\chapter{The SICS Server Client Protocol}
This short chapter describes the command protocol between the SICS server
and  possible SICS clients. All this is very simple.

\section{Logging in to the SICS Server}
In order to log in to the SICS server it needs to be known on which
machine the server runs and at which port number the server listens for
connection requests. Also needed is a valid username/ password pair for the
SICS server in question. Given that the procedure for connecting to a SICS
server requires the following steps:
\begin{enumerate}
\item Open a TCP/IP connection to the SICS server port at the machine 
where it is running.
\item Immediately after opening the connection send the username/password
pair. If everything is OK, a string OK is sent. Else the server will break
the connection again. 
\end{enumerate}

\section{Sending Commands}
After login, two means of communications exist. The communication
protocoll is choosen through the server port the client connects too. 
 The recommended way is
to adhere to the telnet protocoll as described in RFC-854. Just a
basic NVT (Network Virtual Terminal) with no options is
implemented. Binary communication is not possible on a telnet port. 

The older way of communication is to send commands directly on the
TCP/IP port.  Commands are strings terminated by a \verb+\n+. Return 
messages from the server have the same format.  This scheme is
obsolete but it has been left in because the need for a binary
communication may arise and this would help implement such a thing. 



For a list of possible commands consult the
user documentation.

\section{Issuing Interrupts}
In some cases a user might want to stop the operation of the server. In
order to do this the client must send an interrupt to the SICS server. There
are two ways to do this: through the normal connection socket and through
the a special UDP message to the server. Let us first discuss the first
method, which uses the normal command socket for sending an interrupt.

In order to send an interrupt through the normal command socket a special
message must be sent, consisting of a special command string. The command
string is again all ASCII. The first part of this command string must read:
INT1712, followed by a number denoting the interrupt. The meaning of these
numbers is resolved in file interrupt.h. For example, the string 'INT1712 2'
will abort the current scan. The presence of an interrupt string will be
checked for already by the network reader. This means that such interrupts
will even be honored when the client connection is waiting for some hardware
to finish its job and would not accept normal commands. \label{prot1}

The SICS server also checks a special TCP/IP port for UDP messages which
denote an interrupt. In order to send an interrupt through this channel, a
UDP connection to the SICS servers UDP port must be opened. Then an special
ASCII string of the form: {\bf SICSINT num} must be sent. num must be
replaced by the number of the interrupt to issue. Again interrupt codes are
resolved in file interrupt.h.