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Extension protocols and network applications 79
4.9.3 NTP
NTP (Network Time Protocol) is a protocol with which computers can set their internal clock
to the same time of another computer. NTP is based on the predictability of the network
latency. The computer network is hierarchically organised whereby the computer with the
most accurate time source is indicated as ’stratum 0’. The computer systems that derive
their time from this computer, via NTP, are by definition ’stratum 1’. The protocol has some
intelligent functions. An NTP Client can use several NTP servers whereby the NTP Client sorts
out by itself server works the best. Based on a number of decision criteria, an NTP Client
selects a server and synchronises with this server. Minor differences in time between server
and client are updated by the client by having the time processing on the client’s computer
to function a bit faster or slower. This means that the difference can be eliminated without
jumps in time.
4.9.4 SSH
Secure Shell is located in the application layer of the TCP/IP protocol. SSH replaces older
protocols such as telnet and rlogin with a secure version of these. The protocol uses TCP port
22.
SSH permits secure login to another computer and the execution of remote commands on
the other computer via a Shell. The encryption used makes it difficult for third parties to
detect the original commands.
One major advantage of SSH is that authentication is also possible with the aid of a private/public
key. As a result, SSH applications may be used automatically without the need
for a password in the code. With the use of the private key, it is also possible to log into any
system that recognises the public key.
4.9.5 CLI (Command Line Interface)
If an operating system provides a Command Line Interface, the user can make the system
execute one or more tasks by means of a command line. Once the task has been executed,
the user once more has an opportunity to input a subsequent command line. Commands
are usually finished with the key. Well-known CLIs include command.com (DOS) and
bash (UNIX).
In addition to operating systems, there are also other programs that can run with a CLI. FTP
client and Microsoft’s Telnet Client, for example, use a command line.
Most commercial switches are also configurable via a CLI.