\section{Discussion}\label{sec:discussion}
%\todo
\pages{3}

\subsection{Future Work}

In addition to the XEPs covered above, there are a few additional XEPs which can
be implemented to further increase the effictivity of Chatty Things. Especially
the documents XEP-0323 through XEP-0326 (which are currently in Experimental
status) are targeted to the Internet of Things.

\paragraph{Concentrators (XEP-0326)~\cite{xep0326}}
In contrast to sensor nodes which are focused on collecting data, concentrators
can be used to control and serve as proxy for a subset of the network. The XEP
defines messages to query a sensor node for data sources, and subscribing to
them, while subscription is loosely modeled after the Publish-Subscribe
mechanism (XEP-0060). It also specifies how clients can request data from or
control certain nodes over a concentrator.

This approach can be practical in large-scale sensor networks, where usually not
every sensor node can be reached directly, and where sensor nodes only have a
very limited amount of storage. Individual concentrators can then be equipped
with larger storage and serve as a facility to aggregate data from sensor nodes.
This structure can be implemented on several levels, forming a hierarchy. A user
interested in specific values then only needs to communicate with a single node
in the network.

\paragraph{Sensor Data (XEP-0323)~\cite{xep0323}}
%\begin{figure}
  %\caption{Example stream between a sensor node and a client}
  %\label{fig:streamexample}
  %\begin{verbatim}
%Client               Device
%<stream>

  %\end{verbatim}
%\end{figure}
This XEP specifies a way of reading out values from a
sensor node. It allows to specify multiple data sources (e.~g. temperature,
humidity) as well as multiple types of data (e.~g., momentary values, historical
values, peak values).
As a simple use case, the client sends an \code{<iq>} stanza containing the request and a
sequence number used to identify the request. The sensor node then rejects or
accepts the request by returning a corresponding \code{<iq>} stanza. If it has
accepted the request, it reads out the requested data and returns it in a
subsequent \code{<message>} stanza to the client.

\paragraph{Control (XEP-0325)~\cite{xep0325}}
In this document, a way of controlling sensor nodes is specified, which allows a
client to get and set control values on the node over
\code{<message>} or \code{<iq>} stanzas. As an example, in this way a sensor
node can be instructed to return data in a different unit or range, or can be put
into power-safe mode.

\paragraph{Provisioning (XEP-0324)~\cite{xep0324}}
To protect the integrity of a sensor network and securing the data being
collected, this document specifies a way of implementing access rights and user
privileges. Since a single sensor node is usually very restricted in user input and
output, the approach is very simple and can be implemented e.~g. using a button
and an LED for interaction, while presentation of data takes places on a
provisioning server with a rich user interface (which can be, for example, a
concentrator).

When integrating a new sensor node into the network, the user instructs the
provisioning server to generate a \term{friendship} request for the new node.
The node can e.~g. symbolize this request by blinking its LED and requesting a
button press in the next 30 seconds. If the user presses the button, the node
confirms the friendship to the server. The server then remembers this sensor
node and generates a token which must be used in all further communication
between the server and the sensor node or the communication is rejected.

\paragraph{Efficient XML Interchange Format (EXI, XEP-0322)~\cite{xep0322}}
Finally, EXI describes how XMPP stanzas sent between nodes can be compressed,
thereby effectively reducing the overhead in message size introduced by XML.
XMPP nodes can negotiate a compressed stream inside their existing XMPP streams
and exchange \code{<compress>} stanzas which then contain the payload. However,
it is to be noted that this requires further implementation of compression
algorithms as well as additional CPU and memory resources and thus might
decrease message throughput and increase power consumption on embedded systems.


