sec-xmpp.tex

   1 \subsection{XMPP}
   2 \todo
   3 \pages{3-4}
   4
   5 The \term{Extensible Messaging and Presence Protocol (XMPP)} is a distributed,
   6 XML-based protocol for real-time communication. Its core functionalities are
   7 specified in RFCs~6120~\cite{rfc6120} and RFC~6122~\cite{rfc6121}, while protocol
   8 extensions are usually defined by the XMPP community in \term{XMPP Extension
   9 Proposals (XEPs)}.
  10
  11 \subsubsection{Addressing}
  12
  13 Every user account in XMPP is addressed by a globally unique identifier, called
  14 the \term{Jabber ID (JID)}~\cite{rfc6122}. It has the form
  15 \code{localpart@domainpart/resource}, where \code{domainpart} is the DNS name of
  16 an XMPP server, and \code{localpart} is the name of a user account on that
  17 server. Since a user can be logged in from multiple clients, the \code{resource}
  18 part is a string chosen by the user to distinguish those clients. Only the part
  19 \code{localpart@domainpart} (the \term{bare JID}) is needed to identify a user,
  20 the resource is only needed for routing between client and server.
  21
  22 \subsubsection{Architecture}
  23 \begin{wrapfigure}{r}{0.4\textwidth}
  24 %\begin{figure}[htop]
  25   \centering
  26   \includegraphics[width=0.4\textwidth]{fig-xmpp-architecture-mock.jpg}
  27   \caption{XMPP architecture}
  28   \label{fig:xmpparch}
  29 %\end{figure}
  30 \end{wrapfigure}
  31
  32 The original architecture underlying XMPP strongly leans on the established
  33 design of Internet Mail, and an example is depicted in Fig.~\ref{fig:xmpparch}.
  34 The distributed network is formed by \term{XMPP servers} on one hand, which make
  35 up the always-on backbone of the network used for routing message, and manage
  36 user accounts and statuses. On the other hand, \term{XMPP clients} represent a
  37 single logged-in user and are the interface for communication with other users.
  38
  39 Every client communicates only with the server that manages the respective user
  40 account which is configured in the client, as given in the user's JID. The
  41 server then routes the messages to their recipients, using the JID to determine
  42 the correct server for a message to be sent to. Finally, the receiving server
  43 sends the message to a client where the receiving JID is logged in. If the user
  44 is not logged in at the time the message is sent, the server can store it for
  45 the user and deliver it on the next login.
  46
  47 XMPP strongly relies on DNS Service Discovery (see Section~\ref{sec:dnssd}) to
  48 determine the server being in charge of a domain. For example, the server who
  49 manages the users for the domain \code{example.org} is given by the SRV record
  50 \code{\_xmpp-server.\_tcp.example.org}.
  51
  52 \subsubsection{Communication primitives}
  53
  54 All communication over XMPP is based on XML. To minimize communication overhead,
  55 only fragments of XML, called \term{stanzas}, are sent between hosts. A stanza
  56 is always well-formed as a whole; it consist of a root element, which also
  57 includes routing attributes (\code{to} and \code{from}), and its optional child
  58 elements.
  59
  60 On top of that, living connections between hosts are represented by \term{XML
  61 streams}. The client initiates a connection by sending an XML declaration
  62 followed by an opening \code{<stream>} tag. The server then responds also with
  63 an opening \code{<stream>} tag. The client then performs SASL authentication and
  64 binds its stream to a resource for proper addressing. If this process succeeded,
  65 both client and server can send an unlimited number of stanzas, until the
  66 connection is closed by one side by sending an closing \code{</stream>} tag. The
  67 other side then has the chance to send all outstanding stanzas and likewise
  68 closes its stream. If both streams are closed, the underlying TCP connection is
  69 terminated.
  70
  71 \todo[Example stream]
  72
  73 \subsubsection{Multi-User Chats}
  74
  75 \cite{xep-0045}
  76
  77 \subsubsection{Publish/Subscribe, Presence and the Roster}
  78
  79 \cite{xep-0060}
  80
  81
  82 \cite{rfc6121}
  83
  84 \subsubsection{XMPP Serverless Messaging}\label{sec:xsm}
  85 \todo
  86
  87 To overcome the need for a central server and authentication, XMPP Serverless
  88 Messaging~\cite{xep-0174} allows XMPP clients on a network to build a
  89 peer-to-peer mesh network and chat directly with each other. This feature was
  90 first introduced by Apple as part of their \term{Bonjour} project, and nowadays
  91 it is also available in many other XMPP clients.
  92
  93 With XMPP Serverless Messaging, XMPP clients simply open a port on the host, and
  94 then rely on mDNS and DNS-SD (see Section~\ref{sec:dns})
  95 to publish instance names in the domain \code{\_presence.\_tcp.local}. For
  96 example, if Juliet uses her machine (named \code{capulet}) with serverless
  97 messaging, her client would publish the following four mDNS records:
  98
  99 \begin{itemize}
 100   \item an A record \code{capulet.local}, specifying her IP address,
 101   \item an SRV record \code{juliet@capulet.\_presence.\_tcp.local}, specifying
 102     the port on which her XMPP client listens, and refering to
 103     \code{capulet.local} as the host name
 104   \item a PTR record \code{\_presence.\_tcp.local} for service discovery,
 105     pointing to \code{juliet@capulet.\_presence.\_tcp.local}
 106   \item and a TXT record \code{juliet@capulet.\_presence.\_tcp.local} specifying
 107     more information about her (e.~g. her online status, contact data, etc.)
 108 \end{itemize}
 109
 110 When other clients in the same network enumerate the available services by
 111 querying \code{\_presence.\_tcp.local}, they notice Juliet's presence and add
 112 her to the roster automatically. In that way, XMPP users can see who is
 113 currently available for communication. To start a chat session, clients initiate
 114 a TCP connection over the advertised ports, open their XML streams, and send
 115 message or IQ stanzas like they would to an XMPP server. Presence is managed
 116 over the corresponding TXT record in the mDNS. To go offline, a client
 117 announces the deletion of its mDNS records.
 118
 119 \pages{1}
 120
 121 % vim: set ft=tex et ts=2 sw=2 :