presentation.tex

   1 \documentclass{beamer}
   2 \usepackage[utf8]{inputenc}
   3 \usepackage[english]{babel}
   4 \usepackage{todonotes}
   5 \usepackage{inconsolata}
   6 %\usepackage{multicol}
   7 \usepackage{array}
   8 \usepackage{tikz}
   9 \usetikzlibrary{arrows}
  10 \usetikzlibrary{arrows}
  11 \usetikzlibrary{positioning}
  12 \usetikzlibrary{shapes.symbols}
  13 %\usepackage[usenames,dvipsnames]{color}
  14
  15 %\presetkeys{todonotes}{inline}{}
  16 \newcommand{\code}[1]{\texttt{#1}}
  17 \definecolor{green}{RGB}{137,164,0}  % from tubslatex
  18
  19 \title{Chatty Things}
  20 \subtitle{Making the Internet of Things Readily Usable for the Masses with XMPP}
  21 \author{Roland Hieber}
  22 \institute[IBR, TU-BS]{Institute of Operating Systems and Computer Networks}
  23 \date[SKM-13/14]{Seminar Communications and Multimedia, WS 2013/14}
  24
  25 \begin{document}
  26
  27 \begin{frame}[plain]
  28 \maketitle
  29 \end{frame}
  30
  31 \begin{frame}{Contents}
  32 \tableofcontents
  33 \end{frame}
  34
  35 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  36 \section{Motivation}
  37 \begin{frame}{Motivation}
  38 \begin{block}{The IoT Vision}
  39         \begin{itemize}
  40                 \item plentitude of smart objects
  41                 \item interoperability between devices
  42                 \item easy accessibility for users
  43         \end{itemize}
  44 \end{block}
  45 \end{frame}
  46
  47 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  48 \begin{frame}<1>[label=subgoals]{Motivation: Subgoals}
  49         \begin{block}{But how do we\ldots}
  50                 \begin{itemize}
  51                         \item configure our devices?
  52                                 \begin{itemize}
  53                                         \invisible<1-1>{\item[\color{green}\checkmark]\color{green}
  54                                                 Link-Local Addressing, Stateless Address Autoconfiguration}
  55                                 \end{itemize}
  56                         \item find other nodes to talk to?
  57                                 \begin{itemize}
  58                                         \invisible<1-2>{\item[\color{green}\checkmark]\color{green}
  59                                                 DNS-SD + mDNS}
  60                                 \end{itemize}
  61                         \item talk to other nodes or users?
  62                                 \begin{itemize}
  63                                         \invisible<1-3>{\item[\color{green}\checkmark]\color{green} XMPP
  64                                                 Serverless Messaging}
  65                                 \end{itemize}
  66                         \item filter relevant information?
  67                                 \begin{itemize}
  68                                         \invisible<1-4>{\item[\color{green}\checkmark]\color{green} ``Traffic
  69                                                 Lights'', Temporary Subscription for Presence}
  70                                 \end{itemize}
  71                 \end{itemize}
  72         \end{block}
  73 \end{frame}
  74
  75 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  76 \section{Technologies for Distributed Networking}
  77 \begin{frame}{Address Allocation}
  78 IPv4 Link-Local Addessing (``APIPA'', ``Zeroconf'', RFC 3927)
  79 \begin{itemize}
  80         \item subnet \code{169.254.0.0/16}
  81 \end{itemize}
  82 IPv6 Stateless Address Autoconfiguration (RFC 4862)
  83 \begin{itemize}
  84         \item subnet \code{fe80::/64} (link-local)
  85         \item subnet \code{fc00::/11} (unique-local, if configured)
  86         \item or global address (if configured)
  87 \end{itemize}
  88
  89 \begin{block}{Algorithm}
  90 \begin{enumerate}
  91         \item choose (random) IP address in subnet
  92         \item ask if anyone uses that address
  93         \item if not, we're fine
  94         \item else, retry
  95 \end{enumerate}
  96 \end{block}
  97 \end{frame}
  98
  99 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 100 \againframe<2>{subgoals}
 101
 102 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 103 \begin{frame}{Multicast DNS (RFC 6762)}
 104 \begin{itemize}
 105 \item distributed DNS database
 106 \item uses multicast address \code{224.0.0.251} (IPv4) and \code{ff02::fb}
 107         (IPv6), UDP port 5353
 108 \item standard DNS packet format