\todo{example XML stream with sensor data}

\subsection{Related Approaches}

Chatty Things is not the only approach to implement communication in embedded
networks. This section gives a short overview of related protocols for the
Internet of Things and shows their advantages and disadvantages, which are
summarized in Table~\ref{tab:comparison}.

\begin{table}
\small\centering
\caption{Comparison of related approaches}
\label{tab:comparison}
\begin{tabular}{|l||l|l|l|l|}
  \hline
  Feature & Chatty Things & CoAP & MQTT & WS4D \\
  \hline\hline
  application gateways neccessary & - & yes & yes & - \\ \hline
  usable with standard clients & yes & - & - & (yes) \\ \hline
  discovery support & yes & yes & - & yes\\ \hline
  IPv6/6LoWPAN ready & yes & yes & ? & partial \\ \hline
  asynchronous messages & yes & yes & & \\ \hline
  protocol overhead & moderate & small & small & high \\ \hline
\end{tabular}
\end{table}

\paragraph{Constrained Application Protocol (CoAP)}

The Constrained Application Protocol~\cite{draft-ietf-core-coap-18} focuses on
machine-to-machine communication and originates from the IETF Constrained
Resources Working Group, but still has been only in draft status since 2010.\ It
allows a mapping to HTTP, and is therefore stateless, but it specifies a binary
protocol, which makes it neccessary to deploy application-level gateways and
special client software to communicate with its environment. It relies on UDP,
but emulates congestion control, message confirmation and message IDs, since –
in contrast to HTTP – messages can be sent asynchronously. Discovery is also
specified and done over multicast, service discovery is then done over a
well-known URI on the host. Since it is a binary protocol and mostly
self-contained, it has low protocol overhead and parsing complexity.

\paragraph{MQ Telemetry Transport (MQTT)}

Specified by IBM as a binary protocol, the MQ Telemetry Transport~\cite{mqtt}
has been proposed as an OASIS standard for machine-to-machine communication. It
also relies on TCP/IP, and its fixed message header is only 2 bytes in size, but
can contain further variable headers. Since it is also only used in embedded
networks, application gateways and appropriate client software are necessary.
Its main feature is a publish-subscribe mechanism with topic names, discovery is
not specified.

\paragraph{Web Service for Devices (WS4D)}

As a different approach to avoid application-level gateways, WS4D has been
specified as a Devices Profile for Web Services~\cite{zeeb-moritz-ws4d}. Since
Web Services are wide-spread in the business world, this approach can probably
be used in existing infrastructures, and is also focused on multiple platforms
like embedded systems and servers. Web Services can be very flexible and
composable, and discovery is already specified, however, this also comes at a
cost: web services are enclosed in SOAP, which is enclosed in HTTP, which is
transported over TCP, which introduces very much overhead, especially with SOAP
being based on verbose XML. IPv6 support is only partially implemented.
For communication, standard APIs can be used.

\subsection{Conclusion}

 With the XMPP
protocol, there is the need to implement at least an XML parser on each node,
which comes with protocol overhead and increased code size. However Klauck and
Kirsche show that with good optimization (in the code as well as in the
procotol), a complete stack can be implemented in 12 kByte of ROM, which leaves
enough space for other applications to be built onto it. As compared to Web
Services, Chatty Things are probably not as flexible, but they have less
overhead, even when using XML, while MQTT and CoAP provide less flexibility for
future enhancement, but less protocol overhead and easier parsing.

With TCP, mDNS, DNS-SD and XMPP as foundation, the proposed architecture builds
on reliable and established standards, which allows it to reuse Chatty Things in
various contexts without the need for central infrastructure.

Nonetheless, a drawback is the virtual dependency from a centralized XMPP
server in order to use Temporary Subscription for Presence for topic filtering,
which is caused by the lack of support for Multi-User Chats in XEP-0174
(Serverless Messaging). If this gap can be closed, or a different way for topic
filtering in distributed networks is found, the server can be
eliminated and what remains is truly distributed network without the need for
any central infrastructure, therefore eliminating most single points of failure
in the system.

It is always hard to trade flexibility and accessibility for efficiency. The
Chatty Things approach is probably not one of the most efficient, and not the
most flexible, but it has the big advantage that users can interact with their
things using standard chat clients, without the need for application gateways or
specialized software. In terms of efficiency, it chooses a common ground between
binary protocols and Web Services, which were originally developed for servers
with much less resource constraints as embedded systems.

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