sync: Temporary Subscription for Presence
[skm-ma-ws1314.git] / sec-chatty-things.tex
1 \section{System Architecture of ``Chatty Things''}
2
3 After the underlying techniques have been explained, we can have a look at the
4 system architecture which Klauck and
5 Kirsche~\cite{Klauck:2012:BCC:2352852.2352881} use to build Chatty Things.
6
7 \todo
8 \pages{3}
9 \subsection{Service Provisioning Sublayer}
10
11 Considering the application in deeply embedded systems and the special needs of
12 the Internet of Things on the one hand, the protocol stack needs to fulfil
13 certain technical requirements.
14
15 First, memory, computing resources and bandwith on embedded systems are limited,
16 which demands for a lightweight protocol stack without too much overhead and
17 predictable memory consumption, while also retaining enough flexibility for
18 future development. The authors therefore provide only the most essential
19 functions of XMPP (presence and message exchange, multi-user chats) to achieve
20 basic communication, grouping of devices, and information filtering to prevent
21 information overflow.
22
23 Their solution builds on the \term{Contiki} operating system on an MSP430 board,
24 and uses the \term{uXMPP} project, which already implements core XMPP features
25 and serves as a starting point for implementing further XEPs. However, as the
26 uXMPP project was still in early development, they first needed to enhance its
27 functionality to comply with the limited resources. In particular, they
28 optimized the message flow and enabled uXMPP to fully use the available payload
29 instead of sending one TCP packet per message and reduced the code footprint
30 through compiler flags and refactoring.
31
32 Furthermore, Klauck and Kirsche implemented new features for uXMPP, which were
33 realized as separate modules to allow enabling and disabling them at runtime,
34 thus further reducing the memory footprint of a running system:
35
36 \todo[minimize space between list item]
37 \begin{itemize}
38 \item support for IPv6
39 \item support for Multi-User Chats (XEP-0045), which are used for information
40 filtering
41 \item support for SASL ANONYMOUS login for XMPP servers \ref{xep-0175}
42 \item a new publish-subscribe mechanism called Temporary Subscription for
43 Presence (see \ref{sec:tsp})
44 \item XMPP Serverless Messaging (XEP-0174), using \term{uBonjour} as
45 underlying mDNS/DNS-SD implementation for Contiki.
46 \end{itemize}
47
48 The resulting implementation (uXMPP and uBonjour) gets by with 12{.}2\ kBytes of
49 ROM and 0{.}63\ kBytes of RAM, which was about the size of the original,
50 unoptimized uXMPP implementation while also implementing new features.
51
52 \todo[figure of example network structure with and without central server]
53
54 In order to react to different network infrastructures, their implementation
55 allows both communication with a central XMPP server as well as peer-to-peer
56 communication over XMPP Serverless Messaging. When a central XMPP server is
57 detected over uBonjour, it is used instead with the ANONYMOUS login method, and the
58 XEP-0174 module is disabled. The ANONYMOUS login method is chosen since TLS
59 encryption is not yet implemented, and it assigns random JID to the client,
60 which do not need to exist on the server. However, a server must exist and must
61 be configured in order to use this method.
62
63 With a server, information filtering is achieved by creating topic-based
64 Multi-User Chats where multiple devices can be grouped. A user can then simply
65 join the chat with a standard XMPP client on her machine and interact with all
66 devices of a topic, or she can also interact with them directly.
67
68 In scenarios without an XMPP server, the XEP-0174 module is activated and
69 devices talk directly with the user or with other devices. This method has the
70 drawback that Multi-User Chats cannot be used for topic filtering, since no
71 method is specified to do XEP-0045 and XEP-0174 at the same time. \todo[temporary subscription presence here?]
72 In this case,
73 a user must have an XEP-0174-compliant chat client, but it also gives her the
74 opportunity to interact with things spontaneously on an ad-hoc basis (e.~g. when
75 entering a room) without need for any additional gateway on the application
76 level.
77
78 %low bandwith in IEEE~802.15.4 (less than 250 kib/s at a maximum of 122 byte per
79 %packet\footnote{according to the maximum MAC payload size in the IEEE~802.15.4-2011
80 %specification}
81
82 \subsection{Bootstrapping}
83
84 In a distributed context like the Internet of Things, devices need to be ready
85 to use out of the box. Users often do not want to set up configurations for each
86 device they use, and when using several of those devices, it is often not
87 reasonable having to configure every single one.
88
89 With the given approach, bootstrapping new Chatty Things is easy and no
90 configuration is neccessary: on the network layer, IP addresses can simply be
91 obtained using IPv4 Link-Local Addressing or IPv6 Stateless Address
92 Autoconfiguration. On the transport layer, all needed ports can be obtained over
93 DNS-Based Service Discovery. Finally, on the application layer, host names can
94 be resolved over Multicast DNS; and for the actual communication between devices
95 it is possible to use auto-generated JIDs with the ANONYMOUS login method on an
96 existing XMPP server, or if no server is found, use peer-to-peer communication
97 over XMPP Serverless Messaging.
98
99 Bootstrapping a Chatty Thing therefore incorporates three steps:
100
101 \begin{enumerate}
102 \item Activate uBonjour and try to discover an XMPP server
103 \item If a server is found, connect to it using ANONYMOUS login, join
104 topic-based Multi-User Chats, deactivate the uBonjour client
105 \term(Infrastructure mode).
106 \item If no server is found, activate the XEP-0174 client \term{Ad-hoc mode}.
107 \end{enumerate}
108
109 During runtime, a device can then react to changes in network infrastructure:
110
111 \begin{itemize}
112 \item In Infrastructure mode: when connection to the server is lost, enable
113 the uBonjour client, try to find a server, and when none is found, enable
114 Serverless Messaging.
115 \item In Ad-hoc mode: if uBonjour detects a new XMPP server joining the
116 network, try to connect to it. If this succeeds, disable Serverless
117 Messaging and uBonjour and join topic-based Multi-User Chats.
118 \end{itemize}
119
120 \todo[short conclusion?]
121
122 \subsection{Temporary Subscription for Presence}\label{sec:tsp}
123
124 To further reduce the message overhead and allow more fine-grained controls over
125 information filtering, \term{Temporary Subscription for Presence} is introduced.
126 This technique builds on top of presence stanzas as defined in core XMPP. These
127 presence stanzas are sent without a \code{to} or \code{from} attribute, and
128 therefore fit into a single TCP/IP packet over IEEE~802.15.4. However, to be
129 able to receive these stanzas, a client must manually subscribe to those
130 information in their roster, which requires further communication between nodes. As the network
131 can change rapidly, subscriptions would be often outdated, and there would be
132 much overhead of subscriptions and unsubscription packets, which would inhibit
133 the flow of the actual information.
134
135 To solve this problem, a dynamic, topic-based roster is implemented on top of
136 Multi-User Chats (XEP-0045). Every topic corresponds with a chat room, and nodes
137 join the chat rooms which they are interested in. This allows nodes to inform
138 only interested nodes about updates. This has the advantage that existing
139 clients supporting Multi-User Chats can be used by a user, but Chatty Things and
140 XMPP servers need to be adapted to the new subscription model. Also, this
141 mechanism does not work with Serverless Messaging.
142
143 % vim: set ft=tex et ts=2 sw=2 :
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