[skm-ma-ws1314.git] / sec-xmpp.tex
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)}.
10 \subsubsection{Addressing}
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 \tikzstyle{label}=[font=\footnotesize]
25 \begin{tikzpicture}[node distance=0pt,scale=1.5,>=stealth,thick]
26 \def\nodelist{
27 juliet/{(-1,-1)}/XMPP client \code{juliet@example.net}/below/computer,
28 examplenet/{(-1,1)}/XMPP server \code{example.net}/above/server,
29 imexampleorg/{(1,1)}/XMPP server \code{im.example.org}/above/server,
30 romeo/{(1,-1)}/XMPP client \code{romeo@im.example.org}/below/computer%
31 }
32 \foreach \name/\pos/\text/\tpos/\icon in \nodelist {
33 \node (\name) at \pos { \includegraphics[width=1cm]{icon-\icon.pdf} };
34 \node[\tpos=of \name,iconlabel] (\name text) { \text };
35 }
36 \draw[<->,dashed] (juliet) -- node[anchor=east,label]{s2c} (examplenet);
37 \draw[<->] (examplenet) -- node[anchor=south,label]{s2s} (imexampleorg);
38 \draw[<->,dashed] (imexampleorg) -- node[anchor=west,label]{s2c} (romeo);
39 \end{tikzpicture}
40 \centering
41 \caption{XMPP architecture, showing server-to-server (s2s) and
42 server-to-client (s2c) connections}
43 \label{fig:xmpparch}
44 \end{wrapfigure}
46 The original architecture underlying XMPP strongly leans on the established
47 design of Internet Mail, and an example is depicted in Fig.~\ref{fig:xmpparch}.
48 The distributed network is formed by \term{XMPP servers} on one hand, which make
49 up the always-on backbone of the network used for message routing, and manage
50 user accounts and statuses. On the other hand, \term{XMPP clients} represent a
51 single logged-in user and make up the interface for communication with other
52 users.
54 Every client communicates only with the server that manages the respective user
55 account which is configured in the client, as given in the user's JID. The
56 server then routes the messages to their recipients, using the JID to determine
57 the correct server for a message to be sent to. Finally, the receiving server
58 sends the message to a client where the receiving JID is logged in. If the user
59 is not logged in at the time the message is sent, the server can store it for
60 the user and deliver it on the next login.
62 XMPP strongly relies on DNS Service Discovery (see Section~\ref{sec:dnssd}) to
63 determine the server being in charge of a domain. For example, the server who
64 manages the users for the domain \code{example.org} is given by the SRV record
65 \code{\_xmpp-server.\_tcp.example.org}.
67 \subsubsection{Communication primitives}
69 All communication over XMPP is based on XML. To minimize communication overhead,
70 only fragments of XML, called \term{stanzas}, are sent between hosts. A stanza
71 is always well-formed as a whole; it consists of a root element, which in most
72 cases also includes routing attributes (\code{to} and \code{from}), and its
73 optional child elements.
75 On top of that, living connections between hosts are represented by \term{XML
76 streams}. The client initiates a connection by sending an XML declaration
77 followed by an opening \code{<stream>} tag. The server then responds also with
78 an opening \code{<stream>} tag. The client then performs SASL authentication and
79 binds its stream to a resource for proper addressing. If this process succeeded,
80 both client and server can send an unlimited number of stanzas, until the
81 connection is closed by one side by sending an closing \code{</stream>} tag. The
82 other side then has the chance to send all outstanding stanzas and then likewise
83 closes its stream. If both streams are closed, the underlying TCP connection is
84 terminated.
86 \subsubsection{Publish/Subscribe and Presence}
88 Typically, a user wants to chat with a more or less fixed set of other users,
89 whose JIDs she needs to know, so she needs some kind of ``address book'' that
90 remembers the JIDs for her. In XMPP, this is address book is called
91 \term{roster}, and it also shows the users' willingness to chat (``presence'').
92 In order to see their chat status (which can be one of ``online'', ``offline'',
93 and several ``away'' or ``do not disturb'' states), a user needs to subscribe to
94 the other user's status. The mechanism behind this is called
95 \term{Publish-Subscribe} and is specified in XEP-0060~\cite{xep0060}. It can
96 be used to notify interested users about changes in personal information, and
97 implements the well-known Observer pattern.
99 A user publishes information by creating a \term{node} on the XMPP server, which
100 acts as a handle for the data. Interested users can then query the server for
101 nodes, and request subscription to them. When the owner of the node confirms the
102 subscription request, subscribers get notified whenever the owner updates the
103 respective node.
105 All communication takes place between the client and the server over \code{<iq>}
106 (``information query'') stanzas.
108 \subsubsection{Multi-User Chats}
110 Besides one-to-one messaging, XMPP also allows users to create multi-user chat
111 rooms, which is specified in XEP-0045~\cite{xep0045}. Each chat room is given a
112 unique JID to which the users send their messages to. Each incoming message is
113 then dispatched to all users which have joined the room.
115 To join a room, the user sends a \code{<presence>} stanza to the room JID, where
116 the resource part of the room JID specifies the desired nick name.
118 \subsubsection{XMPP Serverless Messaging}\label{sec:xsm}
119 \pages{1}
121 To overcome the need for a central server and authentication, XMPP Serverless
122 Messaging~\cite{xep0174} allows XMPP clients on a network to build a
123 peer-to-peer mesh network and chat directly with each other. This feature was
124 first introduced by Apple as part of their \term{Bonjour}\footnote{see
125 \url{https://developer.apple.com/bonjour/}} project, and nowadays it is also
126 available in many other XMPP clients.
128 With XMPP Serverless Messaging, XMPP clients simply open a port on the host, and
129 then rely on mDNS and DNS-SD (see Section~\ref{sec:dns})
130 to publish instance names in the domain \code{\_presence.\_tcp.local}. For
131 example, if Juliet uses her machine (named \code{capulet}) with serverless
132 messaging, her client would publish the following four mDNS records:
134 \begin{itemize}
135 \item an A record \code{capulet.local}, specifying her IP address,
136 \item an SRV record \code{juliet@capulet.\_presence.\_tcp.local}, specifying
137 the port on which her XMPP client listens, and refering to
138 \code{capulet.local} as the host name
139 \item a PTR record \code{\_presence.\_tcp.local} for service discovery,
140 pointing to \code{juliet@capulet.\_presence.\_tcp.local}
141 \item and a TXT record \code{juliet@capulet.\_presence.\_tcp.local} specifying
142 more information about her (e.~g. her online status, contact data, etc.) in
143 standardized key-value pairs.
144 \end{itemize}
146 When other clients in the same network enumerate the available services by
147 querying \code{\_presence.\_tcp.local}, they notice Juliet's presence and add
148 her to the roster automatically. In that way, XMPP users can see who is
149 currently available for communication. To start a chat session, clients initiate
150 a TCP connection over the advertised ports, open their XML streams, and send
151 message or IQ stanzas like they would to an XMPP server. Presence is managed
152 over the corresponding TXT record in the mDNS. To go offline, a client
153 announces the deletion of its mDNS records.
155 % vim: set ft=tex et ts=2 sw=2 :
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