 109 \item hosts announce their own resources
 110 \item hosts respond to queries if queried resource is known
 111 \end{itemize}
 112 \end{frame}
 113
 114 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 115 \begin{frame}{DNS-Based Service Discovery (RFC 6763)}
 116 Two-step process:
 117 \begin{enumerate}
 118         \item Service Instance Enumeration
 119         \begin{itemize}
 120                 \item query PTR records of form \code{\_service.\_proto.domain}
 121                 \item results: instance names of form \code{name.\_service.\_proto.domain}
 122         \end{itemize}
 123         \item Service Instance Resolution
 124         \begin{itemize}
 125                 \item query instance names as SRV records
 126                 \item result gives host name, port, priority, weight
 127         \end{itemize}
 128 \end{enumerate}
 129 \end{frame}
 130
 131 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 132 \begin{frame}[fragile]{Example: mDNS + DNS-SD}
 133 \tikzset{font=\fontsize{7.5pt}{1em}\selectfont, >=latex, thick, y=3.5em}
 134 %\tikzstyle{iconlabel}=[text width=2cm, align=center,
 135         %font=\fontsize{7.5pt}{1em}\selectfont]
 136 \tikzstyle{label}=[font=\fontsize{7pt}{1em}\selectfont]
 137 \tikzstyle{pkt}=[font=\fontsize{6pt}{0.7em}\selectfont,
 138         draw,fill=white,dash pattern=on 2pt off 2pt, align=center,
 139         inner sep=2pt]
 140 \def\xhalf{.45\textwidth}
 141 \def\nodeskip{2.2em}
 142 \begin{enumerate}
 143         \item Service Instance Enumeration
 144         \begin{tikzpicture}
 145                 \node[cloud,draw,fill=gray!20,aspect=2] (thecloud) at (0,0) {multicast};
 146                 \def\nodelist{
 147                         node/{(-\xhalf,0)}/{},
 148                         juliet/{(\xhalf,0)}/juliet,
 149                         romeo/{(\xhalf,-1)}/romeo,
 150                         mercutio/{(\xhalf,1)}/mercutio%
 151                 }
 152                 \foreach \name/\pos/\text in \nodelist {
 153                         \node (\name) at \pos { \includegraphics[width=3em]{icon-computer.pdf} };
 154                         \node[below=\nodeskip of \name.north,label] (\name text) { \text };
 155                 }
 156                 \draw[->] (node) -- (thecloud) node[pkt,pos=0.5]{
 157                         \verb+_presence._tcp.local+\\PTR? };
 158                 \draw[->] (juliet) -- (thecloud) node[pkt,pos=0.4]{
 159                         \verb+juliet._presence.+\\\verb+_tcp.local+ };
 160                 \draw[->] (romeo) -- (thecloud) node[pkt,pos=0.4]{
 161                         \verb+romeo._presence.+\\\verb+_tcp.local+ };
 162                 \draw[->] (mercutio) -- (thecloud) node[pkt,pos=0.4]{
 163                         \verb+mercutio._presence.+\\\verb+_tcp.local+ };
 164         \end{tikzpicture}
 165
 166         \item Service Instance Resolution
 167         \begin{tikzpicture}
 168                 \node[cloud,draw,fill=gray!20,aspect=2] (thecloud) at (0,0) {multicast};
 169                 \node (node) at (-\xhalf,0) { \includegraphics[width=3em]{icon-computer.pdf} };
 170                 \node (juliet) at (\xhalf,0) { \includegraphics[width=3em]{icon-computer.pdf} };
 171                 \node[below=\nodeskip of juliet.north,label] (juliet text) { juliet };
 172                 \draw[->] (node) -- (thecloud) node[pkt,pos=0.5]{
 173                         \verb+juliet._presence._tcp.local+\\SRV? };
 174                 \draw[->] (juliet) -- (thecloud) node[pkt,pos=0.5]{
 175                         \verb+juliet.local, port 5222,+\\\verb+prio 0, weight 0+ };
 176         \end{tikzpicture}
 177 \end{enumerate}
 178 \end{frame}
 179
 180 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 181 \againframe<3>{subgoals}
 182
 183 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 184 \begin{frame}{XMPP (RFC 6122)}
 185 \begin{columns}[c]
 186         \column{.55\textwidth}
 187         \begin{onlyenv}<1>
 188                 \begin{block}{Extensible Messaging and Presence Protocol}
 189                 \begin{itemize}
 190                         \item XML-based
 191                         \item Federated architecture
 192                         \begin{itemize}
 193                                 \item modeled after E-Mail
 194                         \end{itemize}
 195                         \item publish-subscribe mechanism
 196                         \item XMPP Extension Protocols (XEPs), e.\,g.
