Target Skills:


By the end of this module you will be able to...

  • Describe the basics of messaging, in terms of the protocols used to send and receive messages.

  •  

Messaging Basics

Let's examine the basics of messaging, in terms of the main components and the protocols they use to communicate with each other process messages.
 

Main Components

The main components of a messaging system are the Messaging Servers, the Clients, and the Directory Server.

The Messaging Servers are used to send and receive messages from other servers, store messages for local users, and allow users through their Clients to send and retrieve their messages. 

The Clients (such as Netscape Communicator) are used by users to send and retrieve  messages from the Messaging Servers.

The Directory Server is used to store all information about user accounts, and all the configuration information for the Messaging Servers.  It ultimately determines where a message goes once it has been sent to a Messaging Server.
 

Basic Messaging Flow

When a message is sent off from a client to a messaging server, the messaging server accepts it, examines the recipient's address, then looks in ("queries") the Directory Server to see which messaging server the recipient uses to store their messages. 
  • If the recipient uses this messaging server to store messages, the recipient is "local", and the server stores the message on its hard disk in the appropriate folder. 

  •  
  • If the recipient does not use this messaging server, is "non-local", then the messaging server sends the message off to the recipient's messaging server, which will then accept it, repeat the query process, and eventually store the message.
The recipient then uses a client to access their messaging server, and retrieves the stored message.
  • tquery the Directory Server, determin store the message in the appropriate folder.delivers it to the servertries to deliver it to t

Basic Roles of the Messaging Server

The Messaging server performs 2 basic roles:

1) Message Transfer Agent (MTA)

Accepts messages from clients and other servers, and transfers non-local messages to other servers, using the SMTP protocol.  Delivers local messages into the users inbox folders in the local message store.  The server determines where to deliver messages by sending a query to the Directory Server using the LDAP protocol.
2) Message Access Agent: 
Allows local users client access to their stored messages, using either the POP3 or IMAP4 protocols.

Protocols

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 an puts it in the queue to be processed

SMTP Deliver: A server or client sends a message to a server.

POP3 (Post Office Protocol 3) and IMAP4 (Internet Mail Access Protocol 4)

Used for retrieving messages from a server.  The basic difference is that IMAP4 allows users to store their messages in folders on the server itself,
while POP3 users store messages in folders on their local hard drives.
 

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.

TCP/IP

Used to establish a network connection between to points on a network.

Before a TCP/IP connection can be established, the initiating server/client must know the IP address of the server it wants to contact.  It gets the IP address by calling on a Domain Name Service (DNS) that translates names to IP addresses.  This process is called a "DNS lookup".

SSL (Secure Sockets Layer)

An encryption protocol that functions on a level between the TCP/IP protocol and the SMTP and IMAP4 protocols. When used together, they are referred to them as SMTP over SSL (SMTP/SLL), or IMAP4 over SSL (IMAP4/SSL).

On the next page, you'll trace a message flow from sender to recipient using these protocols. 
 

Interaction
 
 

     

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