- Electronic mail, or e-mail, has been around for over two decades. Before 1990, it was mostly used in academia. During the 1990s, it became known to the public at large.
- E-mail, like most other forms of communication, has its own conventions and styles. In particular, it is very informal and has a low threshold of use.
- E-mail is full of jargon such as BTW (By The Way), ROTFL (Rolling On The Floor Laughing), and IMHO (In My Humble Opinion). Many people also use little ASCII symbols called smileys or emoticons in their e-mail.
Figure1. Some smileys. They will not be on the final exam :-)
- The first e-mail systems simply consisted of file transfer protocols, with the convention that the first line of each message (i.e., file) contained the recipient's address. As time went on, the limitations of this approach became more obvious.
- Some of the complaints were as follows:
1. Sending a message to a group of people was inconvenient. Managers often need this facility to send memos to all their subordinates.
2. Messages had no internal structure, making computer processing difficult. For example,
if a forwarded message was included in the body of another message, extracting the forwarded part from the received message was difficult.
3. The originator (sender) never knew if a message arrived or not.
4. If someone was planning to be away on business for several weeks and wanted all incoming e-mail to be handled by his secretary, this was not easy to arrange.
5. The user interface was poorly integrated with the transmission system requiring users
first to edit a file, then leave the editor and invoke the file transfer program.
6. It was not possible to create and send messages containing a mixture of text, drawings, facsimile, and voice.
- As experience was gained, more elaborate e-mail systems were proposed. In 1982, the ARPANET e-mail proposals were published as RFC 821 (transmission protocol) and RFC 822 (message format). Minor revisions, RFC 2821 and RFC 2822, have become Internet standards, but everyone still refers to Internet e-mail as RFC 822.
- In 1984, CCITT drafted its X.400 recommendation. After two decades of competition, e-mail systems based on RFC 822 are widely used, whereas those based on X.400 have disappeared.
- The reason for RFC 822's success is not that it is so good, but that X.400 was so poorly designed and so complex that nobody could implement it well. Given a choice between a simple-minded, but working, RFC 822-based e-mail system and a supposedly truly wonderful, but nonworking, X.400 e-mail system, most organizations chose the former.
Architecture and Services
- E-mail systems consist of two subsystems: the user agents, which allow people to read and send e-mail, and the message transfer agents, which move the messages from the source to the destination
- The message transfer agents are typically system daemons, that is, processes that run in the background. Their job is to move e-mail through the system. Typically, e-mail systems support five basic functions.
- Composition refers to the process of creating messages and answers. Although any text editor can be used for the body of the message, the system itself can provide assistance with addressing and the numerous header fields attached to each message.
- Transfer refers to moving messages from the originator to the recipient. In large part, this requires establishing a connection to the destination or some intermediate machine, outputting the message, and releasing the connection.
- Reporting has to do with telling the originator what happened to the message. Was it delivered? Was it rejected? Was it lost? Numerous applications exist in which confirmation of delivery is important and may even have legal significance.
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- E-mail, like most other forms of communication, has its own conventions and styles. In particular, it is very informal and has a low threshold of use.
- E-mail is full of jargon such as BTW (By The Way), ROTFL (Rolling On The Floor Laughing), and IMHO (In My Humble Opinion). Many people also use little ASCII symbols called smileys or emoticons in their e-mail.
Figure1. Some smileys. They will not be on the final exam :-)
- The first e-mail systems simply consisted of file transfer protocols, with the convention that the first line of each message (i.e., file) contained the recipient's address. As time went on, the limitations of this approach became more obvious.
- Some of the complaints were as follows:
1. Sending a message to a group of people was inconvenient. Managers often need this facility to send memos to all their subordinates.
2. Messages had no internal structure, making computer processing difficult. For example,
if a forwarded message was included in the body of another message, extracting the forwarded part from the received message was difficult.
3. The originator (sender) never knew if a message arrived or not.
4. If someone was planning to be away on business for several weeks and wanted all incoming e-mail to be handled by his secretary, this was not easy to arrange.
