Internet Services - Journal of Nuclear Medicine Technology

Internet Services - Journal of Nuclear Medicine Technology
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CONTINUING EDUCATION
An Introduction to the Internet, Part 3: Internet Services
Jerry V. Glowniak
Nuclear Medicine SeiVice, VA Medical Center, Portland, Oregon
In this third article in a four-part series on the Internet,
Internet services are described and examples are given on
how to use them. After reading this article, a technologist
should be able to name the major Internet services, understand how they function and use them to obtain information
from the Internet.
Key Words: Internet; computers; electronic mail; USENET;
telnet; ftp; archie; gopher; veronica; World Wide Web
J Nucl Med Techno/ 1995; 23:231-248
The resources available on the Internet are accessed by various
methods which are referred to as Internet services. Each of
these services has distinctive features. Internet services are
implemented by application programs or protocols, the most
important of which are telnet, ftp, gopher, email, WAIS, archie
and the World Wide Web. These are discussed in detail below.
The user interacts directly with these programs which are at
the highest level of the protocol stacks discussed in the first
part of this series. The application program formats user input
and transfers it to lower levels of the protocol stack, TCP/IP
software, then routes the information onto the Internet. The
information is received by a remote computer and passed up to
an application program that then performs some prescribed
function and returns the results to the local application program.
All Internet services function in a client-server mode. As
described in Part I, this method of interaction between computers involves splitting an application program into two parts:
the client program that accepts input from the user and displays results; and the server program that resides on the remote computer and performs the tasks requested by the client.
Each of the common Internet services is defined by a standard
protocol that describes how connections between computers
are set up, what information is passed between them and how
a particular service functions.
In the traditional model of the client-server architecture, a
client program for a particular service can only communicate
For correspondence or reprints contact: Jerry V. Glowniak. MD. VA
Medical Center ( 115). PO Box 1034. Portland. OR 97207.
VOLUME 23, NUMBER 4, DECEMBER 1995
with its corresponding server. Thus, a gopher client can only
communicate with a gopher server, not with a World Wide
Web or ftp server 1• While there are several reasons for making
resources available by the different services, this approach
complicates access to information. To access all the resources
on the Internet, a user must know how to use the different
services and must have the client software for each of the
different services. In addition, a user must know what type of
server is available on a remote computer.
To circumvent the problem of a client having access to only
a portion of the resources on the Internet, two partial solutions
have evolved. One method of increasing access to different
types of services is to allow servers to interact through special
programs called gateways that permit different protocols to
communicate with one another. For example, an individual
who sets up a gopher server may want to provide information
that is available on an ftp server, such as to allow downloading
files. One way to do this is to create a program that can
translate requests from a gopher server to a format understood
by an ftp server. Another method of accessing multiple services
is to incorporate multiple client applications into a single client
program. The most widespread application of this approach is
with Web browsers. These are client programs whose primary
function is to contact World Wide Web servers. The World
Wide Web is a distributed system of documents written in
hypertext format (see below for further details). Web browsers
have built-in capabilities to access ftp, WAIS, gopher servers
and, depending on the particular browsers, other services such
as email, telnet or USENET.
SERVICES, PROTOCOLS, AND SOFTWARE
Before describing particular Internet services, it is important
to understand the terminology. Strictly speaking, there is a
distinction between an Internet service, the protocol describing
that service and the software program that implements the
protocol.
' Telnet clients are the single major exception to this rule. Using a telnet
client alone a user can send email and download information from gopher.
World Wide Weh and other servers. hut it is heyond the scope of this article
to explain how to do this.
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TABLE 1
Common Internet Services and Their Protocols
and Port Numbers
Service
Directory service
Directory service
Electronic mail
File transfer
Finger
Gopher
IP address lookup
Remote login
USE NET
WAIS
World Wide Web
Protocol
cso
who is
SMTP
ftp*
finger
gopher
DNS
tel net
nntp
Z39.50
http
Port
number
105
43
25
21,20
79
70
53
23
119
210
80
Key: DNS = domain name service: ftp = file transfer protocol; http
= hypertext transport (transfer) protocol; nntp = network news
transfer protocol; SMTP = simple mail transfer protocol; and WAIS
= Wide Area Information Service.
*ftp uses two ports: port 21 is the control channel for sending and
receiving commands and port 20 is the data channel used for
transferring files.
An Internet service is a description of some function performed between two computers. Examples are electronic mail
service, remote login, transferring files between computers or
accessing distributed document systems (gopher and the World
Wide Web). Table I lists the common Internet services, their
related protocols and port numbers.
Since the Internet services involve communication between
computers, two computers must speak the same language if
they are to understand each other. The rules of communication
are specified in documents called protocols. Each protocol
covers a particular aspect of computer communications, and
protocols are generally written to conform to specific levels in
the Internet protocol model. The documents describing Internet protocols are called RFCs (Requests for Comments) and
are available at several locations around the Internet. All the
RFCs can be obtained by ftp from the InterNIC (Internet
Network Information Center) at ftp.internic.net. The common
Internet services are specified by protocols that are described
in RFCs.
A protocol describes what is to be done, not how to do it.
Thus, the ftp protocol describes, among other things, what
control messages are exchanged between computers when files
are being transferred between them. The protocol does not
state how the computer generates these messages, which is the
task of software. Unlike the protocols for the Internet services,
which are freely available and independent of any type of
computer or operating system, the software implementing the
protocols must be written for each type of computer and/or
operating system. For example. although there are many different types of commercial and noncommercial email programs, they all use the simple mail transfer protocol (SMTP)
for sending and receiving messages on the Internet. The Internet protocols standardize all the important Internet functions.
232
Certain aspects of communication are not standardized, however, such as the display of files. Different software programs
can produce considerably different appearances to downloaded data. Furthermore, software for the various Internet
services may have options that allow a user to change parameters within the program that affect the display and execution
of the program. This is especially true of Web browsers. These
options increase the usefulness of the program, but they also
increase its complexity. Given the large amount of Internet
software, it is worthwhile to evaluate different packages in
order to find one that best suits a user's needs.
The terms for describing Internet services, protocols and the
software implementing these protocols are often used interchangeably which can occasionally lead to confusion. In addition, the commands used to run software programs are often
the same as the protocol names, which is a common feature of
UNIX systems. To make matters even more confusing, some
computers are named according to the service they provide.
For example, the National Institutes of Health have a computer called gopher.nih.gov that can be reached using a gopher
client program. The command for accessing the gopher software on a UNIX system is simply "gopher." Thus, to reach this
computer, one would give the command gopher gopher.nih.gov on a UNIX system. Finally, the names of common Internet services are often used as verbs. The statement "Telnet to
liberty.uc.wlu.edu to use the netlink service" means that the
user should use his telnet software to connect to the computer
liberty.uc.wlu.edu which offers a service called netlink.
From the user's point of view, one of the major differences
between software programs for the Internet services is the way
data is displayed and commands are entered. There are two
basic types of interfaces: graphical user interfaces (GUis) and
text-based system (also called command line interfaces). In
general, GUis are easier to use but require more complex
computer hardware and software. GUis use the entire computer screen to display text and graphics. Common examples of
these systems are Macintosh computers, IBM's OS/2 and OS/2
Warp operating systems, Microsoft Windows and Windows 95,
and the UNIX X Window system. Commands and options can
be entered with a mouse by clicking on menu items or icons.
With text-based systems, commands are typed at a prompt or
chosen from a menu. Examples of these systems are DOS for
IBM and IBM-compatible PCs, standard UNIX systems, and
VMS/VAX by Digital Equipment Corporation. Software for
all the common Internet services is available for either textbased or GUI systems. The visual appeal and some of the
functionality of the World Wide Web, however, is lost using
text-based systems. On the other hand, certain services are text
oriented, such as telnet, and GUis have few advantages over
text-based systems for these services.
IN GENERAL
Internet resources are accessed by choosing an Internet
service and supplying the computer name or IP address of a
computer on the Internet that offers that service. When a
TCP/IP packet arrives at the destination computer, there must
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be some means of identifying to which application program the
data is to be delivered. This is accomplished by including a
number in the TCP header which identifies the required Internet service. This number is called the port or port number.
An Internet computer may designate any number in the range
0 through 65535 as a port for a specific service it offers. If
number assignments were completely random, it would be
difficult for a user to reach services at remote sites since the
user would need to know how a particular computer assigns
port numbers to its services. Fortunately, the commonly used
Internet services are assigned standard port numbers that are
referred to as "well known ports." If a user fails to specify a
port number for one of these services in a command, the
standard port number for that particular service will be used.
These standard port numbers are assigned to servers.
A client also functions through a port on the local system,
but the local system randomly assigns port numbers to clients
for each connection to a remote system. The TCP header in a
TCP/IP packet contains the port number that the local client
uses in addition to the server port number. The remote server,
when it communicates with a client, identifies a particular
connection by means of the randomly assigned port number.
