jarkom 1 - application layer-1 (kuroseross) - principle, http v2015.pdf

8/19/2019 Jarkom 1 - Application Layer-1 (kuroseRoss) - Principle, HTTP V2015.pdf

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2: Application Layer 1

Chapter 2

Application Layer

Computer Networking:A Top Down Approach,5th edition.Jim Kurose, Keith RossAddison-Wesley, April2009.

Jaringan Komputer Jurusan Ilmu Komputer/ Informatika

Universitas Diponegoro

2014/2015


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Chapter 2: Application Layer

Our goals: conceptual,

implementationaspects of network

application protocols transport-layer

service models

client-server

paradigm peer-to-peer

paradigm

learn about protocolsby examining popularapplication-levelprotocols

HTTP FTP

SMTP / POP3 / IMAP

DNS

programming networkapplications

socket API


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Some network apps

e-mail

web

instant messaging

remote login P2P file sharing

multi-user networkgames

streaming stored videoclips

social networks

voice over IP

real-time video

conferencing grid computing


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Creating a network app

write programs that run on (different) end

systems

communicate over network

e.g., web server softwarecommunicates with browsersoftware

No need to write softwarefor network-core devices Network-core devices do

not run user applications

applications on end systemsallows for rapid appdevelopment, propagation

applicationtransportnetworkdata link

physical

applicationtransportnetwork

data linkphysical

applicationtransportnetworkdata linkphysical


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Chapter 2: Application layer

Principles of networkapplications

Web and HTTP


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Application architectures

Client-server Including data centers / cloud computing

Peer-to-peer (P2P)

Hybrid of client-server and P2P


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Client-server architecture

server: always-on host

permanent IP address

server farms forscaling

clients: communicate with server

may be intermittently

connected may have dynamic IP

addresses

do not communicatedirectly with each other

client/server


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Google Data Centers

Estimated cost of data center: $600M

Google spent $2.4B in 2007 on new data centers

Each data center uses 50-100 megawatts of power


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Pure P2P architecture

no always-on server

arbitrary end systemsdirectly communicate

peers are intermittentlyconnected and change IPaddresses

Highly scalable butdifficult to manage

peer-peer


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Hybrid of client-server and P2P

Skype voice-over-IP P2P application centralized server: finding address of remote

party: client-client connection: direct (not through

server)Instant messaging

chatting between two users is P2P centralized service: client presence

detection/location• user registers its IP address with central

server when it comes online• user contacts central server to find IP

addresses of buddies


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Processes communicating

Process: program runningwithin a host.

within same host, twoprocesses communicate

using inter-processcommunication (definedby OS).

processes in different

hosts communicate byexchanging messages

Client process: processthat initiatescommunication

Server process: process

that waits to becontacted

Note: applications with

P2P architectures haveclient processes &server processes


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Sockets

process sends/receivesmessages to/from itssocket

socket analogous to door sending process shoves

message out door

sending process relies ontransport infrastructureon other side of door whichbrings message to socketat receiving process

process

TCP with

buffers,

variables

socket

host or server

process

TCP with

buffers,

variables

socket

host or server

Internet

controlled by OS

controlled by

app developer

API: (1) choice of transport protocol; (2) ability to fixa few parameters (lots more on this later)


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Addressing processes

to receive messages,process must haveidentifier

host device has unique32-bit IP address

Exercise: use ipconfigfrom command prompt toget your IP address(Windows)

Q: does IP address of

host on which processruns suffice foridentifying the process?

A: No, many processes

can be running onsame

Identifier includes bothIP address and portnumbers associated withprocess on host.

Example port numbers: HTTP server: 80

Mail server: 25


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App-layer protocol Defines:

1. Types of messagesexchanged, e.g., request, response

2. Message syntax: what fields in messages &

how fields are delineated

3. Message semantics meaning of information in

fields4. Rules for when and how

processes send &respond to messages

Public-domain protocols:

defined in RFCs

allows for

interoperability e.g., HTTP, SMTP,

BitTorrent

Proprietary protocols:

e.g., Skype, ppstream


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Transport service requirements of common apps

