2011-03-5 chapter 2 - application layer

7/28/2019 2011-03-5 Chapter 2 - Application Layer

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Application LayerOur goals:

conceptual,implementation

aspects of networkapplication protocols

transport-layerservice models

client-serverparadigm

peer-to-peer paradigm

learn about protocols byexamining popularapplication-level

protocols HTTP

FTP

SMTP / POP3 / IMAP

DNS programming network

applications socket API


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Chapter 2: outline 2.1 Principles of app layer

protocols clients and servers

app requirements

2.2 Web and HTTP

2.3 FTP

2.4 Electronic Mail SMTP, POP3, IMAP

2.5 DNS

2.6 Socket programmingwith TCP

2.7 Socket programmingwith UDP

2.8 Building a Web server

2.9 Content distribution Network Web caching

Content distributionnetworks

P2P file sharing


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Principles of App layer protocolsClients and servers

Protocols support remote access

Software is distributed among two or more systemsApplication software

Electronic mail (sendmail, qmail, )

Web (apache, tomcat, )

File transfer (vsftpd, Serv-U, )Remote access to computer (telnet, ssh, )


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Network application: terminologiesProcess: program running

within a host.

within same host, two

processes communicateusing interprocesscommunication (definedby OS).

processes running indifferent hostscommunicate with anapplication-layer protocol

user agent: interfaces withuser above and networkbelow.

implements userinterface & application-level protocol Web: browser

E-mail: mail reader streaming audio/video:

media player


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Application and application layer protocolsApplication: communicating,

distributed processes e.g., e-mail, Web, P2P file

sharing, instant messaging

running in end systems(hosts)

exchange messages toimplement application

Application-layer protocols one piece of an app

define messages exchanged byapps and actions taken

use communication servicesprovided by lower layer

protocols (TCP, UDP)

applicationtransportnetworkdata linkphysical




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Application layer protocols 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 forinteroperability

eg, HTTP, SMTP

Proprietary protocols:

eg, KaZaA


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Clients Servers paradigm

Client:

initiates contact with server(speaks first) typically requests service from

server, Web: client implemented in

browser; e-mail: in mail reader

Server:

provides requested service to client e.g., Web server sends requested Web

page, mail server delivers e-mail

Typically has two parts or sidesClient side and server side



request

reply


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Processes Communicating across network

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 fix a few parameters (lotsmore on this later)

process sends/receivesmessages to/from its socket

socket analogous to door sending process shoves

message out door

sending process asssumestransport infrastructure onother side of door whichbrings message to socket atreceiving process


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Addressing processesTo send a message from host A to host B, the sending

prosess must identify the receiving process The name or address of the host machine

The port number for receiving process on destination host Identify both of IP address and port number associated

with the process on the host Every host has a unique 32-bit IP address

Port number serves for the purpose in the InternetEx:HTTP server: 80

Mail Server: 25


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User AgentsThe interface between user and network application

Ex: the web application is running on Web server, user

uses browser (Netscape, Internet Explorer, Firefox, )Navigate in the web

view web pages

Web scripts (java scripts, perl script, )

Interact with the web server (input to forms, send email,receive email, )


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What services does an Application Need?

Data loss

some apps (e.g., audio) cantolerate some loss

other apps (e.g., file transfer,telnet) require 100% reliabledata transfer

Timing some apps are real-time

apps (e.g., Internettelephony, interactivegames) require low delay

to be effective

Bandwidth

some apps (e.g.,multimedia , Inter nettelephony) requireminimum amount of

bandwidth to beeffective

other apps (elasticapps) make use of

whatever bandwidth theyget

Network applications may need from a transport protocol, services:


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Transport service requirements of

common apps

Application

file transfere-mail

Web documents

real-time audio/video

stored audio/video

interactive games

instant messaging

Data loss

no lossno loss

no loss

loss-tolerant

loss-tolerant

loss-tolerant

no loss

Bandwidth

elasticelastic

elastic

audio: 5kbps-1Mbps

video:10kbps-5Mbps

same as above

few kbps up

elastic

Time Sensitive

nono

no

yes, 100s msec

yes, few secs

yes, 100s msecyes and no


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Internet Transport Protocol Services