This mechanism allows multiple simultaneous connections to
be maintained between users of a client on a particular system
and a remote server. Although the users are all using the same
local client program, they each have a different local port
number. Table 1 lists the standard port numbers for common
Internet services. Occasionally, a particular computer may assign a nonstandard port number to one of its services. In that
case, the user must specify the port number in order to access
the service. An example is weather information from the University of Michigan's Weather Underground on port 3000. A
user can access this service by telnet at downwind.sprl.umich.edu 3000.
With some software programs, like Trumpet Winsock, a
TCP/IP program for Windows systems, a user may encounter
the term "socket." A socket is defined as the combination of
the IP address of a computer that offers a particular service
and the port number of that service. Communications on the
Internet take place through connections. A connection is a pair
of communicating sockets: the local socket of the client and the
corresponding socket of the remote server. Thus, a connection
is defined by 4 numbers, the IP addresses and port numbers of
the client and server. Each data packet exchanged between the
client and server contains these 4 numbers. This ensures that
each client-server interaction on the Internet at any given time
is unique.
The method of accessing a remote computer in a text-based
system is:
command (computer name) [port number]
The IP address can be substituted for (computer name), and
[port number] can usually be omitted. (computer name) can be
entered in upper or lower case. For DOS or VMS systems, all
text is case insensitive and commands can be entered as either
upper or lower case. UNIX commands are case sensitive,
VOLUME 23, NUMBER 4, DECEMBER 1995
almost all of which must be in lower case. In systems that use
GUis, point and click methods are used and destination computers can be chosen from a predefined list of computers, or
the computer name can be entered manually. For UNIX systems, the command for a particular service is often the same as
the protocol name. This is the case for telnet, ftp, gopher and
finger; but electronic mail, USENET and the World Wide
Web have different commands, depending on the particular
software program used to access that service.
In the description of Internet services that follows, examples
will be given in UNIX format. While most readers are probably
unfamiliar with UNIX, I have chosen this approach for several
reasons. UNIX is the most widely used operating system on the
Internet and a user can gain more information about resources
at many sites with a basic knowledge of UNIX. Secondly,
although text-based systems are not as easy to use as GUis,
they are easier to describe because they have a more standardized format than GUis. Lastly, a large proportion of Internet
access providers offers UNIX-based shell accounts. Since these
accounts can be purchased for very low prices, this may be a
cost-effective way of accessing the Internet for users who don't
require or cannot use the graphical or audio capabilities of
services such as the World Wide Web. It should be pointed
out, however, that many UNIX sites offer an optional menu
system that allows a user to access specific services by choosing
menu items so that a knowledge of UNIX is not required.
Commands and responses in the examples given below are
indicated by indented text. Where computer prompts and user
inputs are shown together, user input is in boldface type. The
reader should enter a carriage return after typing the indicated
text. Computer prompts and responses are indicated by ordinary text. Items enclosed in square brackets [ ] indicate optional user input, while items enclosed in angled brackets ( )
indicate mandatory user input.
ELECTRONIC MAIL (EMAIL)
One of the most widely used services on the Internet is
electronic mail, or email. It is also one of the more difficult
services to describe in detail because of the large number of
programs in existence. Email serves such a useful purpose that
several networks with no other connections to the Internet
have Internet email gateways. Since these networks often employ different addressing conventions than those used on the
Internet, addressing mail to users on some of these networks
can be complicated. A reader having trouble sending mail to or
receiving mail from one of these networks should consult with
the network administrator or a text that describes mail addressing conventions. The standard Internet format for addressing
mail is:
(username)@(computer name)
where (username) is the way an individual is identified on his
local computer, and (computer name) is the computer running
the mail server that receives and distributes mail on the remote
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network2 • No spaces are allowed anywhere within the address.
As an example, the author's email address at the Veterans
Affairs Medical Center in Portland, Oregon is:
compliant mail programs can incorporate images, graphics,
sound or even video clips in a mail message.
MAILING LISTS
[email protected]
On UNIX systems, the user name is often the same as the login
name.
Mail programs differ considerably. Although some mail programs come as single software packages, especially programs
for personal computers, most mail programs on the Internet
have a basic two-part structure: a program with which the user
interacts, the mail user agent (MUA); and a program that
sends and receives mail messages, the mail transfer agent
(MTA). The MUA is similar to a word processor. An individual uses this program to create, modify, read and delete mail
messages. Messages are stored in files in a directory that is
usually referred to as a user's mailbox. Messages a user creates
are stored in a subdirectory called an out box and received
messages are stored in an in box. The MTA is actually responsible for sending and receiving messages. The MTA deposits
received messages in the user's in box and retrieves and sends
messages from the out box when instructed to do so by the
MUA. MTAs employ the standard Internet mail protocol, the
Simple Mail Transfer Protocol (SMTP). The most common
MTAs on the Internet are the programs send mail and smail.
The two-part structure of mail programs offers a great deal
of flexibility in installing mail systems on a computer used by
many individuals. Different MUAs (the programs usually identified by the user as the mail program) can be employed that all
use the same MTA. This simplifies implementing and updating
mail programs since only the MUAs need to be changed rather
than the MTAs.
One of the major differences between mail programs is the
type of information they can send and receive. Mail programs
were originally developed to handle simple text. Thus, computer programs, image files and files with special nontext
formatting codes (e.g., spreadsheets and many word processing
files) could not be sent directly by email. One solution to this
problem is to encode the contents of these files into text
equivalent characters. These text equivalent versions of a file
appear as ordinary characters in a mail message, but the file is
completely unreadable as ordinary text. The recipient of such
a file must decode the file to use it. The most common method
of encoding and decoding files on a UNIX system is with the
uuencode and uudecode commands.
Another method for sending nontext files is with an extended version of the standard Internet mail protocol, the
Multipurpose Internet Mail Extensions (MIME) protocol. This
protocol describes how various file types can be sent by email.
To use this method of file transfer, both the sending and
receiving mail programs must be MIME compliant. MIME-
' In actual practice. (computer name) is often not the name of the
computer with the mail server but. rather. an alias that is a shorter or easier
to remember name than that of the mail server. This alias is frequently just
the network portion of the mail server's name. The domain name system.
described later. knows how to locate the actual mail server given an alias.
234
A mailing list is a specialized form of email. Mailing lists
define a group of individuals who correspond through a program that delivers mail to all members on the list. Mailing lists
require no special software for members on the list aside from
a mail program on the local system. The program responsible
for maintaining the mailing list is called a mailing list manager
(MLM) and is an MUA. The most common MLMs on the
Internet are LISTSERV (a contraction of list server),
Majordomo and ListProcessor. These programs are also referred to as mail bursters or mail exploders.
The program that runs the mailing list has two addresses.
One address is used for administrative functions such as subscribing or unsubscribing to the list and providing information
on how the program works. The other part of the program is
the address to which members post messages and which sends
the postings to all members on the list. A good example of a
nuclear medicine mailing list is the nucmed mailing list at the
University of Western Ontario. To subscribe to this list, a user
sends a message to the address:
[email protected]
with the message: subscribe nucmed (name), where (name) is
the user's real name. To unsubscribe to the mailing list, the
message should be: unsubscribe nucmed. Postings to the list
are sent to the address:
[email protected]
A message sent to the above address is sent out to all subscribers on the list.
USE NET
A service related to mailing lists is USENET (a contraction
of User's Network). USENET is a service devoted entirely to
discussion groups or newsgroups, as they are more commonly
called. Originally, USENET was an independent network of
UNIX-based computers that exchanged messages over phone
lines. USENET gradually began to use the Internet for exchanging messages so that at present nearly all USENET
traffic travels over the Internet. Unlike mailing lists where
there is a computer to which all messages are sent, messages to
USENET are sent to newsgroups which can be located on a
large number of computers on the Internet. Newsgroups are
devoted to specific topics and are named in a manner similar to
Internet computers. Newsgroup names are strings of descriptive characters separated by periods with the most general
category on the left and the most specific category on the right
of the string. The major general categories are listed in Table
2. Each general category has a number of subcategories that
are not as well-defined. The alt and mise categories are for
newsgroups that do not precisely fall into the other categories.
Specific newsgroups can be very general such as:
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TABLE 2
Usenet Categories
alt
comp
mise
news
rec
sci
soc
talk
alternate
computers
miscellaneous
newsgroups
recreation
science
society
debate oriented
sci.chem
for chemistry or very specific such as:
alt.sports.baseball.ny-yankees
for a newsgroup discussing the New York Yankees.
There are thousands of newsgroups world wide. Newsgroups
can be local, regional, national or international in distribution.
