sync: XMPP Serverless Messaging

[skm-ma-ws1314.git] / dns-extensions.tex

diff --git a/dns-extensions.tex b/dns-extensions.tex
index bf8bdc8..c4c84e3 100644 (file)
--- a/dns-extensions.tex
+++ b/dns-extensions.tex
@@ -1,20 +1,30 @@
-\subsection{Extensions to the DNS protocol}
+\subsection{Extensions to the Domain Name System}\label{sec:dns}
+
+In a distributed context, it is often not feasible to rely on a central,
+authoritative DNS server, and there is usually no easy way to discover services.
+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.

 
 \subsubsection{Multicast DNS}
 
 
 \subsubsection{Multicast DNS}
 

-\term{Multicast DNS}~\cite{rfc6762} (mDNS) describes an extension of the Domain
+\term{Multicast DNS}~\cite{rfc6762} (mDNS) describes an extension to the Domain

 Name System that allows DNS resource records to be distributed on multiple hosts
 Name System that allows DNS resource records to be distributed on multiple hosts

-in a network, therefore avoiding the need of a central server.
+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.

 
 

-Software that supports resolution of mDNS records listens on the reserved
+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.
 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.

-Queries sent to those multicast address and port are standard DNS queries, and
-if a host receiving a query knows about the queried resource, it responds to the
+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
 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 querying for \code{SRV} records to obtain a
-list of services which are present in the network.
+latter is typically the case when a record can have multiple values, as it is
+the case with \code{SRV} and \code{PTR} records.

 
 Another feature of Multicast DNS is the reduction of traffic through
 \term{Known-Answer Suppression}. It allows a querying host to specify already
 
 Another feature of Multicast DNS is the reduction of traffic through
 \term{Known-Answer Suppression}. It allows a querying host to specify already


@@ -29,18 +39,19 @@ the network of new services available on a host.
 
 \subsubsection{DNS-Based Service Discovery}\label{sec:dnssd}
 
 
 \subsubsection{DNS-Based Service Discovery}\label{sec:dnssd}
 

-As a recent extension for the Domain Name System, \term{DNS-Based Service
-Discovery (DNS-SD)}~\cite{rfc6763} uses DNS records of type
+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}.
 
 \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}[htop]
+\begin{figure}[top]

   \centering
   \includegraphics[width=0.9\textwidth]{dnssd-mock.jpg}
   \centering
   \includegraphics[width=0.9\textwidth]{dnssd-mock.jpg}

-  \caption{DNS-SD: Service Instance Enumeration and Resolution}
+  \caption{DNS-SD: Service Instance Enumeration and Resolution
+    \todo[XMPP is a bad example here, use IPP]}

   \label{fig:dnssd}
 \end{figure}
 
   \label{fig:dnssd}
 \end{figure}
 


@@ -49,14 +60,18 @@ 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
 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}, all XMPP clients in the domain
-\code{example.org} are enumerated, and as a result, the instance names of three
-hosts are returned.
+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
 
 \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 XMPP client is running at host \code{juliet.example.org} on port 5222.
+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
 
 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