Internet Security - Dang Thanh Binh's Page

52 INTERNET SECURITY
SMTP is usually implemented to operate over TCP port 25. The details of SMTP are
in RFC 2821 of the Internet Engineering Task Force (IETF). An alternative to SMTP that
is widely used in Europe is X.400.
2.5.2 Post Office Protocol Version 3 (POP3)
The most popular protocol used to transfer e-mail messages from a permanent mailbox
to a local computer is known as the Post Office Protocol version 3 (POP3). The user
invokes a POP3 client, which creates a TCP connection to a POP3 server on the mailbox
computer. The user first sends a login and a password to authenticate the session. Once
authentication has been accepted, the user client sends commands to retrieve a copy of one
or more messages and to delete the message from the permanent mailbox. The messages
are stored and transferred as text files in RFC 2822 standard format.
Note that computers with a permanent mailbox must run two servers – an SMTP
server accepts mail sent to a user and adds each incoming message to the user’s permanent
mailbox, and a POP3 server allows a user to extract messages from the mailbox and delete
them. To ensure correct operation, the two servers must coordinate with the mailbox so
that if a message arrives via SMTP while a user extracts messages via POP3, the mailbox
is left in a valid state.
2.5.3 Internet Message Access Protocol (IMAP)
The Internet Message Access Protocol (IMAP) is a standard protocol for accessing email
from your local server. IMAP4 (the latest version) is a client–server protocol in
which e-mail is received and held for you by your Internet server. You (or your e-mail
client) can view just the subject and the sender of the e-mail and then decide whether to
download the mail. You can also create, manipulate and delete folders or mailboxes on
the server, delete messages or search for certain e-mails. IMAP requires continual access
to the server during the time that you are working with your mail.
A less sophisticated protocol is Post Office Protocol 3 (POP3). With POP3, your mail is
saved for you in your mailbox on the server. When you read your mail, it is immediately
downloaded to your computer and no longer maintained on the server.
IMAP can be thought of as a remote file server. POP can be thought of as a ‘storeand-forward’
service.
POP and IMAP deal with receiving e-mail from your local server and are not to be
confused with SMTP, a protocol for transferring e-mail between points on the Internet.
You send e-mail by SMTP and a mail handler receives it on your recipient’s behalf. Then
the mail is read using POP or IMAP.
2.5.4 Multipurpose Internet Mail Extension (MIME)
The Multipurpose Internet Mail Extension (MIME) is defined to allow transmission of
non-ASCII data via e-mail. MIME allows arbitrary data to be encoded in ASCII and then
transmitted in a standard e-mail message. SMTP cannot be used for languages that are
not supported by seven-bit ASCII characters. It cannot also be used for binary files or to
send video or audio data.