5. The user interface was poorly integrated with the transmission system requiring users
first to edit a file, then leave the editor and invoke the file transfer program.
6. It was not possible to create and send messages containing a mixture of text, drawings, facsimile, and voice.
- As experience was gained, more elaborate e-mail systems were proposed. In 1982, the ARPANET e-mail proposals were published as RFC 821 (transmission protocol) and RFC 822 (message format). Minor revisions, RFC 2821 and RFC 2822, have become Internet standards, but everyone still refers to Internet e-mail as RFC 822.
- In 1984, CCITT drafted its X.400 recommendation. After two decades of competition, e-mail systems based on RFC 822 are widely used, whereas those based on X.400 have disappeared.
- The reason for RFC 822's success is not that it is so good, but that X.400 was so poorly designed and so complex that nobody could implement it well. Given a choice between a simple-minded, but working, RFC 822-based e-mail system and a supposedly truly wonderful, but nonworking, X.400 e-mail system, most organizations chose the former.
Architecture and Services
- E-mail systems consist of two subsystems: the user agents, which allow people to read and send e-mail, and the message transfer agents, which move the messages from the source to the destination
- The message transfer agents are typically system daemons, that is, processes that run in the background. Their job is to move e-mail through the system. Typically, e-mail systems support five basic functions.
- Composition refers to the process of creating messages and answers. Although any text editor can be used for the body of the message, the system itself can provide assistance with addressing and the numerous header fields attached to each message.
- Transfer refers to moving messages from the originator to the recipient. In large part, this requires establishing a connection to the destination or some intermediate machine, outputting the message, and releasing the connection.
- Reporting has to do with telling the originator what happened to the message. Was it delivered? Was it rejected? Was it lost? Numerous applications exist in which confirmation of delivery is important and may even have legal significance.
- Displaying incoming messages is needed so people can read their e-mail. Sometimes conversion is required or a special viewer must be invoked, for example, if the message is a PostScript file or digitized voice.
- Disposition is the final step and concerns what the recipient does with the message after receiving it. Possibilities include throwing it away before reading, throwing it away after reading, saving it, and so on.
- When people move or when they are away for some period of time, they may want their e-mail forwarded, so the system should be able to do this automatically.
- Corporate managers often need to send a message to each of their subordinates, customers, or suppliers. This gives rise to the idea of a mailing list, which is a list of e-mail addresses. When a message is sent to the mailing list, identical copies are delivered to everyone on the list.
- A key idea in e-mail systems is the distinction between the envelope and its contents. The envelope encapsulates the message. It contains all the information needed for transporting the message, such as the destination address, priority, and security level, all of which are distinct from the message itself.
- The message transport agents use the envelope for routing, just as the post office does. The message inside the envelope consists of two parts: the header and the body. The header contains control information for the user agents. The body is entirely for the human recipient.
Figure2. Envelopes and messages. (a) Paper mail. (b) Electronic mail.
- Disposition is the final step and concerns what the recipient does with the message after receiving it. Possibilities include throwing it away before reading, throwing it away after reading, saving it, and so on.
- When people move or when they are away for some period of time, they may want their e-mail forwarded, so the system should be able to do this automatically.
- Corporate managers often need to send a message to each of their subordinates, customers, or suppliers. This gives rise to the idea of a mailing list, which is a list of e-mail addresses. When a message is sent to the mailing list, identical copies are delivered to everyone on the list.
- A key idea in e-mail systems is the distinction between the envelope and its contents. The envelope encapsulates the message. It contains all the information needed for transporting the message, such as the destination address, priority, and security level, all of which are distinct from the message itself.
- The message transport agents use the envelope for routing, just as the post office does. The message inside the envelope consists of two parts: the header and the body. The header contains control information for the user agents. The body is entirely for the human recipient.
Figure2. Envelopes and messages. (a) Paper mail. (b) Electronic mail.
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