TCP service: connection-oriented: setup

required between client andserver processes

reliable transport betweensending and receiving process,guarantees delivery of all data.

flow control:sender wontoverwhelm receiver

congestion control: throttlesender when network overloaded

does not providing: timing,minimum bandwidth guarantees

UDP service: unreliable data transfer

between sending andreceiving process

does not provide: connectionsetup, reliability, flowcontrol, congestion control,timing, or bandwidthguarantee

Q: why bother? Why is there aUDP? (be faster withoutacknowledgements andretransmissions real-timeapplications)


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Internet apps: applications,

transport protocols

Application

e-mailremote terminal access

Web

file transfer

streaming multimedia

Internet telephony

Application

layer protocol

SMTP [RFC 2821]Telnet [RFC 854]

HTTP [RFC 2616]

FTP [RFC 959]

proprietary

(e.g. RealNetworks)proprietary

(e.g., Dialpad)

Underlying

transport protocol

TCPTCP

TCP

TCP

TCP or UDP

typically UDP


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Network Apps covered in this

chapterFocus on a number of important and popular

applications:

The Web

File Tranfers

Electronic mail

Directory service (Domain Name System DNS)

Invoke through other application indirectly


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Chapter 2: Outline 2.1 Principles of app layer

protocols

clients and servers app requirements

2.2 Web and HTTP

2.3 FTP


2.5 DNS






P2P file sharing


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Introduction In the 1980s, the Internet was used by researchers,

academics and university students (academic and researchcommunities only) Login to remote host Transfer files from local to remote hosts, vice versa Send, and receive news or electronic email.

In the 1990s, the World Wide Web arrived on the scence,the Web an Internet application

Everyone can become a publisherDisplay graphics and animated graphicsAudio and video materials can be access on demand


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Web and HTTPSome Terminologies

Web pages consists ofobjects

Objects can be HTML files, JPEG file, Java applet, audiofile, video file,

Web page consists of a base HTML file and severalreference objects

Each object has addressed by a URL (Uniform ResourceLocator)

www.someschool.edu/someDept/pic.gif

host namepath name


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Overview of HTTPHTTP: hypertext transferprotocol

Webs application layer

protocol client/server model

client: browser thatrequests, receives,displays Web objects

server: Web server sendsobjects in response torequests

PC runningExplorer

Serverrunning

Apache Webserver

Mac runningNavigator


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HTTP Overview (cont.)HTTP definesHow web client (browsers) request web pages from

server (Web servers)

How the server transfer Web pages to clientsHTTP/1.0 [RFC 1945] in 1997

HTTP/1.1 [RFC 2068] in 1998

Both HTTP/1.0 and HTTP/1.1 use TCP as transport

protocolDefine communication protocols between

HTTP client program

HTTP server program


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HTTP Overview (Cont.)Uses TCP: 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 Web server(HTTP server)

TCP connection closed

Protocols that maintain stateare complex!

past history (state) must be

maintained if server/client crashes, theirviews of state may beinconsistent, must bereconciled

aside

HTTP is stateless protocols Without storing any state

information about past client

requests


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HTTP ConnectionsNon-persistent HTTP

At most one object is

sent over a TCPconnection.

HTTP/1.0 is default modeof non-persistent HTTP

Persistent HTTP

Multiple objects can besent over single TCPconnection betweenclient and server.

HTTP/1.1 uses persistentconnections in default

mode


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Non-Persistent HTTPSuppose user enters URL www.someSchool.edu/someDepartment/home.index

(contains text,

references to 10

jpeg images)

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

on port 80

2. HTTP client sends HTTP requestmessage (containing URL) intoTCP connection socket. Messageindicates that client wants objectsomeDepartment/home.index

1b. HTTP server at hostwww.someSchool.edu

waitingfor TCP connection at port 80.accepts connection, notifyingclient

3. HTTP server receives requestmessage, retrieves the requestedobjects from its storage, formsresponse message (encapsulate srequested objects in a HTTPresponse message, and sends

message into its socket

time


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Non-Persistent HTTP (Cont.)