 197                         \begin{itemize}
 198                                 \item Multi-User Chats % (XEP-0045)
 199                                 \item Serverless Messaging %(XEP-0174)
 200                                 \item recently, also XEPs for the IoT %(XEP-0323 through -0326)
 201                         \end{itemize}
 202                 \end{itemize}
 203                 \end{block}
 204         \end{onlyenv}
 205         \begin{onlyenv}<2>
 206                 \begin{block}{Serverless XMPP (XEP-0174)}
 207                 \begin{itemize}
 208                         \item Clients communicate directly, no server needed
 209                         \item service discovery via mDNS/DNS-SD
 210                 \end{itemize}
 211                 \end{block}
 212         \end{onlyenv}
 213
 214         \column{.45\textwidth}
 215         \tikzset{node distance=0pt, >=stealth, thick, x=3.5em, y=2.8em}
 216         \tikzstyle{iconlabel}=[text width=2.5cm, align=center,
 217                 font=\fontsize{7.5pt}{1em}\selectfont]
 218         \tikzstyle{label}=[font=\fontsize{7pt}{1em}\selectfont]
 219         \begin{overprint}
 220                 \begin{figure}
 221                 \begin{tikzpicture}[remember picture]
 222                         \input{fig-xmpparch.tex}
 223                 \end{tikzpicture}
 224                 \end{figure}
 225                 \tikz[remember picture,overlay]
 226                         \draw<2>[<->,dotted] (juliet) -- node[anchor=south,label]{serverless} (romeo);
 227         \end{overprint}
 228 \end{columns}
 229 \end{frame}
 230
 231 \begin{frame}{XMPP (RFC 6122)}
 232         \begin{block}{XEPs for the Internet of Things}
 233                 \begin{itemize}
 234                         \item XEP-0323 Sensor Data
 235                         \begin{itemize}
 236                                 \item format for sensor data, query modes
 237                         \end{itemize}
 238                         \item XEP-0324 Provisioning
 239                         \begin{itemize}
 240                                 \item defining access rights and user privileges
 241                         \end{itemize}
 242                         \item XEP-0325 Control
 243                         \begin{itemize}
 244                                 \item get/set control prameters on a (group of) sensor node(s)
 245                         \end{itemize}
 246                         \item XEP-0326 Concentrators
 247                         \begin{itemize}
 248                                 \item implement proxies for a subnet of the WSN
 249                         \end{itemize}
 250                 \end{itemize}
 251         \end{block}
 252 \end{frame}
 253
 254 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 255 \againframe<4>{subgoals}
 256
 257 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 258 \section{Chatty Things}
 259 \begin{frame}{Chatty Things}
 260 \begin{itemize}
 261         \item use serverless XMPP + mDNS + DNS-SD for communication
 262         \item interaction using a standard XMPP client
 263         \item prevent information overflow
 264         \begin{itemize}
 265                 \item ``Traffic lights'': status icon in roster represents threshold value
 266                 \item \emph{Temporary Subscription for Presence (TSP)}
 267         \end{itemize}
 268         %\item working prototype for Contiki on MSP430
 269 \end{itemize}
 270 \begin{block}{Prototype}
 271         \begin{itemize}
 272                 \item \emph{uBonjour} for mDNS + DNS-SD
