วิชา Network

1. Retrieving the Mail
• The SMTP delivery process described in the preceding
section is not designed to deliver mail to a user but
only to deliver mail to the user's mailbox. The user
must then access the mailbox and download the mail.
This additional step might complicate the process, but
it offers the following advantages:
• The server will continue to receive mail for the user
even when the user's computer is not on the network.
• The email delivery system is independent of the
recipient's computer or location.

2. • The latter advantage is a feature with which many
email users are well acquainted. This feature enables
the user to check an email from multiple locations.
Theoretically, any computer with access to the
Internet and an email reader application can be
configured to check the user's mailbox for messages.
You can check your mail from home, from an office, or
from a hotel room. This process of accessing the
mailbox and downloading messages requires a mail
retrieval protocol. In the following sections, you'll learn
about Post Office Protocol (POP) and Internet Message
Access Protocol (IMAP).

3. By the Way
• In reality, network security structures such as
firewalls sometimes prevent the user from
checking email from any random spot on the
Internet.

4. • The email server that holds user mailboxes
typically must support both the SMTP service (for
receiving incoming messages) and a mail retrieval
protocol service (for giving users access to the
mailbox). This process is depicted in Figure 18.3.
This interaction requires coordination and
compatibility between the SMTP service and the
mail retrieval service so that data doesn't
become lost or corrupt when the services access
the same mailbox simultaneously.

5. Figure 18.3. The SMTP service application and the mail retrieval
service application must coordinate access to the mailbox.

6. POP3
• Post Office Protocol version 3 (POP3) is a widely used
message retrieval protocol. If you are using Internet
email now, the chances are good that you are using a
POP3-compliant email client.
• POP3 is described in RFC 1939. The client initiates a
TCP connection to the POP3 server application on the
email server. The POP3 server listens for connections
on TCP port 110. After the connection has been
established, the client application must send username
and password information to the email server. If the
login credentials are accepted, the user can access the
mailbox to download or delete messages.

7. • Like the SMTP client, the POP3 client uses a series of
four-character commands for communicating with the
server. The server responds with a very small number
of alphabetic responses, such as +OK (indicating that
the command was executed) and -ERR (indicating that
the command resulted in an error). The responses
might also include additional arguments or
parameters. Each message in the mailbox is referenced
by a message number. The client sends a RETR
(retrieve) command to the server to download a
message. The DELE command deletes a message from
the server.

8. • The messages sent between the POP3 client and server are
invisible to the user. These commands are issued by the
email reader application as a response to the user's
activities within the email reader user interface.
• One disadvantage of POP3 is the limited number of
functions that can take place at the server. The user can
only list the messages in the mailbox, delete messages, and
download messages. Any manipulation of the message
contents must occur on the client side. This limitation can
cause delays and increase network traffic as messages are
downloaded from the server to the client. The newer and
more sophisticated IMAP protocol was developed to
address some of these shortcomings.

9. IMAP4
• Internet Message Access Protocol version 4
(IMAP4) is a message retrieval protocol similar to
POP3. IMAP4, however, offers several new
features that aren't available with POP3. With
IMAP4, you can browse server-based folders and
move, delete, and view messages without first
copying the messages to your local computer.
IMAP4 also allows you to save certain settings
such as client window appearance or search
messages on the server for a specified search
string. You can also create, remove, and rename
mailboxes on the server computer.

10. • Most recent email readers support both POP3
and IMAP4. Although POP3 currently has a
wider user base, the many advantages of
IMAP ensure that email installations will
continue converting to the IMAP4 protocol.

11. Network Address Translation (NAT)
• A network address translation device obscures all details of the local
network and hides the very existence of the local network. Figure 9.9
shows a network address translation device on the Internet.
• The NAT device serves as a gateway for computers on the local network to
access the Internet. Behind the NAT device, the local network can use any
network address space.
• The network does not have to use specially assigned Internet addresses
because the local network is not even part of the Internet.
• The NAT device instead acts as a proxy for the local network on the
Internet.

12. NAT (continue)
• When a local computer attempts to connect to an Internet resource, the
NAT device makes the connection instead.
• Any packets received from the Internet resource are translated into the
address scheme of the local network and forwarded to the local computer
that initiated the connection.

13. NAT (continue)