1. CPET 355Data Communications & Networking 7. The Application Layer: Email Paul I-Hai Lin, Professor Electrical and Computer Engineering Technology Purdue University, Fort Wayne Campus Prof. Paul Lin

2. Electronic Mail • ARPANET e-mail proposals (1982) • RFC 821 – transmission protocol • RFC 822 – (message format) • RFC 2821 and RFC 2822 • Minor revisions of RFC 821 and 822 • Internet e-mail standards Prof. Paul Lin

3. E-Mail Architecture and Services • Two Subsystems • User agents – local programs • Message transfer agents • Five Basic Functions • Email composition • Transfer • Reporting • Displaying • Disposition Prof. Paul Lin

4. E-Mail Architecture and Services • Email Envelope • Destination address • Priority level • Security level • Message (inside the envelope) • Header (control info for user agent programs) • Body – the message Prof. Paul Lin

5. RFC 2822 • Header (related to message transport) • TO: Primary recipients • Cc: Secondary recipients • Bcc: Blind carbon copy • From: Person who created email • Sender: E-mail address of the actual sender • Received: Transfer agent added info • Return-Path: A path back to the sender Prof. Paul Lin

6. RFC 2822 • Message Header • Date: the date and time the message was sent • Reply-To: the email address to be replied • Message-Id: unique message reference ID • In-Reply-To: message ID • References: other relevant message IDs • Keywords: user-chosen keywords • Subject: short message for one-line display Prof. Paul Lin

7. MIME- Multipurpose Internet Mail Extensions • Five Message Headers • MIME-Version: 1.0 • Content-Description: Human-readable string telling what is in the message • Content-Id: Unique Identifier • Content-Transfer-Encoding: 5 schemes • Content-Type: 7-types defined in RFC 2045 Prof. Paul Lin

8. MIME- Multipurpose Internet Mail Extensions • Content-Type: 7-types defined in RFC 2045 • Text: plain, enriched • Image: gif, jpeg • Audio: basic • Video: Mpeg • Application: Octet-stream, Postscript • Message: Rfc822, Partial, External-body • Multipart: Mixed, Alternative, Parallel, Digest Prof. Paul Lin

9. MIME- Multipurpose Internet Mail Extensions • Content-Type:text/subtype Examples • Content-Type:text/plain • Content-Type:text/enriched • Content-Type:text/html - RFC 2854 • Content-Type:text/xml - RFC 3023 Prof. Paul Lin

10. MIME- Multipurpose Internet Mail Extensions • Content-Type:image/subtype Examples • Content-Type:image/gif • Content-Type:text/jpg • Content-Type:audio/subtype Examples • Content-Type:audio/basic • Content-Type:audio/mpeg - RFC 3003 Prof. Paul Lin

11. Message Transfer • SMTP – Simple Mail Transfer Protocol • TCP Port 25 • < 64 KB message length • Time-out issue • Assume that the hosts are on-line all the time • ESMTP – Extended SMTP (RFC 2821) • To get around these SMTP related problems Prof. Paul Lin

12. Final Message Delivery • POP3 – Post Office Protocol Version 3 • RFC 1939 • ISP’s POP server port 110 • Download emails to the user machine • IMAP – Internet Message Access Protocol • RFC 2060 • Port 143 • For creating, destroying, and manipulating multiple mailboxes on the server • Webmail Prof. Paul Lin

13. Delivery Features • Filters (rules – conditions, actions) • Spam (junk-email) • Forwarding emails (vacation daemon) Prof. Paul Lin