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Chapter 2Application LayerComputer Networking:A Top Down Approach,4th edition.Jim Kurose, Keith RossAddison-Wesley, July2007.

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Last Lecture

DNSSMTP

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Todays Lecture

P2P

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Electronic Mail

Asynchronous CommunicationMedium

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, Mozilla

Thunderbird Sends message to the mail

server

user mailboxoutgoing

message queue

mail

server

user

agent

user

agent

user

agentmailserver

user

agentuser

agent

mail

server

user

agent

SMTP

SMTPSMTP

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Electronic Mail

Mail Servers

Each recipient has a mailboxlocated in one of the mail servers

Mailbox manages and maintains themessages that have been sent tohim.

Messagequeue of outgoing (to besent) mail messages

Simple Mail Transfer Protocol(SMTP) between mail servers tosend email messages

Client and Server Both client and sever sides of

SMTP run on every mail server.

Runs on TCP at port 25.

RFC 5321

mailserver

useragent

useragent

user

agentmail

serveruseragent

useragent

mailserver

useragent

SMTP

SMTPSMTP

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Sample SMTP InteractionClient SMTP establishes a TCP connection with ServerSMTPApplication Layer handshaking to introduce themselves

Sample SMTP InteractionS: 220 hamburger.edu (Service Ready)C: HELO crepes.frS: 250 Hello crepes.fr, pleased to meet you

C: MAIL FROM: S: 250 alice@crepes.fr... Sender okC: RCPT TO: S: 250 bob@hamburger.edu ... Recipient okC: DATA (All lines after DATA command are treated as the

mail message)S: 354 Enter mail, end with "." on a line by itselfC: Do you like ketchup?C: How about pickles?C: .S: 250 Message accepted for deliveryC: QUITS: 221 hamburger.edu closing connection

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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]

IMAP: Internet Mail Access Protocol [RFC 1730]

HTTP: Gmail, Hotmail, Yahoo! Mail, etc. User agent is ordinary web browser

HTTP to send email to web server

HTTP to access mail box

useragent

senders mailserver

useragent

SMTP SMTP accessprotocol

receivers mailserver

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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 serversamazon.com

DNS servers

pbs.org

DNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com:

Client first queries a root server

The root server returns the IP addresses for TLD servers forthe top level domain com

The client then contacts one of the TLD servers which returnsthe IP address of an authoritative server for amazon.com

The authoritative server then returns the IP address for the

host name www.amazon.com

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requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

2 345

6

authoritative DNS serverdns.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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requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

2

45

6

authoritative DNS serverdns.cs.umass.edu

7

8

TLD DNS server

3Recursive Query: Puts burden of name

resolution on otherserver

DNS Caching Extensively used Cache entries timeout

(disappear) after sometime

TLD servers typicallycached in local nameservers Thus root name

servers not oftenvisited

DNS nameresolution example

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Pure P2P Architecture

noalways-on server

Arbitrary end systemsdirectly communicate

Peers are intermittentlyconnected and change IPaddresses

P2P is a class of applicationsthat takes advantage of

resources e.g. storage, content,human presence, available at theedges of the Internet

peer-peer

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Examples of P2P Technologies

File sharing Programs:

Gnutella ,Napster ,LimeWire, Kazaaetc.

Instant messaging:

ICQ, Jabber etc

ConferencingNetmeeting, Voice over IP (VoIP) etc

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P2P File Sharing

Alice runs P2P clientapplication on her

notebook computer

Intermittently

connects to Internet;

gets new IP address

for each connection

Registers her content

in P2P system

Asks for some music fileApplication displays other peersthat have copy of that file. Alice chooses one of the peers,Bob.File is copied from Bobs PC toAlices notebook: P2P While Alice downloads, otherusers uploading from Alice.

How a peer determines which peers have the desired contentThree approaches for organizing and searching for contentsDifferent approaches are used by different P2P file sharingsystems.

