application layer.pdf

Application Layer

CC&N-CE-402 1

Application Layer

Computer Communication & Networking— CE - 402

Department of Computer Engineering, SSUET

1Instructor: M. Rehan RasheedCE - 402

Instructor: M. Rehan Rasheed 2

Network applications:

Process: program running within a host.

within same host, two processes communicate using interprocess communication (defined by OS).

processes running in different hosts communicate with an application-layer protocol

User agent: interfaces with user “above” and network “below”.

implements user interface & application-level protocol

Web: browser

E-mail: mail reader

streaming audio/video: media player

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Application Layer

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Applications and application-layer protocols

Application: communicating, distributed processes

e.g., e-mail, Web, P2P file sharing, instant messaging

running in end systems (hosts)

exchange messages to implement application

Application-layer protocols

one “piece” of an app

define messages exchanged by apps and actions taken

use communication services provided by lower layer protocols (TCP, UDP)

applicationtransportnetworkdata linkphysical



CE - 402 Instructor: M. Rehan Rasheed

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App-layer protocol defines

Types of messages exchanged, eg, request & response messages

Syntax of message types: what fields in messages & how fields are delineated

Semantics of the fields, ie, meaning of information in fields

Rules for when and how processes send & respond to messages

Public-domain protocols:

defined in RFCs

allows for interoperability

eg, HTTP, SMTP

Proprietary protocols:

eg, KaZaA

THREE PARADIGMS

• Client server

• Peer-to-Peer

• Hybrid


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Client-server paradigm

Client:

initiates contact with server

(“speaks first”)

typically requests service from

server,

Web: client implemented in

browser; e-mail: in mail reader

Typical network app has two pieces: client and server

Server:

provides requested service to client

e.g., Web server sends requested Web

page, mail server delivers e-mail





request

reply



Peer-to-Peer paradigm

Peers communicate with each other

Peers are desktop/laptops controlled by users

Communication is without passing through a dedicatedserver

Examples: File distribution application (Bit Torrent,Lime wire, eMule), Internet Telephony (Skype), IPTV(PPLive)

Hybrid Applications are instant messaging services(MSN, Yahoo)

Servers are used to track peer IP address andmessages are exchanged between users withoutpassing through intermediate servers

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Application Layer

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Internet apps: their protocols and transport

protocols

Application

e-mail

remote terminal access

Web

file transfer

streaming multimedia

remote file server

Internet telephony

Application

layer protocol

smtp

telnet

http

ftp

proprietary

(e.g. RealNetworks)

NSF

proprietary

(e.g., Vocaltec)

Underlying

transport protocol

TCP

TCP

TCP

TCP

TCP or UDP

TCP or UDP

typically UDP

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WWW: the http protocol

http: hypertext transfer

protocol

WWW’s application layer

protocol

client/server model

client: browser that

requests, receives,

“displays” WWW objects

server: WWW server

sends objects in response

to requests

PC runningExplorer

Server runningNCSA Webserver

Mac runningNavigator


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The WWW today is a distributed client/server service, inwhich a client using a browser can access a service using aserver.

However, the service provided is distributed over manylocations called sites.

Each site holds one or more documents, referred to asWeb pages.

The request, among other information, includes theaddress of the site and the Web page, called the URL

WWW: the http protocol

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Architecture of WWW


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Client (Browser)

Each browser usually consists of three parts: a controller, client protocol, and

interpreters.

The controller receives input from the keyboard or the mouse and uses the

client programs to access the document.

After the document has been accessed, the controller uses one of the

interpreters to display the document on the screen.

The client protocol can be one of the protocols described previously such as

FTP or HTTP.

The interpreter can be HTML, Java, or JavaScript, depending on the type of

document.


URL

To facilitate the access of documents distributedthroughout the world, HTTP uses locators.

The uniform resource locator (URL) is a standard forspecifying any kind of information on the Internet.

The URL defines four things: protocol, host computer,port, and path


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Cookies

A cookie, also known as an HTTP cookie, web cookie, or

browser cookie, is a small piece of data sent from a website and

stored in a user's web browser while the user is browsing that

website.