5. HTTP client receives responsemessage containing html file,extract file from responsemessage , and then displays html.Parsing html file, finds 10referenced jpeg objects

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

4. HTTP server closes TCPconnection after flush out

response message to client.

time

In this example, when a user requests theWeb page, there are 1 text , and 10 referenceimages, hence, 11 TCP connections are

generated. 1 text over 1 TCP connection 10 images over ten serial TCP connections


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Response Time ModelingRound-Trip Time (RTT):Time to send a small

packet to travel from clientto server and back.

Response time:one RTT to initiate TCP

connectionone RTT for HTTP request

and first few bytes ofHTTP response to return

file transmission timetotal = 2RTT+transmit time

time to

transmit

file

initiate TCP

connection

RTT

request

file

RTT

file

received

time time


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Persistent HTTPNonpersistent HTTP issues: requires 2 RTTs per object

OS must work and allocate hostresources for each TCP

connection but browsers often open

parallel TCP connections tofetch referenced objects

Persistent HTTP

server leaves connection openafter sending response

subsequent HTTP messagesbetween same client/server aresent over single connection

Persistent without pipelining:

client issues new request onlywhen previous response hasbeen received

one RTT for each referencedobject

Persistent with pipelining:

default in HTTP/1.1

client sends requests as soonas it encounters a referencedobject

as little as one RTT for all thereferenced objects


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HTTP message formats 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: text/html, image/gif, image/jpegAccept-language:fr

(extra carriage return, line feed)

request line(GET, POST,HEAD commands)

headerlines

Carriage return,line feed

indicates endof message


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HTTP request message: general format

Method: POST


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Uploading form inputPost method:Web page often includes

form input

Input is uploaded toserver in entity body

URL method:

Uses GET method

Input is uploaded in URLfield of request line:

www.somesite.com/animalsearch?monkeys&banana


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Method TypesHTTP/1.0

GET

POST

HEAD asks server to leave

requested object out of

response

HTTP/1.1

GET, POST, HEAD

PUT uploads file in entity body

to path specified in URLfield

DELETE deletes file specified in the

URL field


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HTTP Response Message

HTTP/1.1 200 OK

Connection closeDate: 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(protocol

status code

status phrase)

headerlines

data, e.g.,requestedHTML file


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HTTP Response Status Code200 OK

request succeeded, requested object later in this message

301 Moved Permanently

requested object moved, new location specified later in thismessage (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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HTTP response message: general format


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Trying out HTTP request with Telnet1. Telnet to your favorite Web server:

telnet 10.33.6.10 80

2. Type in a GET HTTP request:

GET index.html HTTP/1.0 By typing this in (hit carriage

return twice), you sendthis minimal (but complete)GET request to HTTP server

3. Look at response message sent by HTTP server!

Opens TCP connection to port 80(default HTTP server port) at 10.33.6.10Anything typed in sent

to port 80 at 10.33.6.10


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HTTP Response message


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User-Server Interaction: AuthorizationAuthorization : control access to

server content

authorization credentials:

typically name, password stateless: client must present

authorization in each request

authorization: header line ineach request

if no authorization: header,server refuses access, sends

WWW authenticate:

header line in response

client server

usual http request msg

401: Authorization required.

WWW-Authenticate: header

Authorization: username &

password

usual http response msg

usual http request msg+ Authorization:


time


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Cookies: keeping stateMany major Web sites use

cookies

Four components:1) cookie header line in the

HTTP response message

2) cookie header line inHTTP request message

3) cookie file kept on usershost and managed byusers browser

4) back-end database at Website

Example: Susan access Internet

always from same PC

She visits a specific e-commerce site for firsttime

When initial HTTPrequests arrives at site, site

creates a unique ID andcreates an entry in backenddatabase for ID


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Cookies (cont.)client server

usual http request msg

usual http response +

Set-cookie: 1678

usual http request msgcookie: 1678


usual http request msgcookie: 1678


cookie-specificaction

cookie-spectificaction

servercreates ID

1678 for user

Cookie file

amazon: 1678

ebay: 8734

Cookie file

ebay: 8734

Cookie file

amazon: 1678

ebay: 8734

one week later:


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Cookies (Cont.)What cookies can bring:

authorization

shopping carts recommendations

user session state (Webe-mail)

