\begin{slide}{}
\begin{center}
{\Large \bf Zephyr}

A Notification and Location Service for a Global Network

Marc Horowitz \\
April 10, 1990
\end{center}
\end{slide}

\begin{note}
\begin{enumerate}
\item Audience: Project Athena Sys \& Ops
\item Goal: My design of a system for rapid communication over a large
scale network
\item Zephyr Notification and Location System: my solution
\item Provides messages transmission and location services
\end{enumerate}
\end{note}

\begin{slide}{}
Issues
\begin{items}
\item Transmission
\item Fragmentation
\item Fault Tolerance
\item Authentication
\item Scaling
\end{items}
\end{slide}

% gross ordering so that the right thing happens.  Ick.

\begin{note}
How to make the system useful and functional.  What issues had to be
addressed.
\addtolength{\textheight}{1.5in}
\end{note}

\addtolength{\topmargin}{-0.75in}
\begin{slide}{}
\epsfile{structure.ps}
\end{slide}

\addtolength{\topmargin}{0.75in}
\begin{note}
\begin{enumerate}
\item Sender \to\ HM \to\ Server (\to\ Server) \to\ Recipient
\item broadcast spreading: remote server dispatch
\item Personal vs. Broadcast
\end{enumerate}
\end{note}

\begin{slide}{}
\stitle{Packet Fragmentation}

A message contains three different kinds of data:

\begin{items}
\item Identification data: {\invisible Sent only once, in the first
packet}
\item Routing data: {\invisible Sent with each packet}
\item Message data: {\invisible Divided among all the packets.  This
is usually the largest section of the message}
\end{items}

\invisible
The data is divided as shown above.  This allows each packet to be as
small as possible and independent of the other packets.
\end{slide}

\begin{overlay}{black}
\invisible
\stitle{Packet Fragmentation}

A message contains three different kinds of data:

\begin{items}
\item Identification data: {\black Sent only once, in the first
packet}
\item Routing data: {\black Sent with each packet}
\item Message data: {\black Divided among all the packets.  This
is usually the largest section of the message}
\end{items}

\black
The data is divided as shown above.  This allows each packet to be as
small as possible and independent of the other packets.
\end{overlay}

\begin{overlay}{}
\stitle{Packet Fragmentation}

A message contains three different kinds of data:

\begin{items}
\item Identification data: Sent only once, in the first
packet
\item Routing data: Sent with each packet
\item Message data: Divided among all the packets.  This
is usually the largest section of the message
\end{items}

The data is divided as shown above.  This allows each packet to be as
small as possible and independent of the other packets.
\end{overlay}


\begin{note}
\begin{enumerate}
\item{Contents}
 \begin{itemize}
 \item Id data: sender, auth
 \item Routing: recipient, uid
 \item Data: message content
 \end{itemize}
\item{Why this is efficient}
\item{Size ratios}
\end{enumerate}
\end{note}

\begin{slide}{}
\stitle{Fault Tolerance}

Because it is depended on for instantaneous communication, the zephyr
system must be resistant to network or computer failure.  In order to
prevent loss of service, the system is designed to automatically
recover from failure.

\begin{items}
\item Server replication
\item Hostmanager operation
\end{items}
\end{slide}

\begin{note}
\begin{enumerate}
\item Server replication details:  data replication
\item Hostmanager---Client split: auto-search for servers
\end{enumerate}
\end{note}

\begin{slide}{}
\stitle{Authentication}

In a large network, it is impossible to know all the users, and to
trust them.  Because of this, it is necessary to have some kind of
authentication so that the recipient of a message can be sure of its
sender.
\end{slide}

\begin{slide}{}
\stitle{Scaling}

On a network of global scale there are issues not present on smaller
networks which must be addressed:

\begin{items}
\item Distribution
\item Delay
\item Unreliablility
\item Insecurity
\end{items}
\end{slide}

\begin{note}
\begin{enumerate}
\item Conclusion: How and why this all fits together
\item Distribution: efficient routing of information
\item Delay: packets more efficient than large notices
\item Unreliability: built-in safeguards against failure
\item Insecurity: authentication over networks
\end{enumerate}
\end{note}
