Merge branch 'presentation'
[skm-ma-ws1314.git] / 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}
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