Cookies and privacy: cookies permit sites to learn a lot

about you

you may supply name and e-mailto sites search engines use redirection &

cookies to learn yet more advertising companies obtain info

across sites


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Web cache: Example

Browser requests an uncached object from Web sever

Web server sends a response message with object to client

GET /fruit/kiwi.gif HTTP/1.0

User-agent: Mozilla/4.0Accept: text/html, image/gif, image/jpeg

HTTP/1.0 200 OKDate: Wed, 12 Aug 1998 15:39:29Server: Apache/1.3.0 (Unix)Last-Modified: Mon, 22 Jun 1998 09:23:24Content-Type: image/gifdata data data data data ...

Last-modified datealong with the object

Client saves object inlocal cache.


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Web cache: ExampleOne week later, user requests the same object is still in cacheGET /fruit/kiwi.gif HTTP/1.0User-agent: Mozilla/4.0

Accept: text/html, image/gif, image/jpegIf-modified-since: Mon, 22 Jun 1998 09:23:24

If-modified-since = Last-Modified

Conditional GET is enable, and Web server only send the

objects which has been modified since specified dateHTTP/1.0 304 Not ModifiedDate: Wed, 19 Aug 1998 15:39:29Server: Apache/1.3.0 (Unix)(empty entity body)


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Chapter 2: outline2.1 Principles of app layer


app requirements

2.2 Web and HTTP

2.3 FTP


2.5 DNS






P2P file sharing


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FTP: File Transfer Protocolfile transfer

FTPserver

FTPuser

interface

FTPclient

local filesystem

remote filesystem

userat host

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


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FTP: separate control, data connections FTP client contacts FTP server at

port 21, specifying TCP astransport protocol

Client obtains authorization over

control connection

Client browses remote directoryby sending commands overcontrol connection.

When server receives acommand for a file transfer, theserver opens a TCP dataconnection to client

After transferring one file, servercloses connection.

Server opens a second TCP dataconnection to transfer anotherfile.

Control connection: out ofband

FTP server maintains state:current directory, earlierauthentication

FTPclient FTPserver

TCP control connectionport 21

TCP data connection

port 20


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FTP commands, responsesSample return codes status code and phrase (as in

HTTP)

331 Username OK,password required

125 data connection

already open;

transfer starting

425 Cant open dataconnection

452 Error writing

file

Sample commands: sent as ASCII text over control

channel

USERusername

PASS password

LISTreturn list of file incurrent directory

RETR filenameretrieves(gets) file

STOR filenamestores(puts) file onto remote host


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FTP: Example


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Electronic MailThree 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., Eudora, Outlook, elm,

Netscape Messenger

outgoing, incoming messagesstored on server

user mailbox

outgoingmessage queue

mailserver

useragent

useragent

user

agentmail

server

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP


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Mail Server

mailserver

useragent

useragent

useragent

mail

server

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Mail Servers mailbox contains incoming

messages for user

messagequeue of outgoing (tobe sent) mail messages

SMTP protocol between mailservers to send email messages

client: sending mail server

server: receiving mailserver


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SMTP [RFC 5321]Simple Mail Transfer Protocol: Internet standard for

electronic mail (e-mail)

RFC 821 first defined

RFC 2821 updated in 2001

RFC 5321 last updated in 2008

Include extended SMTP (ESMTP)

Transmission across Internet Protocol (IP)Using TCP port 25


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RFC 5321 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

command/response interaction

commands: ASCII text

response: status code and phrase

messages must be in 7-bit ASCII


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ASCII Code


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Example1) Alice uses UA to compose messageand to [email protected]

2) Alices UA sends message to hermail server; message placed inmessage queue

3) Client side of SMTP opens TCPconnection with Bobs mail server

4) SMTP client sends Alicesmessage over the TCPconnection

5) Bobs mail server places the

message in Bobs mailbox

6) Bob invokes his user agent toread message

useragent

mailserver

mailserver user

agent

1

2 3 4 56


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Example


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SMTP: final word SMTP uses persistent

connections

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

ASCII

SMTP server uses CRLF.CRLFto determine end of message

Comparison with HTTP:

