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2: Application Layer 1

Chapter 2

Application Layer

Computer Networking:A Top Down Approach,4thedition.Jim Kurose, Keith RossAddison-Wesley, July2007.

A note on the use of these ppt slides:Were making these slides freely available to all (faculty, students, readers).

Theyre in PowerPoint form so you can add, modify, and delete slides

(including this one) and slide content to suit your needs. They obviously

represent a lotof work on our part. In return for use, we only ask the

following:

If you use these slides (e.g., in a class) in substantially unaltered form,that you mention their source (after all, wed like people to use our book!)

If you post any slides in substantially unaltered form on a www site, that

you note that they are adapted from (or perhaps identical to) our slides, and

note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2007

J.F Kurose and K.W. Ross, All Rights Reserved

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Chapter 2: Application layer

2.1 Principles ofnetwork applications

2.2 Web and HTTP

2.3 FTP 2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS

2.6 P2P applications

2.7 Socket programmingwith TCP

2.8 Socket programmingwith UDP

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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

voice over IP

real-time videoconferencing

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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2.2 Web and HTTP

2.3 FTP 2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS




2.9 Building a Webserver

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Application architectures

Client-server

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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Pure P2P architecture

noalways-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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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 orserver

process

TCP with

buffers,

variables

socket

host orserver

Internet

controlledby 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

Q:does IP address ofhost suffice foridentifying the process?

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Addressing processes

to receive messages,process must haveidentifier

host device has unique32-bit IP address

Q:does IP address ofhost on which processruns suffice foridentifying the

process? A:No, many

processes can berunning on same host

identifierincludes bothIP addressand portnumbersassociated withprocess on host.

Example port numbers: HTTP server: 80

Mail server: 25

to send HTTP messageto gaia.cs.umass.edu web

server: IP address:128.119.245.12

Port number:80

more shortly

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App-layer protocol defines

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

Message syntax: what fields in messages &

how fields are delineated

Message semantics meaning of information in

fields Rules for when and how

processes send &respond to messages

Public-domain protocols:

defined in RFCs

allows for

interoperability e.g., HTTP, SMTP

Proprietary protocols:

e.g., Skype

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What transport service does an app need?

Data loss some apps (e.g., audio) can

tolerate some loss other apps (e.g., file

transfer, telnet) require

100% reliable datatransferTiming some apps (e.g.,

Internet telephony,interactive games)require low delay to beeffective

Throughput

some apps (e.g.,multimedia) requireminimum amount ofthroughput to be

effective other apps (elastic apps)

make use of whateverthroughput they get

Security Encryption, data

integrity,

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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, 100s msec

yes, few secs

yes, 100s 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 wontoverwhelm 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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HTTP overview

HTTP: hypertexttransfer protocol

Webs 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 maintainstate 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.HTTPclient sends HTTPrequest message(containingURL) into TCP connection

socket. Message indicatesthat client wants objectsomeDepartment/home.index

1b.HTTPserver at hostwww.someSchool.edu waiting

for TCP connection at port 80.accepts connection, notifyingclient

3.HTTPserver receives request

message, forms responsemessagecontaining 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.HTTPserver closes TCPconnection.

time

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Persistent HTTP

Nonpersistent HTTP issues: requires 2 RTTs per object

OS overhead for eachTCPconnection

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 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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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 usercreate

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 aninstitutions accesslink.

Internet dense withcaches: enables poor

content providers toeffectively delivercontent (but so doesP2P file sharing)

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Caching example (cont)

possible solution increase bandwidth of access

link to, say, 10 Mbps

consequence utilization on LAN = 15%

utilization on access link = 15%

Total delay = Internet delay +access delay + LAN delay

= 2 sec + msecs + msecs

often a costly upgrade

origin

servers

publicInternet

institutionalnetwork 10 Mbps LAN

10 Mbpsaccess link

institutionalcache

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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

file transferFTP

server

FTPuser

interface

FTPclient

local filesystem

remote file

system

userat host

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FTP: separate control, data connections

FTP client contacts FTP serverat port 21, TCP is transportprotocol

client authorized over controlconnection

client browses remotedirectory by sending commandsover control connection.

when server receives file

transfer command, serveropens 2ndTCP connection (forfile) to client

after transferring one file,server closes data connection.