 273                 \item \emph{uXMPP} for XMPP
 274                 \item 12~kB of ROM, 0.6 kB of RAM with Contiki on MSP-430
 275         \end{itemize}
 276 \end{block}
 277 \end{frame}
 278
 279 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 280 \begin{frame}{Temporary Subscription for Presence}
 281         \begin{block}{Problem}
 282                 \begin{itemize}
 283                         \item node must manually subscribe to get information
 284                         \item users can move quickly out of the network
 285                         \item subscriptions become outdated
 286                         \item renewing/canceling subscriptions needs bandwidth
 287                         \item data publishers also get updates
 288                 \end{itemize}
 289         \end{block}
 290 \end{frame}
 291
 292 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 293 \begin{frame}{Temporary Subscription for Presence}
 294 \begin{block}{Solution: Multi-User Chats}
 295         \begin{itemize}
 296                 \item create one chat room per topic
 297                 \item users subscribe to information by entering the chat room
 298                 \item server only sends information to nodes who want it
 299                 \begin{itemize}
 300                         \item Chatty Things send a flag that they're uninterested
 301                 \end{itemize}
 302         \end{itemize}
 303 \end{block}
 304 \begin{block}{Drawbacks}
 305         \begin{itemize}
 306                 \item only works with central XMPP server
 307                 \begin{itemize}
 308                         \item XEP-0045 is not (yet) specified for serverless XMPP
 309                 \end{itemize}
 310                 \item XMPP server needs to handle TSP
 311         \end{itemize}
 312 \end{block}
 313 \end{frame}
 314
 315 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 316 \begin{frame}{Bootstrapping}
 317 \begin{block}{At Boot}
 318         \begin{enumerate}
 319                 \item activate uBonjour
 320                 \item try to discover a central XMPP server
 321                 \begin{itemize}
 322                         \item DNS-SD: \code{\_xmpp-client.\_tcp.local}
 323                 \end{itemize}
 324                 \item if an XMPP server is discovered: \emph{Infrastructure mode}
 325                 \begin{itemize}
 326                         \item connect with ANONYMOUS login (XEP-0175)
 327                         \item join topic-based chats
 328                         \item deactivate uBonjour
 329                 \end{itemize}
 330                 \item if no server is found: \emph{Ad hoc mode}
 331                 \begin{itemize}
 332                         \item activate serverless messaging
 333                 \end{itemize}
 334         \end{enumerate}
 335 \end{block}
 336 \end{frame}
 337 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 338 \begin{frame}{Bootstrapping}
 339 \begin{block}{During Runtime}
 340         \begin{itemize}
 341                 \item if server is lost, change to Ad hoc mode
 342                 \item if new server is found in Ad hod mode, try changing to Infrastructure
 343                         mode
 344                 \begin{itemize}
 345                         \item if that fails, stay in Ad hoc mode
 346                 \end{itemize}
 347         \end{itemize}
 348 \end{block}
 349 \end{frame}
 350
 351 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 352 \againframe<5>{subgoals}
 353
 354 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 355 \section{Related Approaches}