Every time the user loads the website, the browser sends the

cookie back to the server to notify the website of the user's

previous activity.

Cookies were designed to be a reliable mechanism for websites

to remember stateful information (such as items in a shopping

cart) or to record the user's browsing activity (including clicking

particular buttons, logging in, or recording which pages were

visited by the user as far back as months or years ago).


Cookies

The World Wide Web was originally designed as a stateless

entity. A client sends a request; a server responds. Their

relationship is over.

The original design of WWW, retrieving publicly available

documents, exactly fits this purpose.

Today the Web has other functions; some are listed here.

1. Some websites need to allow access to registered clients only.

2. Websites are being used as electronic stores that allow users to browse

through the store, select wanted items, put them in an electronic cart,

and pay at the end with a credit card.

3. Some websites are used as portals: the user selects the Web pages he

wants to see.

4. Some websites are just advertising.

For these purposes, the cookie mechanism was devised


http://en.wikipedia.org/wiki/Web_browser

http://en.wikipedia.org/wiki/Program_state

http://en.wikipedia.org/wiki/Authentication

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WEB DOCUMENTS

The documents in the WWW can be grouped into three

broad categories: static, dynamic, and active.

The category is based on the time at which the contents of

the document are determined.


Static document

When a client accesses the document, a copy of the document is

sent. The user can then use a browsing program to display the

document


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Dynamic document using CGI

A dynamic document is created by a Web server whenever a

browser requests the document.

When a request arrives, the Web server runs an application

program or a script that creates the dynamic document. The

server returns the output of the program or script as a response

to the browser that requested the document.


Dynamic document using server-site script

Dynamic documents are sometimes referred to as server-site

dynamic documents.


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Active document using Java applet

Active document using client-site script

Active documents are

sometimes referred to as

client-site dynamic

documents.


HTTP

The Hypertext Transfer Protocol (HTTP) is a protocol used

mainly to access data on the World Wide Web. HTTP functions

as a combination of FTP and SMTP.

It is similar to FTP because it transfers files and uses the

services of TCP.

However, it is much simpler than FTP because it uses only one

TCP connection. There is no separate control connection; only

data are transferred between the client and the server.

HTTP uses the services of TCP on well-known port 80.


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Request and response messages


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The http protocol: more

http: TCP transport service:

client initiates TCP connection

(creates socket) to server, port 80

server accepts TCP connection

from client

http messages (application-layer

protocol messages) exchanged

between browser (http client) and

WWW server (http server)

TCP connection closed

http is “stateless”

server maintains no

information about past client

requests

so that the communication

consists of independent pairs

of request and response

Protocols that maintain “state”

are complex!


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http example

Suppose user enters URL

www.SSUET.edu/CEDt/home.index

1a. http client initiates TCP

connection to http server

(process) at www.SSUET.edu.

Port 80 is default for http server.

2. http client sends http request message (containing URL) into TCP connection socket

1b. http server at host www.SSUET.edu waiting for TCP connection at port 80. “accepts” connection, notifying client

3. http server receives request message, forms response message containing requested object (CED/home.index), sends message into sockettime

(contains text, references

to 10 jpeg images)

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http example (cont.)

5. http client receives response

message containing html file,

displays html. Parsing html file,

finds 10 referenced jpeg

objects

6. Steps 1-5 repeated for each of 10 jpeg objects

4. http server closes TCP connection.

time

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HTTP persistent connection

HTTP persistent connection, also called HTTP keep-alive, or

HTTP connection reuse, is the idea of using a single TCP

connection to send and receive multiple HTTP

requests/responses, as opposed to opening a new connection

for every single request/response pair

HTTP 1.1

In HTTP 1.1, all connections are considered persistent unless

declared otherwise. The HTTP persistent connections do not use

separate keepalive messages, they just allow multiple requests

to use a single connection.


Advantages Persistent connection

Lower CPU and memory usage (because fewer connections are

open simultaneously).

Enables HTTP pipelining of requests and responses.

Reduced network congestion (fewer TCP connections).

Reduced latency in subsequent requests (no handshaking).