An individual can start a newsgroup by posting a message to
news.announce.newgroups, a newsgroup specifically established for setting up newsgroups. A name for the newsgroup is
proposed and its function described. Individuals who are interested can then vote on whether or not to create the newsgroup. If enough interest is expressed, a new newsgroup is
created.
Postings to newsgroups are stored in specific files on a
computer offering USENET. Access to these files is through a
special program called a newsreader. An individual uses the
newsreader program to create a list of newsgroups he is interested in reading. When the individual wishes to read postings
in a newsgroup, he activates the newsreader program which
then displays the newsgroups to which he has subscribed. The
user picks a newsgroup, and the newsreader displays the subjects of messages in that newsgroup. The user then chooses
which messages he wishes to read. There are different types of
newsreader programs, but all programs allow a user to subscribe and unsubscribe to newsgroups, reply to postings and
save messages to his mailbox. Unlike mailing lists, postings to
newsgroups are not stored in a user's mailbox, but rather in
files accessed by the newsreader. USENET uses its own protocol, the network news transfer protocol (nntp), to transfer
messages between computers.
Each USENET site has an administrator, often the same
individual who maintains the computer on which the newsreader is located. The administrator is responsible for deciding
which newsgroups are offered on the local system. He can also
create a number of newsgroups for discussing issues pertinent
to the local network, institution or geographical area. Messages
posted on the local system are sent to other USENET sites that
accept posting for the same newsgroups. An individual can
specify when posting a message how far the message should be
distributed, e.g. local, regional or worldwide, and to what
newsgroups the message should go. USENET is a very popular
service and postings can take up large amounts of space on a
computer system. Few computers have the resources to make
VOLUME 23, NUMBER 4, DECEMBER 1995
all USENET newsgroups available, and the administrator selects which groups are offered on the local system. If a newsgroup is not available on the user's system, he can request that
the administrator make the group available.
TELNET
The principal method of logging onto remote computers on
the Internet is by means of the telnet protocol. This versatile
protocol is a built-in feature of UNIX systems. Telnet turns a
user's computer into a terminal of the remote computer, and
only commands understood by the remote computer can be
properly interpreted. Of all the Internet services, telnet gives a
user the greatest potential access to a remote system's resources. Most publicly accessible tel net sites, however, limit the
resources a user can access and a user is usually presented with
a menu system after logging on. Depending on the site, the
user may be prompted for a login name and, occasionally, a
password. As an example, entering the command:
telnet rs.internic.net
will connect the user to the registration service at the InterNIC
which offers several large databases of Internet related information. As with many sites, an information screen appears
showing the services available and commands for accessing
those services. Telnet can also be used to access services at
remote sites that may not be available on the local computer.
There are public sites that offer archie clients, gopher clients
and text-based Web browsers programs. An example of the
latter, at the University of North Carolina, allows public access
to the text-based Web browser called lynx. To access this
service, a user enters:
telnet sunsite.unc.edu
login: lynx
FILE TRANSFER PROTOCOL (nP)
The file transfer protocol (ftp) is the primary method for
transferring files on the Internet. There are thousands of publicly accessible sites on the Internet from which files can be
downloaded to the user's local computer. The types of files
available cover a wide spectrum from text files and documents
to computer software of every imaginable variety. There are
large archives on the Internet that specialize in certain types of
files. such as software archives for Macintosh, Windows and
UNIX systems, reference sources for specific subjects, and
government documents to name a few. Unlike public telnet
sites, all ftp sites require a login and password. For public ftp
sites, the login name is "anonymous" while the password is
usually the user's email address. With text-based interfaces, the
user sees a prompt, usually ftp>, at which a command must be
entered.
The file and directory structure of the remote system will
determine how the data is arranged, but with UNIX systems
(by far the majority of systems on the Internet) files and
directories are arranged in a hierarchical structure similar to
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that of DOS or Windows systems. With a UNIX client program, the following commands are available:
dir or Is
cd (name)
get (filename)
cdup
quit
help
binary
ascii
list the contents of the current directory
change current directory to (name)
download (filename) to the local computer
move up one directory
close the connection to the remote computer
list available commands
switch to binary mode file transfer
switch to text (ASCII) mode file transfer
A directory listing using the "dir" command will display file
and directory names as well as other information while the "Is"
command lists just file and directory names. For ftp servers
that employ the UNIX operating system, a directory is identified by the letter "d" in the first column of an entry and a file
by a "-". See examples below. There are two methods of
downloading files: binary mode and text (or ASCII) mode·\
The mode of transfer is set by typing "binary" or "ascii" at the
ftp prompt. ASCII mode is the preferred method for downloading text files while binary mode is used for downloading
binary files. Binary files are computer programs and any files
that contain nontext characters. These latter files include image, audio and video files as well as text files that contain
control or formatting codes which are commonly added to
many types of spreadsheet and word processing files. A binary
transfer of a text file will usually cause no problems, but a
text-mode transfer of a binary file will produce a severely
garbled file. When in doubt about file type, perform a binary
transfer first. If this does not produce a usable file, an "ascii"
transfer can be tried.
An example of how to use "ftp" is shown below. To download the very popular Web browser, Mosaic, from the University of Illinois at Urbana-Champaign, the following commands
are entered:
ftp ftp.ncsa.uiuc.edu
Name > anonymous
password > [email protected]
The author's email address is entered as a password. After
logging in, the user is at the main, or root, directory. A "dir"
command is given to display its contents:
ftp> dir
-rw-r--r-- 16557 Jan 3
README
drwxr-xr-x 2048 Aug 22 Web
Only two of 35 lines of output are shown, and the output has
been shortened to show particular items. The first string of
characters describes file attributes. The first character in this
string denotes the file type. README is an ordinary file (a"-"
is the first character) while Web is a directory (a "d" is the first
ASCII stands for American Standard Code for Information Interchange.
ASCII is the standard method of encoding letters. numbers and punctuation
into seven-bit binary numbers. This method of text encoding is used by all
computer systems.
J
238
character). This format also indicates that the remote computer is a UNIX system. The next three groups of characters
are the file length in bytes, the date the file was last altered and
the file name. The next step is to change directories:
ftp> cd Web
ftp> dir
drwxr-xr-x
2048 Jan 6
Mosaic
Only the relevant directory listing is shown. After changing to
the Mosaic directory, another "dir" command shows the subdirectories "Windows," "Mac" and "Unix." The Windows version of Mosaic will be downloaded. Changing to the Windows
and Win31x directories and giving a "dir" command shows
several files. To install the Mosaic program, two files are
needed: mosaic20.exe and w32sOLE.exe. Installation instructions are in the README.TXT file in the Win31x directory.
To retrieve these files, first change to binary mode:
ftp> binary
ftp> get mosaic20.exe
ftp> get w32sOLE.exe
ftp> quit
ARCHIE
Archie is a search program, written at McGill University in
Montreal, Canada, for locating files at public ftp sites. Approximately once a week, archie searches most public ftp sites on
the Internet that can be reached by anonymous ftp and creates
a complete listing of all files and directories at these sites. This
listing is added to a database that can be searched for keywords. Archie functions in a typical client-server mode and, for
best results, the client should be run on the user's local system.
Archie, however, is not a standard Internet service and the
user's local system may not have an archie client. In this case,
a user can reach a public archie client by telnet. There are over
a dozen sites on the Internet that offer public archie clients.
It is important to realize that archie searches for directory
and file names, not for contents of a file. If a user knows the
name of a file, archie will locate that file, but if a user is looking
for information on a specific topic, a certain amount of guesswork will be required. For example, if a user wants information
on hepatitis, he can try search terms such as "liver," "hepatitis"
or "hepatic." Each file that contains the search term in the file
name or in the name of a directory leading to the file will be
displayed along with the directory path to the file and the
computer where the file is located. The user then needs to
download and examine the files to see what information they
contain.
On a UNIX system with an archie client, archie is accessed
in the following manner:
archie [- h archie server] (search term)
where "archie server" is name of a computer with a server and
(search term) is a string of characters. The "-h" characters
indicate the following string of characters is a host computer
name. If the (- h archie server] term is omitted, as is usually the
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case, the archie client will connect to a predefined archie
server. Depending on how the server is set up, archie can
search for arbitrary strings of characters or whole file names.
The user can specify which type of search he wants. If a user
uses a search mode that looks for arbitrary strings, the search
term "hepat" will locate "hepatitis" and "hepatic." An example of an archie search with a public archie client is shown
below. The search will be performed at McGill University in
Montreal, Canada for a Windows program, hopchkw.exe
(Windows hop check), that implements a service called traceroute:
telnet archie.mcgill.ca
login: archie
A password, "archie," may be required at other sites. After
logging in, the archie prompt is displayed. Typing "help" activates the help program that explains how the program works.
Typing "?" at the help prompt will display a list of commands.
