USO OBRIGATORIO DE MÁSCARA

response from a Web server to a Web browser using the HTTP standard

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The response body in this example shows a Web page in Hypertext Markup Language (HTML). The response body can be in any format, such as text, Microsoft Word, Adobe PDF, or a host of other formats, but the most commonly used format is HTML. HTML was developed by CERN at the same time as the first Web browser and has evolved rapidly ever since. HTML is covered by standards produced by the IETF, but Microsoft keeps making new additions to HTML with every release of its browser, so the HTML standard keeps changing.

2.4 ELECTRONIC MAIL

Electronic mail (or email) was one of the earliest applications on the Internet and is still among the most heavily used today. With email, users create and send messages to one user, several users, or all users on a distribution list. Most email software enables users to send text messages and attach files from word processors, spreadsheets, graphics programs, and so on. Many email packages also permit you to filter or organize messages by priority.

Several standards have been developed to ensure compatibility between different email software packages. Any software package that conforms to a certain standard can

send messages that are formatted using its rules. Any other package that understands that particular standard can then relay the message to its correct destination; however, if an email package receives a mail message in a different format, it may be unable to process it correctly. Many email packages send using one standard but can understand messages sent in several different standards. The most commonly used standard is SMTP (Simple Mail Transfer Protocol). Other common standards are X.400 and CMC (Common Messaging Calls). In this book, we will discuss only SMTP, but CMC and X.400 both work essentially the same way. SMTP, X.400, and CMC are different from one another (in the same way that English differs from French or Spanish), but several software packages are available that translate between them, so that companies that use one standard (e.g., CMC) can translate messages they receive that use a different standard (e.g., SMTP) into their usual standard as they first enter the company and then treat them as “normal” email messages after that.

2.4.1 How Email Works

The Simple Mail Transfer Protocol (SMTP) is the most commonly used email standard simply because it is the email standard used on the Internet. Email works similarly to how the Web works, but it is a bit more complex. SMTP email is usually implemented as a two-tier thick client-server application, but not always. We first explain how the normal two-tier thick client architecture works and then quickly contrast that with two alternate architec-tures.

Two-Tier Email Architecture With a two-tier thick client-server architecture, each client computer runs an application layer software package called a mail user agent, which is usu-ally more commonly called an email client (Figure 2-12). There are many common email client software packages such as Eudora and Outlook. The user creates the email message using one of these email clients, which formats the message into an SMTP packet that includes information such as the sender’s address and the destination address.

The user agent then sends the SMTP packet to a mail server that runs a special appli-cation layer software package called a mail transfer agent, which is more commonly called mail server software (see Figure 2-13).

This email server reads the SMTP packet to find the destination address and then sends the packet on its way through the network—often over the Internet—from mail server to mail server, until it reaches the mail server specified in the destination address (see Figure 2-13). The mail transfer agent on the destination server then stores the message in the receiver’s mailbox on that server. The message sits in the mailbox assigned to the user who is to receive the message until he or she checks for new mail.

The SMTP standard covers message transmission between mail servers (i.e., mail server to mail server) and between the originating email client and its mail server. A different stan-dard is used to communicate between the receiver’s email client and his or her mail server.

Two commonly used standards for communication between email client and mail server are Post Office Protocol (POP) and Internet Message Access Protocol (IMAP). Although there are several important technical differences between POP and IMAP, the most notice-able difference is that before a user can read a mail message with a POP (version 3) email client, the email message must be copied to the client computer’s hard disk and deleted from the mail server. With IMAP, email messages can remain stored on the mail server after they are read. IMAP therefore offers considerable benefits to users who read their email from many different computers (e.g., home, office, computer labs) because they no longer need to worry about having old email messages scattered across several client computers; all email is stored on the server until it is deleted.

In our example in Figure 2-13, when the receiver next accesses his or her email, the email client on his or her computer contacts the mail server by sending an IMAP or a POP packet that asks for the contents of the user’s mailbox. In Figure 2-13, we show this as an

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IMAP packet, but it could just as easily be a POP packet. When the mail server receives the IMAP or POP request, it converts the original SMTP packet created by the message sender into a POP or an IMAP packet that is sent to the client computer, which the user reads with the email client. Therefore, any email client using POP or IMAP must also understand SMTP to create messages. POP and IMAP provide a host of functions that enable the user to manage his or her email, such as creating mail folders, deleting mail, creating address books, and so on. If the user sends a POP or an IMAP request for one of these functions, the mail server will perform the function and send back a POP or an IMAP response packet that is much like an HTTP response packet.

Three-Tier Thin Client-Server Architecture The three-tier thin client-server email archi-tecture uses a Web server and Web browser to provide access to your email. With this architecture, you do not need an email client on your client computer. Instead, you use your Web browser. This type of email is sometimes called Web-based email and is provided by a variety of companies such as Hotmail and Yahoo!.