Errors can be reported without the penalty of closing the TCP

connection.


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Persistent V/s Non persistent

non-persistent

connection: one object

in each TCP

connection

some browsers

create multiple

TCP connections

simultaneously -

one per object

persistent connection:

multiple objects

transferred within one

TCP connection


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http message format: request

two types of http messages: request, response

http request message:

ASCII (human-readable format)

GET /somedir/page.html HTTP/1.1

Connection: close

User-agent: Mozilla/4.0

Accept: text/html, image/gif,image/jpeg

Accept-language:fr

(extra carriage return, line feed)

request line(GET, POST, HEAD commands)

headerlines

Carriage return, line feed indicates end of message


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http request message: general format

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http message format: reply

HTTP/1.1 200 OK

Connection: close

Date: Thu, 06 Aug 2011 12:00:15 GMT

Server: Apache/1.3.0 (Unix)

Last-Modified: Mon, 22 Jun 2011 …...

Content-Length: 6821

Content-Type: text/html

data data data data data ...

status line(protocolstatus codestatus phrase)

headerlines

data, e.g., requestedhtml file

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http reply status codes

200 OK

request succeeded, requested object later in this message

301 Moved Permanently

requested object moved, new location specified later in this

message (Location:)

400 Bad Request

request message not understood by server

404 Not Found

requested document not found on this server

505 HTTP Version Not Supported

In first line in server->client response message.

A few sample codes:

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User-server interaction: authentication

Authentication goal: control

access to server documents

stateless: client must present

authorization in each request

authorization: typically name,

password

authorization: header line in

request

if no authorization presented,

server refuses access, sends

WWW authenticate:

header line in response

client server

usual http request msg

401: authorization req.

WWW authenticate:

usual http request msg+ Authorization:line

usual http response msg

usual http request msg+ Authorization:line

usual http response msgtime

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User-server interaction: conditional GET

Goal: don’t send object if

client has up-to-date stored

(cached) version

client: specify date of cached

copy in http request

If-modified-since:

<date>

server: response contains no

object if cached copy up-to-

date:

HTTP/1.0 304 Not

Modified

client server

http request msgIf-modified-since:

<date>

http responseHTTP/1.0

304 Not Modified

object not modified

http request msgIf-modified-since:

<date>

http responseHTTP/1.1 200 OK

…

<data>

object modified


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Web Caches (proxy server)

user sets browser:

WWW accesses via

web cache

client sends all http

requests to web cache

if object at web cache,

web cache immediately

returns object in http

response

else requests object

from origin server, then

returns http response to

client

Goal: satisfy client request without involving origin server

client

Proxyserver

clientorigin server

origin server


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Why WWW Caching?

Assume: cache is “close” to

client (e.g., in same

network)

smaller response time:

cache “closer” to client

decrease traffic to distant

servers

link out of institutional/local

ISP network often

bottleneck

originservers

publicInternet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutional

cache



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 server: port 21

file transferFTPserver

FTPuserinterface

FTPclient

local filesystem

remote filesystem

user at host

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ftp: separate control, data connections

ftp client contacts ftp server at

port 21, specifying TCP as

transport protocol

two parallel TCP connections

opened:

control: exchange

commands, responses

between client, server.

“out of band control”

data: file data to/from server

ftp server maintains “state”:

current directory, earlier

authentication

FTPclient

FTPserver

TCP control connectionport 21

TCP data connectionport 20

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ftp commands, responses

Sample commands:

sent as ASCII text over control

channel

USER username

PASS password

LIST return list of files in

current directory

RETR filename retrieves

(gets) file

STOR filename stores (puts)

file onto remote host

Sample return codes

status code and phrase (as in

http)

331 Username OK,

password required

125 data connection

already open; transfer

starting

425 Can’t open data

connection

452 Error writing file

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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., Microsoft Outlook,

Pegasus Mail,Gmail, Yahoo

outgoing, incoming messages

stored on server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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Electronic Mail: mail servers

Mail Servers

mailbox contains incoming

messages (yet to be read) for user

message queue of outgoing (to be

sent) mail messages

smtp protocol between mail server

to send email messages

client: “sending mail server”

server: “receiving mail server”

