IT 424 Networks2 IT 424 Networks2 Ack.: Slides are adapted from the slides of the book: “Computer Networking” – J. Kurose, K. Ross Chapter 2: Application

IT 424

Networks2IT 424

Networks2

Ack.: Slides are adapted from the slides of the book: “Computer Networking” – J. Kurose, K. Ross

Chapter 2: Application Layer

Chapter 2: Application Layer

Part 2: Client Server

Part 2: Client Server

- FTP & Email

- FTP & Email

File

Trans

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FTP)

Electro

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Applic

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Simple Mail Transfer Protocol

Mail Access Protocols (POP3, IMAP)

Overview

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Learning Outcomes

1 To describe the FTP protocol

2To differentiate between in-band and out of band protocols

3To explain the different components and protocols incorporated in e-mail application

4 To define the role of the SMTP Protocol

5To describe the main architecture of the email application

6To compare between push and pull protocols and define the role of POP3 & IMAP Protocols3

File

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FTP)

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Overview

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Transfer file to/from remote host Client/server model

Client: side that initiates transfer (either to/from remote) Server: remote host

FTP: RFC 959 FTP server: port 21

FTP: The File Transfer Protocol

file transferFTP

server

FTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host

File Transfer Protocol (FTP)

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http://www.ietf.org/rfc/rfc0959.txt

FTP: Separate control, Data connections

FTP client contacts FTP server at port 21, using TCP

Client authorized over control connection

Client browses remote directory, sends commands over control connection

When server receives file transfer command, server opens 2nd TCP data connection (for file) to client Server opens another TCP

data connection to transfer another file

Control connection: “out of band”

After transferring one file, server closes data connection

FTP server maintains “state”: current directory, earlier authentication

File Transfer Protocol – How FTP Works?

FTPclient

FTPserver

TCP control connection,server port 21

TCP data connection,server port 20

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FTP Commands, Responses

Sample commands:Sent as ASCII text over control channel

USER usernamePASS passwordLIST: return list of file in current directoryRETR filename: retrieves (gets) fileSTOR filename: stores (puts) file onto remote host

Sample return codesStatus code and phrase (as in HTTP)

331 Username OK, password required125 data connection already open; transfer starting425 Can’t open data connection452 Error writing file

File Transfer Protocol - Commands, Responses

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Overview

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Three major components: User agents Mail servers Simple mail transfer protocol: SMTP

1. User AgentA.K.A. “Mail reader”Composing, editing, reading mail messagesE.G., Outlook, thunderbird, iphone mail clientOutgoing, incoming messages stored on server

Electronic Mail

Electronic Mail Application

user mailbox

outgoing message queue

mailserver

mailserver

mailserver

SMTP

SMTP

SMTP

useragent

useragent

useragent

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2. Mail servers: which consists of:Mailbox contains incoming messages for userMessage queue of outgoing (to be sent) mail messages

3. SMTP protocol between mail servers to send email messages Client: sending mail server “Server”: receiving mail server

Electronic Mail

Electronic Mail Application

mailserver

mailserver

mailserver

SMTP

SMTP

SMTP

useragent

useragent

useragent

useragent

useragent

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File

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Overview

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Electronic Mail: SMTP [RFC 2821]

Uses TCP to reliably transfer email message from client to server, port 25 Direct transfer: sending server to receiving server

Three phases of transfer:1. Handshaking (greeting)2. Transfer of messages3. Closure

Command/response interaction (like HTTP, FTP) Commands: ASCII text Response: status code and phrase

Messages (header + body) must be in 7-bit ASCII


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Scenario: Alice Sends Message To Bob

1) Alice uses UA to compose message “to” [email protected]

2) Alice’s UA sends message to her mail server; message placed in message queue

3) client side of SMTP opens TCP connection with Bob’s mail server


4) SMTP client sends Alice’s message over the TCP connection

5) Bob’s mail server places the message in Bob’s mailbox

6) Bob invokes his user agent to read message

useragent

mailserver

mailserver

1

2 3 4

5

6

Alice’s mail server Bob’s mail server

useragent

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S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

Sample SMTP Interaction

Simple Mail Transfer Protocol - Interaction

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SMTP: protocol for exchanging email msgsRFC 822: standard for text message format:

1.Header lines:Some required and optional keywords followed by collen, e.g.To:From:Subject:

different from SMTP MAIL FROM, RCPT TO: commands!

