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 \pages{3}
   8 \subsection{Service Provisioning Sublayer}
   9
  10 Considering the application in deeply embedded systems and the special needs of
  11 the Internet of Things on the one hand, the protocol stack needs to fulfil
  12 certain technical requirements.
  13
  14 First, memory, computing resources and bandwith on embedded systems are limited,
  15 which demands for a lightweight protocol stack without too much overhead and
  16 predictable memory consumption, while also retaining enough flexibility for
  17 future development. The authors therefore provide only the most essential
  18 functions of XMPP (presence and message exchange, multi-user chats) to achieve
  19 basic communication, grouping of devices, and information filtering to prevent
  20 information overflow.
  21
  22 Their solution builds on the \term{Contiki} operating system on an MSP430 board,
  23 and uses the \term{uXMPP} project, which already implements core XMPP features
  24 and serves as a starting point for implementing further XEPs. However, as the
  25 uXMPP project was still in early development, they first needed to enhance its
  26 functionality to comply with the limited resources. In particular, they
  27 optimized the message flow and enabled uXMPP to fully use the available payload
  28 instead of sending one TCP packet per message and reduced the code footprint
  29 through compiler flags and refactoring.
  30
  31 Furthermore, Klauck and Kirsche implemented new features for uXMPP, which were
  32 realized as separate modules to allow enabling and disabling them at runtime,
  33 thus further reducing the memory footprint of a running system:
  34
  35 \todo{minimize space between list items}
  36 \begin{itemize}
  37   \item support for IPv6
  38   \item support for Multi-User Chats (XEP-0045), which are used for information
  39     filtering
  40   \item support for SASL ANONYMOUS login for XMPP servers \cite{xep0175}
  41   \item a new publish-subscribe mechanism called Temporary Subscription for
  42     Presence (see \ref{sec:tsp})
  43   \item XMPP Serverless Messaging (XEP-0174), using \term{uBonjour} as
  44     underlying mDNS/DNS-SD implementation for Contiki.
  45 \end{itemize}
  46
  47 The resulting implementation (uXMPP and uBonjour) gets by with 12{.}2\ kBytes of
  48 ROM and 0{.}63\ kBytes of RAM, which was about the size of the original,
  49 unoptimized uXMPP implementation while also implementing new features.
  50
  51 \todo{figure of example network structure with and without central server}
  52
  53 In order to react to different network infrastructures, their implementation
  54 allows both communication with a central XMPP server as well as peer-to-peer
  55 communication over XMPP Serverless Messaging. When a central XMPP server is
  56 detected over uBonjour, it is used instead with the ANONYMOUS login method, and the
  57 XEP-0174 module is disabled. The ANONYMOUS login method is chosen since TLS
  58 encryption is not yet implemented, and it assigns random JID to the client,
  59 which do not need to exist on the server. However, a server must exist and must
  60 be configured in order to use this method.
  61
  62 With a server, information filtering is achieved by creating topic-based
  63 Multi-User Chats where multiple devices can be grouped. A user can then simply
  64 join the chat with a standard XMPP client on her machine and interact with all
  65 devices of a topic, or she can also interact with them directly.
  66
  67 In scenarios without an XMPP server, the XEP-0174 module is activated and
  68 devices talk directly with the user or with other devices. This method has the
  69 drawback that Multi-User Chats cannot be used for topic filtering, since no
  70 method is specified to do XEP-0045 and XEP-0174 at the same time. In this case,
  71 a user must have an XEP-0174-compliant chat client, but it also gives her the
  72 opportunity to interact with things spontaneously on an ad-hoc basis (e.~g. when
  73 entering a room) without need for any additional gateway on the application
  74 level.
  75
  76 %low bandwith in IEEE~802.15.4 (less than 250 kib/s at a maximum of 122 byte per
  77 %packet\footnote{according to the maximum MAC payload size in the IEEE~802.15.4-2011
  78 %specification}
  79
  80 \subsection{Bootstrapping}
  81
  82 In a distributed context like the Internet of Things, devices need to be ready
  83 to use out of the box. Users often do not want to set up configurations for each
  84 device they use, and when using several of those devices, it is often not
  85 reasonable having to configure every single one.
  86
  87 With the given approach, bootstrapping new Chatty Things is easy and no
  88 configuration is neccessary: on the network layer, IP addresses can simply be
  89 obtained using IPv4 Link-Local Addressing or IPv6 Stateless Address
  90 Autoconfiguration. On the transport layer, all needed ports can be obtained over
  91 DNS-Based Service Discovery. Finally, on the application layer, host names can
  92 be resolved over Multicast DNS; and for the actual communication between devices
  93 it is possible to use auto-generated JIDs with the ANONYMOUS login method on an
  94 existing XMPP server, or if no server is found, use peer-to-peer communication
  95 over XMPP Serverless Messaging.
  96
  97 Bootstrapping a Chatty Thing therefore incorporates three steps:
  98
  99 \begin{enumerate}
 100   \item Activate uBonjour and try to discover an XMPP server
 101   \item If a server is found, connect to it using ANONYMOUS login, join
 102     topic-based Multi-User Chats, deactivate the uBonjour client
 103     \term(Infrastructure mode).
 104   \item If no server is found, activate the XEP-0174 client \term{Ad-hoc mode}.
 105 \end{enumerate}
 106
 107 During runtime, a device can then react to changes in network infrastructure:
 108
 109 \begin{itemize}
 110   \item In Infrastructure mode: when connection to the server is lost, enable
 111     the uBonjour client, try to find a server, and when none is found, enable
 112     Serverless Messaging.
 113   \item In Ad-hoc mode: if uBonjour detects a new XMPP server joining the
 114     network, try to connect to it. If this succeeds, disable Serverless
 115     Messaging and uBonjour and join topic-based Multi-User Chats.
 116 \end{itemize}
 117
 118 \todo{short conclusion?}
 119
 120 \subsection{Temporary Subscription for Presence}\label{sec:tsp}
 121
 122 To further reduce the message overhead and allow more fine-grained controls over
 123 information filtering, \term{Temporary Subscription for Presence} is introduced.
 124 This technique builds on top of presence stanzas as defined in core XMPP. These
 125 presence stanzas are sent without a \code{to} or \code{from} attribute, and
 126 therefore fit into a single TCP/IP packet over IEEE~802.15.4. However, to be
 127 able to receive these stanzas, a client must manually subscribe to those
 128 information in their roster, which requires further communication between nodes. As the network
 129 can change rapidly, subscriptions would be often outdated, and there would be
 130 much overhead of subscriptions and unsubscription packets, which would inhibit
 131 the flow of the actual information.
 132
 133 To solve this problem, a dynamic, topic-based roster is implemented on top of
 134 Multi-User Chats (XEP-0045). Every topic corresponds with a chat room, and nodes
 135 join the chat rooms which they are interested in. This allows nodes to inform
 136 only interested nodes about updates. This has the advantage that existing
 137 clients supporting Multi-User Chats can be used by a user, but Chatty Things and
 138 XMPP servers need to be adapted to the new subscription model. Also, this
 139 mechanism does not work with Serverless Messaging.
 140
 141 % vim: set ft=tex et ts=2 sw=2 :