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Email

Email. SMTP - Simple Mail Transfer Protocol RFC 821 POP - Post Office Protocol RFC 1939 Also: RFC 822 Standard for the Format of ARPA Internet Text Messages RFCs 1521, 1522 Mime. Terminology. User Agent (UA) : end-user mail program

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Email

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  1. Email • SMTP - Simple Mail Transfer Protocol • RFC 821 • POP - Post Office Protocol • RFC 1939 • Also: • RFC 822 Standard for the Format of ARPA Internet Text Messages • RFCs 1521, 1522 Mime Netprog: Email Protocols

  2. Terminology • User Agent (UA): end-user mail program • Message Transfer Agent (MTA): responsible for communicating with remote hosts and transmitting/receiving email (both a client and server). • Mail Exchanger: host that takes care of email for a domain. Netprog: Email Protocols

  3. SMTP Used to exchange mail messages between mail servers (Message Transfer Agents). MTA MTA MTA SMTP SMTP File System SMTP UA UA Netprog: Email Protocols

  4. SMTP Protocol • SMTP sender is the client • SMTP receiver is the server. • The communication use NVT ASCII. • Alternating dialogue: • client sends command and server responds with command status message. • Order of the commands is important! • Status messages include ascii encoded numeric status code (like 250, 220) and text string. Netprog: Email Protocols

  5. SMTP Commands • HELO- identifies sender • MAIL FROM: - starts a mail transaction and identifies the mail originator • RCPT TO: - identifies individual recipient. There may be multiple RCPT TO: commands. • DATA - sender ready to transmit a series of lines of text, each ends with \r\n. A line containing only a period ‘.’ indicates the end of the data. Netprog: Email Protocols

  6. Data Format • ASCII only- must convert binary to an ASCII representation to send via email. • What if we want to send a line containing only a period? • Sender prepends a period to any line staring with a period (in the message). • Receiver strips the leading period in any line that starts with a period and has more stuff. Netprog: Email Protocols

  7. Leading Period DATA 354 Start mail input; end with <CRLF>.<CRLF> this message is a test of SMTP .. ..foo .. . 250 2.6.0 <c2b1c1a0-3951-4fd6-b6fb-675c0ee4a0dd> Queued mail for delivery Resulting Message: this message is a test of SMTP . .foo . Netprog: Email Protocols

  8. Typical Exchange wyxu@canopus % telnet hub0.engr.sc.edu 25 Trying 129.252.21.22... Connected to hub0.engr.sc.edu. Escape character is '^]'. 220 HUB0.engr.sc.edu Microsoft ESMTP MAIL Service ready at Sat, 17 Nov 2007 20:18:49 -0500 HELO 250 HUB0.engr.sc.edu Hello [129.252.130.106] MAIL from:<wyxu@engr.sc.edu> 250 2.1.0 Sender OK RCPT to:<wyxu@cse.sc.edu> 250 2.1.5 Recipient OK DATA 354 Start mail input; end with <CRLF>.<CRLF> Subject:test from:sender@bla.com to:receiver@bla.com this is a test mail . 250 2.6.0 <c2b1c1a0-3951-4fd6-b6fb-675c0ee4a0dd> Queued mail for delivery Netprog: Email Protocols

  9. Other SMTP Commands VRFY - confirm that a name is a valid recipient. S: VRFY Smith R: 250 Fred Smith <Smith@USC-ISIF.ARPA> EXPN - expand an alias (group email address). S: EXPN Example-People R: 250-Jon Postel <Postel@USC-ISIF.ARPA> R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA> R: 250-Sam Q. Smith <SQSmith@USC-ISIQ.ARPA> R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA> R: 250-<joe@foo-unix.ARPA> R: 250 <xyz@bar-unix.ARPA> TURN - switch roles (sender <=> receiver). Netprog: Email Protocols

  10. more Commands SOML - Send Or Mail if recipient is logged in, display message on terminal, otherwise email. SAML - Send and Mail NOOP - send back a positive reply code. RSET - abort current transaction. Netprog: Email Protocols

