The first problem is addressed with \term{Multicast DNS}, and since DNS is
basically a key-value store, it can also be used for service discovery, which is
achieved using \term{DNS-Based Service Discovery}. Both techniques were first
-developed by Apple as part of the \term{Bonjour} project, and are now maintained
-by the IETF Zeroconf working group.
+developed by Apple as part of the \term{Bonjour}
+project\footnote{\url{https://developer.apple.com/bonjour/}}, and are now
+maintained by the IETF Zeroconf working
+group\footnote{\url{http://zeroconf.org}}.
\subsubsection{Multicast DNS}
-\term{Multicast DNS}~\cite{rfc6762} (mDNS) describes an extension to the Domain
+\term{Multicast DNS} (mDNS)~\cite{rfc6762} describes an extension to the Domain
Name System that allows DNS resource records to be distributed on multiple hosts
in a network, therefore avoiding central authorities and enabling every host to
-publish its own entries. For that purpose, a special domain, usually
-named \code{.local}, is used.
+publish its own entries. For that purpose, a special top-level domain, is used,
+usually named \code{.local}, which contains those entries.
Software that supports mDNS listens on the reserved
link-local multicast address \code{224.0.0.251} (for IPv4 queries) or
-\code{FF02::FB} (for IPv6 queries) on UDP port 5353 for incoming queries.
+\code{ff02::fb} (for IPv6 queries) on UDP port 5353 for incoming queries.
Queries sent to those multicast address and port are standard DNS queries.
If a host receives a query and knows about the queried resource, it responds to the
querying host with a standard DNS response. The querying host can then simply
finish and use the result, or wait until other hosts respond to its query. The
latter is typically the case when a record can have multiple values, as it is
-the case with \code{SRV} and \code{PTR} records.
+the case with \code{SRV} and \code{PTR} records (which will be discussed in the
+next section).
Another feature of Multicast DNS is the reduction of traffic through
\term{Known-Answer Suppression}. It allows a querying host to specify already
known resources in its query when querying resources that could exist on more
-than one host (e.~g., SRV records). The hosts matching those resources then do
-not generate a response.
+than one host (e.\,g., SRV records). The hosts matching those resources then do
+not generate a response, thus reducing the messages in the network and saving
+bandwidth, which is usually a scarce resource in wireless networks.
+\enlargethispage{2\baselineskip}
Finally, hosts may also send unsolicited responses. This can be used to notify
the network of new services available on a host.
-\pages{1}
-
\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]{fig-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}
+SRV~\cite{rfc2782} and PTR~\cite{rfc1035} in a way that allows hosts to browse
+for services in a domain. While SRV records specify the location of services on
+a host, PTR records hold a reverse mapping from IP address to host name.
+DNS-SD now relies on a two-step process, consisting of
+\term{Service Instance Enumeration} and \term{Service Instance Resolution}.
-\paragraph{Service Instance Enumeration} At first, to enumerate the available
+\paragraph{1. 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.
+PTR resources 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 point to the hosts providing the
+service. For example, by querying for \code{\_ipp.\_tcp.\_example.org}, the
+instance names for all printers supporting the IPP protocol in the domain
+\code{example.org} 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
+\paragraph{2. Service Instance Resolution} As a second step, the returned instance
+names are resolved as SRV records to retrieve the actual host names and
+port numbers of a service. For 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
+Additionally, an optional 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
+Through the usage of 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}.
+with Multicast DNS, this makes it easy to deploy decentralized systems for the
+Internet of Things. \cite{Klauck:2012:BCC:2352852.2352881}
-\pages{1}
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