The command "prog" followed by a keyword is used to search
for files:
archie> prog hopchkw.exe
ftp://ftp.isri.unlv.edu/private/dos/winsock!hopchkw.exe
Size: 27392 bytes
Date: 20:00 11 May 1994
Archie found the program on the computer ftp.isri.unlv.edu
which is at the University of Nevada at Las Vegas. The computer, directory path and file name are given in the URL
format (see below under World Wide Web for an explanation
of URLs). The date the file was put on the system and its size
are also specified.
GOPHER
One of the major drawbacks of telnet and ftp is that one
needs to know what information is located at what sites. To be
able to use the Internet effectively, a user would have to know
a lot about the location of resources. To overcome this problem, the gopher program was written. This program, developed
at the University of Minnesota and named after its mascot, was
made freely available to the Internet community in 1991.
Gopher is one solution to the problem of finding resources at
widely separated sites without knowing where these resources
are located.
Superficially, a text-based gopher program appears similar
to telnet. Gopher operates in the typical client-server mode. A
user accesses a gopher client by giving the gopher command
followed by the name of a site that runs a gopher server. If the
gopher command is given without specifying a server, the client
will connect to a predetermined, or default, gopher server. At
many institutions, the default server is the institution's own
gopher server. Unlike telnet, no login or password is required.
After connecting to a gopher server, a menu is returned that
appears very similar to menus seen at public telnet sites. Picking a menu item retrieves another menu or a document in a
manner similar to telnet. The difference, however, is that the
menu item may be located anywhere on the Internet. AssociVOLUME 23, NUMBER 4, DECEMBER 1995
ated with each menu item is a location and a path to a file or
other directory located either on the original gopher server or
some other gopher server on the Internet. When a menu item
is chosen, the local gopher client makes a connection to the
associated location, downloads the file or menu and then
terminates the connection. If a menu is downloaded, its items
may be linked to other gopher servers. Thus, by specifying a
single gopher server, a user could potentially have access to all
the gopher servers on the Internet. Whether or not this is
possible will depend upon the links the original gopher server
has to other servers. These links are established by the individual who sets up the gopher server and creates the menus.
Some gopher servers have very few links to other servers while
others, such as the gopher server at the University of Minnesota, have links to thousands of other servers. A gopher client
connecting to this server can access a large proportion of all
the publicly available gopher servers on the Internet.
Because all gopher servers can be linked together, the total
amount of information on the Internet that can be accessed by
the gopher program is often referred as gopherspace.
Gopherspace defines a distributed document system where
information is located using descriptive terms (i.e., the menu
items) rather than by stating where the information is located.
One can think of gopherspace as a tree with branches and
leaves. The trunk of the tree is the first gopher server accessed.
The branches are menu items. If a menu item is another menu,
that branch gives off further branches. If a menu item is a file,
the branch ends in a leaf where information is located. The
gopher client keeps track of a user's path along this tree and
allows a user to retrace his steps backward to the trunk.
The types of files that gopher can access are quite general.
The gopher program has built-in abilities to display files from
ftp sites and can retrieve information using search programs
such as WAIS and CSO. Although gopher can download image, sound and movie files, it does not have the ability to
display or play these files by itself. External programs, called
viewers, can be linked to the gopher client to perform these
functions. The computer on which the client resides must, of
course, also have the hardware capabilities for image, audio
and video support. Gopher programs can access documents on
the World Wide Web if a Web browser is linked to the gopher
client as a viewer.
As an example of how to use the gopher program, a session
will be described with the University of Minnesota gopher
server. A user gives the following command:
gopher gopher.tc.umn.edu
The UNIX shell activates the gopher client which contacts
the University of Minnesota gopher server. The server returns
the main menu shown below and then terminates the connection:
Internet Gopher Information Client v2.1.3
Home Gopher server: gopher.tc.umn.edu
-->
l. Information About Gopher/
2. Computer Information/
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3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Discussion Groups/
Fun & Games/
Internet file server (ftp) sites/
Libraries/
News/
Other Gopher and Information Servers/
Phone Books/
Search Gopher Titles at the University of Minnesota (?)
Search lots of places at the University of Minnesota (?)
University of Minnesota Campus Information/
Press ? for Help, q to Quit
Page: 1/1
Some of this text is added by the client, such as the title
"Internet Gopher Information Client v2.1.3" that identifies the
version of the client software that was obtained from the
University of Minnesota. The client identifies the gopher
server in the second line and numbers the menu items returned
from the server. The client adds identifying information at the
end of each menu item. The following symbols are used:
I
(?)
(Picture)
(Movie)
a directory
a keyword search item
an image file
a video clip
No symbol at the end of an item indicates a text file 4 • The client
also provides a help line at the bottom of the display screen.
Typing "?" will display a list of commands that control how
gopher functions. Gopher clients that use GUis often provide
similar functions in pull-down menus. Finally, the client lists, in
the bottom right-hand corner, the current and total number of
screens of text in the menu. A user chooses an item by moving
the arrow on the left of the menu to a desired item and then
hits return or types an item number followed by a return.
Choosing item I, "Information About Gopher," from the
main menu causes the client to reconnect to the University of
Minnesota gopher and download another directory.
View item number: 1 Connecting. . . Retrieving Directory ...
I.
2.
3.
4.
5.
6.
7.
8.
-->
9.
10.
11.
12.
Gopher T shirt on MTV movie (big) (Movie)
Gopher T shirt on MTV movie (small) (Movie)
Gopher T-shirt on MTV #1 (Picture)
Gopher T-shirt on MTV #2 (Picture)
How to get your information into Gopher
Reporting Problems or Feedback
Press ? for Help, q to Quit, u to go up a menu
Page: 1/1
Notice that in the help line at the bottom of the display, a new
command "u" appears which allows a user to return to the
previous menu. Item 6, "Commercial Gopher Software/", is
chosen next.
View item number: 6 Connecting. . . Retrieving Directory ...
Internet Gopher Information Client v2.1.3
-->
I.
2.
3.
4.
5.
Commercial Gopher Software
AIRGopher
Chameleon
GUIDE for Macintosh
GUIDE for Windows
WinGopher
Press ? for Help, q to Quit, u to go up a menu
Really quit (y/n) ? y
Page: 1/1
This menu has five items, all of which are text files describing
various types of proprietary gopher client programs. A user
exits the program by typing "q" and then "y" to confirm his
choice.
One important feature of gopher clients is the ability to
make entries, called bookmarks, to a user-created list of
gopher menu items. With the particular client shown, a user
simply types "a" when the arrow is next to a menu item to add
it to the list. The list is created automatically when the first
bookmark is chosen. This bookmark list, which can be viewed
at any time by typing "v", is used like any other gopher menu.
The bookmark list is, in effect, a user's own personalized
gopher server.
Internet Gopher Information Client v2.1.3
VERONICA
Information About Gopher
Although gopher allows a user to find resources throughout
the Internet without needing to know where they are located,
the process of finding information is not very efficient. A user
has to follow links on menus, but there is no guarantee that the
links lead to what the user wants or that the links are the most
appropriate resources. A partial solution to this problem was
the creation of a program called veronica. Veronica is to
gopher as archie is to ftp. As with archie, the veronica program
periodically searches all of gopherspace (approximately once
per month) and creates a database of all menu items it finds. In
order to do this, veronica starts at the University of Minnesota
gopher and walks through the menus recording each item and
following each link. When a link leads to another gopher
server, veronica connects to that server and follows a similar
process, eventually finding all the menu items of all gophers
linked directly or indirectly to the University of Minnesota
gopher.
About Gopher
Gopher News Archive/
GopherCON '95/
GopherCON '94/
Gopher Software Distribution/
Commercial Gopher Software/
Gopher Protocol Information/
University of Minnesota Gopher software licensing
policy
Frequently Asked Questions about Gopher
Gopher+ example server/
comp.infosystems.gopher ( USENET newsgroup )/
Adding Information to Gopher Hotel
4 GUI clients typically identify menu items by icons placed at the left of
each item.
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13.
14.
15.
16.
17.
18.
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Veronica functions in a client-server mode, but there are no
specific veronica clients. A veronica server is accessed through
a gopher client that links to gopher sites with a veronica server.
A user chooses the veronica menu item and is asked for a
keyword search term. Veronica searches its database and returns a menu whose items contain the keyword along with the
links for these items. A user chooses an item from this menu
and is connected to its gopher server. One can think of a
veronica server as a gopher server that creates customized
gopher menus. Veronica was created at the University of
Nevada and there are now about two dozen veronica servers on
the Internet. Any gopher server can contain links to these sites
and there are several gopher servers that contain a list of all the
known veronica sites. Some of these gopher servers provide
information on which veronica servers are currently active.
Unfortunately, veronica servers tend to crash frequently. Many
gopher servers have menu items that link them to veronica
servers. Two examples are the servers at the University of
Minnesota, gopher.tc.umn.edu (under "Other Gopher and Information Servers/" on the main menu) and the server at the
University of Nevada at Reno, futique.scs.unr.edu.