  11. Envelopes, Headers, and Body • Envelopes are used by MTAs for delivery • Specified by MAIL and RCPT commands • MAIL from:<wyxu@engr.sc.edu> • RCPT to:<wyxu@cse.sc.edu> • Headers are used by user agents • RFC 822 specifies header format • Each header field contains a name, a colon, and field value • Body is content of message from sending user to receiving user • When transferred using DATA command, headers are sent first, followed by a blank line, followed by body Netprog: Email Protocols

  12. Mail Headers • Email messages contain many headers, some headers are created by the UA, some are automatically added by the MTA. • Every MTA adds (at least) a “Received:” header. • Some of the headers are read by (parsed) intermediate MTAs, but the content is ignored and passed on transparently. Netprog: Email Protocols

  13. Transmission of a Mail • User agent takes body, adds some headers, and passes to MTA • MTA adds some headers, adds envelope, and sends to another MTA Netprog: Email Protocols

  14. Transmission of a mail to foo@cs.vu.nl • MTA asks the DNS for CNAME records for cs.vn.nl and the response says non exist. • MTA asks the DNS for MX records for cs.vn.nl and get one record zephyr.cs.vn.nl Portion of a possible DNS database zephyr.cs.vu.nl. 86400 IN A 130.37.16.112 top.cs.vu.nl.86400 IN A 130.37.16.6 Netprog: Email Protocols

  15. Transmission of a mail to foo@cs.vu.nl • MTA asks the DNS for an A record for zephyr.cs.vn.nl and get back the value of 130.37.16.112. • An SMTP connection is initiated to 130.37.16.112 and the mail is sent. Portion of a possible DNS database zephyr.cs.vu.nl. 86400 IN A 130.37.16.112 top.cs.vu.nl.86400 IN A 130.37.16.6 Netprog: Email Protocols

  16. Improvements on SMTP • Envelope changes: Extended SMTP • Header changes: Non-ASCII characters • Body changes: Multipurpose Internet Mail Extensions (MIME) Netprog: Email Protocols

  17. Extended SMTP • Framework for adding extensions to SMTP • Client wishing to use new features can use EHLO, instead of HELO, to initiate session • Server lists extended commands in the 250 reply to EHLO command Netprog: Email Protocols

  18. Non-ASCII Characters • Allow additional characters in sender and receiver names and in subject • Header can contain encoded words =?charset ? encoding ? encoded-text ?= • Charset can be us-ascii or iso-8859-1 • Encoding can be quoted-printable (Q) or base-64 (B) • Using base-64, three consecutive bytes are encoded as four 6-bit values From: =?US-ASCII?Q?Keith_Moore?= <moore@cs.utk.edu> To:=?ISO-8859-1?Q?Keld_j=F8rn_Sim?= <test@gmail.com> Subject:=?ISO-8859-1?B?SWYgeW91IGNhbiByZWFkIHRoaXMgeW8=?==?ISO-8859-2?B?dSB1bmR1cnN0YW5kIHRoZSBleGFtcGxlLg==?= Netprog: Email Protocols

  19. Multipurpose Internet Mail Extensions • Problems: • Messages in languages with accents ( French and German) • non-Latin alphabets (Russian and Hebrew) • languages without alphabets (Chinese and Jamanese) • not containing text at all (audio or images) • Need to add structure and define encoding rules for non-ASCII messages. Netprog: Email Protocols

  20. Multipurpose Internet Mail Extensions • Allow structures in body • Five new header fields Mime-Version: Content-Type:(Type and format of the content) Content-Transfer-Encoding:(How body is wrapped for transmission) Content-ID: Content-Description:(Human-readable string telling what is in the message) • Seven content types: text, multipart, message, application, image, audio, video Mime-Version:1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII TEXT/ENRICHED, IMAGE/JPEG, VIDEO/MPEG, MESSAGE/EXTERNAL-BODY, MULTIPART/ALTERNATIVE… (more examples in the book) • Five encoding formats: 7bit, quoted-printable, base64, 8bit, binary Netprog: Email Protocols

