Browser Cache Settings

Most Web browsers have adjustable settings that specify how Web pages should be updated. (At any time, however, a user can manually update a Web page, typically by clicking on a button named Refresh or Reload.) For example, Internet Explorer provides several options for specifying when the browser will go back to the originating page to check for a newer version than the one stored in the browser cache. These options, which are shown in Figure 6.4, include:
• Every visit to the page—Every time you visit a page, the browser checks to see if the page has changed since the last visit. If the page has changed, the browser displays the new page and stores a copy in the cache. This option increases the time it takes for Web pages to appear between pages you have already viewed.
• Every time you start Internet Explorer—The browser checks to see if the page has changed only when the page was viewed in an earlier session. Otherwise, all visits to the same page, in the same work session, pull data from the browser cache. This option increases performance for pages you have already viewed in a single session.
• Automatically—The browser checks to see if the page has changed only when the page was viewed in an earlier session. The browser also determines if images on the page are changing infrequently over time and check for newer images even less frequently. This option provides the best performance for pages you have already viewed.
• Never—The browser pulls all previously viewed pages from the cache and never checks to see whether pages have changed. This option increases performance for pages you have already viewed. To see the most current version of a page when using this option, you will be required to refresh the page.

Most browsers also provide an option to quickly clear the cache. Deleting the cache slows performance upon each initial visit to a Web page but provides the advantage of restoring disk space that was being used to store the cached pages. 

The Protocol

The protocol portion of the URL specifies the Internet protocol used to access network resources. Most common is the HTTP protocol, which is used to access Web pages, but other Internet protocols are also supported. Table 6.1 lists many of these protocols. This lesson focuses on the HTTP protocol because it is the most common, but you do need to know how to specify the use of other Internet services in a URL. The next two lessons offer detailed information on other Internet services. 

The protocol portion of our sample URL is shown in boldface type: 

http://www.comptia.org:80/certification/inetplus/faqs.htm

Note: When you're typing Web addresses, you can usually omit the "http://" because Web browsers accept this as the default. 

The Uniform Resource Locator (URL)

The Uniform Resource Locator (URL)
The ability to easily locate and link to resources on the Web is made possible by the Uniform Resource Locator (URL)—also called an address. The general format for a URL is:
protocol://hostname:port/path/filename.ext
The following is an example address that points to the page containing Frequently Asked Questions (FAQs) about the i-Net+ certification. This page is located on CompTIA's Web server:
http://www.comptia.org:80/certification/inetplus/faqs.htm
Next, we will use this example to examine each part of a URL.

Hypertext Transfer Protocol (HTTP)

The Hypertext Transfer Protocol (HTTP) is the protocol used over TCP/IP to carry requests and responses between a Web server and a Web client. It can be said that HTTP is to the Web what TCP/IP is to the Internet.
Note: HTTP is not a secure protocol. There are, however, a couple of variations of HTTP that provide for encrypted transmission. These variations include the Secure Hypertext Transfer Protocol (SHTTP) and Hypertext Transfer Protocol Secure (HTTPS).
HTTP is a stateless protocol, which means that it cannot remember what occurred previously. (Programs that are stateful can remember what happened the last time the program was run and keep track of various configuration settings.)
HTTP uses a series of requests and responses to transfer data. Basically, the client sends a request, and the server responds to the request. This transfer of data on the Web, using HTTP, usually occurs in the following four stages:

1. Connection—The client connects to the Web server.
2. Request—The client sends a resource request message to the server.
3. Response—The server delivers a response to the client.
4. Close—The connection is closed between the client and the server.
Note: Under the original HTTP specifications, the connection closes after the server response. However, a newer version of HTTP introduces persistent connections; this version maintains the connection and continues to exchange multiple requests until the connection is explicitly closed.
In addition to HTTP, another Web standard called Hypertext Markup Language (HTML) plays a crucial role in the Web's functionality. After the client makes its request and the server responds, the client uses a Web browser to interpret and display the information

The Client/Server Model

Web services, as well as most other Internet services, use a client/server model. The server provides the services, which are usually requested by the client. A Web client typically uses a Web browser to make requests, and the server responds to those requests by delivering Web pages, for example. While end users (the clients) are generally concerned only with what's going on at their end, you, as an Internet professional, may need to be knowledgeable about both ends. Figure 6.1 shows an example of a Web server responding to client requests.

ftp

FTP (File Transfer Protocol) is the most widely used file-transfer mechanism. First defined in RFC 959, FTP runs on port 21. To use FTP to send or receive files, your systems must meet the following requirements:
• The client computer must have FTP client software.
• The user must have a user name and password on the remote system. In many cases, a user name of anonymous with no password is used.
• The remote system must be running an FTP daemon (Unix, VMS, etc.) or service (Windows NT/2000, etc.).
• Both systems must be running the TCP/IP protocol.
Figure 8.1 shows an FTP utility (WS_FTP Pro) that can transfer files between remote and local locations.
You can use FTP in either a command-line mode or a command-interpreter mode. The following options are available from the command line:
• -a—Use any local interface when binding data connections.
• -d—Enables debugging.
• -g—Disables file-name globbing (globbing permits the use of wildcard characters such as the asterisk and question mark in local file or path names).
• -i—Turns off interactive prompting during multiple file transfers.
• -n—Suppresses auto-login upon initial connection.
• -s:filename—Specifies a text file containing FTP commands; the commands will automatically run after FTP starts.
• -v—Suppresses the display of remote server responses.
• -w:buffersize—Overrides the default transfer buffer size of 4096K.

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Swap files

A swap file (or swap space or, in Windows NT, a pagefile) is a space on a hard disk used as the virtual memory extension of a computer's real memory (RAM). Having a swap file allows your computer's operating system to pretend that you have more RAM than you actually do. The least recently used files in RAM can be "swapped out" to your hard disk until they are needed later so that new files can be "swapped in" to RAM. In larger operating systems (such as IBM's OS/390), the units that are moved are called pages and the swapping is called paging.
One advantage of a swap file is that it can be organized as a single contiguous space so that fewer I/O operations are required to read or write a complete file.

In general, Windows and Unix-based operating systems provide a default swap file of a certain size that the user or a system administrator can usually change.