A restricted version of veronica is jughead. Jughead performs the indexing and search functions of veronica on a single
gopher server. The administrator of the gopher server decides
whether to make jughead searches available at his site.
WORLD WIDE WEB
The Internet service that is responsible for the explosive
growth of the Internet in the last two years is the World Wide
Web (WWW). Like gopher, the WWW is a distributed document system with links between documents. One of the main
differences between these two services is how information is
accessed. Gopher systems are strictly hierarchical. Menu items
are either directories or files that are arranged in a tree-like
structure with all the information stored in files. A user moves
sequentially down the tree until a file is reached. If one wants
more information about a topic in a file, one can retrace the
path along the tree to see if there are related files on the topic
of interest. This information may or may not exist at some
point further up or down the tree but, in general, there is no
well-defined method for finding related information on a topic.
Although files on WWW servers are arranged in a fashion
identical to those on gopher servers, the method of accessing
WWW files is more flexible. A user can jump to any document
on the WWW without first accessing the intervening directories. Unlike gopherspace where the information is contained in
files that are at isolated end points, all documents in the WWW
can contain links to other documents making the structure
more analogous to a Web than a tree with branches and leaves.
WWW documents can contain text, images, graphics, video
and sound clips, or any combination of these elements. Within
a document, words, phrases or even pictures are highlighted.
Choosing the highlighted item, by clicking on it with a mouse,
retrieves and displays a linked document that itself may contain
highlighted items that are linked to other documents. Text
written in this format is called hypertext, while documents
VOLUME 23, NUMBER 4, DECEMBER 1995
contammg images, videos or sound files are referred to as
hypermedia, although the term hypertext is commonly used to
describe both types. Documents available on the WWW are
called Web pages. The main entry point at a Web site is called
the home page, a term without a precise meaning since any
document at a site could be so designated.
Another major difference between gopher and the WWW is
the format of documents. While both services can display text,
image or video files, gopher can only display one type at a time.
WWW pages, on the other hand, can contain text and multimedia in the same document because Web browsers for GUI
systems have built-in capabilities for displaying the most common types of image files. Images that are displayed as part of
a Web page are called inline images.
The idea of hypertext did not originate with the WWW. The
Hypercard program on Macintosh computers first used this
system extensively, while the on-line help system of Microsoft
Windows is also based on hypertext. The WWW, however, was
the first Internet service to use hypertext as the basis of a
distributed document system. Hypertext is a more natural way
of linking documents together. For example, a document describing wildlife in the North America may have the words
"cougar," "robin," and "North America" highlighted because
a description of these terms may not be appropriate to the rest
of the document. Choosing the highlighted word "cougar"
would return another document that might describe cougars in
more detail. This text itself could have highlighted words, along
with pictures of cougars or a map of their range in North
America.
WWW documents are written in a special format called the
hypertext markup language (html). See Appendix A for a
discussion of html. This language inserts special formatting
codes into the documents that allow WWW client programs to
display Web pages properly and permits the embedding of
links to other files within the document. This contrasts with
documents that can be displayed by telnet, ftp and gopher
clients which are simple text. The method of transferring Web
documents between computers is defined by the hypertext
transport (or transfer) protocol, http, which normally functions
on port 80. As with gopher, as soon as a document is transferred from a Web server, the connection to the client is
terminated.
When the WWW was first introduced on the Internet in
1991, it was not initially widely used for two main reasons.
While it is a straightforward procedure to make simple text
documents available by telnet, ftp and gopher, the same documents would have to be reformatted in html in order to have
links within the document. Web browsers can display simple
text files but, without links within a document, a Web browser
is not much different than a gopher client. Until a large
number of sites began offering html documents through
WWW servers, there was simply not a lot of information
accessible through the WWW. Secondly, the first WWW clients were text-based interfaces and, in many cases, these did
not have an obvious advantage over the menu-based format of
gopher programs.
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The number of sites on the WWW gradually increased until
November 1993 when the WWW entered an explosive growth
phase. The sudden increase in the size and popularity of the
WWW was due to the release of a program, called Mosaic, by
the National Center for Supercomputing Applications (NCSA)
at the University of Illinois at Urbana-Champaign. Mosaic is a
WWW client program that can download and display not only
hypertext but linked image files. When properly linked with
additional programs, Mosaic can also play sound and video
files. Mosaic was made freely available to the entire Internet
community in Windows, Macintosh and UNIX versions. The
response was enormous. Within a few months, millions of
copies of Mosaic were downloaded from NCSA. The number
of people using the WWW greatly accelerated the creation of
Web sites, especially in the commercial sector. NCSA continues to improve Mosaic, offering new versions at regular intervals. The commercial potential of Mosaic was quickly realized
and several companies licensed the technology from NCSA to
create proprietary versions of Mosaic. The most popular of
these proprietary products is Netscape by Netscape Corporation. Some programs related to Mosaic can be found in Appendix B.
Mosaic and its related programs are called Web browsers
and they are more than just WWW client programs. Web
browsers have built-in capabilities to access gopher, WAIS and
ftp servers. Depending on the browser, built-in capabilities for
accessing telnet, USENET and email are also available or can
be linked to the program using additional software that can
perform these functions. Web browsers have built-in capabilities to display the more common types of image files, such as
the widely used GIF and JPEG files.
In general, Web browsers have more extensive hardware and
software requirements than software for the other Internet
services. To run Mosaic and related programs, a computer
must have image display capabilities. These features are standard on Macintosh computers and on any system that can run
Windows or IBM's OS/2 operating system. UNIX systems
require a special display environment called the X Window
system. For personal computers, the minimum memory requirement is 4 megabytes of random access memory (RAM),
although 8 megabytes are recommended. For personal computers accessing the Internet through a modem, the TCP/IP
software must be run on the user's computer rather than on the
remote computer of the Internet access provider.
Web browsers use a special notation for accessing resources
on the Internet called Uniform Resource Locators (URLs).
Every file on the Internet can be described in terms of its
access method, location on a given computer, port number,
directory path to the file and the file name itself. URLs incorporate this information into a character string in a standardized format. The formal names used in URLs for the access
methods are telnet, ftp, gopher, http (for the World Wide
Web), wais, news (for USENET) and mailto (for email).
An example of a URL for a Web page on the WWW at the
University of Utah is:
http://www-medlib.med.utah.edu:80/WebPath/webpath.html
240
The main components of a URL are illustrated above: ( 1) an
access method, http, followed by a colon and two forward
slashes; (2) the name of the computer on which the file resides
followed by a colon, a port number and a forward slash (the
colon and port number are included for illustrative purposes
only; since the WWW normally resides at port 80, they could
be omitted in the above example); (3) a directory path to the
file of interest followed by a forward slash; and (4) a file name 5 •
The computer name can be entered in upper or lower case.
The access method is read by the browser while the directory
paths and file names are read by the server. Some browsers,
such as Netscape, will accept the access method in upper or
lower case while others, such as Mosaic, accept only lower case.
The directory paths and file names are case sensitive for UNIX
servers but case insensitive for Macintosh (e.g., http:
//www.apple.com) and Windows (e.g., http://www.microsoft
.com) servers. Since a user will rarely know the type of server
he is accessing, it is best to enter URLs exactly as given. A
UNIX server will return a message that the file cannot be
located if a path or file name is entered in the wrong case. No
spaces are permitted in a URL. If the file name and directory
path are omitted, a Web browser will display a default document that is often the home page at the Web site. Each
directory in the directory path can have a default document
associated with it (called index.html on UNIX systems) that is
displayed when only the directory path, or a portion of it, is
entered. If no default document is associated with a directory,
a directory listing is returned for the last directory listed. When
a Web browser is used to access a gopher or ftp server and the
directory path is not specified, the main menu at that site is
displayed. Some browsers and some sites require a trailing "/"
if only a computer name or a computer name and a directory
path are given.
Web browsers have a host of interesting features only a few
of which will be described here. Web browsers differ somewhat
from each other, so it's not possible to give a detailed generic
description of a Web browser. In the following discussion, the
Windows version of Mosaic will be described. The Macintosh
and UNIX versions are very similar. Since Mosaic is freely
available, anyone can download and run this program. In
addition, most commercial Web browsers are based on Mosaic
and have functions similar to those described below.
An example of a Web page from Loyola University as displayed by Mosaic is shown in Figure 1. There are six main parts
to this display. From the top downward, they are: (1) the title
line; (2) the menu bar; (3) the toolbar; (4) the URL indicator
line; (5) the display area for Web pages; and (6) the status line.
The Web document title is displayed on the title Iine 6 • Each
word in the menu bar has an associated pull-down menu.