  21. Multipart/mixed From: Nathaniel Borenstein <nsb@bellcore.com> To: Ned Freed <ned@innosoft.com> Subject: Sample message MIME-Version: 1.0 Content-type: multipart/mixed; boundary="simple boundary" This is the preamble. It is to be ignored, though it is a handy place for mail composers to include an explanatory note to non-MIME compliant readers. --simple boundary This is implicitly typed plain ASCII text. It does NOT end with a linebreak. --simple boundary Content-type: text/plain; charset=us-ascii This is explicitly typed plain ASCII text. It DOES end with a linebreak. --simple boundary-- This is the epilogue. It is also to be ignored. Netprog: Email Protocols

  22. Multipart/alternative From: Nathaniel Borenstein <nsb@bellcore.com> To: Ned Freed <ned@innosoft.com> Subject: Formatted text mail MIME-Version: 1.0 Content-Type: multipart/alternative; boundary=boundary42 --boundary42 Content-Type: text/plain; charset=us-ascii ...plain text version of message goes here.... --boundary42 Content-Type: text/richtext .... <bold>richtext</bold> version of same message goes here ... --boundary42 Content-Type: text/x-whatever .... fanciest formatted version of same message goes here ... --boundary42 -- Netprog: Email Protocols

  23. POP – Post Office Protocol • Used to transfer mail from a mail server to a UA. Mail Server POP UA File System Netprog: Email Protocols

  24. POP (version 3) • Similar to SMTP command/reply lockstep protocol. • Used to retrieve mail for a single user • requires authentication • Commands and replies are ASCII lines. • Replies start with “+OK” or “-ERR”. • Replies may contain multiple lines. Netprog: Email Protocols

  25. POP-3 Commands • USER - specify username • PASS - specify password • STAT - get mailbox status • number of messages in the mailbox. • LIST - get a list of messages and sizes. • One per line, termination line contains ‘.’ only. • RETR - retrieve a message Netprog: Email Protocols

  26. More POP-3 Commands • DELE - mark a message for deletion from the mailbox. • NOOP - send back positive reply • RSET - reset. All deletion marks are unmarked. • QUIT - remove marked messages and close the (TCP) connection. Netprog: Email Protocols

  27. Optional Commands • TOP - send header lines from messages. • APOP - alternative authentication • APOP name digest • a string identifying a mailbox and a MD5 digest string • `digest‘: is calculated by applying the MD5 algorithm to a string consisting of the timestamp (sent from POP server) followed by a shared secret. • Requires shared secret! • No clear text password on the network. • Does not authenticate the server!!!! Netprog: Email Protocols

  28. A Pop3 Exchange > telnet hub0.engr.sc.edu 110 Trying 129.252.21.22... Connected to hub0.engr.sc.edu. Escape character is '^]'. +OK Microsoft Exchange Server 2007 POP3 service ready … quit +OK Microsoft Exchange Server 2007 POP3 server signing off. Netprog: Email Protocols

  29. A Pop3 Exchange S: <wait for connection on TCP port 110> C: <open connection> S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us> C: USER mrose S: +OK User accepted C: PASS mrosepass S: +OK Pass accepted C: STAT S: +OK 2 320 C: LIST S: +OK 2 messages (320 octets) S: 1 120 S: 2 200 S: . C: RETR 1 S: +OK 120 octets S: <the POP3 server sends message 1> S: . C: DELE 1 S: +OK message 1 deleted C: RETR 2 S: +OK 200 octets S: <the POP3 server sends message 2> S: . C: DELE 2 S: +OK message 2 deleted C: QUIT S: +OK dewey POP3 server signing off (maildrop empty) Netprog: Email Protocols

  30. APOP S: +OK POP3 server ready 1896.697170952@dbc.mtview.ca.us C: APOP mrose c4c9334bac560ecc979e58001b3e22fb S: +OK maildrop has 1 message (369 octets) C: LIST S: +OK 1 messages (320 octets) S: 1 320 S: . C: RETR 1 S: +OK 320 octets S: <the POP3 server sends message 1> S: . C: DELE 1 S: +OK message 1 deleted C: QUIT Netprog: Email Protocols

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