 356 \begin{frame}{Related Approaches}
 357 \begin{overprint}
 358         \onslide<1>
 359         \begin{block}{Chatty Things}
 360         \end{block}
 361         \onslide<2>
 362         \begin{block}{Constrained Application Protocol (CoAP)}
 363                 \begin{itemize}
 364                         \item binary mapping to HTTP
 365                         \item UDP with confirmation and congestion control
 366                 \end{itemize}
 367         \end{block}
 368         \onslide<3>
 369         \begin{block}{MQ Telemetry Transport (MQTT)}
 370                 \begin{itemize}
 371                         \item binary, only 2-byte header
 372                         \item focused on M2M communication
 373                 \end{itemize}
 374         \end{block}
 375         \onslide<4>
 376         \begin{block}{Web Service for Devices (WS4D)}
 377                 \begin{itemize}
 378                         \item SOAP (XML-based) over HTTP
 379                 \end{itemize}
 380         \end{block}
 381 \end{overprint}
 382 \begin{table}
 383         \small\centering
 384         \begin{tabular}{|>{\onslide<1->}l||>{\onslide<1->}l|>{\onslide<2->}l|
 385                 >{\onslide<3->}l|>{\onslide<4->}l<{\onslide}|}
 386                 \hline
 387                 Feature & Chatty Things & CoAP & MQTT & WS4D \\
 388          \hline\hline
 389                 application gateways & - & yes & yes & - \\ \hline
 390                 usable with standard clients & yes & - & - & (yes) \\ \hline
 391                 discovery support & yes & yes & - & yes\\ \hline
 392                 IPv6/6LoWPAN ready & yes & yes & ? & partial \\ \hline
 393                 asynchronous messages & yes & yes & ? & ? \\ \hline
 394                 protocol overhead & moderate & low & low & high \\ \hline
 395         \end{tabular}
 396 \end{table}
 397 \end{frame}
 398
 399 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 400 \section{Conclusion}
 401 \begin{frame}{Conclusion}
 402 \begin{block}{Advantages of Chatty Things}
 403         \begin{itemize}
 404                 \item no need for central infrastructure
 405                 \item self-configuration and auto-discovery
 406                 \item interaction over standard chat clients
 407                 \item protocol flexibility for enhancements
 408         \end{itemize}
 409 \end{block}
 410 \begin{block}{Disadvantages of Chatty Things}
 411         \begin{itemize}
 412                 \item XMPP introduces some complexity
 413                 \item topic filtering only possible with central server
 414         \end{itemize}
 415 \end{block}
 416 \end{frame}
 417
 418 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 419 \begin{frame}
 420 \vfill
 421 \centering\LARGE{Questions?}
 422 \vfill
 423 \end{frame}
 424
 425 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 426 \section*{Example}
 427 \def\bot{-7.5}
 428 \def\nearlybot{-7}
 429 \def\top{0}
 430 \def\nearlytop{-0.5}
 431 \def\dev{-5}
 432 \def\cli{5}
 433 \newlength{\ypos}
 434 \newlength{\yypos}
 435 \newcommand{\advanceby}[1]{%
 436         \addtolength{\ypos}{#1}%
 437         \addtolength{\yypos}{#1}%
 438         \global\ypos=\ypos% // see https://tex.stackexchange.com/questions/119730
 439         \global\yypos=\yypos%
 440 }
 441 \tikzstyle{pkt}=[
 442   fill,
 443   below=-.5\baselineskip,
 444   font=\scriptsize,
 445   draw,
 446   dash pattern=on 2pt off 2pt,
 447 ]
 448 \tikzstyle{clipkt}=[pkt,
 449   below left=-.5\baselineskip and 0cm,
 450   pos=0.05,