HTTP: pull

SMTP: push

both have ASCIIcommand/responseinteraction, status codes

HTTP: each object

encapsulated in its ownresponse msg

SMTP: multiple objects sent inmultipart msg


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Mail Message FormatSMTP: protocol for exchangingemail 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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MIME Extension for Non-ASCII Data MIME: multimedia mail extension, RFC 2045, 2056

additional lines in msg header declare MIME content type

From: [email protected]: [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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MIME Types

Content-Type: type/subtype; parametersText example subtypes:plain,html

Image example subtypes: jpeg,gif

Audio exampe subtypes:basic (8-

bit mu-law encoded),32kadpcm(32 kbps coding)

Video example subtypes:mpeg,quicktime

Application other data that must be

processed by reader beforeviewable

example subtypes:msword,octet-stream


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Multipart TypeFrom: [email protected]

To: [email protected]

Subject: Picture of yummy crepe.

MIME-Version: 1.0

Content-Type: multipart/mixed; boundary=StartOfNextPart

--StartOfNextPart

Dear Bob, Please find a picture of a crepe.

--StartOfNextPart

Content-Transfer-Encoding: base64

Content-Type: image/jpeg

base64 encoded data ..............................

......base64 encoded data

--StartOfNextPart

Do you want the reciple?


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Mail access protocol

useragent

senders mail

server

useragent

SMTP SMTP accessprotocol

receivers mail

server

SMTP: delivery/storage to receivers 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: Hotmail , Yahoo! Mail, etc.


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POP3 Protocolauthorization phase client commands:

user: declare username

pass: password

server responses +OK

-ERR

transaction phase, client: list: list message numbers

retr: retrieve message bynumber

dele: delete

quit

S: +OK POP3 server ready

C: user bobS: +OK

C: pass hungry

S: +OKuser successfully logged on

C: list

S: 1 498

S: 2 912S: .

C: retr 1

S:

S: .

C: dele 1

C: retr 2

S:

S: .

C: dele 2

C: quit

S: +OKPOP3 server signing off


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POP3 more and IMAPMore about POP3Previous example uses

download and delete

mode.Bob cannot re-read e-

mail if he changes client

Download-and-keep:

copies of messages ondifferent clients

POP3 is stateless acrosssessions

IMAP

Keep all messages in oneplace: the server

Allows user to organizemessages in folders

IMAP keeps user stateacross sessions:

names of folders andmappings betweenmessage IDs and foldername


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Chapter 2: Outline2.1 Principles of app layer


app requirements

2.2 Web and HTTP

2.3 FTP


2.5 DNS


2.7 Socket programming

with UDP 2.8 Building a Web server



P2P file sharing


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DNS (Domain Name System)People: many identifiers:

SSN, name, passport #

Internet hosts, routers: IP address (32 bit) - used

for addressing datagrams

name, e.g.,gaia.cs.umass.edu - used

by humansQ: map between IP

addresses and name ?

Domain Name System: distributed database

implemented in hierarchy of

manyname servers application-layer protocolhost,

routers, name servers tocommunicate to resolvenames(address/name translation)

note: core Internet function,implemented as application-layer protocol

complexity at networks edge


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DNS name serversWhy not centralize DNS? single point of failure

traffic volume

distant centralized database

maintenance

doesnt scale!

no server has all name-to-IPaddress mappings

local name servers:

each ISP, company has local(default) name server

host DNS query first goes tolocal name server

authoritative name server:

for a host: stores that hosts IPaddress, name

can perform name/addresstranslation for that hostsname


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DNS: Root name servers contacted by local name server that can not resolve name root name server:

contacts authoritative name server if name mapping not known

gets mapping

returns mapping to local name server

b USC-ISI Marina del Rey, CA

l ICANN Marina del Rey, CA

e NASA Mt View, CA

f Internet Software C. Palo Alto,CA

i NORDUnet Stockholm

k RIPE London

m WIDE Tokyo

a NSI Herndon, VA

c PSInet Herndon, VA

d U Maryland College Park, MD

g DISA Vienna, VA

h ARL Aberdeen, MDj NSI (TBD) Herndon, VA

13 root name serversworldwide


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Simple DNS Example

requesting hostsurf.eurecom.fr gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