You use your browser to connect to a page on a Web server that lets you write the email message by filling in a form. When you click the send button, your Web browser sends the form information to the Web server inside an HTTP request (Figure 2-14). The Web server runs a program (written in C or Perl, for example) that takes the information from the HTTP request and builds an SMTP packet that contains the email message. Although not important to our example, it also sends an HTTP response back to the client. The Web server then sends the SMTP packet to the mail server, which processes the SMTP packet as though it came from a client computer. The SMTP packet flows through the network in the same manner as before. When it arrives at the destination mail server, it is placed in the receiver’s mailbox.

When the receiver wants to check his or her mail, he or she uses a Web browser to send an HTTP request to a Web server (see Figure 2-14). A program on the Web server (in C or

FIGURE 2-14

Inside the Web. HTTP = Hypertext Transfer

Perl, for example) processes the request and sends the appropriate POP request to the mail server. The mail server responds with a POP packet, which a program on the Web server converts into an HTTP response and sends to the client. The client then displays the email message in the Web browser Web-based email.

TECHNICAL FOCUS

2-2 SMTP Transmission

SMTP (Simple Mail Transfer Protocol) is an older protocol, and transmission using it is rather complicated. If we were going to design it again, we would likely find a simpler transmission method. Conceptually, we think of an SMTP packet as one packet. However, SMTP mail transfer agents transmit each element within the SMTP packet as a sepa-rate packet and wait for the receiver to respond with an

“OK” before sending the next element.

For example, in Figure 2-15, the sending mail transfer agent would send the from address and wait for an OK from the receiver. Then it would send the to address and wait for an OK. Then it would send the date, and so on, with the last item being the entire message sent as one element.

A simple comparison of Figures 2-13 and 2-14 will quickly show that the three-tier approach using a Web browser is much more complicated than the normal two-tier approach. So why do it? Well, it is simpler to have just a Web browser on the client computer rather than to require the user to install a special email client on his or her computer and then set up the special email client to connect to the correct mail server using either POP or IMAP. It is simpler for the user to just type the URL of the Web server

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FIGURE 2-15

An example of an email message using the SMTP (Simple Mail Transfer Protocol) standard

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providing the mail services into his or her browser and begin using mail. This also means that users can check their email from a public computer anywhere on the Internet.

It is also important to note that the sender and receiver do not have to use the same architecture for their email. The sender could use a two-tier client-server architecture, and the receiver, a host-based or three-tier client-server architecture. Because all communica-tion is standardized using SMTP between the different mail servers, how the users interact with their mail servers is unimportant. Each organization can use a different approach.

In fact, there is nothing to prevent one organization from using all three architectures simultaneously. At Indiana University, email is usually accessed through an email client (e.g., Microsoft Outlook) but is also accessed over the Web because many users travel internation-ally and find it easier to borrow a Web browser with Internet access than to borrow an email client and set it up to use the Indiana University mail server.

2.4.2 Inside an SMTP Packet

SMTP defines how message transfer agents operate and how they format messages sent to other message transfer agents. An SMTP packet has two parts:

◾ The header, which lists source and destination email addresses (possibly in text form [e.g., “Pat Smith”]) as well as the address itself (e.g.,

[email protected]), date, subject, and so on.

◾ The body, which is the word DATA, followed by the message itself.

Figure 2-15 shows a simple email message formatted using SMTP. The header of an SMTP message has a series of fields that provide specific information, such as the sender’s email address, the receiver’s address, date, and so on. The information in quotes on the from and to lines is ignored by SMTP; only the information in the angle brackets is used in email addresses. The message ID field is used to provide a unique identification code so that the message can be tracked. The message body contains the actual text of the message itself.

2.4.3 Attachments in Multipurpose Internet Mail Extension

As the name suggests, SMTP is a simple standard that permits only the transfer of text mes-sages. It was developed in the early days of computing, when no one had even thought about using email to transfer nontext files such as graphics or word processing documents. Several standards for nontext files have been developed that can operate together with SMTP, such as Multipurpose Internet Mail Extension (MIME), uuencode, and binhex.

Each of the standards is different, but all work in the same general way. The MIME software, which exists as part of the email client, takes the nontext file such as a PowerPoint graphic file, and translates each byte in the file into a special code that looks like regular text. This encoded section of “text” is then labeled with a series of special fields understood

by SMTP as identifying a MIME-encoded attachment and specifying information about the attachment (e.g., name of file, type of file). When the receiver’s email client receives the SMTP message with the MIME attachment, it recognizes the MIME “text” and uses its MIME software (that is part of the email client) to translate the file from MIME “text” back into its original format.

2.5 OTHER APPLICATIONS

There are literally thousands of applications that run on the Internet and on other networks.

Most application software that we develop today, whether for sale or for private internal use, runs on a network. We could spend years talking about different network applications and still cover only a small number.