+\subsubsection{DNS-Based Service Discovery}\label{sec:dnssd}
+
+As another recent extension for the Domain Name System, \term{DNS-Based Service
+Discovery (DNS-SD)}~\cite{rfc6763} uses DNS records of types
+\code{SRV}~\cite{rfc2782} and \code{PTR} in a way that allows hosts to browse
+for services in a domain. As an example, Figure~\ref{fig:dnssd} shows the
+process of browsing for all XMPP clients in the domain \code{example.org}.
+This is a two-step process, consisting of \term{Service Instance Enumeration}
+and \term{Service Instance Resolution}.
+
+\begin{figure}[top]
+ \centering
+ \includegraphics[width=0.9\textwidth]{dnssd-mock.jpg}
+ \caption{DNS-SD: Service Instance Enumeration and Resolution
+ \todo[XMPP is a bad example here, use IPP]}
+ \label{fig:dnssd}
+\end{figure}
+
+\paragraph{Service Instance Enumeration} At first, to enumerate the available
+services in a domain for a given protocol, a DNS-SD-enabled client queries
+resources of type \code{PTR} of the form \code{\_service.\_proto.domain}. The
+result of this query is then a list of \term{instance names} of the form
+\code{name.\_service.\_proto.domain} which provide the specified service. For
+example, in Figure~\ref{fig:dnssd}, by querying for
+\code{\_ipp.\_tcp.\_example.org}, all printers supporting the IPP protocol in the
+domain \code{example.org} are enumerated, and as a result, the instance names of
+three hosts are returned.
+
+\paragraph{Service Instance Resolution} As a second step, the returned instance
+names are resolved as \code{SRV} records to retrieve the actual host names and
+port numbers of a service. In the example, resolution of one instance name shows
+that an IPP server is running at host \code{gutenberg.example.org} on port 5222.
+Additionally, an optional \code{TXT} record with the same instance name can
+contain further information about the service (e.~g., information about the
+supported paper sizes).
+
+Through the usage of \code{SRV} records, it is easily possible for a service to
+inform clients about non-standard port numbers, and especially in connection
+with Multicast DNS makes it easy to deploy decentralized systems for the
+Internet of Things~\cite{Klauck:2012:BCC:2352852.2352881}.