sec-xmpp.tex

   1 \subsection{XMPP}
   2
   3 The \term{Extensible Messaging and Presence Protocol (XMPP)} is a distributed,
   4 XML-based protocol for real-time communication. Its core functionalities are
   5 specified in RFC~6120~\cite{rfc6120} and RFC~6122~\cite{rfc6121}, while protocol
   6 extensions are usually defined by the XMPP community in \term{XMPP Extension
   7 Proposals (XEPs)}.
   8
   9 \subsubsection{Addressing}
  10 \enlargethispage{2\baselineskip}
  11
  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@domain/resource}, where \code{domain} is the DNS name of an XMPP
  15 server, and \code{localpart} is the name of a user account on that server. Since
  16 a user can be logged in from multiple clients at the same time, the
  17 \code{resource} part is a string chosen by the user to distinguish those
  18 clients. Only the part \code{localpart@domain} (the \term{bare JID}) is
  19 needed to identify a user, the resource is only needed for routing between
  20 client and server.
  21
  22 \subsubsection{Architecture}
  23 \begin{wrapfigure}{r}{0.5\textwidth}
  24   \tikzstyle{iconlabel}=[text width=3cm, align=center, font=\footnotesize]
  25   \tikzstyle{label}=[font=\footnotesize]
  26   \begin{tikzpicture}[node distance=0pt,scale=1.5,>=stealth,thick]
  27     \input{fig-xmpparch.tex}
  28   \end{tikzpicture}
  29   \centering
  30   \caption{XMPP architecture, showing server-to-server (s2s) and
  31     server-to-client (s2c) connections}
  32   \label{fig:xmpparch}
  33 \end{wrapfigure}
  34
  35 The original architecture underlying XMPP strongly leans on the established
  36 design of Internet Mail, and an example is depicted in Fig.~\ref{fig:xmpparch}.
  37 The distributed network is formed by \term{XMPP servers} on one hand, which make
  38 up the always-on backbone of the network used for message routing, and which
  39 manage user accounts and statuses. On the other hand, \term{XMPP clients}
  40 represent a single logged-in user and make up the interface for communication
  41 with other users.
  42
  43 Every client communicates only with the server that manages the respective user
  44 account which is configured in the client, as given in the user's JID. The
  45 server then routes the messages to their recipients, using the JID to determine
  46 the correct server for a message to be sent to. Finally, the receiving server
  47 sends the message to a client where the receiving JID is logged in. If the user
  48 is not logged in at the time the message is sent, the server can store it for
  49 the user and deliver it on the next login.
  50
  51 XMPP strongly relies on DNS Service Discovery (see Section~\ref{sec:dnssd}) to
  52 determine the server being in charge of a domain. For example, the server who
  53 manages the users for the domain \code{example.org} is given by the SRV record
  54 \code{\_xmpp-server.\_tcp.example.org}.
  55
  56 \subsubsection{Communication primitives}
  57
  58 All communication over XMPP is based on XML. To minimize communication overhead,
  59 only fragments of XML, called \term{stanzas}, are sent between hosts. A stanza
  60 is always well-formed as a whole; it consists of a root element, which in most
  61 cases also includes routing attributes (\code{to} and \code{from}), and its
  62 optional child elements.
  63
  64 On top of that, living connections between hosts are represented by \term{XML
  65 streams}. The client initiates a connection by sending an XML declaration
  66 followed by an opening \code{<stream>} tag. The server then responds also with
  67 an opening \code{<stream>} tag. The client then performs SASL authentication and
  68 binds its stream to a resource for proper addressing. If this process succeeded,
  69 both client and server can send an unlimited number of stanzas, until the
  70 connection is closed by one side by sending a closing \code{</stream>} tag. The
  71 other side then has the chance to send all outstanding stanzas and then likewise
  72 closes its stream. If both streams are closed, the underlying TCP connection is
  73 terminated.
  74
  75 \subsubsection{Publish/Subscribe and Presence}
  76
  77 Typically, a user wants to chat with a more or less fixed set of other users,
  78 whose JIDs she needs to know, so she needs some kind of ``address book'' that
  79 remembers the JIDs for her. In XMPP, this address book is called
  80 \term{roster}, and it also shows the users' willingness to chat (``presence'').
  81 In order to see their chat status (which can be  one of ``online'', ``offline'',
  82 and several ``away'' or ``do not disturb'' states), a user needs to subscribe to
  83 the other user's status. The mechanism behind this is called
  84 \term{Publish-Subscribe} and is specified in XEP-0060~\cite{xep0060}. It can
  85 be used to notify interested users about changes in personal information, and
  86 implements the well-known Observer pattern~\cite{GOF95}.