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P2P: Centralized Indexoriginal Napster design

When peer connects, it informscentral server:

IP address

Content that it is makingavailable

Central Server collects info frompeers that becomes active

Creates a centralized dynamicdatabase that maps each objectname to a set of IP addresses

Hybrid of P2P and client-server

File distribution is P2P

Search is client-server

centralizeddirectory server

peers

Alice

Bob

1

1

1

12

3

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www.napster.com

Main ServerFile List:UserC song.mp3

UserD another.mp3

..

User A

2. User A

searches forsong.mp3

User C(Song.mp3)

1. Construct Database

Users connect to Napster Server

Server builds up a list of available

songs and locations

User D(Another.mp3)

User B

3. Server searchesdatabase. Finds song

on User Cs machine

4. Server informsUser A of the location

of song.mp3

5. User A connects to

User C and downloadssong.mp3

File Sharing with Napster

http://www.napster.com/http://www.napster.com/

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P2P: Problems with Centralized Directory

Single Point of Failure If directory server crashes, the

entire P2P application crashes.

Performance Bottleneck Thousands of connected users

Server must maintain a hugedatabase and must respond tothousands of queries per second

Copyright Infringement Easy to obtain copyrighted

material for free Heavy metal rock group Metallica

sued Napster for copyrightinfringement in April 2000.

file transfer isdecentralized, but

locating content ishighly centralized

D l d D

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Decentralized Directory

Query Flooding

Directory/Index isfully distributed overthe community ofpeers No central server

Used by Gnutella Each peer indexes the

files it makes availablefor sharing (and no

other files)

Overlay Network:

Peers form an abstract,logical network called anoverlay network

Edge between peer X and Yif theres a TCP connection

All active peers and edgesform overlay network

edge: virtual (notphysical)link

Overlay network may have

thousands of participatingpeers but

Given peer typicallyconnected with < 10overlay neighbors

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Query Flooding

Query

Query Hit

Query

QueryHit

File transfer Query messagesent over existingTCP connections Peers forwardQuery message Query-Hit

message sent overreverse path

Non-scalable

Significant amount oftraffic among the peersin the underlyingnetwork connecting thepeers

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Limited Query Flooding

Apeer-countfield in the message is set to specificlimit (say, 7).

Each time the query message reaches a new peer,the peer decrements thepeer-countfield beforeforwarding the query to its overlay neighbor.

Stops forwarding the query whenpeer-countfieldset to zero

Flooding is localized to a region of the overlaynetwork

Reduces the query traffic Peer seeking the content may not be able to locate

that content.

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Gnutella: Peer Joining

1. Joining peer X must find another peer in the overlaynetwork Maintain a list of peers (IP addresses ) that are often up

in the overlay network Peer X can also contact a tracker site that maintains such

a list

2. Peer X sequentially attempts TCP connections withcandidate peers until connection setup with some peer Y.3. Peer X sends Pingmessage to Peer Y that forwards this

message to his overlay neighbors (who then forward totheir neighbors.)

4. Peer Z on receiving Pingmessage respond to Peer X withPongmessage (IP address)

5. Peer X receives many Pong messages, and can then setupadditional TCP connections Creating multiple edges from itself into the overlay

network

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Hierarchical Overlay Between centralized index and query

flooding approaches

No dedicated server for indexing files

Super Peers: Peers with high bandwidthconnections into internet and highavailability.

Ordinary peer is assigned as a childto super peer.

A new peer Establishes a TCP connection with one of

super peers

Informs all the files it is sharing Super peer maintains an index of IP

addresses of children holding differentfiles.

Significant more peers can be checked

for a match without creating anexcessive quantity of query traffic

ordinary peer

group-leader peer

neighoring relationships

in overlay network

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Home Assignment

Bit Torrent ProtocolSee your text book and paper on LMS

Anti-Snubbing, Pipelining, Endgamemode, Peer Churn?

Home Assignment