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9 TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
9 TCP/IP Key Applications and Application Protocols
9 TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
9 TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
9 TCP/IP Electronic Mail System Overview and Concepts
TCP/IP Electronic Mail Message Communication Model and Device and Protocol Roles
(Page 2 of 3)
The Delayed Delivery Model and Device Roles
To allow the type of communication
needed for electronic mail, the entire system is designed to facilitate
the delayed delivery of e-mail messages from one user to another.
To see how this works, let's look again at the example communication
we discussed in the
previous topic, but this time, considering
the roles of the different devices in the exchange (as
shown in Figure 301):
- Sender's Client Host: The sender of the
mail composes an electronic mail message, generally using a mail client
program on his or her local machine. The mail, once composed, is not
immediately sent out over the Internet; it is held in a buffer area
called a spool. This allows the user to be detached
for the entire time that a number of outgoing messages are created.
When the user is done, all of the messages can be sent at once.
- Sender's Local SMTP Server: When the user's
mail is ready to be sent, he or she connects to the internetwork. The
messages are then communicated to the user's designated local SMTP
server, normally run by the user's Internet Service Provider (ISP).
The mail is sent from the client machine to the local SMTP server using
SMTP. (It is possible in some cases for the sender to be working directly
on a device with a local SMTP server, in which case sending is simplified.)
- Recipient's Local SMTP Server: The sender's
SMTP server sends the e-mail using SMTP to the recipient's local SMTP
server over the Internetwork. There, the e-mail is placed into the recipient's
incoming mailbox (inbox). This is comparable to the outgoing
spool that existed on the sender's client machine. It allows the recipient
to accumulate mail from many sources over a period of time, and retrieve
them when it is convenient.
- Recipient's Client Host: In certain cases
the recipient may access his or her mailboxes directly on the local
SMTP server. More often, however, a mail access and retrieval protocol,
such as POP3 or IMAP, is used to read the mail from the SMTP server
and display it on the recipient's local machine. There, it is displayed
using an e-mail client program, similar to the one the sender used to
compose the message in the first place.
Figure 301: Electronic Mail (E-Mail) Communication Model
This diagram shows the four devices that are involved in a typical e-mail communication between two users. (Yes, they are identical twins, imagine that! J) Each device consists of a number of different elements, which communicate as indicated by the black arrows. Note the inherent asymmetry, because the method used to send an e-mail from a user is not the same as that used to retrieve it from the server. The large green arrows show a typical transaction: the sender composes mail and it goes to her local e-mail spool. It is sent to the senders local SMTP server using SMTP, and then to the recipients SMTP server, where it goes into that users inbox. It is then retrieved, usually using a protocol such as POP or IMAP.
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Version 3.0 - Version Date: September 20, 2005
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