FTPclient

FTPserver

TCP control connectionport 21

TCP data connectionport 20

server opens another TCPdata connection to transferanother file.

control connection: out of

band FTP server maintains state:

current directory, earlierauthentication

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FTP commands, responses

Sample commands: sent as ASCII text over

control channel

USER username

PASS password LISTreturn list of file in

current directory

RETR filenameretrieves

(gets) file STOR filenamestores

(puts) file onto remotehost

Sample return codes status code and phrase (as

in HTTP) 331 Username OK,

password required 125 data connection

already open;

transfer starting

425 Cant open data

connection 452 Error writing

file

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2.2 Web and HTTP

2.3 FTP

2.4 Electronic Mail SMTP, POP3, IMAP

2.5 DNS




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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., Eudora, Outlook, elm,

Mozilla Thunderbird

outgoing, incoming messagesstored on server

user mailbox

outgoingmessage queue

mail

server

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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Electronic Mail: mail servers

Mail Servers mailboxcontains incoming

messages for user

messagequeueof outgoing

(to be sent) mail messages SMTP protocolbetween mail

servers to send emailmessages

client: sending mail

server server: receiving mail

server

mailserver

useragent

useragent

useragent

mail

server

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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Electronic Mail: SMTP [RFC 2821]

uses TCP to reliably transfer email message from clientto 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

S i Ali d t B b

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Scenario: Alice sends message to Bob

1) Alice uses UA to compose

message and [email protected]

2) Alices UA sends messageto her mail server; messageplaced in message queue

3) Client side of SMTP opensTCP connection with Bobsmail server

4) SMTP client sends Alices

message over the TCPconnection

5) Bobs mail server places themessage in Bobs mailbox

6) Bob invokes his user agent

to read message

useragent

mailserver

mailserver user

agent

1

2 3 4 56

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Sample SMTP interactionS: 220 hamburger.edu

C: HELO crepes.frS: 250 Hello crepes.fr, pleased to meet you

C: MAIL FROM:

S: 250 [email protected]... Sender ok

C: RCPT TO:

S: 250 [email protected] ... Recipient okC: 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

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Try SMTP interaction for yourself:

telnet servername 25 see 220 reply from server

enter HELO, MAIL FROM, RCPT TO, DATA, QUITcommands

above lets you send email without using email client(reader)

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SMTP: final words

SMTP uses persistentconnections

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

SMTP server usesCRLF.CRLFto determineend of message

Comparison with HTTP:

HTTP: pull

SMTP: push

both have ASCIIcommand/responseinteraction, status codes

HTTP: each objectencapsulated in its own

response msg SMTP: multiple objects

sent in multipart msg

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Mail message format

SMTP: protocol forexchanging email msgs

RFC 822: standard for textmessage format:

header lines, e.g., To:

From:

Subject:

differentfrom SMTPcommands!

body the message, ASCII

characters only

header

body

blankline

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Message format: multimedia extensions

MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content

type

From: [email protected]

To: [email protected]: Picture of yummy crepe.

MIME-Version: 1.0

Content-Transfer-Encoding: base64

Content-Type: image/jpeg

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

......base64 encoded data

multimedia data

type, subtype,parameter declaration

method usedto encode data

MIME version

encoded data

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Mail access protocols

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: gmail, Hotmail, Yahoo! Mail, etc.

useragent

senders mailserver

useragent

SMTP SMTP accessprotocol

receivers mailserver

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POP3 protocol

authorization 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

C: list

S: 1 498

S: 2 912

S: .C: retr 1

S:

S: .

