AppendixD. Definitions of terms
StaticRouting
Afixed path for the contact between computers. With a static routing, traffic cannot be redirected to another
pathif the connection is broken. This would require dynamic routing, for example, with RIP.
Syslog
Syslogis a service for logging data. In UNIX, regular programs do not log any information; they send all data
toa syslog server that saves data in a log file. One example is a web server that sends data over the computers
thatconnects to the server and sends error messages for web pages that it could not locate. Messages to a
syslogserver can also be sent over the network. Syslog uses the UDP protocol. A syslog server listens to port
514for syslog messages.
TCPprotocol
TCPconnects two computers and makes sure that all data gets through and in the right order. TCP uses IP. IP
managesaddresses and makes sure that data is sent out to the network. When TCP connects, it receives a
responsefrom the TCP protocol layer on the receiving end. The recipient sends a little data along with a
confirmationthat the sender’s data arrived. When a connection is made, a confirmation is always sent with all
datapackets. This can be compared with Peter and Christy sending postcards and, along with their message,
commentingthat they received the other’s postcard. TCP shortens this confirmation to ACK
(acknowledgment).
Youknow if a TCP packet is a connection attempt if it does not have ACK.
TCPkeeps track of connections for different services using different port numbers. See Ports.
UDPprotocol
UDPdoes not make a connection. It examines data that comes from outside for accuracy, by checksums. This
islike examining a postcard to ensure that it has not been torn up. UDP does not keep track of whether or not
alldata gets through or if it is in the right order; this is the job of the application. So the data does not have an
ACKconfirmation. Peter and Christy, sending postcards, have to keep track of their own postcards and Peter
hasto tell Christy the order in which they should be read. UDP keeps track of the contacts using port numbers,
justlike TCP.
UUCP
UNIXto UNIX Copy, an old protocol for copying files between two UNIX computers. This is sometimes used
tosend e-mail between two computers.
WWW,World Wide Web
TheWWW is currently the best known Internet service. The World Wide Web consists of millions of
documentsthat are interconnected all over the world. A document can contain text, pictures, sound, and even
videosequences. The WWW is based on the client-server concept. This means that each document is in a
databaseon a web server. The user runs a client program, such as Netscape or Internet Explorer, that connects
toa server, which could be anywhere in the world, and request a document. This document is displayed on the
user’sscreen and the user can use his client program to click on other documents to display them. WWW
usuallyruns on the HTTP and HTTPS protocols, using ports 80 and 443, respectively.
XWindow System
Awindow system that is used by several work stations. A similar window system is Open Windows. The X
WindowSystem 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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