CALMET 99 - Workshop C: Web-based instruction (introduction)

1. What a browser does and how the Web works

The World Wide Web operates according to a client/server model. Web clients (browsers) can access multiprotocol and hypermedia information from servers.

Protocol of the network ; an address for each computer :

Internet is based on TCP/IP protocol, which enables computers to exchange data.
Each computer has an IP address. 137.129.254.14 is the IP address of the EuroMET server in Météo-France.
By the way, it's easier for users to use names than IP addresses, so each computer may have a name. (euromet.meteo.fr)
This name is constituted by the main domain name (.fr ; .uk ; .com ; .net...) the company name (meteo ; microsoft ; netscape) , the computer name.
A DNS (Domain Name Server) shows the correspondance between the name and the IP address. Your browser has to know at least one DNS.
The name isn't an obligation, so that many clients do have only an IP address, but servers generally have a name.

Protocols of datas :

The World Wide Web is an information system on the Internet that uses standard protocols.
HTTP (hypertext transfer protocol) is a protocol using a standard format for describing the structure of documents ( HTML : hypertext markup language).
FTP (File Transfer Protocol) and telnet are the two other main protocol first used on the Web. Many others exist (NNTP, WAIS, gopher, ...) and each of
them has a corresponding data format. The URL (Uniform Resource Locator) has been developed to simplify the access to these differents protocols corresponding to different resources. It includes the type of protocol and the address / name of the server.

http://euromet.meteo.fr
http://137.129.254.14
ftp://ftp.netscape.com
telnet://www.euromet.met.ed.ac.uk

are different examples of URL.

Helpers and Plug-in :

World Wide Web pages were previously limited to text and static graphics. Sound and video files had to be downloaded and played with "helper" applications.
Helper applications are those programs called upon by the browser to display those content types that it cannot handle internally. When the server informs the
browser of the MIME content type of the following object, the browser consults its internal tables to see if the MIME type is known to it. If so, it either displays
the object inside the browser window, or it invokes the helper application to handle the object.
Essentially, a plug-in is a helper application that runs within the browser and. can directly control the browser, but will not run as stand-alone applications.

How to know what plug-in and what helpers are installed on your system ?
For Netscape :

The useful way to know about plug-in installed is to go to about:plugins
Helpers installed are described in Preferences/Navigator/Applications.

What about security :

Your browser is clearly a client and isn't able to give any access to the servers on your computor. Of course you have to be careful if you are installing some software (like Frontpage) which will install Web servers or ftp servers on your computer.

Some new protocols like Secure Sockets Layer (SSL) protocol enable a high level of security for commercial purpose for example.

- What is a proxy ?

A proxy server is a computor running a special software which will manage all the exchanges between your institution and the Web. It's a way to allow or not members of an institution to visit all the Web or to restrict the access. You have to declare it in your browser preferences.

- What is a cookie ?

Cookies are the result of a  mechanism that the servers use  to store and retrieve information on the client side of the connection. A piece of information is stored in the cookie and can be read later. The cookie is only readable by one URL or domain. The cookie has an expiration date. This is used by servers tohelp the navigation, saving the language your are using for example. The only information a cookie can have is what you gave it yourself, in a dialog box. Cookies don' t give your name or your e-mail address. Cookies cannot read other datas on your hard disk.

The only bad point is that cookies can be readable by all computers of the same domain, and that some companies put banners on many servers, so that they are able to know your habits and they use this for marketing.

You can stop the cookies on your browser. In Netscape

Notice that some information is available to the server without the use of cookies (name of you computer, IP address, Oprerating System, Browser).

The cookie.txt on my computer :

- What is readable in my cache ?

Cache enables your browser to save some pages, images, etc. in the memory of your computer or on the hard disk to increase the speed of you browser. Cache isn't a problem for security. It's useful to empty the cache frequently.

- What about viruses ?

The main danger on the Web is the possibility of downloading and running a virus. A virus can be loaded on your computer when doing an ftp, when downloading directly a file from an html page, or when receiving a file in attachment in your mail. When having a network connection, it's really necessary to protect the computers by a software anti-virus, updating it frequently.

Practice