C: dele 1

C: retr 2

S:

S: .

C: dele 2

C: quit

S: +OK POP3 server signing off

S: +OK POP3 server ready

C: user bob

S: +OK

C: pass hungry

S: +OKuser successfully logged on

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POP3 (more) and IMAP

More about POP3 Previous example uses

download and deletemode.

Bob cannot re-read e-mail if he changesclient

Download-and-keep:copies of messages ondifferent clients

POP3 is statelessacross sessions

IMAP Keep all messages in

one place: the server

Allows user to

organize messages infolders

IMAP keeps user stateacross sessions:

names of folders andmappings betweenmessage IDs and foldername

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2.2 Web and HTTP

2.3 FTP


2.5 DNS

2.6 P2P applications 2.7 Socket programming

with TCP


2.9 Building a Webserver

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DNS: Domain Name System

People:many identifiers: SSN, name, passport #

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

used for addressingdatagrams

name, e.g.,ww.yahoo.com - used byhumans

Q:map between IPaddresses and name ?

Domain Name System: distributed database

implemented in hierarchy ofmany name servers

application-layer protocolhost, routers, name servers tocommunicate to resolvenames(address/name translation)

note: core Internet

function, implemented asapplication-layer protocol

complexity at networksedge

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DNS

Why not centralize DNS? single point of failure

traffic volume

distant centralized

database maintenance

doesnt scale!

DNS services hostname to IP

address translation

host aliasing Canonical, alias names

mail server aliasing

load distribution replicated Web

servers: set of IPaddresses for onecanonical name

Distributed Hierarchical Database

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Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.edu

DNS servers

umass.edu

DNS serversyahoo.com

DNS servers

amazon.com

DNS servers

pbs.org

DNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1stapprox:

client queries a root server to find com DNS server

client queries com DNS server to get amazon.comDNS server

client queries amazon.com DNS server to get IPaddress for www.amazon.com

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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

13 root nameservers worldwide

b USC-ISI Marina del Rey, CA

l ICANN Los Angeles, CA

e NASA Mt View, CA

f Internet Software C. PaloAlto,CA (and 36 other locations)

i Autonomica, Stockholm (plus

28 other locations)

k RIPE London (also 16 other locations)

m WIDE Tokyo (also Seoul,

Paris, SF)

a Verisign, Dulles, VA

c Cogent, Herndon, VA (also LA)

d U Maryland College Park, MD

g US DoD Vienna, VA

h ARL Aberdeen, MDj Verisign, ( 21 locations)

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TLD and Authoritative Servers

Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all

top-level country domains uk, fr, ca, jp. Network Solutions maintains servers for com TLD

Educause for edu TLDAuthoritative DNS servers:

organizations DNS servers, providingauthoritative hostname to IP mappings for

organizations servers (e.g., Web, mail). can be maintained by organization or service

provider

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Local Name Server

does not strictly belong to hierarchy each ISP (residential ISP, company,

university) has one.

also called default name serverwhen host makes DNS query, query is sent

to its local DNS server acts as proxy, forwards query into hierarchy

DNDNS name

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requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

2 3

4

5

6

authoritative DNS server

dns.cs.umass.edu

78

TLD DNS server

DNS nameresolution example

Host at cis.poly.eduwants IP address forgaia.cs.umass.edu

iterated query: contacted server

replies with name ofserver to contact

I dont know this

name, but ask thisserver

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DNS: caching and updating records

once (any) name server learns mapping, it cachesmapping

cache entries timeout (disappear) after sometime

TLD servers typically cached in local nameservers Thus root name servers not often visited

update/notify mechanisms under design by IETF

RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html

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DNS records

DNS:distributed db storing resource records (RR)

Type=NS nameis domain (e.g.

