 romeo/{(1,-1)}/XMPP client \code{romeo@im.example.org}/below/computer%
30 }
31 \foreach \name/\pos/\text/\tpos/\icon in \nodelist {
32 \node (\name) at \pos { \includegraphics[width=1.2cm]{icon-\icon.pdf} };
33 \node[\tpos=of \name,iconlabel] (\name text) { \text };
34 }
35 \draw[<->,dashed] (juliet) -- node[anchor=east]{s2c} (examplenet);
36 \draw[<->] (examplenet) -- node[anchor=south]{s2s} (imexampleorg);
37 \draw[<->,dashed] (imexampleorg) -- node[anchor=west]{s2c} (romeo);
38 \end{tikzpicture}
39 \centering
40 \caption{XMPP architecture, showing server-to-server (s2s) and
41 server-to-client (s2c, dashed) connection types}
42 \label{fig:xmpparch}
43 \end{wrapfigure}
45 The original architecture underlying XMPP strongly leans on the established
46 design of Internet Mail, and an example is depicted in Fig.~\ref{fig:xmpparch}.
47 The distributed network is formed by \term{XMPP servers} on one hand, which make
48 up the always-on backbone of the network used for routing message, and manage
49 user accounts and statuses. On the other hand, \term{XMPP clients} represent a
50 single logged-in user and are the interface for communication with other users.
52 Every client communicates only with the server that manages the respective user
53 account which is configured in the client, as given in the user's JID. The
54 server then routes the messages to their recipients, using the JID to determine
55 the correct server for a message to be sent to. Finally, the receiving server
56 sends the message to a client where the receiving JID is logged in. If the user
57 is not logged in at the time the message is sent, the server can store it for
58 the user and deliver it on the next login.
60 XMPP strongly relies on DNS Service Discovery (see Section~\ref{sec:dnssd}) to
61 determine the server being in charge of a domain. For example, the server who
62 manages the users for the domain \code{example.org} is given by the SRV record
63 \code{\_xmpp-server.\_tcp.example.org}.
65 \subsubsection{Communication primitives}
67 All communication over XMPP is based on XML. To minimize communication overhead,
68 only fragments of XML, called \term{stanzas}, are sent between hosts. A stanza
69 is always well-formed as a whole; it consist of a root element, which also
70 includes routing attributes (\code{to} and \code{from}), and its optional child
71 elements.
73 On top of that, living connections between hosts are represented by \term{XML
74 streams}. The client initiates a connection by sending an XML declaration
75 followed by an opening \code{<stream>} tag. The server then responds also with
76 an opening \code{<stream>} tag. The client then performs SASL authentication and
77 binds its stream to a resource for proper addressing. If this process succeeded,
78 both client and server can send an unlimited number of stanzas, until the
79 connection is closed by one side by sending an closing \code{</stream>} tag. The
80 other side then has the chance to send all outstanding stanzas and likewise
81 closes its stream. If both streams are closed, the underlying TCP connection is
82 terminated.
84 \subsubsection{Publish/Subscribe and Presence}
86 Typically, a user wants to chat with a more or less fixed set of other users,
87 whose JIDs she needs to know, so she needs some kind of address book'' that
88 remembers the JIDs for her. In XMPP, this is address book is called
89 \term{roster}, and it also shows the users' willingness to chat (presence'').
90 In order to see their chat status (which can be one of online'', offline'',
91 and several away'' or do not disturb'' states), a user needs to subscribe to
92 the other user's status. The mechanism behind this is called
93 \term{Publish-Subscribe} and is specified in XEP-0060~\cite{xep0060}. It can
94 be used to notify interested users about changes in personal information, and
95 implements the classic Observer pattern.
97 A user publishes information by creating a \term{node} on the XMPP server, which
98 acts as a handle for the data. Interested users can then query the server for
99 nodes, and request subscription to them. When the owner of the node confirms the
100 subscription request, subscribers get notified whenever the owner updates the
101 respective node.
103 All communication takes place between the client and the server over \code{<iq>}
104 (information query'') stanzas.
106 \subsubsection{Multi-User Chats}
108 Besides one-to-one messaging, XMPP also allows users to create multi-user
109 chat rooms, which is specified in \cite{xep0045}. Each chat room is given a
110 unique JID to which the users send their messages to. Each incoming message is
111 then dispatched to all users which have joined the room.
113 To join a room, the user sends a \code{<presence>} stanza to the room JID, where
114 the resource part of the room JID specifies the desired nick name.
116 \subsubsection{XMPP Serverless Messaging}\label{sec:xsm}
117 \pages{1}
119 To overcome the need for a central server and authentication, XMPP Serverless
120 Messaging~\cite{xep0174} allows XMPP clients on a network to build a
121 peer-to-peer mesh network and chat directly with each other. This feature was
122 first introduced by Apple as part of their \term{Bonjour} project, and nowadays
123 it is also available in many other XMPP clients.
125 With XMPP Serverless Messaging, XMPP clients simply open a port on the host, and
126 then rely on mDNS and DNS-SD (see Section~\ref{sec:dns})
127 to publish instance names in the domain \code{\_presence.\_tcp.local}. For
128 example, if Juliet uses her machine (named \code{capulet}) with serverless
129 messaging, her client would publish the following four mDNS records:
131 \begin{itemize}
132 \item an A record \code{capulet.local}, specifying her IP address,
133 \item an SRV record \code{juliet@capulet.\_presence.\_tcp.local}, specifying
134 the port on which her XMPP client listens, and refering to
135 \code{capulet.local} as the host name
136 \item a PTR record \code{\_presence.\_tcp.local} for service discovery,
137 pointing to \code{juliet@capulet.\_presence.\_tcp.local}
138 \item and a TXT record \code{juliet@capulet.\_presence.\_tcp.local} specifying