  87
  88 A user publishes information by creating a \term{node} on the XMPP server, which
  89 acts as a handle for the data. Interested users can then query the server for
  90 nodes, and request subscription to them. When the owner of the node confirms the
  91 subscription request, subscribers get notified whenever the owner updates the
  92 respective node.
  93
  94 All communication takes place between the client and the server over \code{<iq>}
  95 (``information query'') stanzas.
  96
  97 \subsubsection{Multi-User Chats}
  98
  99 Besides one-to-one messaging, XMPP also allows users to create multi-user chat
 100 rooms, which is specified in XEP-0045~\cite{xep0045}. Each chat room is given a
 101 unique JID on the server managing the room to which the users send their
 102 messages to. Each incoming message is then dispatched to all users which have
 103 joined the room.
 104
 105 To join a room, the user sends a \code{<presence>} stanza to the room JID, where
 106 the resource part of the room JID specifies the desired nick name.
 107
 108 \subsubsection{XMPP Serverless Messaging}\label{sec:xsm}
 109
 110 To overcome the need for a central server and authentication, XMPP Serverless
 111 Messaging~\cite{xep0174} allows XMPP clients on a network to build a
 112 peer-to-peer mesh network and chat directly with each other. This feature was
 113 first introduced by Apple as part of their \term{Bonjour} project, and nowadays
 114 it is also available in many other XMPP clients.
 115
 116 %\begin{wrapfigure}{R}{0.4\textwidth}
 117   %\tikzstyle{iconlabel}=[text width=2cm, align=center, font=\footnotesize]
 118   %\tikzstyle{label}=[font=\footnotesize]
 119   %\begin{tikzpicture}[node distance=0pt,scale=1.2,>=stealth,thick]
 120     %\def\nodelist{
 121       %juliet/{(-1,-1)}/\code{juliet@\ balcony.local}/below/computer,
 122       %tybalt/{(-1,1)}/\code{tybalt@\ montague.local}/above/computer,
 123       %mercutio/{(1,1)}/\code{mercutio@\ capulet.local}/above/computer,
 124       %romeo/{(1,-1)}/\code{romeo@\ romeo.local}/below/computer%
 125     %}
 126     %\foreach \name/\pos/\text/\tpos/\icon in \nodelist {
 127       %\node (\name) at \pos { \includegraphics[width=1cm]{icon-\icon.pdf} };
 128       %\node[\tpos=of \name,iconlabel] (\name text) { \text };
 129     %}
 130     %\draw[<->,dashed] (juliet) -- (tybalt);
 131     %\draw[<->,dashed] (juliet) -- (romeo);
 132     %\draw[<->,dashed] (juliet) -- (mercutio);
 133     %\draw[<->,dashed] (romeo) -- (mercutio);
 134     %\draw[<->,dashed] (romeo) -- (tybalt);
 135     %\draw[<->,dashed] (mercutio) -- (tybalt);
 136   %\end{tikzpicture}
 137   %\centering
 138   %\caption{XMPP architecture with Serverless Messaging}
 139   %\label{fig:xmpparch2}
 140 %\end{wrapfigure}
 141
 142 With XMPP Serverless Messaging, XMPP clients simply open a port on their host,
 143 and then rely on mDNS and DNS-SD (see Section~\ref{sec:dns}) to publish instance
 144 names in the domain \code{\_presence.\_tcp.local}. For example, if Juliet uses
 145 her machine (named \code{capulet}) with serverless messaging, her client would
 146 publish the following four mDNS records:
 147
 148 \begin{itemize}
 149   \item an A record \code{capulet.local}, specifying her IP address,
 150   \item an SRV record \code{juliet@capulet.\_presence.\_tcp.local}, specifying
 151     the port on which her XMPP client listens, and referring to
 152     \code{capulet.local} as the host name
 153   \item a PTR record \code{\_presence.\_tcp.local} for service discovery,
 154     pointing to \code{juliet@capulet.\_presence.\_tcp.local}
 155   \item and a TXT record \code{juliet@capulet.\_presence.\_tcp.local} specifying
 156     more information about her (e.~g. her online status, contact data, etc.) in
 157     standardized key-value pairs.
 158 \end{itemize}
 159
 160 When other clients in the same network enumerate the available services by
 161 querying \code{\_presence.\_tcp.local}, they notice Juliet's presence and add
 162 her to the roster automatically. In that way, XMPP users can see who is
 163 currently available for communication. To start a chat session, clients initiate
 164 a TCP connection over the advertised ports, open their XML streams, and send
 165 message or IQ stanzas like they would to an XMPP server. Presence is managed
 166 over the corresponding TXT record in the mDNS. To go offline, a client
 167 announces the deletion of its mDNS records.
 168
 169 % vim: set ft=tex et ts=2 sw=2 :