These menus can be seen by moving the pointer to an item in
the menu bar and clicking the left mouse button. A menu item
• For email and USENET, the format of a URL is somewhat different.
The URL format for email is mailto:(address) where (address) is an email
address. For USENET, the format is news:(newsgroup) where (newsgroup) is
name of a newsgroup such as sci.chem mentioned above.
6 A title is an optional element of a Web page, although it is strongly
recommended that authors include titles for Web pages.
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FIGURE 1. The home page of LUNIS, the Loyola University Nuclear Information System, a
nuclear medicine bulletin board. The URL of
this Web page, http://www.lunis.luc.edu, is
shown immediately above the display area.
Text highlighted in blue is linked to another
document. The actual Web page is considerably longer than the displayed text and can be
viewed by using the scroll bar on the right. The
Web browser used here is NCSA Mosaic, final
beta version 2.0 for Windows.
is selected by moving the pointer to that item and clicking on
it. The toolbar contains buttons that perform various functions
by clicking the mouse on the buttons. For example, the arrow
buttons allow a user to move forward or backward to previously viewed Web pages. The URL indicator line shows the
complete URL of the currently displayed Web page. A user
can manually enter a URL on this line to access a new Web
page. The main portion of the screen is used to display the
Web page. The user can remove some or all of the other
elements mentioned above to create a larger display area for
the Web page. Many Web pages are too large to be displayed
on a single screen, so the scroll bars to the right and at the
bottom of the screen can be used to view different parts of the
document. The last element in the display is the status bar at
the bottom. As a document is being transferred, information
appears in this area describing the status of the transfer, such
as the size of the document and what portion of the document
has been received.
One very important feature of Web browsers is the hotlist.
This is a list of URLs a user creates that can be accessed
through pull-down menus from the menu bar. Hotlists are the
equivalent of bookmark lists with gopher clients. Figure 1
shows several hotlists I have created: Internet, Nonmedical,
Medical, Fun Stuff, Science and Maps. Starting Points is also a
hotlist that comes preinstalled with Mosaic. A user adds an
item to a hotlist by choosing a hotlist and clicking on the "~dd
To Current Hotlist" item under the "~avigate" menu. The title
of the document is then added to the hotlist. As an example,
Figure 2 shows my Medical hotlist, opened from the menu bar.
Moving the pointer to any item on the list and clicking the left
mouse button transfers that Web page to the browser. In
Figure 2, I have moved the pointer to "Welcome to MedSearch
America", the main Web Page for a company advertising
available positions in several medical related fields, including
nuclear medicine technology. After clicking the left mouse
button, the Web page in Figure 3 is retrieved and displayed.
When a Web browser is first started, it reads information
from a start-up file. This start-up file, called mosaic.ini in
Windows systems, contains a large number of parameters that
control the function of the browser. These parameters can be
changed by using the "Qptions" item on the menu bar and
selecting "~references ..." One piece of information a browser
needs is the initial Web page to be displayed when the browser
is started. This page is specified by a URL in the start-up file.
Most browsers are preconfigured to display the home page of
the company or institution that created the browser. A user,
however, may want to display a different Web page when
starting the browser, such as the Web page I have created for
my own use shown in Figure 4. With Mosaic, the initial Web
page can be changed using the "~references... " selection
mentioned above. To use custom-made Web pages, a user
creates an html document, as described in Appendix A, and
stores it on the computer's hard drive. The browser is then
instructed to use this document as the initial Web page.
One of the functions of a URL is to tell a Web browser
which server to contact to access a file. A Web browser,
however, can also directly access files on its own computer
without passing the files through a server. In order to do this,
any reference to a local file, such as a link in a Web page, must
use the following syntax:
file:/1/drive/path/filename
or
file://localhost/drive/path/filename
In this syntax "drive/path/filename" is how a file is identified by
the local operating system; file:/// or file://localhost tells the
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FIGURE 2. A pull-down menu associated with
the hotlist "Medical." This list is displayed by
placing the arrow over the word "Medicine" in
the menu bar and clicking the left mouse button. The items in this list are the titles of the
Web pages, not their URLs. Sliding the indicator line over an item and clicking the mouse
button connects to the title's URL. In this example, Welcome to MedSearch America was
chosen and is displayed in Figure 3. Notice item
10, "untitled." This Web page did not have a
title.
browser the following file is on the local computer. For example, a text file called phones. txt in the windows directory on the
C drive of a PC could contain a list of phone numbers. If a user
wanted to link this file to a local Web page, the file would be
referred to as:
file:///C:/windows/phones. txt
or
file:/!Iocalhost/C:/windows/phones. txt
Some browsers, such as Mosaic, do not require "driver' if the
file is located on the same drive as the browser. One could use
Mosaic to display files and images located on the local computer without having a connection to the Internet or any other
network. This could be done by creating Web pages with links
only to local files. In fact, most Web browsers have options that
allow a user to select and display files directly from local disk
drives even if the files are not linked to any Web pages.
For any Web browser to display image or video files or to
play audio files, it must have appropriate software for handling
these files. These multimedia files are written in multiple
different formats and corresponding software programs are
needed for handling each type of format. Most Web browsers
have built-in capabilities to display the more common types of
FIGURE 3. The home page of MedSearch
America, URL http://www.medsearch.com. In
order to show as much of the page as possible,
presentation mode is used which removes from
the screen all graphic elements of the Mosaic
program except for the scroll bar on the right.
Presentation mode is activated and deactivated by pressing the Alt and P keys simultaneously. MedSearch America is an information
service for the allied health professions. The
circled text in the image is linked to other Web
pages. As in Figure 1, this page is too long to be
displayed in its entirety.
242
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FIGURE 4. A Web page created for use as the
initially displayed document when Mosaic is
started. The Options item from the menu bar
has been usid to tum off the display of the
status bar and the tool bar. The six lines at the
bottom are links to other sites. Note that since
this page is read directly from the computer's
hard disk. its format starts with file:///. The html
document for this page is shown in Figure 5.
image files: GIF and JPEG. There are, however, many other
image formats that cannot be directly handled by Mosaic and
most video and audio formats are also not supported. In order
to display these other types of files, Web browsers can be
linked to programs, generically referred to as external viewers,
that can display these files. When a browser downloads a file it
cannot directly display, it checks its start-up file to see if there
is a viewer on the local system to which it can send the file. The
user must specify the name of the external viewer and its
location on the local system in the start-up file. These viewers
may or may not be available as part of the software that comes
with the operating system. If they are not, a user has to find and
install the viewers he desires. External viewers are required for
more than just multimedia files. For example, some text files.
such as word processing files, are also written in special formats
and, for these types of files to be displayed, the word processing program must be linked to the browser through the start-up
file. Fortunately, the procedure for linking a viewer is now a
fairly automated proces.-; with most browsers, but it is still one
of the more difficult tasks in making a browser a highly versatile display tool.
There are a large number of other Internet services. Some of
these are defined by standard Internet protocols and others are
implemented by various commercial and noncommercial software programs. A few of these services are described below.
Finger is a standard Internet protocol that provides information about users on a system. Finger is a built-in feature of
all UNIX systems. The format of the command is:
finger [user name]@[computer name]
If the finger command is given without arguments, a list of
users who are currently signed onto the local system and the
VOL- U, ,.._... 4, -~- 18H
programs they are running are displayed. If only [username] is
given as an argument of the finger command, finger returns
more complete information about a user on the local system.
On UNIX systems, this includes the contents of a file, the .plan
file, that can contain any information a user may wish to make
available. The file may contain simple text or a list of commands that are performed whenever that user name is accessed. If the user is on a remote system, [computer name]
must be specified. If only @[computer name] is given, users
who are currently signed onto the remote system are listed. As
an example of how finger can be used to disseminate information, the command:
ftnger [email protected]
returns the .plan file created by the user "copi" on the computer oddjob.uchicago.edu at the University of Chicago. The
file lists significant events that have occurred on the current
date. Because finger can provide important information about
a system and its users, many sites have disabled this service for
remote users.
The domain name system (DNS) in one of the most important standard Internet protocols. The primary function of DNS
is to provide IP addresses for computer names. As discussed in
the first article in this series, communication on the Internet is
done in terms of IP addresses; computer names are a convenience for humans who find it difficult to remember IP addresses. DNS clients obtain IP addresses from DNS servers
that are usually referred to as name servers. These are specifically defined computers that maintain lists of computer names
and their associated IP addresses. The servers return the IP
addresses of computer names. Name service is such an important and widely used function, that DNS clients are set up to
automatically query name servers and return IP addresses
whenever computer names are given in a command. Some
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software programs, such as TCP/IP software for personal computers, may require a user to enter the IP address of a name
server when installing the program. For most TCP/IP software,
the DNS client functions automatically and is invisible to the
user. Name servers provide more information than just IP
addresses for computer names. Some systems have commands
that allow a user to directly access a name server for information about the computers and the computer networks that they
serve. On UNIX systems, these commands are nslookup and
dig.
