sec-chatty-things.tex

   1 \section{System Architecture of ``Chatty Things''}\label{sec:arch}
   2
   3 After the underlying techniques have been explained, we can now have a look at
   4 the system architecture which Klauck and
   5 Kirsche~\cite{Klauck:2012:BCC:2352852.2352881} use to build Chatty Things.
   6
   7 \subsection{Service Provisioning Sublayer}
   8
   9 Considering the application in deeply embedded systems and the special needs of
  10 the Internet of Things on the one hand, the protocol stack needs to fulfill
  11 certain technical requirements.
  12
  13 First, memory, computing resources and bandwidth on embedded systems are limited,
  14 which demands for a lightweight protocol stack without too much overhead and
  15 predictable memory consumption, while also retaining enough flexibility for
  16 future development. The authors therefore provide only the most essential
  17 functions of XMPP (presence and message exchange, multi-user chats) to achieve
  18 basic communication, grouping of devices, and information filtering to prevent
  19 information overflow.
  20
  21 Their solution builds on the \term{Contiki} operating system on an MSP430 board,
  22 and uses the \term{uXMPP} project, which already implements core XMPP features
  23 and serves as a starting point for implementing further XEPs. However, as the
  24 uXMPP project was still in early development, they first needed to enhance its
  25 functionality to comply with the limited resources. In particular, they
  26 optimized the message flow and enabled uXMPP to fully use the available payload
  27 instead of sending one TCP packet per message and reduced the code footprint
  28 through compiler flags and refactoring.
  29
  30 Furthermore, Klauck and Kirsche implemented new features for uXMPP, which were
  31 realized as separate modules to allow enabling and disabling them at runtime,
  32 thus further reducing the memory footprint of a running system:
  33
  34 \begin{itemize}
  35   \item support for IPv6
  36   \item support for Multi-User Chats (XEP-0045), which are used for information
  37     filtering
  38   \item support for SASL ANONYMOUS login for XMPP servers \cite{xep0175}
  39   \item a new publish-subscribe mechanism called Temporary Subscription for
  40     Presence (see Section~\ref{sec:tsp})
  41   \item XMPP Serverless Messaging (XEP-0174), using \term{uBonjour} as
  42     underlying mDNS/DNS-SD implementation for Contiki.
  43 \end{itemize}
  44
  45 The resulting implementation (uXMPP and uBonjour) gets by with 12{.}2\ kBytes of
  46 ROM and 0{.}63\ kBytes of RAM, which was about the size of the original,
  47 unoptimized uXMPP implementation while also implementing new features.
  48
  49 In order to react to different network infrastructures, their implementation
  50 allows both communication with a central XMPP server as well as peer-to-peer
  51 communication over XMPP Serverless Messaging. When a central XMPP server is
  52 detected over uBonjour, it is used instead with the ANONYMOUS login method, and
  53 the XEP-0174 module is disabled. The ANONYMOUS login method is chosen since TLS
  54 encryption is not yet implemented, and with this method, the server assigns a
  55 random JID to the client, which does not need to exist on the server. However, a
  56 server must exist and must be configured to supply this login method in order
  57 for this approach to work.
  58
  59 With a server, information filtering is achieved by creating topic-based
  60 Multi-User Chats where multiple devices can be grouped. A user can then simply
  61 join the chat with a standard XMPP client on her machine and interact with all
  62 devices of a topic, or she can also interact with single devices directly.
  63
  64 In scenarios without an XMPP server, the XEP-0174 module is activated and
  65 devices talk directly with the user or with other devices. This method has the
  66 drawback that Multi-User Chats cannot be used for topic filtering, since no
  67 method is specified to do XEP-0045 and XEP-0174 at the same time. In this case,
  68 a user must have an XEP-0174-compliant chat client, but it also gives her the
  69 opportunity to interact with things spontaneously on an ad hoc basis (e.~g. when
  70 entering a room) without need for any additional gateway on the application
  71 level.
  72
  73 %low bandwith in IEEE~802.15.4 (less than 250 kib/s at a maximum of 122 byte per
  74 %packet\footnote{according to the maximum MAC payload size in the IEEE~802.15.4-2011
  75 %specification}
  76
  77 \subsection{Bootstrapping}
  78
  79 With the given approach, bootstrapping new Chatty Things is easy and no
  80 configuration is necessary: on the network layer, IP addresses can simply be
  81 obtained using IPv4 Link-Local Addressing or IPv6 Stateless Address
  82 Autoconfiguration. On the transport layer, all needed ports can be obtained over
  83 DNS-Based Service Discovery. Finally, on the application layer, host names can
  84 be resolved over Multicast DNS; and for the actual communication between devices
  85 it is possible to use auto-generated JIDs with the ANONYMOUS login method on an
  86 existing XMPP server, or if no server is found, use peer-to-peer communication
  87 over XMPP Serverless Messaging.
  88
  89 Bootstrapping a Chatty Thing therefore incorporates three steps:
  90
  91 \begin{enumerate}
  92   \item Activate uBonjour and try to discover an XMPP server.
  93   \item If a server is found, connect to it using ANONYMOUS login, join
  94     topic-based Multi-User Chats, deactivate the uBonjour client
  95     \term{(Infrastructure mode).}
  96   \item If no server is found, activate the XEP-0174 client \term{(Ad hoc
  97     mode)}.
  98 \end{enumerate}
  99
 100 During runtime, a device can then react to changes in network infrastructure by
 101 changing from one mode to the other:
 102
 103 \begin{itemize}
 104   \item In Infrastructure mode: when connection to the server is lost, enable
 105     the uBonjour client, try to find a server, and when none is found, enable
 106     Serverless Messaging.
 107   \item In Ad hoc mode: if uBonjour detects a new XMPP server joining the
 108     network, try to connect to it. If this succeeds, disable Serverless
 109     Messaging and uBonjour and join topic-based Multi-User Chats.
 110 \end{itemize}
 111
 112 \subsection{Temporary Subscription for Presence}\label{sec:tsp}
 113
 114 To further reduce the message overhead and allow more fine-grained control over
 115 information filtering, \term{Temporary Subscription for Presence} is introduced.
 116 This technique builds on top of presence stanzas as defined in core XMPP, which
 117 are sent by default without a \code{to} or \code{from} attribute, and therefore
 118 fit into a single TCP/IP packet over IEEE~802.15.4. However, a drawback of the
 119 presence mechanism defined by core XMPP is the fact that a client must manually
 120 subscribe to presence information of another client in order to receive it,
 121 which requires further communication between the clients. Since the network can
 122 change rapidly, and clients can frequently join and leave the network,
 123 subscriptions would often be outdated and must be renewed, leading to overhead
 124 of subscriptions and unsubscription messages, which would inhibit the flow of
 125 the actual information.
 126
 127 To solve this problem, a dynamic, topic-based roster is implemented on top of
 128 Multi-User Chats (XEP-0045). Every topic corresponds with a chat room, and nodes
 129 join the chat rooms which they are interested in. This allows nodes to inform
 130 only interested nodes about updates. This has the advantage that existing
 131 clients supporting Multi-User Chats can be used by a user, but Chatty Things and
 132 XMPP servers need to be adapted to the new subscription model. Also, this
 133 mechanism does not work with Serverless Messaging.
 134
 135 % vim: set ft=tex et ts=2 sw=2 :