1. © MMII JW RyderCS 428 Computer Networks1 Electronic Mail  822, SMTP, MIME, POP  Most widely used application service  Sometimes only way a person ever uses a network  Mail delivery is a new concept  differs fundamentally from other network transfers

2. © MMII JW RyderCS 428 Computer Networks2 Difference  Network send packets directly to destinations  timeout, retransmission, ack  E-mail must provide for when remote machine is unavailable  Senders don’t want to wait until the other person is there  E-mail systems use a technique called spooling

3. © MMII JW RyderCS 428 Computer Networks3 Spooling  User sends a message  Mail system places a copy in private storage with  sender id  recipient  destination machine  time of deposit  Initiates transfer of mail as a background activity

4. © MMII JW RyderCS 428 Computer Networks4 Simple Diagram outgoing mail spool area client (background process) User Inter- face incoming mail spool area client (background process) User reads mail User sends mail TCP connection for incoming mail TCP connection for outgoing mail

5. © MMII JW RyderCS 428 Computer Networks5 Transfer Process  A client  Use DNS to map destination system to an IP Address  Attempt to form a TCP connection to the mail server on destination machine  If succeed, pass a copy of message to remote server  Remote server stores to copy in its spool area

6. © MMII JW RyderCS 428 Computer Networks6 Transfer Process  Once client and server agree that the message was successfully passed, client removes copy from spool area  If cannot form TCP connection or connection fails  record time delivery attempted then terminate  Background sweeps through spool area at time intervals to try again

7. © MMII JW RyderCS 428 Computer Networks7 Transfer Process  If can’t deliver after a while (few days) sends message back to the sender

8. © MMII JW RyderCS 428 Computer Networks8 Mailbox Names and Aliases  Specify recipients by  mail destination machine name  mailbox address  Names used in above specifications are independent of other names assigned to machines  Usually mailbox name is same as user login id and destination machine name same as domain name but not required!

9. © MMII JW RyderCS 428 Computer Networks9 Mailboxes  Could use ‘dept-head’ as mailbox name  DNS includes separate query type for mail destinations  Mail to machine.com may go to a different machine than telnet connection  Simple diagram idea too simple

10. © MMII JW RyderCS 428 Computer Networks10 Alias Expansion, Forwarding  Mail forwarding software includes a mail alias expansion mechanism  Map mail addresses in mail to a new set of addresses  usually mail interface program consults local aliases to replace recipient with real address  Mapping can be many to one  Associate groups of users with a single id!

11. © MMII JW RyderCS 428 Computer Networks11 Aliases  See Figure 27.2 on page 514  x->y, y->x  Can map to wrong machine a user mail address

12. © MMII JW RyderCS 428 Computer Networks12 Internetworking and Mail  2 claims  TCP/IP internet makes possible universal delivery system  Mail systems built on TCP/IP inherently more reliable than those built on arbitrary networks

13. © MMII JW RyderCS 428 Computer Networks13 Why  Universal interconnection among machines a good idea (1)  More reliable? Yes. TCP provides end to end connectivity  Only after client has sent successfully and server has received successfully will the message be erased from client machine.

14. © MMII JW RyderCS 428 Computer Networks14 Mail Gateways  Sender’s machine does not contact recipient’s machine  Less reliable  Chief advantage to mail gateways is interoperability

15. © MMII JW RyderCS 428 Computer Networks15 TCP/IP Standards  One standard for format of mail messages  Another standard for details of electronic exchange of messages  Memos divided into two parts  header  body  separated by a blank line

16. © MMII JW RyderCS 428 Computer Networks16 Mail Standards  Message standard specifies exact header content leaves body open  Headers must contain readable text with  keyword -> colon -> value  To: email address  From: email address  [Reply-to:] address for replies  Straightforward format good for heterogeneous machines

17. © MMII JW RyderCS 428 Computer Networks17 SMTP  Simple Mail Transfer Protocol  Standard transfer protocol  MTP was first try  All readable ASCII text  Rigid definition of command format  See Figure 27.3 on page 520  Bottom of page 520 is example definition

18. © MMII JW RyderCS 428 Computer Networks18 MIME  To allow non-ASCII data to be sent in mail  Multipurpose Internet Mail Extension  No change to SMTP  Allows arbitrary data to be encoded in ASCII then transmitted in plain ASCII

19. © MMII JW RyderCS 428 Computer Networks19 MIME Example  From: bill@acollege.edu  To: john@somewhere.com  MIME-Version: 1.0  Content-Type: image/gif  Content-Transfer-Encoding: base64  … data for image...

20. © MMII JW RyderCS 428 Computer Networks20 MIME  MIME-Version declares message composed with version 1.0 of MIME protocol  Content-Type has type and subtype  See MIME types in figure 27.5 on page 523  See example in figure 27.6 on page 524

21. © MMII JW RyderCS 428 Computer Networks21 Summary  Client/server paradigm  822 is mail message format - RFC  SMTP defines how a mail system on a one machine transfers mail to a server on another  MIME provides mechanism that allows arbitrary data to be transferred by SMTP

22. © MMII JW RyderCS 428 Computer Networks22 Summary  RFC 821 - SMTP with examples by Postel  RFC 822 - Exact format of mail messages by Crocker  RFC 1521 - standard for MIME