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Chapter 22

Simple MailTransfer Protocol

(SMTP)

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SMTP concept(RFC 821, ISI, USC 1982)

Can send text, graphics, voice, video

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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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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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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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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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Format of an E-mail

USER AGENT (cont.)

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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

marko@cs.sdsu.edu

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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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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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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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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22-12One-to-many expansion

alias group john@nett.com joe@deanza.edu janet@mcgraw.com

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22-13Many-to-one expansion

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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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22-15Commands and responses

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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 masc0860@rohan.sdsu.edu Send and mail; to terminal (if logged on) and to mboxSAML FROM: marko@cs.sdsu.edu

Send or mail; to terminal if logged on, otherwise to mboxSOML FROM: marko@cs.sdsu.eduMessage to be delivered to recipient's terminal, not to mboxSEND FROM: marko@cs.sdsu.eduAbort 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: masc0860@rohan.sdsu.edu Initiate the mail transactionMAIL FROM: marko@cs.sdsu.eduClient identifies itself, receiver sends back its domain n.HELO cs.sdsu.edu

SMTP CommandsMandatory commands (every implementation must support these)

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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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Mail transfer phases:

Connection establishmentMessage transferConnection termination

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Connection establishment

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22-21Message transfer

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22-22Message transfer (cont.)

End of the text

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Connection termination

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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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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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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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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22-29

Base64 Encoding Table

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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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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