Lecture 9: Electronic Mail Representation and TransferElectronic Mail Paradigm, SMTP, POP/ IMAP
Outline Introduction Architecture User Agent Message Transfer Agent: SMTP Message Access Agent: POP and IMAP Web-Bas...
Electronic Mail At beginning email were short and text  only, today email much more complex. It allows a message to incl...
Architecture   To explain the architecture of e-mail we will    use four scenarios
First scenario in electronic mailAlice (user) send a message to Bob (another user) using User Agentprogram in order to pre...
First scenario   Sender and receiver of email are users on the    same system (mail server).   They directly connected t...
Second scenario in electronic mailAlice needs to use UA program to send her message to the system (mail server)at her own ...
Second scenario Sender and receiver of the email are users  on two different system Message needs to be sent over the In...
Third scenario in electronic mail                        Bob as in 2nd scenario.                        Alice is separated...
Fourth scenario in electronic mail                         This is most common                         scenario.          ...
User Agent
User Agent   Compose Message       To write message in proper format       Provide template, built-in editor that can s...
User Agent   Reply Message       Reply to the sender   Forward Message       Send the message to third party   Handle...
User Agent   Command Driven     Accepts one character command from keyboard      to perform its task. E.g. r to reply   ...
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   B...
Receiving Mail UA is triggered by user (or timer) If user has mail, UA inform user with notice. If user ready to read t...
E-mail address Local part: define name of special file (user mailbox) where all mail received for user is stored for retri...
Mailing List Allow one name (alias) to represent  different e-mail addresses. Every time message to be sent system  chec...
MIME – Multipurpose Internet MailExtensions E-mail can only send message in NVT 7-bit  ASCII. MIME – supplementary proto...
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     Mai...
SMTP   SMTP is simple ASCII protocol where     sender makes TCP connection to port 25 and      waits     receiver ident...
SMTP: Commands and responses                                      Command FormatSMTP uses commands and responses to transf...
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 ...
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 p...
Push versus pull in electronic email
POP3   POP3 client is installed in recipient computer.   POP3 server is installed in mail server   Mail access start wh...
The exchange of commands andresponses in POP3                  Client opens a connection to                  server on TCP...
IMAP4   IMAP4 is more sophisticated protocol that    offers     Check email header prior to downloading     Search the ...
Web-based Mail Yahoo Mail, Gmail, Hotmail and etc.. Mail transfer from sender’s browser to mail  server through HTTP. T...
Web-based Mail
Summary   Several program (protocol) including SMTP, POP3 and IMAP4, are used    in the Internet to provide e-mail servic...
Review Questions Describe the addressing system used by  SMTP In e-mail, what are the tasks of User agent? What is MIME...
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Lecture 9 electronic_mail_representation_and_transfer

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Lecture 9 electronic_mail_representation_and_transfer

  1. 1. Lecture 9: Electronic Mail Representation and TransferElectronic Mail Paradigm, SMTP, POP/ IMAP
  2. 2. Outline Introduction Architecture User Agent Message Transfer Agent: SMTP Message Access Agent: POP and IMAP Web-Based Mail
  3. 3. 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
  4. 4. Architecture To explain the architecture of e-mail we will use four scenarios
  5. 5. First scenario in electronic mailAlice (user) send a message to Bob (another user) using User Agentprogram 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 UserAgent
  6. 6. 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
  7. 7. Second scenario in electronic mailAlice 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 besent.Bob also needs a UA to retrieve messages stored in the mailbox of the mail serverat 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
  8. 8. 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)
  9. 9. Third scenario in electronic mail Bob as in 2nd scenario. Alice is separated from her mail server, connected to server through LAN or WAN Alice still need to use UA to prepare the message. Then she can send it through LAN/WAN to MTA
  10. 10. Fourth scenario in electronic mail This 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)
  11. 11. User Agent
  12. 12. 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
  13. 13. 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
  14. 14. 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
  15. 15. User Agent
  16. 16. Sending email Format of an e-mail
  17. 17. 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
  18. 18. 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.
  19. 19. E-mail address Local part: define name of special file (user mailbox) where all mail received for user is stored for retrieval by MAA Domain name: host to receive and send e-mail. Mail server of exchanger
  20. 20. 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
  21. 21. MIME – Multipurpose Internet MailExtensions 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
  22. 22. MIME
  23. 23. MIME header
  24. 24. Data types and subtypes in MIME
  25. 25. Message Transfer Agent: SMTP SMTP – define how commands and responses must be sent back and forth
  26. 26. 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
  27. 27. 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
  28. 28. SMTP: Commands and responses Command FormatSMTP uses commands and responses to transfer messagesbetween MTA client and MTA server
  29. 29. SMTP: Commands
  30. 30. SMTP: Responses
  31. 31. SMTP: Responses (cont.)
  32. 32. 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, forouzanb@adelphia.net 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.
  33. 33. SMTP session$ telnet mail.adelphia.net 25Trying 68.168.78.100 . . .Connected to mail.adelphia.net (68.168.78.100).
  34. 34. SMTP session
  35. 35. Message Access Agent: POP3 and IMAP4
  36. 36. 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
  37. 37. Push versus pull in electronic email
  38. 38. 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
  39. 39. The exchange of commands andresponses 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.
  40. 40. 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
  41. 41. 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
  42. 42. Web-based Mail
  43. 43. 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.
  44. 44. 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?

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