23

4

5

6

host surf.eurecom.frwants IP address ofgaia.cs.umass.edu

1. contacts its local DNS server,dns.eurecom.fr

2.dns.eurecom.fr contactsroot name server, if necessary

3. root name server contactsauthoritative name server,dns.umass.edu, ifnecessary

authorititive name serverdns.umass.edu


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DNS Example

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server


1

23

4 5

6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

Root name server: may not know

authoritative nameserver

may know intermediatename server: who tocontact to findauthoritative nameserver


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DNS: iterated query

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server


1

23

4

5 6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

iterated queryrecursive query: puts burden of name

resolution on contacted

name server heavy load?

iterated query: contacted server replies

with name of server tocontact

I dont know this name,but ask this server


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DNS: caching and updating recordsonce (any) name server learns mapping, it caches

mapping

cache entries timeout (disappear) after some time

update/notify mechanisms under design by IETF RFC 2136

http://www.ietf.org/html.charters/dnsind-charter.html


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DNS recordsDNS: distributed db storing resource records (RR)

Type=NS name is domain (e.g.

foo.com)

value is IP address ofauthoritative name server forthis domain

RR format: (name, value, type, ttl)

Type=A name is hostname value is IP address

Type=CNAME name is alias name for some

cannonical (the real) namewww.ibm.com is reallyservereast.backup2.ibm.com

value is cannonical name

Type=MX value is name of mailserver

associated with name


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DNS protocol, messagesDNS protocol :queryand reply messages, both with same message format

msg header

identification: 16 bit # forquery, reply to query usessame #

flags: query or reply

recursion desired recursion available reply is authoritative


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DNS protocol, messagesName, type fields

for a query

RRs in reponseto query

records for

authoritative servers

additional helpfulinfo that may be used


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Chapter 2: Outline2.1 Principles of app layerprotocols clients and servers

app requirements

2.2 Web and HTTP

2.3 FTP

2.4 Electronic Mail

SMTP, POP3, IMAP2.5 DNS






P2P file sharing


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Socket Programming

Socket API

introduced in BSD4.1 UNIX,1981

explicitly created, used,released by apps

client/server paradigm

two types of transport servicevia socket API: unreliable datagram

reliable, byte stream-oriented

a host-local,application-created,

OS-controlledinterface (adoor) into which

application process canboth send and

receive messages to/fromanother application process

socket

Goal: learn how to build client/server application that communicate usingsockets


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Socket-programming using TCP

process

TCP with

buffers,variables

socket

controlled byapplication

developer

controlled by

operatingsystem

host orserver

process

TCP with

buffers,variables

socket

controlled byapplicationdeveloper

controlled byoperatingsystem

host orserver

internet

Socket: a door between application process and end-end-transport protocol (UCP or TCP)

TCP service: reliable transfer ofbytesfrom one process toanother


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Socket Programming with TCPClient must contact server server process must first be

running

server must have created

socket (door) that welcomesclients contact

Client contacts server by:

creating client-local TCPsocket

specifying IP address, portnumber of server process

When client creates socket:client TCP establishesconnection to server TCP

When contacted by client,server TCP creates new socketfor server process tocommunicate with client

allows server to talk withmultiple clients

source port numbers used todistinguish clients (more inChap 3)

TCP provides reliable, in-ordertransfer of bytes (pipe)between client and server

application viewpoint


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Stream typesAstream is a sequence of characters that flow intoor out of a process.

An input stream is attached to some input sourcefor the process, eg, keyboard or socket.

An output stream is attached to an output source,eg, monitor or socket.


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Socket programming with TCPExample client-server app:1) client reads line from standard

input (inFromUser stream) ,

sends to server via socket(outToServer stream)

2) server reads line from socket

3) server converts line to uppercase,sends back to client

4) client reads, prints modified linefrom socket (inFromServerstream) o

utToServe

r

to network from network

inFromServer

inFromUser

keyboard monitor

Process

clientSocket

input

stream

input

stream

output

stream

TCP

socket


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Client/server socket interaction: TCP

wait for incoming

connection requestconnectionSocket =

welcomeSocket.accept()

create socket,port=x, for

incoming request:welcomeSocket =

ServerSocket()

create socket,connect to hostid, port=xclientSocket =

Socket()

close

connectionSocket

read reply from

clientSocket

close

clientSocket

Server (running on hostid) Client

send request using

clientSocketread request from

connectionSocket

write reply to

connectionSocket

TCPconnection setup


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Example: Java ClientBufferedReader inFromServer =new BufferedReader(new