Application

file transfer

e-mail

Web documentsreal-time audio/video

stored audio/video

interactive games

instant messaging

Data loss

no loss

no loss

no lossloss-tolerant

loss-tolerant

loss-tolerant

no loss

Throughput

elastic

elastic

elasticaudio: 5kbps-1Mbps

video:10kbps-5Mbps

same as above

few kbps up

elastic

Time Sensitive

no

no

noyes, 100’s msec

yes, few secs

yes, 100’s msec

yes and no


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Internet transport protocols services

TCP service: connection-oriented: setup

required between client andserver processes

reliable transport betweensending and receiving process

flow control: sender won’toverwhelm receiver

congestion control: throttle

sender when networkoverloaded

does not provide: timing,minimum throughputguarantees, security

UDP service: unreliable data transfer

between sending andreceiving process

does not provide:connection setup,reliability, flow control,congestion control, timing,throughput guarantee, or

security

Q: why bother? Why isthere a UDP?


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Internet apps: application, transport protocols

Application

e-mail

remote terminal access

Webfile transfer

streaming multimedia

Internet telephony

Applicationlayer protocol

SMTP [RFC 2821]

Telnet [RFC 854]

HTTP [RFC 2616]FTP [RFC 959]

HTTP (eg Youtube),

RTP [RFC 1889]

SIP, RTP, proprietary

(e.g., Skype)

Underlyingtransport protocol

TCP

TCP

TCPTCP

TCP or UDP

typically UDP


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Chapter 2: Application layer

Principles of networkapplications

Web and HTTP


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Web and HTTP

First some jargon

Web page consists of objects

Object can be HTML file, JPEG image, Java

applet, audio file,… Web page consists of base HTML-file which

includes several referenced objects

Each object is addressable by a URL

Example URL:www.someschool.edu/someDept/pic.gif

host name path name


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HTTP overview

HTTP: hypertexttransfer protocol

Web’s application layerprotocol

client/server model client: browser that

requests, receives,“displays” Web objects

server: Web server

sends objects inresponse to requests

PC runningExplorer

Serverrunning

Apache Web

server

Mac runningNavigator


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HTTP overview (continued)

Uses TCP: client initiates TCP

connection (creates socket)to server, port 80

server accepts TCPconnection from client

HTTP messages (application-layer protocol messages)exchanged between browser

(HTTP client) and Webserver (HTTP server)

TCP connection closed

HTTP is “stateless” server maintains no

information aboutpast client requests

Protocols that maintain“state” are complex!

past history (state) mustbe maintained

if server/client crashes,their views of “state” maybe inconsistent, must bereconciled

aside


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HTTP connections

Nonpersistent HTTP

At most one object issent over a TCPconnection.

Persistent HTTP

Multiple objects canbe sent over singleTCP connectionbetween client andserver.



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Nonpersistent HTTPSuppose user enters URL

www.someSchool.edu/someDepartment/home.index

1a. HTTP client initiates TCPconnection to HTTP server(process) at

www.someSchool.edu on port 80

2. HTTP client sends HTTPrequest message (containingURL) into TCP connection

socket. Message indicatesthat client wants objectsomeDepartment/home.index

1b. HTTP server at hostwww.someSchool.edu waiting

for TCP connection at port 80.“accepts” connection, notifyingclient

3. HTTP server receives request

message, forms responsemessage containing requestedobject, and sends messageinto its socket

time

(contains text,

references to 10

jpeg images)


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Nonpersistent HTTP (cont.)

5. HTTP client receives responsemessage containing html file,displays html. Parsing htmlfile, finds 10 referenced jpegobjects

6. Steps 1-5 repeated for eachof 10 jpeg objects

4. HTTP server closes TCPconnection.

time


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Non-Persistent HTTP: Response time

Definition of RTT: time fora small packet to travelfrom client to server andback.