Ping is one of the simplest services a computer can run. Ping
sends a single, small data packet to a remote computer. The
remote system sends back a reply that the packet was received.
The purpose of ping is to determine if a computer is active and
connected to the Internet. The structure of a ping command is:
ping (computer name)
The response is either that the computer is active or that the
computer cannot be contacted, or no response is received. If a
remote computer does not respond to ping, it will usually not
respond to any other service request from the local computer.
As an example, if a user cannot ping a remote computer name
but can ping the computer's IP address, either the user has
typed an incorrect computer name or there is a problem accessing the name server of the local computer.
Traceroute is a nonstandard Internet service that is a sophisticated use of ping. The trace route command is similar to ping:
traceroute (computer name)
Traceroute displays a listing of all the computers, routers and
networks through which a packet travels on its way to the
remote computer. It performs this function by sending out a
series of ping commands to the intervening computers and
routers. Equivalent commands on other systems are hopchkw
on Windows and tracerte on OS/2 Warp.
One of the most frequently asked questions is how to find
the email address of an individual on the Internet or other
information such as his business address or phone number.
Although efforts are being made to provide the equivalent of
phone books for individuals on the Internet, any comprehensive listing is still a long way off. Nevertheless, a variety of
protocols and indexing methods are available, and only a superficial discussion is presented here. One of the first attempts
to provide directory service at local institutions was through
CSO servers, sometimes called qi servers, named after the
Computing Systems Office of the University of Illinois. This
service was introduced as a type of electronic phone book. It is
commonly found on gopher servers at many universities and
allows keyword searching. The problem with this service is that
a user must know where an individual is located. More comprehensive directory services are provided by a number of
protocols such as the whois and X.SOO protocols. A good
example of the whois service is available at the interNIC which
can be reached by telnet (rs.internic.net, login: whois), gopher
(gopher.internic.net) and the WWW (http://www.internic.net).
244
The main Internet search programs described so far, archie
and veronica, have one major drawback-they search for file
names and menu items, respectively, not content. What most
users want is a method for searching files or documents for
specific information. This is accomplished by a service called
WAIS (Wide Area Information Service). Each WAIS server
has a database of related documents, such as files on Asian
culture. WAIS creates an index for each document that lists
the number of occurrences of each nontrivial word in the
document. Trivial words are words that have little or no specific information content, such as and, or, the, not, which, etc.
A WAIS client accepts keywords and sends them to a user
selected server. The server returns not only a list of documents
with the keyword, but also a ranking based upon several criteria such as how often the keyword appears in the document.
This ranking is a method of determining the relevance of a
specific keyword in a particular document. A user can pick a
document in the list to display its contents. The value of a
WAIS client is dependent upon the number of WAIS servers it
can contact, each of which is dedicated to a specific topic.
There are now about 1,000 such servers on the Internet. It
would be impossible for a user to be familiar with all these
servers and a database of WAIS servers, the directory of
servers, has been created to help search for a particular server.
A public WAIS client is available on the WWW at http://
www.wais.com.
WAIS is one of the most important search functions on the
Internet, but it is also one of the most difficult to use. Many
keyword search capabilities on telnet, gopher and WWW servers access WAIS servers through gateways in a manner that is
transparent to a user. For users who need more in-depth
search capabilities of these WAIS databases, commercial
WAIS clients are available.
A large number of Internet services are available as homegrown programs on specific computers that can be reached by
telnet. There are too many of these programs to describe even
superficially. Two examples have been listed previously, the
University of Michigan's Underground Weather Service and
netlink. The WWW provides a rich source of search programs
and other services that will be described in more detail in the
last article in this series, "Resources on the Internet."
APPENDIX A
Creating Hypertext Markup Language (htmll
Documents
One of the characteristics of html documents that makes
them easy to create is that they are simple text files. Although
they contain special formatting codes, tags, these tags are
themselves text. This contrasts with most word processing
programs that add nonprintable, proprietary formatting codes
to documents they create. These codes can be read and have
meaning only to the word processing program or printers. A
user can create an html document using one of these word
processors, but the program must be instructed to save the
document as a simple text (or ASCII) file, in other words
without the proprietary formatting codes. The word processing
JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY
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<TITLE> My Home Page<fTITLE>
<H1 >Welcome to my Home Page</H1 >
<P>
< IMG SAC= "file:/I /c:/internet/gif/glacier_.gif" >
This is the jumping off point to the Internet's World Wide Web.<BR>
Next stop: Cyberspace!
<H2>Links to the lnternet</H2>
<UL>
< L1><A HREF=" http://www. ncsa. uiuc.edu/SDG/Software/Mosaic/Docs/whatsnew.html">
What's New on the Web</A>
<L 1 ><A HREF="http://www.ukans.edu/uroulette.html">
Uroulette</A>
<L 1 ><A HREF="http://cuiwww.unige.ch/meta-index.html">
We Search Engines</A>
<L 1 ><A HREF="http://gamma.wustl.edu:80/tf/caic.html">
Computer and Instrumentation Council, Society of Nuclear Medicine</A>
<L 1 ><A HREF="http://www.lunis.luc.edu">
LUNIS</A>
<L 1 ><A HREF="http://wings.buffalo.edu/world">
Virtual Tourist</A>
</UL>
FIGURE 5. The html file, home.htm, for the Web page shown in Figure 4. See Appendix A for an explanation of symbols. For Web pages
stored on Windows systems, html files are identified by the file extension "htm."
program Notepad with Windows creates simple text files. Figure 5 is an example of a Web document that I created and
Figure 4 shows how this document is displayed by Mosaic. Tags
are enclosed within angled brackets ( ).
The tags in Figure 5 are of two general types. Tags that
perform a simple function, such as indicating where a paragraph begins (P) or where a carriage return (BR) should be
inserted within text, are single symbols called standalone tags.
Most tags, however, occur in pairs such as (UL) and (IUL)
indicating where an unordered list of items, each identified by
(LI), begins and ends. An ending tag always contains a forward
slash. The most important of these tags are the anchor tags.
The beginning anchor tag requires another piece of information to define the anchor-a reference to linked information.
Only one reference type is used in Figure 5, the hypertext
reference HREF. This indicates that the following text in
quotes is a URL or a local file. The text between the anchor
tags is displayed by the Web browser as a highlighted item. In
Figure 4, the text is highlighted in blue. The wording of the text
is arbitrary but descriptive of the underlying link. In fact, the
text could be replaced by the name of an image file, in which
case a picture is displayed on the Web page. When the user
clicks on the picture, the underlying link in the anchor tag is
retrieved. See below for a further explanation. An item in a list
VOLUME 23, NUMBER 4, DECEMBER 1995
does not need to be linked to a URL. Text after an (LI) tag
without a link is displayed as ordinary, nonhighlighted text.
The tags in Figure 5 are as follows:
(TITLE) Name (!TITLE)
(Hl) Heading (/Hl)
(P)
(IMG SRC="file")
(BR)
(H2) Subheading (!H2)
(UL) Items (IUL)
(LI)
(A reference) text (/A)
Document title
Main document heading
Begin a paragraph
Insert an image specified by
"file" (file is a URL or a local
image file)
Break; insert a carriage return
Secondary document heading
Unordered list of items
List item
Anchor
The title tag identifies the enclosed text as the title of the
document. The title is different from the URL of the document
and is a readable version of what the document contains.
Mosaic and other browsers display the document title in different ways and use the title to identify documents in hotlists.
The (H) tags (header tags) are a method of creating subdivisions with a document. Note that simple text, such as "Welcome to Cyberspace," does not have any tags. It is displayed
245
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sequentially after the previous tag or text. The important part
of this document is the list of links or anchors. For brevity, I
have inserted only six links in this document, but many more
could be added. Each beginning anchor tag contains a reference. In this document only one type, the hypertext reference,
is used. Its format is HREF and a following URL, such as
HREF="http://cuiwww.unige.ch/meta-index.html". The immediately following text is highlighted and displayed by the
browser.
In this document, one image is included, a GIF file showing
the scene of a glacier. This image could have been linked to
another document if the reference to the image file had been
placed between anchor tags. For example, assume there is an
explanation of glaciers in the file glaciers.txt on the computer
earth.geo.edu and that the URL of this file is http://earth.geo
.edu/land/glaciers.txt. Then, if line 4 in Figure 5
(IMG SRC
=
"file:/1/c:/internet/gif/glacier_.gif)
was replaced with
(A HREF = "http://earth.geo.edu/land/glaciers.txt")
(IMG SRC = "file:/1/c:/internet/gif/glacier_.gif)(/A)
the resulting document would be displayed exactly like the
original one except for a blue border around the image. This
would indicate that the image is linked to another document,
and clicking on the image would retrieve and display the file
glaciers.txt from the computer earth.geo.edu.