InputStreamReader(clientSocket.getInputStream()));

sentence = inFromUser.readLine();

outToServer.writeBytes(sentence + '\n');

modifiedSentence = inFromServer.readLine();

System.out.println("FROM SERVER: " + modifiedSentence);

clientSocket.close();

}

}

Createinput stream

attached to socket

Send lineto server

Read linefrom server


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Example: Java Server (TCP)import java.io.*;import java.net.*;class TCPServer {

public static void main(String argv[]) throws Exception

{String clientSentence;

String capitalizedSentence;

ServerSocket welcomeSocket = new ServerSocket(6789);

while(true) {

Socket connectionSocket = welcomeSocket.accept();

BufferedReader inFromClient =

new BufferedReader(new

InputStreamReader(connectionSocket.getInputStream()));

Createwelcoming socket

at port 6789

Wait, on welcomingsocket for contactby client

Create inputstream, attached

to socket


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Example: Java Server (TCP)

Read in linefrom socket

Create outputstream, attached

to socket

Write out lineto socket

End of while loop,loop back and wait foranother client connection

DataOutputStream outToClient =

new DataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();

capitalizedSentence = clientSentence.toUpperCase() + '\n';

outToClient.writeBytes(capitalizedSentence);

}

}

}


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Socket programming with UDPUDP: no connection betweenclient and server

no handshaking

sender explicitly attaches IPaddress and port ofdestination to each packet

server must extract IP address,port of sender from receivedpacket

UDP: transmitted data may bereceived out of order, or lost

application viewpointUDP provides unreliable transferof groups of bytes (datagrams)

between client and server


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Client/server socket interaction: UDP

close

clientSocket

Server (running on hostid)

read reply from

clientSocket

create socket,clientSocket =

DatagramSocket()

Client

Create, address (hostid, port=x,

send datagram request

using clientSocket

create socket,port=x, for

incoming request:serverSocket =

DatagramSocket()

read request from

serverSocket

write reply to

serverSocket

specifying client

host address,

port number


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Example: Java client (UDP)

sendPacket

to network from network

receive

Packet

inFromUser

keyboard monitor

Process

clientSocket

UDP

packet

input

stream

UDP

packet

UDP

socket

Output: sendspacket (TCP sent

byte stream)

Input: receivespacket (TCPreceived bytestream)

Clientprocess

client UDPsocket


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Example: Java client (UDP)import java.io.*;import java.net.*;

class UDPClient {

public static void main(String args[]) throws Exception

{

BufferedReader inFromUser =

new BufferedReader(new InputStreamReader(System.in));

DatagramSocket clientSocket = new DatagramSocket();

InetAddress IPAddress = InetAddress.getByName("hostname");

byte[] sendData = new byte[1024];

byte[] receiveData = new byte[1024];

String sentence = inFromUser.readLine();

sendData = sentence.getBytes();

Createinput stream

Createclient socket

Translatehostname to IPaddress using DNS


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Example: Java Server (UDP)import java.io.*;import java.net.*;

class UDPServer {

public static void main(String args[]) throws Exception

{

DatagramSocket serverSocket = new DatagramSocket(9876);

byte[] receiveData = new byte[1024];

byte[] sendData = new byte[1024];

while(true){

DatagramPacket receivePacket =

new DatagramPacket(receiveData, receiveData.length);

serverSocket.receive(receivePacket);

Createdatagram socket

at port 9876

Create space forreceived datagram

Receive

datagram


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Example: Java Server (UDP) cont.String sentence = new String(receivePacket.getData());

InetAddress IPAddress = receivePacket.getAddress();

int port = receivePacket.getPort();

String capitalizedSentence = sentence.toUpperCase();

sendData = capitalizedSentence.getBytes();

DatagramPacket sendPacket =

new DatagramPacket(sendData, sendData.length, IPAddress,

port);

serverSocket.send(sendPacket);