Response time:

one RTT to initiate TCPconnection

one RTT for HTTPrequest and first fewbytes of HTTP responseto return

file transmission time

total = 2RTT+transmit time

time totransmitfile

initiate TCPconnection

RTT

requestfile

RTT

file

received

time time


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Persistent HTTP

Nonpersistent HTTP issues: requires 2 RTTs per object

OS overhead for each TCPconnection

browsers often open parallel

TCP connections to fetchreferenced objects

Persistent HTTP server leaves connection

open after sendingresponse

subsequent HTTP messages

between sameclient/server sent overopen connection

client sends requests assoon as it encounters a

referenced object as little as one RTT for all

the referenced objects


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HTTP request message

two types of HTTP messages: request , response

HTTP request message: ASCII (human-readable format)

GET /somedir/page.html HTTP/1.1

Host: www.someschool.edu

User-agent: Mozilla/4.0

Connection: close

Accept-language:fr

(extra carriage return, line feed)

request line(GET, POST,

HEAD commands)

header

lines

Carriage return,line feed

indicates end

of message


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HTTP request message: general format


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Uploading form input

Post method:

Web page oftenincludes form input

Input is uploaded toserver in entity body

URL method:

Uses GET method

Input is uploaded inURL field of requestline:

www.somesite.com/animalsearch?monkeys&banana


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Method types

HTTP/1.0

GET

POST

HEAD asks server to leave

requested object out ofresponse

HTTP/1.1

GET, POST, HEAD

PUT uploads file in entity

body to path specifiedin URL field

DELETE deletes file specified in

the URL field


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HTTP response message

HTTP/1.1 200 OK

Connection close

Date: Thu, 06 Aug 1998 12:00:15 GMT

Server: Apache/1.3.0 (Unix)

Last- Modified: Mon, 22 Jun 1998 …...

Content-Length: 6821

Content-Type: text/html

data data data data data ...

status line(protocolstatus code

status phrase)

headerlines

data, e.g.,

requestedHTML file


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HTTP response 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 server404 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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Trying out HTTP (client side) for yourself

1. Telnet to your favorite Web server:

Opens TCP connection to port 80(default HTTP server port) at cis.poly.edu.Anything typed in sent

to port 80 at cis.poly.edu

telnet cis.poly.edu 80

2. Type in a GET HTTP request:

GET /~ross/ HTTP/1.1

Host: cis.poly.edu

By typing this in (hit carriagereturn twice), you send

this minimal (but complete)GET request to HTTP server

3. Look at response message sent by HTTP server!


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User-server state: cookies

Many major Web sitesuse cookies

Four components:1) cookie header line of

HTTP response message2) cookie header line inHTTP request message

3) cookie file kept onuser’s host, managed byuser’s browser

4) back-end database atWeb site

Example: Susan always access

Internet always from PC

visits specific e-

commerce site for firsttime

when initial HTTPrequests arrives at site,

site creates: unique ID

entry in backenddatabase for ID


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Cookies: keeping “state” (cont.)

client

server

usual http response msg

usual http response msg

cookie file

one week later:

usual http request msgcookie: 1678 cookie-

specificaction

access

ebay 8734usual http request msg Amazon server

creates ID1678 for user create

entry

usual http responseSet-cookie: 1678

ebay 8734amazon 1678

usual http request msgcookie: 1678 cookie-

spectificaction

accessebay 8734

amazon 1678

backenddatabase


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Cookies (continued)

What cookies can bring: authorization

shopping carts

recommendations

user session state(Web e-mail)

Cookies and privacy: cookies permit sites to

learn a lot about you

you may supply name

and e-mail to sites

aside

How to keep “state”:

protocol endpoints: maintain stateat sender/receiver over multipletransactions

cookies: http messages carry state


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Web caches (proxy server)

user sets browser:Web accesses viacache

browser sends allHTTP requests tocache object in cache: cache

returns object else cache requests

object from originserver, then returnsobject to client

Goal: satisfy client request without involving origin server

client

Proxy

server

clientoriginserver

originserver


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More about Web caching

cache acts as bothclient and server

typically cache isinstalled by ISP

(university, company,residential ISP)

Why Web caching?

reduce response timefor client request

reduce traffic on aninstitution’s accesslink.

Internet dense withcaches: enables “poor”

content providers toeffectively delivercontent (but so doesP2P file sharing)


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Conditional GET

Goal: don’t send object ifcache has up-to-date cachedversion

cache: specify date ofcached copy in HTTP requestIf-modified-since:

server: response contains noobject if cached copy is up-to-date:HTTP/1.0 304 Not

Modified

cache serverHTTP request msgIf-modified-since:

HTTP responseHTTP/1.0

304 Not Modified

objectnot

modified

HTTP request msgIf-modified-since:

HTTP responseHTTP/1.0 200 OK

objectmodified


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Thank You

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