Target Skills:
Ability to describe the basic messaging protocols. |
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Messaging Protocols
Once TCP/IP connections are established, the essential messaging functions
can be performed with additional protocols:
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Sending and receiving messages (SMTP)
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Retrieving stored messages (POP3 or IMAP4)
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Querying the Directory Server (LDAP)
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Setting up encrypted messaging communications (SSL)
A brief description of each protocol is below.
SMTP (Simple Mail Transfer Protocol)
Used for sending and receiving messages between servers, and for sending
from a client to a server. There are 2 distinct phases:
SMTP Accept: A server accepts a message and puts it in the
queue to be processed.
SMTP Deliver: A server or client sends a message to another server.
POP3 (Post Office Protocol 3) and IMAP4 (Internet Mail Access Protocol
4)
Two different protocols for retrieving messages from a server. POP3
downloads all messages in a user's Inbox folder to the user's local disk,
optionally keeping the messages up on the server. IMAP4 sends all
the message headers in a user's Inbox, allowing the user to then choose
which messages they actually want to download and view. IMAP4 also
allows users to organize and store all messages in folders on the server
itself, allowing access to all messages from anywhere.

LDAP (Lightweight Directory Access Protocol)
Used for sending inquiries about user information to the Directory Server.
The messaging server LDAP queries the Directory Server to find where
to send messages it receives.
SSL (Secure Sockets Layer)
An encryption protocol that can function on a level between the TCP/IP
protocol and the SMTP and IMAP4 protocols. It allows encrypted message
transmission to prevent eavesdropping. When encrypting message transmission,
first a TCP/IP connection is established, then an SSL connection, then
an SMTP or IMAP4 connection for sending or retrieving messages. When used
in messaging, these connections are referred to as SMTP over SSL (SMTP/SSL),
and IMAP4 over SSL (IMAP4/SSL).
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