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Appendix D. Definitions of terms
Static Routing
A fixed path for the contact between computers. With a static routing, traffic cannot be redirected to another
path if the connection is broken. This would require dynamic routing, for example, with RIP.
Syslog
Syslog is a service for logging data. In UNIX, regular programs do not log any information; they send all data
to a syslog server that saves data in a log file. One example is a web server that sends data over the computers
that connects to the server and sends error messages for web pages that it could not locate. Messages to a
syslog server can also be sent over the network. Syslog uses the UDP protocol. A syslog server listens to port
514 for syslog messages.
TCP protocol
TCP connects two computers and makes sure that all data gets through and in the right order. TCP uses IP. IP
manages addresses and makes sure that data is sent out to the network. When TCP connects, it receives a
response from the TCP protocol layer on the receiving end. The recipient sends a little data along with a
confirmation that the sender’s data arrived. When a connection is made, a confirmation is always sent with all
data packets. This can be compared with Peter and Christy sending postcards and, along with their message,
commenting that they received the other’s postcard. TCP shortens this confirmation to ACK
(acknowledgment).
You know if a TCP packet is a connection attempt if it does not have ACK.
TCP keeps track of connections for different services using different port numbers. See Ports.
UDP protocol
UDP does not make a connection. It examines data that comes from outside for accuracy, by checksums. This
is like examining a postcard to ensure that it has not been torn up. UDP does not keep track of whether or not
all data gets through or if it is in the right order; this is the job of the application. So the data does not have an
ACK confirmation. Peter and Christy, sending postcards, have to keep track of their own postcards and Peter
has to tell Christy the order in which they should be read. UDP keeps track of the contacts using port numbers,
just like TCP.
UUCP
UNIX to UNIX Copy, an old protocol for copying files between two UNIX computers. This is sometimes used
to send e-mail between two computers.
WWW, World Wide Web
The WWW is currently the best known Internet service. The World Wide Web consists of millions of
documents that are interconnected all over the world. A document can contain text, pictures, sound, and even
video sequences. The WWW is based on the client-server concept. This means that each document is in a
database on a web server. The user runs a client program, such as Netscape or Internet Explorer, that connects
to a server, which could be anywhere in the world, and request a document. This document is displayed on the
user’s screen and the user can use his client program to click on other documents to display them. WWW
usually runs on the HTTP and HTTPS protocols, using ports 80 and 443, respectively.
X Window System
A window system that is used by several work stations. A similar window system is Open Windows. The X
Window System and Open Windows uses port numbers starting at 6000 and upward for traffic to the work
stations. It is a good idea to block ports 6000-6010 from incoming traffic from an insecure outside network.
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