foo.com) valueis hostname of

authoritative nameserver for this domain

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

Type=A

nameis hostname valueis IP address

Type=CNAME

nameis alias name for somecanonical (the real) name

www.ibm.com is reallyservereast.backup2.ibm.com

valueis canonical name

Type=MX valueis name of mailserver

associated with name

l

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DNS protocol, messages

DNS protocol :queryand replymessages, both withsame message format

msg header identification:16 bit #

for query, reply to queryuses same #

flags:

query or reply

recursion desired

recursion available reply is authoritative

DN l

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DNS protocol, messages

Name, type fieldsfor a query

RRs in response

to query

records forauthoritative servers

additional helpfulinfo that may be used

I i d i DNS

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Inserting records into DNS

example: new startup Network Utopia register name networkuptopia.com at DNS registrar

(e.g., Network Solutions) provide names, IP addresses of authoritative name server

(primary and secondary)

registrar inserts two RRs into com TLD server:

(networkutopia.com, dns1.networkutopia.com, NS)

(dns1.networkutopia.com, 212.212.212.1, A)

create authoritative server Type A record forwww.networkuptopia.com; Type MX record fornetworkutopia.com

How do people get IP address of your Web site?

Ch t 2 A li ti l

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2.1 Principles ofnetwork applications app architectures

app requirements

2.2 Web and HTTP 2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS


with TCP

2.8 Socket programming

with UDP

P P2P hit t

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Pure P2P architecture

noalways-on server arbitrary end systems

directly communicate

peers are intermittently

connected and change IPaddresses

Three topics: File distribution

Searching for information

Case Study: Skype

peer-peer

Fil Di t ib ti S Cli t P2P

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File Distribution: Server-Client vs P2P

Question: How much time to distribute filefrom one server to N peers?

us

u2d1d2

u1

uN

dN

Server

Network (withabundant bandwidth)

File, size F

us:server upload

bandwidth

ui

:

peer i uploadbandwidth

di:peer i download

bandwidth

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Fil di t ib ti ti P2P

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File distribution time: P2P

us

u2d1 d2u1

uN

dN

Server

Network (withabundant bandwidth)

F server must send one

copy: F/ustime

client i takes F/di timeto download

NF bits must bedownloaded (aggregate) fastest possible upload rate: us+ Sui

dP2P= max {F/us, F/min(di), NF/(us+ Sui)}i

S li t P2P l

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0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35

N

MinimumD

istributionTime P2P

Client-Server

Server-client vs. P2P: example

Client upload rate = u, F/u = 1 hour, us= 10u, dmin us

Fil di t ib ti BitT t

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File distribution: BitTorrent

tracker:tracks peersparticipating in torrent

torrent:group ofpeers exchangingchunks of a file

obtain listof peers

trading

chunks

peer

P2P file distribution

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BitTorrent (1)

file divided into 256KB chunks.

peer joining torrent:

has no chunks, but will accumulate them over time registers with tracker to get list of peers,

connects to subset of peers (neighbors)

while downloading, peer uploads chunks to other

peers. peers may come and go

once peer has entire file, it may (selfishly) leave or(altruistically) remain

BitT t (2)

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BitTorrent (2)

Pulling Chunks at any given time,

different peers havedifferent subsets of

file chunks periodically, a peer

(Alice) asks eachneighbor for list ofchunks that they have.

Alice sends requestsfor her missing chunks

rarest first

Sending Chunks: tit-for-tat

Alice sends chunks to fourneighbors currentlysending her chunks at thehighest rate

re-evaluate top 4 every

10 secs every 30 secs: randomly

select another peer,starts sending chunks

newly chosen peer mayjoin top 4

optimistically unchoke

BitT t Tit f t t

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BitTorrent: Tit-for-tat(1) Alice optimistically unchokes Bob

(2) Alice becomes one of Bobs top-four providers; Bob reciprocates(3) Bob becomes one of Alices top-four providers

With higher upload rate,can find better tradingpartners & get file faster!

