1. April 5, 2004 Prof. Paul Lin 1 CPET 355 Data Communications & Networking 7. The Application Layer: Email Paul I-Hai Lin, Professor Electrical and Computer Engineering Technology Purdue University, Fort Wayne Campus

2. April 5, 2004 Prof. Paul Lin 2 Electronic Mail ARPANET e-mail proposals (1982) ARPANET e-mail proposals (1982) RFC 821 – transmission protocolRFC 821 – transmission protocol RFC 822 – (message format)RFC 822 – (message format) RFC 2821 and RFC 2822 RFC 2821 and RFC 2822 Minor revisions of RFC 821 and 822Minor revisions of RFC 821 and 822 Internet e-mail standardsInternet e-mail standards

3. April 5, 2004 Prof. Paul Lin 3 E-Mail Architecture and Services Two Subsystems Two Subsystems User agents – local programsUser agents – local programs Message transfer agentsMessage transfer agents Five Basic Functions Five Basic Functions Email compositionEmail composition TransferTransfer ReportingReporting DisplayingDisplaying DispositionDisposition

4. April 5, 2004 Prof. Paul Lin 4 E-Mail Architecture and Services Email Envelope Email Envelope Destination addressDestination address Priority levelPriority level Security levelSecurity level Message (inside the envelope) Message (inside the envelope) Header (control info for user agent programs)Header (control info for user agent programs) Body – the messageBody – the message

5. April 5, 2004 Prof. Paul Lin 5 RFC 2822 Header (related to message transport) Header (related to message transport) TO:Primary recipientsTO:Primary recipients Cc:Secondary recipientsCc:Secondary recipients Bcc:Blind carbon copyBcc:Blind carbon copy From:Person who created emailFrom:Person who created email Sender: E-mail address of the actual senderSender: E-mail address of the actual sender Received: Transfer agent added infoReceived: Transfer agent added info Return-Path: A path back to the senderReturn-Path: A path back to the sender

6. April 5, 2004 Prof. Paul Lin 6 RFC 2822 Message Header Message Header Date:the date and time the message was sentDate:the date and time the message was sent Reply-To: the email address to be repliedReply-To: the email address to be replied Message-Id: unique message reference IDMessage-Id: unique message reference ID In-Reply-To: message IDIn-Reply-To: message ID References: other relevant message IDsReferences: other relevant message IDs Keywords: user-chosen keywordsKeywords: user-chosen keywords Subject: short message for one-line displaySubject: short message for one-line display

7. April 5, 2004 Prof. Paul Lin 7 MIME- Multipurpose Internet Mail Extensions Five Message Headers Five Message Headers MIME-Version: 1.0MIME-Version: 1.0 Content-Description: Human-readable string telling what is in the messageContent-Description: Human-readable string telling what is in the message Content-Id: Unique IdentifierContent-Id: Unique Identifier Content-Transfer-Encoding: 5 schemesContent-Transfer-Encoding: 5 schemes Content-Type: 7-types defined in RFC 2045Content-Type: 7-types defined in RFC 2045

8. April 5, 2004 Prof. Paul Lin 8 MIME- Multipurpose Internet Mail Extensions Content-Type: 7-types defined in RFC 2045 Content-Type: 7-types defined in RFC 2045 Text: plain, enrichedText: plain, enriched Image: gif, jpegImage: gif, jpeg Audio: basicAudio: basic Video: MpegVideo: Mpeg Application: Octet-stream, PostscriptApplication: Octet-stream, Postscript Message: Rfc822, Partial, External-bodyMessage: Rfc822, Partial, External-body Multipart: Mixed, Alternative, Parallel, DigestMultipart: Mixed, Alternative, Parallel, Digest

9. April 5, 2004 Prof. Paul Lin 9 MIME- Multipurpose Internet Mail Extensions Content-Type:text/subtype Examples Content-Type:text/subtype Examples Content-Type:text/plainContent-Type:text/plain Content-Type:text/enrichedContent-Type:text/enriched Content-Type:text/html- RFC 2854Content-Type:text/html- RFC 2854 Content-Type:text/xml- RFC 3023Content-Type:text/xml- RFC 3023

10. April 5, 2004 Prof. Paul Lin 10 MIME- Multipurpose Internet Mail Extensions Content-Type:image/subtype Examples Content-Type:image/subtype Examples Content-Type:image/gifContent-Type:image/gif Content-Type:text/jpgContent-Type:text/jpg Content-Type:audio/subtype Examples Content-Type:audio/subtype Examples Content-Type:audio/basicContent-Type:audio/basic Content-Type:audio/mpeg- RFC 3003Content-Type:audio/mpeg- RFC 3003

11. April 5, 2004 Prof. Paul Lin 11 Message Transfer SMTP – Simple Mail Transfer Protocol SMTP – Simple Mail Transfer Protocol TCP Port 25TCP Port 25 < 64 KB message length< 64 KB message length Time-out issueTime-out issue Assume that the hosts are on-line all the timeAssume that the hosts are on-line all the time ESMTP – Extended SMTP (RFC 2821) ESMTP – Extended SMTP (RFC 2821) To get around these SMTP related problemsTo get around these SMTP related problems

12. April 5, 2004 Prof. Paul Lin 12 Final Message Delivery POP3 – Post Office Protocol Version 3 POP3 – Post Office Protocol Version 3 RFC 1939RFC 1939 ISP’s POP server port 110ISP’s POP server port 110 Download emails to the user machineDownload emails to the user machine IMAP – Internet Message Access Protocol IMAP – Internet Message Access Protocol RFC 2060RFC 2060 Port 143Port 143 For creating, destroying, and manipulating multiple mailboxes on the serverFor creating, destroying, and manipulating multiple mailboxes on the server Webmail Webmail

13. April 5, 2004 Prof. Paul Lin 13 Delivery Features Filters (rules – conditions, actions) Filters (rules – conditions, actions) Spam (junk-email)Spam (junk-email) Forwarding emails (vacation daemon)Forwarding emails (vacation daemon)