 451 ]
 452 \tikzstyle{devpkt}=[pkt,
 453   below right=-.5\baselineskip and 0cm,
 454   pos=0.05,
 455 ]
 456 %%%%%%%%%
 457 \begin{frame}[fragile,plain,t]{Example: Sensor Data}
 458 \begin{tikzpicture}[
 459   every node/.style={fill=white, font=\small},
 460   every picture/.style=ultra thin,
 461   >=latex,
 462   semithick
 463 ]
 464 \setlength{\ypos}{0cm}
 465 \setlength{\yypos}{.5cm}
 466 %% Header
 467 \draw (\cli,\top) node[draw]{Client} -- (\cli,\nearlybot);
 468 \draw[dashed] (\cli,\nearlybot) -- (\cli,\bot);
 469 \draw (\dev,\top) node[draw]{Device} -- (\dev,\nearlybot);
 470 \draw[dashed] (\dev,\nearlybot) -- (\dev,\bot);
 471
 472 \advanceby{.75cm}
 473 \draw[->] (\cli,-\ypos) -- (\dev,-\yypos) node[clipkt]{\verb+<stream>+};
 474 \advanceby{1cm}
 475 \draw[->] (\dev,-\ypos) -- (\cli,-\yypos) node[devpkt]{\verb+<stream>+};
 476
 477 \advanceby{1cm}
 478 \draw[->] (\cli,-\ypos) -- (\dev,-\yypos) node[clipkt,align=left]{
 479 \verb+<iq type='get' id='1'+\\
 480 \verb+    to='dev@device.local/res'+\\
 481 \verb+    from='client@client.local'>+\\
 482 \verb+  <req xmlns='urn:xmpp:sn' seqnr='1'+\\
 483 \verb+      momentary='true'>+\\
 484 \verb+    <node nodeId='Device04'/>+\\
 485 \verb+    <field name='+{\color{red}\code{Energy}}\verb+'/>+\\
 486 \verb+    <field name='+{\color{blue}\code{Power}}\verb+'/>+\\
 487 \verb+  </req> </iq>+
 488 %\end{verbatim}
 489 \advanceby{9\baselineskip}
 490 };
 491
 492 \advanceby{0.3cm}
 493 \draw[->] (\dev,-\ypos) -- (\cli,-\yypos) node[devpkt,align=left]{
 494 \verb+<iq type='result' id='1'+\\
 495 \verb+    from='dev@device.local'+\\
 496 \verb+    to='client@client.local/res'>+\\
 497 \verb+  <accepted xmlns='urn:xmpp:sn' seqnr='1'/>+\\
 498 \verb+</iq>+
 499 \advanceby{4\baselineskip}
 500 };
 501 \end{tikzpicture}
 502 \end{frame}
 503
 504 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 505 \begin{frame}[fragile,plain,t]{Example: Sensor Data (cont.)}
 506 \begin{tikzpicture}[
 507   every node/.style={fill=white, font=\small},
 508   every picture/.style=ultra thin,
 509   >=latex,
 510   semithick
 511 ]
 512 \setlength{\ypos}{0cm}
 513 \setlength{\yypos}{.5cm}
 514 %% Header
 515 \draw[dashed] (\cli,\top) node[draw]{Client} -- (\cli,\nearlytop);
 516 \draw (\cli,\nearlytop) -- (\cli,\bot);
 517 \draw[dashed] (\dev,\top) node[draw]{Device} -- (\dev,\nearlytop);
 518 \draw (\dev,\nearlytop) -- (\dev,\bot);
 519
 520 \advanceby{.75cm}
 521 %\advanceby{1cm}
 522 \draw[->] (\dev,-\ypos) -- (\cli,-\yypos) node[devpkt,align=left]{
 523 \verb+<message from='dev@device.local'+\\
 524 \verb+    to='client@client.local/res'>+\\
 525 \verb+  <fields xmlns='urn:xmpp:sn' seqnr='1' done='true'>+\\
 526 \verb+    <node nodeId='Device04'>+\\
 527 \verb+      <timestamp value='2013-03-07T22:03:15'>+\\
 528 \verb+        <numeric name='+{\color{red}\code{Energy}}\verb+' momentary='true'+\\
 529 \verb+          value='+{\color{red}\code{167.5}}\verb+' unit='+{\color{red}\code{kWh}}\verb+'/>+\\
 530 \verb+        <numeric name='+{\color{blue}\code{Power}}\verb+' momentary='true'+\\
 531 \verb+          value='+{\color{blue}\code{239.4}}\verb+' unit='+{\color{blue}\code{W}}\verb+'/>+\\
 532 \verb+      </timestamp>+\\
 533 \verb+    </node>+\\
 534 \verb+  </fields>+\\
 535 \verb+</message>+
 536 \advanceby{12\baselineskip}
 537 };
 538
 539 \advanceby{1cm}
 540 \draw[->] (\cli,-\ypos) -- (\dev,-\yypos) node[clipkt]{\verb+</stream>+};
 541 \advanceby{1cm}
 542 \draw[->] (\dev,-\ypos) -- (\cli,-\yypos) node[devpkt]{\verb+</stream>+};
 543 \end{tikzpicture}
 544 \end{frame}
 545 \end{document}