-\section{Discussion}
-%\todo
-\pages{3}
-
+\section{Discussion}\label{sec:discussion}
+
+\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 serve as a proxy and control 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 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}}
+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 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 IQ stanza. If it has
+accepted the request, it reads out the requested data and returns it in a
+subsequent message stanza to the client.
+
+An example of this protocol can be seen in Figure~\ref{fig:examplexmpp}: after
+both clients have opened their streams, the client requests the momentary values
+for power and energy from the node named \emph{Device04}. The device first
+acknowledges this request, and, after retrieving the values, sends them back
+to the client.
+
+\input{fig-example-xmpp-stream.tex}
+
+\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 message or IQ stanzas. As an example, in this
+way a sensor node could be instructed to return data in a different unit or
+range, or 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 XEP 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, else 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.
\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}.
+``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
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.
+Resources Working Group\footnote{\url{http://datatracker.ietf.org/wg/core/}},
+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)}
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, except with SOAP
-being based on verbose XML. IPv6 support is only partially implemented.
+cost: messages are enclosed in SOAP, which is enclosed in HTTP, which is
+transported over TCP, which introduces a substantial overhead, especially with
+SOAP being based on verbose XML. IPv6 support is only partially implemented.
For communication, standard APIs can be used.
\subsection{Conclusion}
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 build onto it. As compared to Web
+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, therefore less protocol overhead and easier parsing.
+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
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 every single point of failure
+eliminated and what remains is a highly distributed network without the need for
+much 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.
-
+specialized software. In terms of efficiency, it chooses a compromise between
+binary protocols and Web Services, latter which were originally developed for
+servers with much less resource constraints as embedded systems. However, with
+additional XEPs focusing on the Internet of Things, enough flexibility can be
+achieved for this use case.
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