\section{Discussion}
%\todo
\pages{3}


\subsection{Related Approaches}

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.

Nonetheless, there are a few additional XEPs which can be implemented to further
increase the effictivity of Chatty Things. XEP-0322 (Efficient XML Interchange
Format)~\cite{xep0322} describes how XMPP stanzas sent between a server and a
client can be compressed, therefore effectively reducing the overhead introduced
by XML. A client and a server can then negotiate a compressed stream inside
their existing XMPP stream and exchange \code{<compress>} stanzas which contain
the payload.

The documents XEP-0324 through XEP-0326 contain specifications targeted
especially to the Internet of Things:

\todo{additional XEPs, example XML stream with sensor data}

\begin{itemize}
 \item asdf
 \item asdfasdf
 \item lkjlkj
\end{itemize}

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