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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Electronic Mail: Smtp

uses tcp to reliably transfer email msg from client

to server, port 25

direct transfer: sending server to receiving server

three phases of transfer

handshaking (greeting)

transfer

closure

command/response interaction

commands: ASCI text

response: status code and phrase

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Sample smtp interaction

S: 220 SSUET.edu

C: HELO yahoo.com

S: 250 Hello yahoo.com, 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 SSUET.edu closing connection

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Smtp: http

Comparison with http

http: pull

email: push

both have ASCII command/response interaction, status codes

http: each object encapsulated in its own response (if v.1.0 or so

specified in 1.1)

smtp: multiple message parts sent in one connection (multipart

mess)

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Message format: multimedia extensions

MIME: multimedia mail extension,

additional lines in msg header declare MIME content type

From: [email protected]

To: [email protected]

Subject: Picture of yummy crepe.

MIME-Version: 1.0

Content-Transfer-Encoding: base64

Content-Type: image/jpeg

base64 encoded data .....

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

......base64 encoded data

.

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data

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Mail access protocols

SMTP: delivery/storage to receiver’s server

Mail access protocol: retrieval from server

POP: Post Office Protocol

authorization (agent <-->server) and

download

IMAP: Internet Mail Access Protocol

more features (more complex)

manipulation of stored msgs on

server

receiver’s mail server

useragent

sender’s mail server

useragent

SMTP SMTP POP3 orIMAP

POP IMAP

Relatively straightforward

Slower but more redundant

Faster (sends your e-mails and gets e-mails from you)

Keeps a copy of everything you do on the server

The downside is that if you lose e-mails on your computer you have lost them forever (unless you have saved a copy on the server)

The downside is that it takes up more space



P2P file sharing

Example

Alice runs P2P client application

on her notebook computer

Intermittently connects to

Internet; gets new IP address for

each connection

Asks for “XYZ”

Application displays other peers

that have copy of XYZ.

Alice chooses one of the peers,

Bob.

File is copied from Bob’s PC to

Alice’s notebook: HTTP

While Alice downloads, other

users uploading from Alice.

Alice’s peer is both a Web client

and a transient Web server.

All peers are servers = highly

scalable!

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P2P: centralized directory

original “Napster” design

1) when peer connects, it

informs central server:

IP address

content

2) Alice queries for “XYZ”

3) Alice requests file from

Bob

centralizeddirectory server

peers

Alice

Bob

1

1

1

12

3

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P2P: problems with centralized directory

Single point of failure

Performance bottleneck

Copyright violation file transfer is decentralized, but locating content is highly decentralized

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P2P: decentralized directory

Each peer is either a group

leader or assigned to a

group leader.

Group leader tracks the

content in all its children.

Peer queries group leader;

group leader may query

other group leaders.

ordinary peer

group-leader peer

neighoring relationships

in overlay network

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More about decentralized directory

overlay network

peers are nodes

edges between peers and their

group leaders

edges between some pairs of

group leaders

virtual neighbors

bootstrap node

connecting peer is either

assigned to a group leader or

designated as leader

advantages of approach

no centralized directory server

location service distributed

over peers

more difficult to shut down

disadvantages of approach

bootstrap node needed

group leaders can get

overloaded

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P2P: Query flooding

Gnutella

no hierarchy

use bootstrap node to learn

about others

join message

Send query to neighbors

Neighbors forward query

If queried peer has object,

it sends message back to

querying peer

join

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Skype


Skype is a peer-to-peer VoIP client

developed in 2003 by the

organization that created Kazaa

Skype uses an overlay peer-to-peer

network

An ordinary host must connect to a

super node and must authenticate

itself with the Skype login server

User names, passwords and “buddy

list” are stored at the login server

It uses TCP for signaling and both

UDP and TCP for transporting

media traffic

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application layer.pdf

Documents

web server

file distribution application

mail server

rehan rasheedce

rehan rasheedinstructor

types of messages

lower layer protocols

smtpproprietary protocols