Chapter 2 application layer
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Chapter 2: Application layer. 2.1 Web and HTTP 2.2 FTP. 2- 1. FTP: the file transfer protocol. 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 user interface.

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Chapter 2: Application layer

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Chapter 2 application layer

Chapter 2: Application layer

2.1 Web and HTTP

2.2 FTP

Lecture 5 Application Layer

2-1


Ftp the file transfer protocol

FTP: the file transfer protocol

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

user

interface

FTP

client

FTP

server

file transfer

user

at host

remote file

system

local file

system

Lecture 5 Application Layer

2-2


Ftp separate control data connections

FTP: separate control, data connections

FTP client contacts FTP server at port 21

client authorized over control connection

client browses remote directory by sending commands over control connection.

when server receives file transfer command, server opens 2ndTCP connection (for file) to client

after transferring one file, server closes data connection.

TCP control connection

port 21

TCP data connection

port 20

FTP

client

FTP

server

Lecture 5 Application Layer

2-3


Chapter 2 application layer1

Chapter 2: Application layer

2.1 Web and HTTP

2.2 FTP

2.3 SMTP

Lecture 5 Application Layer

2-4


Electronic mail

Three major components:

user agents

mail servers

simple mail transfer protocol: SMTP

User Agent

a.k.a. “mail reader”

composing, editing, reading mail messages

e.g., Outlook, Mozilla Thunderbird

outgoing, incoming messages stored on server

user

agent

user

agent

user

agent

user

agent

user

agent

user

agent

SMTP

SMTP

SMTP

mail

server

mail

server

mail

server

outgoing

message queue

user mailbox

Electronic Mail

Lecture 5 Application Layer

2-5


Electronic mail mail servers

Mail Servers

mailbox contains incoming messages for user

messagequeue of outgoing (to be sent) mail messages

SMTP protocol between mail servers to send email messages

client: sending mail server

“server”: receiving mail server

user

agent

user

agent

user

agent

user

agent

user

agent

user

agent

SMTP

SMTP

SMTP

mail

server

mail

server

mail

server

Electronic Mail: mail servers

Lecture 5 Application Layer

2-6


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

handshaking (greeting)

transfer of messages

closure

Electronic Mail: SMTP [RFC 2821]

Lecture 5 Application Layer

2-7


Scenario alice sends message to bob

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

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

3) Message placed in message queue

4) Client side of SMTP opens TCP connection with Bob’s mail server

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

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

Bob invokes his user agent to read message

user

agent

user

agent

mail

server

mail

server

Scenario: Alice sends message to Bob

1

2

6

3

4

5

Lecture 5 Application Layer

2-8


Smtp comparison with http

SMTP uses persistent connections

Comparison with HTTP:

HTTP: pull

SMTP: push

HTTP: each object encapsulated in its own response msg

SMTP: multiple objects handled

SMTP: comparison with HTTP

Lecture 5 Application Layer

2-9


Mail access protocols

SMTP: delivery/storage to receiver’s server

Mail access protocol: retrieval from server

POP: Post Office Protocol [RFC 1939]

authorization (agent <-->server) and download

IMAP: Internet Mail Access Protocol [RFC 1730]

more features (more complex)

manipulation of stored msgs on server

HTTP: gmail, Hotmail, Yahoo! Mail, etc.

user

agent

user

agent

sender’s mail

server

SMTP

Mail access protocols

SMTP

access

protocol

receiver’s mail

server

Lecture 5 Application Layer

2-10


Pop3 and imap comparison

More about POP3

Previous example uses “download and delete” mode.

Bob cannot re-read e-mail if he changes client

“Download-and-keep”: copies of messages on different clients

POP3 is stateless across sessions

IMAP

Keep all messages in one place: the server

Allows user to organize messages in folders

IMAP keeps user state across sessions:

names of folders and mappings between message IDs and folder name

POP3 and IMAP: Comparison

Lecture 5 Application Layer

2-11


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