Lecture 9 electronic_mail_representation_and_transfer

Lecture 9: Electronic Mail Representation and Transfer

Electronic Mail Paradigm, SMTP, POP/ IMAP

Outline

Introduction Architecture User Agent Message Transfer Agent: SMTP Message Access Agent: POP and IMAP Web-Based Mail

Electronic Mail

At beginning email were short and text only, today email much more complex.

It allows a message to include text, audio, and video.

It also allows one message to be sent to one or more recipients

Architecture

To explain the architecture of e-mail we will use four scenarios

First scenario in electronic mail

Alice (user) send a message to Bob (another user) using User Agent program in order to prepare the message and store it in Bob’s Mailbox.The message has sender’s and recipient mailbox address (names of files)Bob can retrieve and read the contents on his mailbox at any time using User Agent

First scenario Sender and receiver of email are users on the

same system (mail server). They directly connected to a shared system Administrator created one mailbox for each user

where the received messages are stored A Mailbox is part of local hard drive – a special

file with permission restriction. Only owner of mailbox has access to the mailbox

Second scenario in electronic mail

Alice needs to use UA program to send her message to the system (mail server) at her own site. Mail server at her site uses queue to store messages waiting to be sent.Bob also needs a UA to retrieve messages stored in the mailbox of the mail server at his siteMessage needs to be sent through the internet from Alice’s site to Bob’s site.So two MTAs are needed. MTA Client and MTA server

Second scenario

Sender and receiver of the email are users on two different system

Message needs to be sent over the Internet Here we need to have User Agents (UA)

and Message Transfer Agents (MTA)

Third scenario in electronic mailBob as in 2nd scenario. Alice is separated from her mail server, connected to server through LAN or WANAlice still need to use UA to prepare the message. Then she can send it through LAN/WAN to MTA

Fourth scenario in electronic mailThis is most common scenario.Bob and Alice are connected to Mail server through LAN/WAN.Here we need a set of client/server agent, Message Access Agent (MAA)

User Agent

User Agent Compose Message

To write message in proper format Provide template, built-in editor that can spell checking,

grammar checking etc.

Read Message Read incoming message Each received message contain;

Number field Flag – status of email. E.g. new, read Size of message Sender Optional subject field

User Agent

Reply Message Reply to the sender

Forward Message Send the message to third party

Handle Mailbox UA usually has Inbox and Outbox Inbox – keeps all received emails Outbox – keeps all sent email

User Agent

Command Driven Accepts one character command from keyboard

to perform its task. E.g. r to reply Examples: mail, pine, and elm

GUI- based Microsoft Outlook, Netscape, Eudora,

Thunderbird

User Agent

Sending email

Format of an e-mail

Message

Consist of Header and Body Header Define sender, subject of message and any

other information

Body Contain the actual information to be read by

receiver

Receiving Mail

UA is triggered by user (or timer) If user has mail, UA inform user with notice. If user ready to read the mail a list is

displayed in which each line contain summary of information

User can select any message and displays its contents on the screen.

E-mail address

Local part: define name of special file (user mailbox) where all mail received for user is stored for retrieval by MAADomain name: host to receive and send e-mail. Mail server of exchanger

Mailing List

Allow one name (alias) to represent different e-mail addresses.

Every time message to be sent system checks the recipient name against alias database

MIME – Multipurpose Internet Mail Extensions E-mail can only send message in NVT 7-bit

ASCII. MIME – supplementary protocol to allow

non ASCII data sent through e-mail. Allows transmission of Binary data Multimedia files (video/audio clips) Multiple types in single message Mixed formats

MIME

MIME header

Data types and subtypes in MIME

Message Transfer Agent: SMTP

SMTP – define how commands and responses must be sent back and forth

SMTP - Simple Mail Transfer Protocol

Runs over TCP Used between Mail transfer program on sender’s computer Mail server on recipient’s computer

Specifies how Client interacts with server Recipients specified Message is transferred

SMTP

SMTP is simple ASCII protocol where sender makes TCP connection to port 25 and

waits receiver identifies itself and says if it will receive

e-mail if rejected, sender tries again later

SMTP: Commands and responses

SMTP uses commands and responses to transfer messages between MTA client and MTA server

Command Format

SMTP: Commands

SMTP: Responses

SMTP: Responses (cont.)

SMTP - example Let us see how we can directly use SMTP to send an

e-mail and simulate the commands and responses we described in this section.

We use TELNET to log into port 25 (the well-known port for SMTP). We then use the commands directly to send an e-mail. In this example, [email protected] is sending an e-mail to himself.

The first few lines show TELNET trying to connect to the Adelphia mail server.

After connection, we can type the SMTP commands and then receive the responses, as shown on the next slide.

Note that we have added, for clarification, some comment lines, designated by the “=” signs. These lines are not part of the e-mail procedure.

SMTP session

$ telnet mail.adelphia.net 25Trying 68.168.78.100 . . .Connected to mail.adelphia.net (68.168.78.100).

SMTP session

Message Access Agent: POP3 and IMAP4

POP3 and IMAP4

First and second stages of mail delivery use SMTP. STMP not involve in the third stage because it is a push protocol; client push message to server

The third stage needs a pull protocol; client pull the message from server

POP3 – Post Office Protocol ver. 3 IMAP4 – Internet Mail Access Protocol ver. 4

Push versus pull in electronic email

POP3 POP3 client is installed in recipient computer. POP3 server is installed in mail server Mail access start when a client downloads its e-

mail messages from a server. Two modes

Delete: mail is deleted from mailbox after retrieval Keep: mail remains in mailbox after retrieval

The exchange of commands and responses in POP3

Client opens a connection to server on TCP port 110.Then it send user name and password to access mailbox.After that, user can list and retrieve mail message.

IMAP4

IMAP4 is more sophisticated protocol that offers Check email header prior to downloading Search the content of email for specific string Partially download the email Create, delete or rename mailbox on mail server Create hierarchy of mailboxes in a folder for

email storage

Web-based Mail

Yahoo Mail, Gmail, Hotmail and etc.. Mail transfer from sender’s browser to mail

server through HTTP. Transfer of message from sending mail

server to receiving server still through SMTP Message from receiving server (web server)

to receiver’ browser is done through HTTP Instead of POP3 and IMAP4, HTTP is used

as MAA

Web-based Mail

Summary Several program (protocol) including SMTP, POP3 and IMAP4, are used

in the Internet to provide e-mail service The UA prepares the message, creates the envelope, and puts the

message in the envelope. The email address consists of two parts: a local address (user mailbox)

and a domain name. The form is localname@domainname. The MTA transfers the email across the Internet, LAN or WAN. SMTP uses commands and responses to transfer messages between an

MTA client and an MTA server. The steps in transferring a mail message are connection establishment,

message transfer, and connection termination. POP3 and IMAP4 used for pulling messages from mail server Multipurpose Internet Mail Extension (MIME) is an extension of SMTP

that allows the transfer of multimedia and other non-ASCII messages.

Review Questions

Describe the addressing system used by SMTP

In e-mail, what are the tasks of User agent? What is MIME? Why do we need POP3 or IMAP4 for e-mail?