P2P: searching for information

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P2P searching for information

File sharing (eg e-mule)

Index dynamicallytracks the locations of

files that peers share. Peers need to tell

index what they have.

Peers search index to

determine where filescan be found

Instant messaging

Index maps usernames to locations.

When user starts IMapplication, it needs toinform index of itslocation

Peers search index todetermine IP addressof user.

Index in P2P system: maps information to peer location

(location = IP address & port number).

P2P: centralized index

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P2P: centralized index

original Napster design1) when peer connects, it

informs central server: IP address

content2) Alice queries for Hey

Jude

3) Alice requests file from

Bob

centralizeddirectory server

peers

Alice

Bob

1

1

1

12

3

P2P: problems with centralized directory

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P2P: problems with centralized directory

single point of failure performance bottleneck

copyright infringement:target of lawsuit is

obvious

file transfer isdecentralized, butlocating content ishighly centralized

Query flooding

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Query flooding

fully distributed no central server

used by Gnutella Each peer indexes the

files it makes availablefor sharing (and noother files)

overlay network: graph edge between peer X

and Y if theres a TCPconnection

all active peers andedges form overlay net

edge: virtual (notphysical) link

given peer typicallyconnected with < 10overlay neighbors

Query flooding

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Query flood ng

Query

QueryHit

Query

QueryHit

File transfer:

HTTPQuery message

sent over existing TCPconnections

peers forwardQuery message

QueryHitsent overreversepath

Scalability:

limited scopeflooding

Gnutella: Peer joining

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Gnutella: Peer joining

1. joining peer Alice must find another peer inGnutella network: use list of candidate peers

2. Alice sequentially attempts TCP connections withcandidate peers until connection setup with Bob

3. Flooding:Alice sends Ping message to Bob; Bobforwards Ping message to his overlay neighbors(who then forward to their neighbors.) peers receiving Ping message respond to Alice

with Pong message

4. Alice receives many Pong messages, and can thensetup additional TCP connections

Peer leaving: see homework problem!

Hierarchical Overlay

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Hierarchical Overlay

between centralizedindex, query floodingapproaches

each peer is either a

super nodeor assigned toa super node TCP connection between

peer and its super node.

TCP connections betweensome pairs of super nodes.

Super node tracks contentin its children

ordinary peer

group-leader peer

neighoring relationships

in overlay network

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2.2 Web and HTTP

2.3 FTP


2.5 DNS


with TCP


with UDP

Socket programming

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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

service via socket API: unreliable datagram

reliable, byte stream-oriented

a host-local,

application-created,OS-controlledinterface(a door) into which

application process canboth send and

receivemessages to/fromanother applicationprocess

socket

Goal:learn how to build client/server application that

communicate using sockets

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Socket programming with TCP

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Client must contact server

server process must firstbe running

server must have createdsocket (door) thatwelcomes clients contact

Client contacts server by:

creating client-local TCPsocket

specifying IP address, port

number of server process When client creates

socket: client TCPestablishes connection toserver TCP

When contacted by client,server TCP creates newsocketfor server process tocommunicate with client

allows server to talk withmultiple clients

source port numbersused to distinguishclients (more in Chap 3)

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

application viewpoint

Client/server socket interaction: TCP

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Client/server socket interaction: TCP

wait for incomingconnection 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

Stream jargon

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outToServer

to network from network

inFromServer

inFromUser

keyboard monitor

Process

clientSocket

input

stream

input

stream

output

stream

TCP

socket

Client

process

client TCPsocket

Stream jargon

A streamis a sequence ofcharacters that flow intoor out of a process.

An input streamisattached to some input

source for the process,e.g., keyboard or socket.

An output streamisattached to an outputsource, e.g., monitor or

socket.


