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McGraw-Hill ©The McGraw-Hill Companies, Inc., 2000 22-1 Chapter 22 Simple Mail Transfer Protocol (SMTP)
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-1
Chapter 22
Simple MailTransfer Protocol
(SMTP)
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-2
SMTP concept(RFC 821, ISI, USC 1982)
Can send text, graphics, voice, video
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-3
Filesystem
UAs and MTAsUser agent (mail reader, prepares the message, creates the envelope, puts the message in the envelope)
Mail transfer agent(transfers the commands, replies and mail across the Internet)
Well-known TCP port 25Ephemeral
port
UA periodically checks the milboxes and informs the user by giving a notice
Commonly used UNIX MTA is sendmail
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-4
Relay MTAs
The receivers server may be down and the relaying MTA can store the mail and make repeated attempts later to deliver mail to the server for several days. If the recipient's server is still unavailable, the mail will be returned to the sender.
Filesystem
Filesystem
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-5Mail gateway
Relaying system allows connection between parties who may or may not have TCP/IP protocol suite.
Protocol other than SMTP
Filesystem
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-6USER AGENT (UA)
UA is defined by SMTP but the implementation is left to vendors.Popular UA programs:
MH (Message Handler, Rand corporation)BerkeleyMailElmZmailMush
Some user agents have an extra user interface (GUI):
Microsoft Outlook Express Eudora
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-7
Format of an E-mail
USER AGENT (cont.)
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22-8
E-mail addressUSER AGENT (cont.)
Name of the mail gateway in the DNS data baseMust define the local physical network, the
host computer and the user mailbox
Host ID: sciences.sdsu.edu (130.191.226.112)User mailbox ID: marko
Mail server (Mail Exchanger): cs.sdsu.edu (130.191.226.116)
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-9Sender-site delay
SMTP allows delayed delivery
Messages stored before being sent. The mail transfer system periodically (10 30 min) checks if the mail can be sent (if the IP address of the mail server has been obtained from the DNS). After 3 5 days, the unsent mail is returned to the sender.
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-10Receiver-site delay
Mail received by the SMTP server is stored in mailbox. User can read the mail at her/his convenience.
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-11AliasesSMTP allows that a single name represent several different e-mil addresses (one-to-many-alias expansion)
SMTP also allows that several e-mail addresses are assigned to the same mailbox (many-to-one-alias expansion)
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-12One-to-many expansion
alias group [email protected] [email protected] [email protected]
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-13Many-to-one expansion
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-14Complete Email System
SMTP is a bidirectional e-mail system, therefore both ends must have an MTA client and an MTA server.
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-15Commands and responses
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22-16
Ask the receiver to send an OK and to close the connectionQUIT
DescriptionSMTP Commands
Most of the implementations do not support thisTURNAsk the receiver to send an OKNOOP Ask receiv. to send the information about indicated commandHELP:
Ask receiver to return the membership of the mailing listEXPN Verify the address of the recipientVRFY [email protected] Send and mail; to terminal (if logged on) and to mboxSAML FROM: [email protected]
Send or mail; to terminal if logged on, otherwise to mboxSOML FROM: [email protected] to be delivered to recipient's terminal, not to mboxSEND FROM: [email protected] the current transaction, reset the connectionRSET
The lines following DATA are the message, bufferedDATA text CRLF . CRLFIdentify a single recipient. Multiple RCPT can be sentRCPT TO: [email protected] Initiate the mail transactionMAIL FROM: [email protected] identifies itself, receiver sends back its domain n.HELO cs.sdsu.edu
SMTP CommandsMandatory commands (every implementation must support these)
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-17
User not local, the message will be forwarded
251
Command aborted, insufficient storage
452Request command completed250
Command aborted, local error451Service closing transmission channel221
Mailbox not available450Service ready 220
Service not available421Help message214
Start mail input354System status or help reply211
Descrition)CodeDescritionCode
Responses
2xx Positive Completion Reply3xx Positive Intermediate Reply4xx Transient Negative Completion Reply
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Transaction failed554Command temporarily not implemented
504
Requested action not taken: mailbox name not allowed
553Bad sequence of commands503
Requested action aborted: exceeded storage location
552Command not implemented502
User not local551Syntax error: bad parameters501
Command not executed, mailbox not available
550Syntax error: unrecognized command
500
Descrition)CodeDescritionCode
Responses (cont.)
5xx Permanent Negative Completion Reply
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-19
Mail transfer phases:
Connection establishmentMessage transferConnection termination
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22-20
Connection establishment
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-21Message transfer
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-22Message transfer (cont.)
End of the text
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-23
Connection termination
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-24Multipurpose Internet Mail Extensions (MIME)
SMTP can send messages only in NVT 7-bit ASCII format (problem with foreign languages that are not supported by 7-bit ASCII. Cant send binary files and audio/video.) Therefore the MIME is added on top of SMTP to allow for non-ASCII data to be sent through SMTP.
MIME transforms data between non-ASCII and NVT ASCII
(RFC 2047, Univ. of Tennessee, 1996)
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22-25
MIME headerMIME is a set of software functions that transform non-ASCII data to NVT ASCII data and vice versa. Therefore the transformation parameters have to be defined. This is done through an additional header.
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22-26
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General 8-bit data (8-bit bytes)Octet-Stream
Adobe PostScriptPostScriptApplication
Single channel voice encoding at 8 KHzBasicAudio
Video is in MPEG formatMPEGVideo
Image is in GIF formatGIF
Image is in JPEG formatJPEGImage
Unformatted textPlainText
DescriptionSubtypeType
Data Types and Subtypes in MIME
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6-bit blocks are encoded into 8-bit ASCII charactersBase64
Non-ASCII characters with unlimited-length linesbinary
Non-ASCII characters are encoded as an equal sign followed by an ASCII code
Quoted-printable
Non-ASCII characters and short lines8-bit
NVT ASCII characters and short lines (less than 1000 characters)
7-bit
DescriptionType
Content-Transfer-Encoding
Very popular
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22-28Base64
Has 1 at the beginning
8*3 = 6*4
Binary values are converted into printable ASCII characters(see next slide)
Numbers between0 and 63
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-29
Base64 Encoding Table
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-30Email delivery
The client send the mail to the local server (the remote server may not be available, therefore the mail is stored in local server until it can be sent). Local server acts then as a client and sends the mail to the remote server
The remote server receives mail from the local server, then stores it into users mailbox
Remote server
The remote UA uses a mail access protocol (POP3 or IMAP4) to access the mailbox
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McGraw-Hill The McGraw-Hill Companies, Inc., 2000
22-31Post Office Protocol (POP3)
POP3 is a mail access protocol(mail reader) which is not part of the SMTP.
POP3 is a pull protocol(the receiving user is pulling the mail from the mailbox)
SMTP is a push protocol(it pushes mail from the sender to the receivers mail box.
Another mail access protocol is IMAP4(Internet Mail Access Protocol). Has more features