For individuals who do not want to learn html, there are
several html editors available on the Internet, as well as commercial software that will create Web documents. These editors are similar to word processing programs and automatically
insert appropriate tags in the document when a specific function is requested.
APPENDIX B
Software for Internet Services
There is a large amount of software for the various Internet
services. This software can be obtained from both commercial
and noncommercial sources. Many software programs are
freely available from several sites around the Internet. Although freely available, these programs usually have some
restrictions on their use. Freeware refers to programs that can
be downloaded and used free of charge. Shareware is freely
available with the request that any individual who uses the
program registers with the developers and pays a small fee,
usually less than $50.00. Some programs by commercial developers are offered for free use with the restriction that they are
used only by individuals or on a small scale by educational
institutions. These programs are often early versions of software that will be offered for sale at a later date when a more
mature product is developed. Finally, some programs are freely
available but can only be used in conjunction with a specific
on-line access provider who charges for the time the user is
on-line. Freeware and shareware are usually made available
246
with a requirement that they will not used or sold for commercial purposes without the express consent of the developers.
Software is available for both clients and servers for all the
common Internet services for all types of operating systems.
Since most servers on the Internet use the UNIX operating
system, the majority of server software is written for UNIX
systems. This appendix does not discuss server software, but
those who wish to set up servers on their personal computers
should be aware that such software exists.
Freeware and shareware usually consist of individual programs for each of the Internet services. There are many ftp
sites on the Internet that offer these programs. A listing of a
few of these sites is given below. The user should read the
various README or other documentation files at each site to
get an idea of what programs are available and where they are
located. Programs are often in subdirectories listed in the pub
directory on the main menu. The most widely used TCP/IP
shareware for Windows is Trumpet Winsock (tcpman.exe is
name of the program). Specific programs can be found by
archie searches. The program Winsock FTP for Windows
(ws_ftp.exe) contains a list of sites with Internet software. A
user can download this program and review this list even if he
does not actually connect the program to the Internet. More
general topics can be found with veronica searches. For example, a search term "macintosh archives" will return a list of
sites where Macintosh programs can be found.
Many commercial packages are bundled software, in other
words they contain multiple software programs for Internet
services and TCP/IP software. Some are preconfigured to access a given Internet access provider. All the major online
services offer Internet access. See Part 2 of this series for a
description of these services. A few of the better known commercial software products are listed below. The reader should
be aware that software products change often and more upto-date information is available in many computer-oriented
magazines and publications.
Software Available by ftp on the Internet:
Address
Comments
~-----------------------
ftp.ncsa.uiuc.edu
fatty.law.cornell.edu
ftp.netscape.com
ftp.iastate.edu
National Center for Supercomputing
Applications, University of Illinois.
Many programs available including
viewers. Mosaic is in the Web/Mosaic directory.
Cornell Law School. Cello, a Windows Web browser not based on Mosaic, is found in the pub/LII/Cello
directory.
Netscape, the most popular commercial version of Mosaic, is found
in the netscape directory. This version is free subject to licensing restrictions.
Iowa State University. Change to the
pub directory after logging on. Many
Internet application programs are
found here.
JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY
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wuarchive.wustl.edu
oak.oakland.edu
Commercial Software:
Product
Internet in a Box 2.0
Mosaic in a Box
MacWeb
WinWeb
NetCruiser
OS/2 Warp
Chameleon
Netscape
Washington University at St. Louis
and Oakland University in Rochester, Michigan contain some of the
largest software archives on the Internet.
Comments
----------------------------A complete set of Internet application programs for Windows from
Spry, Inc. Included is the Web
browser, Air Mosaic and TCP/IP
software. The package will automatically connect to an Internet access
provider, SprintLink, or let a user
choose another provider. For more
information, contact http://www
.com puserve. com/prod_ services/
consumer/consumer.html.
Internet in a Box adapted for Windows 95. Contact: as above.
MacWeb and WinWeb are Web
browsers available from EINet galaxy. Alpha versions of these products
are available as shareware by ftp
from ftp.einet.net in the einet subdirectory. Contact http://www.einet
.net.
From NetCom Online Communications. This is a fully integrated package of Internet application programs
for Windows including a Web
browser. The software can be downloaded by modem. This package connects to Netcom's own proprietary
network. New versions of the software can be obtained for free online.
Contact http://www.netcom.com.
IBM's OS/2 latest operating system
includes a complete package of Internet software, including the Web
Explorer. The TCP/IP software is
preconfigured to connect to the Internet through the IBM Global Network. Contact http://www.austin
.ibm.com/pspinfo.
Netmanage, Inc. Provides the most
complete set of Internet and networking tools for Windows including
the ability to run ftp server software.
Contact http://www.netmanage.com.
From Netscape Corporation. The
commercial product provides full
user support and a more enhanced
version of the product than the
shareware mentioned above. Avail-
VOLUME 23, NUMBER 4, DECEMBER 1995
able in Windows and Macintosh version. Contact http://www.netscape
.com.
The operating system comes preconfigured to connect to the Microsoft
Network. TCP/IP and Internet application software are available. Contact http://www.windows95.com.
Macintosh does not have a fully integrated set of Internet tools. The
System 7.5 operating system has
built-in TCP/IP software. There are
application programs for all the individual services such as Turbogopher and Fetch (for ftp) as well as
Web browsers. Contact http://www
.apple.com.
A complete set of Internet applications with preconfigured access to
PSINet, the proprietary network of
PSI, Inc. (Performance Systems International). Both Macintosh and
Windows packages are available.
Contact http://www.pipeline.com.
Windows 95
Macintosh
Pipeline
SUGGESTED READING
There are hundreds of books about the Internet. Some are
devoted to specific topics such as Mosaic, Netscape, html,
USENET, email, etc. The books listed below are recommended because they have been reviewed by the author or
have received favorable reviews in the computer literature.
Beginning Level
Levine JR. Young ML. The Internet for dummies quick reference. Foster City, CA: IDG Books Worldwide, Inc.; 1994.
Seiter, C. The Internet for Macs for dummies quick reference.
Foster City, CA: IDG Books Worldwide, Inc.; 1994.
Duntemann J, Pronk R, Vincent P. Web explorer pocket companion. Scottsdale, AZ: The Coriolis Group, Inc.; 1995.
Kent P. 10-minute guide to the lnternet. Indianapolis, IN: Alpha
Books; 1994.
Intermediate Level
Krol E. The whole Internet: user's guide and catalog, 2nd ed.
Sebastopol, CA: O'Reilly and Associates, Inc.; 1994.
Gilster P. Finding it on the Internet: the essential guide to archie,
veronica, gopher, WAJS, WWW, (including Mosaic), and other
search and browsing tools. New York, NY: John Wiley and
Sons, Inc.; 1994.
Kehoe BP. Zen and the an of the lnternet. A beginner's guide.
Englewood Cliffs, NJ: Prentice-Hall, Inc.; 1994.
Marine A, Kirkpatrick S, Neou V, Ward C. Internet: getting
staned. Englewood Cliffs, NJ: Prentice-Hall, Inc.; 1993.
247
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Advanced Level
Comer DE. lnternetworking with TCP/IP: principles, protocols,
and architecture, 3rd ed. Englewood Cliffs, NJ: Prentice Hall,
Inc.; 1995.
Lynch DC, Rose MT, eds. Internet system handbook. Reading,
MA: Addison-Wesley Publishing Company, Inc.; 1993.
Liu C, Peek J, Jones R, Buus B, Nye A. Managing Internet
248
information services. Sebastopol, CA: O'Reilly and Associates, Inc.; 1994.
Hunt C. TCP!IP network administration. Sebastopol, CA:
O'Reilly and Associates, Inc.; 1992.
Dyson P. Novell's dictionary of networking. San Jose, CA: Novell Press; 1994.
Graham IS. HTML sourcebook: a complete guide to HTML.
New York, NY: John Wiley and Sons, Inc.; 1995.
JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY
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An Introduction to the Internet, Part 3: Internet Services
Jerry V. Glowniak
J. Nucl. Med. Technol. 1995;23:231-248.
This article and updated information are available at:
http://tech.snmjournals.org/content/23/4/231
Information about reproducing figures, tables, or other portions of this article can be found online at:
http://tech.snmjournals.org/site/misc/permission.xhtml
Information about subscriptions to JNMT can be found at:
http://tech.snmjournals.org/site/subscriptions/online.xhtml
Journal of Nuclear Medicine Technology is published quarterly.
SNMMI | Society of Nuclear Medicine and Molecular Imaging
1850 Samuel Morse Drive, Reston, VA 20190.
(Print ISSN: 0091-4916, Online ISSN: 1535-5675)
© Copyright 1995 SNMMI; all rights reserved.
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