1. 1 Electronic Mail u Three major components: u user agents u mail servers u simple mail transfer protocol: SMTP u User Agent u a.k.a. “mail reader” u composing, editing, reading mail messages u e.g., Eudora, Outlook, elm, Netscape Messenger u outgoing, incoming messages stored on server user mailbox outgoing message queue mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP

2. 2 Electronic Mail: Mail Servers u Mail Servers u mailbox contains incoming messages (yet to be read) for user u message queue of outgoing (to be sent) mail messages u SMTP protocol between mail servers to send email messages uclient: sending mail server u“server”: receiving mail server mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP

3. 3 Electronic Mail: SMTP [RFC 821] u Uses TCP to reliably transfer email message from client to server, port 25 u Direct transfer: sending server to receiving server u Three phases of transfer u handshaking (greeting) u transfer of messages u closure u Command/response interaction u Commands: ASCII text u Response: status code and phrase  Messages must be in 7-bit ASCII

4. 4 Sample SMTP interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 alice@crepes.fr... Sender ok C: RCPT TO: S: 250 bob@hamburger.edu... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C:. S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

5. 5 Mail Message Format u SMTP: protocol for exchanging email messages u RFC 822: standard for text message format: u header lines, e.g., uTo: uFrom: uSubject: udifferent from SMTP commands! u body uthe “message”, ASCII characters only header body blank line

6. 6 Message Format: Multimedia Extensions u MIME: multimedia mail extension, RFC 2045, 2056 u additional lines in message header declare MIME content type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data....................................base64 encoded data multimedia data type, subtype, parameter declaration method used to encode data MIME version encoded data

7. 7 MIME Types Content-Type: type/subtype; parameters u Text u example subtypes: plain, html (e.g., text/html) u Image u example subtypes: jpeg, gif (e.g., image/jpeg) u Audio u example subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding) (e.g., audio/basic) u Video u example subtypes: mpeg, quicktime (e.g., video/mpeg) u Application u other data that must be processed by reader before “viewable” u example subtypes: msword, octet-stream, postscript (e.g., application/postscript)

8. 8 Multi-part Type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=98766789 --98766789 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain Dear Bob, Please find a picture of a crepe. --98766789 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data....................................base64 encoded data --98766789--

9. 9 Mail Access Protocols u SMTP: delivery/storage to receiver’s server u Mail access protocol: retrieval from server u POP: Post Office Protocol [RFC 1939] uauthorization (agent server) and download u IMAP: Internet Mail Access Protocol [RFC 1730] umore features (more complex) umanipulation of stored msgs on server u HTTP: Hotmail, Yahoo! Mail, etc. user agent sender’s mail server user agent SMTP POP3 or IMAP receiver’s mail server

10. 10 POP3 Protocol u Authorization phase u client commands:  user: declare username  pass: password u server responses u+OK  -ERR u Transaction phase u client:  list: list message numbers  retr: retrieve message by number  dele: delete uquit C: list S: 1 498 S: 2 912 S:. C: retr 1 S: S:. C: dele 1 C: retr 2 S: S:. C: dele 2 C: quit S: +OK POP3 server signing off S: +OK POP3 server ready C: user alice S: +OK C: pass hungry S: +OK user successfully logged on