UNiT-5 +The TCP/IP protocol supports cron al on the Simple Mail Transfer Protocol a ee system for sending messages to other computer Users based on e-mail addresses ‘+ SMTP provides mall exchange between users on the ‘same or different computers. SMTP ‘= Protocol originated in 1982 (RFCB21, Jon Poste!) = += Standard message format (RFCB22,2822, D. Cracker) ‘= Goal: To transfer mail reliably and efficiently t *SMTP. + SMTP clents and servers have two main components Wen - yetowe- wef Fsrested, Sora) he tern (x ‘Sedma Bor) ‘+ alogous tothe posta SRR ran ‘Some examples of command-driven ‘Some examples of GUI-based user user agents are mail, pine, and elm agents are Eudora, Outlook, and Netscape.to relay the mail = SMTP also allows the use of Relays allowing other MTAS = Mail Gateways are used to relay mail ws prepared by a * protocol other than SMTP and convert it ‘to SMTP Format of an email |= Malis a text file Js Envelope - sender acest Fi 257 Ean aes Pou ce atorFgwe2011 Bees Nee oTTIOO 4 ‘data ASC ‘ata 4 Tio011 [601000] eo0100] 111001 wy | @ | w | on fas ToT TTR Table 204 Commands [setretoe = | able 20.8 Responses Gate 7 a Coop er Tear ety 36 Raping Er | Comal ne nate eaeTable 20.8 Responses (Continued) How SMTP works (A-PDU's) Hoo [Saver wwecopedconmsat 501_| Sy err apart or anes ‘502 | Command wot implemented = The Basics BIS Bal ices of command S04 | Command temporarily not implemented eprom’ a [Set ieee eepneety actions —___—_} HELO [Senders Host Domain Name 59 | Comntnetocrcmat nae | ee faa pe | IRCPT TO: | Email of Intended recipient 22_| Reynaed win aba anleTaonge caiea | ee [83 [Request no en ox ae alow | nessa a jqurr S54 | Tranaon ae How SMTP works (A-PDU's) Status Codes = The Extras = The Server responds with a 3 digit code aaa ae that may be followed by text info en = 2## - Success VRFY ‘Name to be verified = 3## - Command can be accepted with NOOP more information TURN = 4## - Command was rejected, but error EXPN ‘Mailing list to expand condition is temporary HELP ‘Command Name = 5## - Command rejected, Bad User!Connection Establishment Message Progress Connection Termination EXAMPLE 1 sce hw sv dry we STP ed mel an see he Sr Mt sn, Wee TINE hg i prt 28 fc wc prt for SMTP) We ho he maa {tes enc ns ep fraps ‘ima ce fr fo nc show TELNET i coat sip ma re S lett mately 23 1 Mae 6416478 Cameco mk kp 08% 100, Apr cameco, we can De the SAT commands ae tan rece teLimitations in SMTP = Only uses NVT 7 bit ASCII format = How to represent other data types? = No authentication mechanisms « Messages are sent un-encrypted Susceptible to misuse (Spamming, faking sender address) Solution: SMTP extensions = MIME — Multipurpose Internet Mail ExtensionsMIME Headers + Located between the Email Header and Body ‘= MIME-Version: 1.1 += Content-Type: type/subtype + Content-Transfer-Encoding: encoding type + Content-1d: message id += Content Description: textual explanation of non-textual contents, MIME Headers (cont’d) ‘= Content-Type - Type of data used in the Body ~ Tea: pl, urtorrated txt HT, utter Bao cones ferent oa types Nesiage: Bey contains whe, pas, poe oa message Image: Message contains mage PEG, IF) ‘Vis: Message cone on aimed ge (PES) de Hesege conan bse soon ame (82) 1 Appleton: Messoge io ata ype no pres dred ‘= Content-Transfer-Encoding ~ How to encode the message 7 bt = no eneasing nese ot New ASC short es Binary Nan ASCH ete eat es 1 bombs bt Boks ennai ot ASC 1 Ghote preae = send no ASI raacirs as 3 ASCH carci, #8, SF Senet rere oe te Base64 Encoding «= Divides binary data into 24 bit blocks ‘= Each block is then divided into 6 bit chunks = Each 6-bit section is interpreted as one character, 25% overhead Tioa1700 TosGo001 COTTTOGT TigOHT_001000_D60100—TTTOoT , e ‘) (ounioie T o1oo1001 To1o00101 J _oor10101 Quoted-Printable Encoding ‘+ Used when the data has a small non-ASCII portion + Non-ASCIT characters are sent as 3 characters + First is'=', second and third are the hex representation of the byteSMTP AUTH + ‘lows the server to provice features cry to kon users and lim thes = Various authentication methods ‘maybe used (LAIN, LOGIN, GaSe) + yp ih eecommended rete y HT + AIPA < AUTH PLA 1 age nw + Authentication segs Bases eased Email can be faked... HELO simpy secs udled MAILFROM: cisdept@cis deeds RCPTTO: amercs.ude.edu Ta. From: Department Chir mat sates) “Tos br. Foul Amer Sande Potey Famenere SPF) Subject: CTSCBS Dr. Amer, ‘by department decree all students in your CISCES6 TCP/IP cass are hereby tebe given automate As Thank you, Department Chale ur MTAs and Mail Access Protocols = The MTA delivers email to the user’s mailbox = Can be complex with numerous delivery methods, routers, and ACLs = Exim, Postfix, Sendmail = The Mail Access Protocols are used by the users to retrieve the email from the mailbox = POPS: = IMAPPOP vs. IMAP Post Office Protocol v3 = Simple + Allows the user to obtain a list of their Emails = Users can retrieve their emails «= Users can either delete or keep the email on their system = Minimizes server resources Internet Mail Access Protocol v4 = Has more features than POP3 + User can check the email header before downloading = Emails can be accessed from any location = Can search the email for a specific string of characters before downloading «= User can download parts of an email = User can create, delete, or rename mailboxes onasserverAL POP 3 (Post Office Protocol) + Short for Post Office Protocol, a protocol used to retrieve e-mail from a mail server. Most e-mail applications (sometimes called an e:mail client) use the POP protocol, although some can use the newer IMAP (Intemet Message Access Protocol). + There are two versions of POP. The first, called POP2, became a standard in the mid-80's and requires SMTP to send messages. The newer version, POP3, can be used with or without SMTP. POP3 uses TCP/IP port 110. + Workstations interact with the SMTP host, which. receives the mail on behalf of every host in the organization, to retrieve messages by using.aclient- server protocol such as Post Office Protocol, version 3(POP3), Although POP3 is used to download ‘messages from the server, the SMTP client still needed on the desktop to forward messages from the workstation user to its SMTP mail server.Post Office Protocol v3 Simple Allows the user to obtain a list of their Emails Users can retrieve their emails Users can either delete or keep the email on their system Minimizes server resources + Post Office Protocol, version 3 (POP3) is simple and limited in functionality. + The client POPS software is installed on the recipient ‘computer; the server POP3 software is installed on ‘the mail server. + Mail access starts with the client when the user needs ‘to download e-mail from the mailbox on the mail + The client opens a connection to the server on TCP port 110. It then sends its user name and password to access the mailbox. ‘The user can then list and retrieve the mail messages, ‘one by one. Figure shows an example of downloading using POP3. POP3 has two modes: the delete mode and the keep mode. Inthe delete mode, the mailis deleted from the ‘mailbox after each retrieval. In the keep mode, the ‘mail remains in the mailbox after retrieval + The delete mode is normally used when the user is working at her permanent computer and can save ‘and organize the received mail after reading or replying. + The keep mode is normally used when the user ‘accesses her mail away from her primary computer (e.g.,a laptop). The mailis read but kept in the system for later retrieval and organizing