2.Body: the “message” ASCII characters only

Mail Message Format

Simple Mail Transfer Protocol – Mail Message Format

header

body

blankline

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HTTP and SMTP Similarities and Differences

Similarity DifferencesBoth have ASCII command/response interaction, status codes

HTTP: pull SMTP: push

SMTP requires the body to be encoded into 7-bit ASCII.

HTTP does not impose this restriction

SMTP uses persistent connections like the case in persistent HTTP.

HTTP: each object encapsulated in its own response message

SMTP: multiple objects sent in multipart message

SMTP server uses CRLF.CRLF to determine end of message

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File

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FTP)

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Overview

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Mail Access Protocols

Protocol to transfer email message from client to server ,delivery/storage to receiver’s server SMTP

Mail Access Protocol: retrieval from server POP: Post Office Protocol [RFC 1939]: authorization, download IMAP: Internet Mail Access Protocol [RFC 1730]: more features, including

manipulation of stored messages on server HTTP: gmail, Hotmail, Yahoo! Mail, etc.

sender’s mail server

SMTP SMTPmail access

protocol

receiver’s mail server

(e.g., POP, IMAP)

useragent

useragent

Mail Access Protocols

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POP3 Protocol

POP3 progresses through three phases:

1. Authorization,2. Transaction3. Update

Mail Access Protocols - POP3

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POP3 Protocol

1- Authorization phase

Client commands: user username pass password

Server responses +Ok -Err

S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on


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POP3 Protocol

2- Transaction phase,

Client commands: List: list message numbers Retr: retrieve message by

number Dele: delete Quit

Server responses: +Ok -Err

C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 2 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off


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POP3 (more)

3- Update Phase:

After processing the “quit” command, the POP3 server enters the phase and remove messages 1 and 2 from the mailbox


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POP3 (more)

POP3 Configuration Modes:A user agernt using POP3 can be configured (by the user) to:

1-“download and delete” mode: Like in the Previous example, Bob cannot re-read e-mail if he changes client

2- “download-and-keep”mode : Keeps copies of messages on different clients

POP3 is stateless across sessions

POP3 does not provide any means for a user to create remote folders and assign messages to folders.


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IMAP

IMAP is a mail access protocol. It has many more features than POP3, but it is also significantly more complex.

Keeps all messages in one place: at server

Allows user to organize messages in folders Keeps user state across sessions:

Names of folders and mappings between message IDs and folder name

Mail Access Protocols - IMAP

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Web-Based E-mail

Hotmail, Yahoo, Google.

The user agent is an ordinary Web browser

The user communicates with its remote mailbox via HTTP

When a sender wants to send email message, the message is sent from browser to mail server over HTTP rather than SMTP.

When a receiver wants to access a message in his mail box, the email message is sent from server to browser using HTTP protocol rather than POP3 or IMAP

However, communications between mail servers use SMTP

Mail Access Protocols - IMAP

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Conclusion

Conclusion

FTP differs from other client-server applications in that it establishes two connections between the hosts:

One connection is used for data transfer, the other for control information (commands and responses).

Separation of commands and data transfer makes FTP more efficient.

E-mail architecture consists of three major components:1. User Agent2. Mail servers3. Protocols

Message Transfer Protocol: SMTP Message Access Protocol: POP and IMAP

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References

References

Computer Networking: A Top-Down Approach Featuring the Internet by James Kurose and Keith Ross, Addison Wesley, 2012 (chapter 2 )

File Transfer Protocol(RFC959)

Simple Mail Transfer Protocol (RFC2821)

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Documents

IT 424 Networks2 IT 424 Networks2 Ack.: Slides are adapted from the slides of the book: “Computer Networking” – J. Kurose, K. Ross Chapter 2: Application