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Notes derived from ComputerNetworking: A Top Down ApproachFeaturing the Internet

, 2005, 3

rd

edition,Jim Kurose, Keith Ross, Addison-Wesley.

Application Layer: FTP & Email

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File Transfer Protocol (FTP)

FTP client contacts FTPserver at port 21, specifyingTCP as transport protocol

Client obtains authorization

over control connection Client browses remote

directory by sendingcommands over controlconnection.

When server receives acommand for a file transfer,the server opens a new TCPdata connection to client

After transferring one file,

server closes connection.

FTP

client

FTP

server

TCP control connectionport 21

TCP data connectionport 20

Server opens a seperate TCPdata connection to transferanother file.

Therefore, controlconnection is out of band.

FTP server maintains state:current directory, earlierauthentication

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FTP commands, responses

Sample commands: sent as 7-bit ASCII text

over control channel

USERusername

PASS password

LISTreturn list of files incurrent directory

RETR filenameretrieves

(gets) file STOR filenamestores

(puts) file onto remotehost

Sample return codes(n) status code and phrase (as

in HTTP)

331 Username OK,

password required 125 data connection

already open;

transfer starting

425 Cant open data

connection 452 Error writing

file

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Electronic Mail

Three majorcomponents:

user agents

e.g., Eudora, Outlook,Pine

mail servers Incoming, outgoing

messages

Simple Mail TransferProtocol: SMTP

user mailbox

outgoingmessage queue

mailserver

useragent

useragent

useragent

mail

server

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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Electronic Mail: SMTP [RFC 2821](n)

Clients SMTP mail server establishes a TCPconnection to the recipients SMTP server usingPort 25

three phases in messg. transfer

handshaking (greeting) transfer of messages

closure

command/response interaction

commands: ASCII text response: status code and phrase

messages must be in 7-bit ASCII

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Sample SMTP interactionS: 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 okC: DATA

S: 354 Enter mail, end with "." on a line by itself

C: First line in the mail

C: Second line in the mail

C: .

S: 250 Message accepted for deliveryC: QUIT

S: 221 hamburger.edu closing connection

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Try SMTP interaction for yourself(n)

telnet servername 25

see 220 reply from server

enter HELO, MAIL FROM, RCPT TO, DATA, QUIT

commandsabove lets you send email without using email client

(reader)

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SMTP: final words

SMTP uses persistentconnections: multiplemessages can be sent to thedestination with single TCPconnection

SMTP requires message(header & body) to be in 7-bitASCII

SMTP server uses CRLF.CRLFto determine end of message

SMPT is a chatty protocol Header lines are part of the

text message. It is not thepart of the commands

Comparison with HTTP:

HTTP: pull SMTP: push

SMTP command/response7-bit ASCII. SMTPmessages must be ASCII.HTTP does not imposerestrictions for message.

HTTP: each objectencapsulated in its ownresponse msg

SMTP: multiple objectssent in multipart msg

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Mail message format

SMTP: protocol forexchanging email msgs

RFC 822: standard for textmessage format:

header lines, e.g., To:

From:

Subject:

differentfrom SMTP

commands! body

the message, ASCIIcharacters only

header

body

blankline

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Message format: multimedia extensions

MIME: Multipurpose Internet Mail Extensions additional lines in msg header declare MIME content

type

From: alice@crepes.fr

To: bob@hamburger.eduSubject: 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 usedto encode data

MIME version

encoded data

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Mail access protocols

SMTP is a push protocol. How will a user access emails?

Mail access protocol: retrieval from server

POP: Post Office Protocol [RFC 1939]

Users cant create folders on mail server

IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex)

manipulation of stored msgs on server

HTTP: Hotmail , Yahoo! Mail, etc.

useragent

senders mailserver

useragent

SMTP SMTP accessprotocol

receivers mailserver