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Example client-server app:1) client reads line fromstandard input (inFromUserstream) , sends to server viasocket (outToServer

stream)2) server reads line from socket

3) server converts line touppercase, sends back toclient

4) client reads, prints modifiedline from socket(inFromServerstream)

Example: Java client (TCP)

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Example: Java client (TCP)

import java.io.*;import java.net.*;

class TCPClient {

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

{

String sentence;String modifiedSentence;

BufferedReader inFromUser =

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

Socket clientSocket = new Socket("hostname", 6789);

DataOutputStream outToServer =

new DataOutputStream(clientSocket.getOutputStream());

Createinput stream

Create

client socket,connect to server

Createoutput stream

attached to socket

Example: Java client (TCP) cont

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Example: Java client (TCP), cont.

BufferedReader 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) cont

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Example: Java server (TCP), cont

DataOutputStream outToClient =

new DataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();

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

outToClient.writeBytes(capitalizedSentence);

}

}

}

Read in linefrom socket

Create outputstream, attached

to socket

Write out lineto socket

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


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2.2 Web and HTTP

2.3 FTP


2.5 DNS


with TCP


with UDP

Socket programming with UDP

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Socket programming with UDP

UDP: no connection betweenclient and server

no handshaking

sender explicitly attachesIP address and port of

destination to each packet server must extract IP

address, port of senderfrom received packet

UDP: transmitted data may bereceived out of order, orlost

application viewpoint

UDP provides unreliable transferof groups of bytes (datagrams)between client and server

Client/server socket interaction: UDP

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Client/server socket interaction: UDP

Server (running on hostid)

close

clientSocket

read datagram from

clientSocket

create socket,

clientSocket =

DatagramSocket()

Client

Create datagram with server IP andport=x; send datagram via

clientSocket

create socket,

port= x.

serverSocket =

DatagramSocket()

read datagram from

serverSocket

write reply to

serverSocket

specifyingclient address,

port number

Example: Java client (UDP)

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sendPacket

to network from network

receivePacket

inFromUser

keyboard monito r

Process

clientSocket

UDPpacket

inputstream

UDPpacket

UDPsocket

Output: sendspacket (recall

that TCP sentbyte stream)

Input: receivespacket (recall

thatTCP receivedbyte stream)

Client

process

client UDPsocket


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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();

Create

input streamCreate

client socket

Translatehostname to IP

address using DNS

Example: Java client (UDP), cont.

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Example Java client (UDP), cont.

DatagramPacket sendPacket =

new DatagramPacket(sendData, sendData.length, IPAddress, 9876);

clientSocket.send(sendPacket);

DatagramPacket receivePacket =

new DatagramPacket(receiveData, receiveData.length);

clientSocket.receive(receivePacket);

String modifiedSentence =

new String(receivePacket.getData());

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

}

}

Create datagramwith data-to-send,

length, IP addr, port

Send datagramto server

Read datagramfrom server

Example: Java server (UDP)

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Example Java server (UDP)


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

Receivedatagram

Example: Java server (UDP), cont

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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 out

datagramto socket

End of while loop,loop back and wait foranother datagram

Create datagramto send to client

Chapter 2: Summary

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application architectures client-server

P2P

hybrid

application servicerequirements: reliability, bandwidth,

delay

Internet transportservice model connection-oriented,

reliable: TCP

unreliable, datagrams: UDP

our study of network apps now complete!

specific protocols: HTTP

FTP

SMTP, POP, IMAP

DNS P2P: BitTorrent, Skype

socket programming

Chapter 2: Summary

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Chapter 2 Summary

typical request/replymessage exchange: client requests info or

service server responds with

data, status code

message formats:

headers: fields givinginfo about data

data: info beingmm ni t d

Most importantly: learned aboutprotocolsImportant themes:

control vs. data msgs

in-band, out-of-band centralized vs.

decentralized

stateless vs. stateful

reliable vs. unreliablemsg transfer

complexity at network

Documents

Chapter2 4th Ed Oct 1 2007