}

}

}

Get IP addrport #, of

sender

Write outdatagramto socket

End of while loop,loop back and wait foranother datagram

Create datagramto send to client


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Chapter 2: Outline2.1 Principles of app layer


app requirements2.2 Web and HTTP

2.3 FTP

2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS




2.9Content distribution Network Web caching


P2P file sharing


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Web caches (Proxy Server)

client

Proxy

server

clientoriginserver

originserver

user sets browser: Webaccesses via cache

browser sends all HTTPrequests to cache

object in cache: cachereturns object

else cache requests objectfrom origin server, thenreturns object to client

Goal: satisfy client request without involving origin server


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More about Web caching Cache acts as both client and

server

Cache can do up-to-datecheck using If-modified-

since HTTP header Issue: should cache take risk

and deliver cached objectwithout checking?

Heuristics are used.

Typically cache is installed byISP (university, company,residential ISP)

Why Web caching? Reduce response time for

client request.

Reduce traffic on aninstitutions access link.

Internet dense with cachesenables poor contentproviders to effectively deliver

content


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Caching Example (1)Assumptions

average object size = 100,000 bits

avg. request rate frominstitutions browser to originserves = 15/sec

delay from institutional router toany origin server and back torouter = 2 sec

Consequences utilization on LAN = 15%

utilization on access link = 100% total delay = Internet delay + access

delay + LAN delay

= 2 sec + minutes + milliseconds

originservers

publicInternet

institutional

network 10 Mbps LAN

1.5 Mbpsaccess link

institutionalcache


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Example caching (2)origin

servers

publicInternet

institutional

network 10 Mbps LAN

10 Mbpsaccess link

institutionalcache

Possible solution

increase bandwidth of access linkto, say, 10 Mbps

Consequences

utilization on LAN = 15%

utilization on access link = 15%

Total delay = Internet delay + accessdelay + LAN delay

= 2 sec + msecs + msecs

often a costly upgrade


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CDN Example HTTP request forwww.foo.com/sports/sports.html

DNS query for www.cdn.com

HTTP request forwww.cdn.com/www.foo.com/sports/ruth.gif

1

2

3

Origin server

CDNs authoritativeDNS server

NearbyCDN server

origin server

www.foo.com

distributes HTML Replaces:

http://www.foo.com/sports.ruth.gif

withhttp://www.cdn.com/www.foo.com/sports/ruth.gif

CDN company cdn.com distributes gif files uses its authoritative DNS server

to route redirect requests


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P2P File SharingExample

Alice runs P2P clientapplication on hernotebook computer

Intermittently connectsto Internet; gets new IPaddress for eachconnection

Asks for Hey JudeApplication displays

other peers that havecopy of Hey Jude.

Alice chooses one of thepeers, Bob.

File is copied from BobsPC to Alices notebook:HTTP

While Alice downloads,other users uploadingfrom Alice.

Alices peer is both a Webclient and a transient

Web server.

All peers are servers =

highly scalable!


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P2P: Problems with centralized directory

Single point of failure

Performance bottleneck

Copyright infringement

file transfer isdecentralized, butlocating content ishighly decentralized


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P2P: Decentralized directory

Each peer is either agroup leader or assignedto a group leader.

Group leader tracks thecontent in all itschildren.

Peer queries group

leader; group leader mayquery other groupleaders.

ordinary peer

group-leader peer

neighoring relationships

in overlay network

Chapter 2 Summary


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Chapter 2: SummaryOur study of network apps now complete!

application service requirements:

reliability, bandwidth, delay

Ubiquitous client-server paradigm

Internet transport service model

connection-oriented, reliable: TCP

unreliable, datagrams: UDP

specific protocols: HTTP FTP

SMTP, POP, IMAP DNS

socket programming content distribution

Most importantly: learned aboutprotocols

typical request/reply message exchange:

client requests info or service

server responds with data, status code

message formats: headers: fields giving info about data

data: info being communicated

control vs. data msgs

in-band, out-of-band centralized vs. decentralized stateless vs. stateful reliable vs. unreliable msg transfer l k d

2011-03-5 chapter 2 - application layer

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