sec-discussion.tex

   1 \section{Discussion}
   2 %\todo
   3 \pages{3}
   4
   5
   6 \subsection{Related Approaches}
   7
   8 This section gives a short overview of related protocols for the Internet of
   9 Things and shows their advantages and disadvantages, which are summarized in
  10 Table~\ref{tab:comparison}.
  11
  12 \begin{table}
  13 \small\centering
  14 \caption{Comparison of related approaches}
  15 \label{tab:comparison}
  16 \begin{tabular}{|l||l|l|l|l|}
  17   \hline
  18   Feature & Chatty Things & CoAP & MQTT & WS4D \\
  19   \hline\hline
  20   application gateways neccessary & - & yes & yes & - \\ \hline
  21   usable with standard clients & yes & - & - & (yes) \\ \hline
  22   discovery support & yes & yes & - & yes\\ \hline
  23   IPv6/6LoWPAN ready & yes & yes & ? & partial \\ \hline
  24   asynchronous messages & yes & yes & & \\ \hline
  25   protocol overhead & moderate & small & small & high \\ \hline
  26 \end{tabular}
  27 \end{table}
  28
  29 \paragraph{Constrained Application Protocol (CoAP)}
  30
  31 The Constrained Application Protocol~\cite{draft-ietf-core-coap-18} focuses on
  32 machine-to-machine communication and originates from the IETF Constrained
  33 Resources Working Group, but still has been only in draft status since 2010.\ It
  34 allows a mapping to HTTP, and is therefore stateless, but it specifies a binary
  35 protocol, which makes it neccessary to deploy application-level gateways and
  36 special client software to communicate with its environment. It relies on UDP,
  37 but emulates congestion control, message confirmation and message IDs, since –
  38 in contrast to HTTP – messages can be sent asynchronously. Discovery is also
  39 specified and done over multicast, service discovery is then done over a
  40 well-known URI on the host. Since it is a binary protocol and mostly
  41 self-contained, it has low protocol overhead and parsing complexity.
  42
  43 \paragraph{MQ Telemetry Transport (MQTT)}
  44
  45 Specified by IBM as a binary protocol, the MQ Telemetry Transport~\cite{mqtt}
  46 has been proposed as an OASIS standard for machine-to-machine communication. It
  47 also relies on TCP/IP, and its fixed message header is only 2 bytes in size, but
  48 can contain further variable headers. Since it is also only used in embedded
  49 networks, application gateways and appropriate client software are necessary.
  50 Its main feature is a publish-subscribe mechanism with topic names, discovery is
  51 not specified.
  52
  53 \paragraph{Web Service for Devices (WS4D)}
  54
  55 As a different approach to avoid application-level gateways, WS4D has been
  56 specified as a Devices Profile for Web Services~\cite{zeeb-moritz-ws4d}. Since
  57 Web Services are wide-spread in the business world, this approach can probably
  58 be used in existing infrastructures, and is also focused on multiple platforms
  59 like embedded systems and servers. Web Services can be very flexible and
  60 composable, and discovery is already specified, however, this also comes at a
  61 cost: web services are enclosed in SOAP, which is enclosed in HTTP, which is
  62 transported over TCP, which introduces very much overhead, except with SOAP
  63 being based on verbose XML. IPv6 support is only partially implemented.
  64 For communication, standard APIs can be used.
  65
  66 \subsection{Conclusion}
  67
  68  With the XMPP
  69 protocol, there is the need to implement at least an XML parser on each node,
  70 which comes with protocol overhead and increased code size. However Klauck and
  71 Kirsche show that with good optimization (in the code as well as in the
  72 procotol), a complete stack can be implemented in 12 kByte of ROM, which leaves
  73 enough space for other applications to build onto it. As compared to Web
  74 Services, Chatty Things are probably not as flexible, but they have less
  75 overhead, even when using XML, while MQTT and CoAP provide less flexibility for
  76 future enhancement, therefore less protocol overhead and easier parsing.
  77
  78 With TCP, mDNS, DNS-SD and XMPP as foundation, the proposed architecture builds
  79 on reliable and established standards, which allows it to reuse Chatty Things in
  80 various contexts without the need for central infrastructure.
  81
  82 Nonetheless, a drawback is the virtual dependency from a centralized XMPP
  83 server in order to use Temporary Subscription for Presence for topic filtering,
  84 which is caused by the lack of support for Multi-User Chats in XEP-0174
  85 (Serverless Messaging). If this gap can be closed, or a different way for topic
  86 filtering in distributed networks is found, the server can be
  87 eliminated and what remains is truly distributed network without the need for
  88 any central infrastructure, therefore eliminating every single point of failure
  89 in the system.
  90
  91 It is always hard to trade flexibility and accessibility for efficiency. The
  92 Chatty Things approach is probably not one of the most efficient, and not the
  93 most flexible, but it has the big advantage that users can interact with their
  94 things using standard chat clients, without the need for application gateways or
  95 specialized software. In terms of efficiency, it chooses a common ground between
  96 binary protocols and Web Services, which were originally developed for servers
  97 with much less resource constraints as embedded systems.
  98
  99 \todo{additional XEPs, example XML stream with sensor data}
 100
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