Peerto Peer Networks

P2P Networks

Sanjoy Sanyal:www.itforintelligentfolks.blogspot.com

P2P networks

A set of technologies that enable the direct exchange of services of data or services between computers

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Client Server

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P2P Network


Network Effects: Promises & Challenges

Can Have the following advantage… …however

Scalability as there is no central resource to exhaust

Has to overcome to challenge of self organization from a collection of unreliable peers with unreliable connections

Aggregating resources can lead to excellent performance

Has to overcome the choking of the network of overhead or organizing messages

Fault resilience as there is no single point of failure

Has to overcome reliability challenges on account of network congestion, isolated networks, unreachable nodes


Types of P2P Networks

P2P Systems

File Sharing CollaborationDistributed Computing

Napster Limewire (www.limewire.com)Aimster/Madster Gnutella (gnutella.com)Morpheus (morpheus.com)Chord

Instant MessagingGroove Multiplayer Games: Magi

SETI@home (http://setiathome.berkeley.edu/)Grid.org

File Sharing is the one we will delve into in this sessionSanjoy Sanyal:www.itforintelligentfolks.blogspot.com

Locating Content in P2P networksCentralized Directory Approach

Flooded Request Approach Document routing Approach

Peers connect to a central directory where they publish information about the content that they have to share

When the directory receives a request it replies with a peer in the directory that matches the request

Criteria such as proximity, bandwidth, capacity, congestion, health, frequency can guide the decision

Peers broadcast a request to its directly connected peers, each of whom broadcast to their directly connected peers and so on thru the network.

This continues until the request is answered or some broadcast limit is reached.

Each peer has helpful but only partially complete referral information. Each referral moves the requester closer to a peer that can satisfy the query.

The network can scale with a number of central servers

Generates a lot of ineffective network traffic which prevents scaling

Can scale effectively as systems can complete a search within a bounded number of steps


Napster – A quick history

• Jan 1999: Set up in Jan 1999 by Shawn Fanning (then 18)

• December 1999: sued for copyright infringement – file screening system wpreventing downloads of specified files put in place

• July 2001 : shut down file sharing service post court orders

• May 2002: purchased by German media conglomerate– Invested USD 85 million

• October 2003: Napster 2.0 a client server system goes live– Division of Roxio


Case Studies

• Napster

• Gnutella & KaZaA

• BitTorrent


Napster Protocol - Introduction

• Was not documented or published – reverse engineered by OpenNap (opennap.sourceforge.net)

• Uses the centralized directory model to locate content

• Communicates using TCP

• Does not use DNS to name peers: – uses nicknames <nick> (another client) and <mynick> (this client)


Napster Protocol Session

Napster Server Client A Client B

Search Request

Search Response

Download Request

Download Ack

See next two slides for message structures

Establish TCP/IP connection

“1”

“GET”

Peer response

Song data


Napster message structures: server - client

Client announcing to server the files it is willing to share

Code 100 – for this type of message <filename> <md5><size><bitrate><frequency><time>

The MD 5 algorithm identifies the song and ensures that two files have identical content

Client search request

Code 200 – for this type of message <filename> <artist name> <song> <max results><line speed><bitrate><frequency>

Server Search Response

Code 201 – for this message<filename><md5><size><length><nick> <ip><link type>

Download request

Code 203<nick> <file name>

Download ack

Code 204<nick> <ip><port><filename><md5><linespeed>


Napster message structures: peer-peer

“1” Single ASCII characters

“GET” Not HTTP GET – this is the Napster application protocol

Peer response

<mynick> <file name><offset> - allows transfer to be resumed at any place in file


Gnutella

• Jan 1998: Justin Frankel developed Winamp, an audio player– Then he founded Nullsoft

• May 1999: Winamp brand & services acquired by AOL

• Early 2000: Gnutella was developed in 14 days

• March 2000: a protoype was published under a GNU General Public License

• In hours (before AOL could react) the software had been downloaded several times


Gnutella Protocol - Introduction

• Unlike Napster has no centralized service

• Uses the flooded request approach

• Software running in each Gnutella peer is called a servant

• Peers use TCP/IP to communicate with each other

• Servant software was developed by several companies: BearShare, LimeWire, ToadNode


Gnutella Protocol – Finding a Servant

• Specialized hosts that cache IP addresses of servants are run by companies who develop Gnutella software

• Servant wishing to join the network contacts host cache servers and receive a list of prospective addresses


Gnutella message structures: descriptors

The MD 5 algorithm identifies the song and ensures that two files have identical content

Descriptor ID Payload Descriptor

TTL – Time to Live

Hops Payload Length

Uniquely identifies this descriptor message in the network

Code identifying the type of message

Limits the maximum number of hops for this message

0xOO = Connection accept request 0x01 = pong Connect accept OK0x01 = push Push file thru firewall0x80=query File search request 0x81=queryhit Search response OK

Each servant receiving a message decrements TTL count and increments the

Hop count before the message is forwarded. The maximum number of hops

is 7.


Gnutella Protocol Session

Servant 1 - Joining Servant 2 – On Network

Gnutella Connect

Gnutella OK

Ping

Pong<Ip address>,<port>,<shared data>

Query<filename>

Host Cache Server

Queryhit<filename>

File DownloadHTTP GET


Gnutella Network Traffic

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BD

EC

Each peer broadcasts requests to its connected peers and so on.

The Pong descriptors may only be sent along the same path that carried the incoming Ping descriptor

.mp3

.mp3

Get .mp3


KazaA

• Kazaa and FastTrack were created by Niklas Zennström, Janus Friis, and Priit Kasesalu (all of whom were to later invent Skype and later on still Joost).

• KazaA is owned by Sharman Networks, headquartered in Australia


KaZaA

• Based on Guntella

• Uses SuperNodes powerful processors with high bandwidth connections

• Peers connect to their local SuperNodes to upload information about files that they are sharing and to search

• Hybrid system between Napster and Gnutella with similarities to the DNS system


BitTorrent

• April 2001: Developed by Bram Cohen

• Become very popular

• CBC is first public broadcaster in North America to make a full show available for download by BitTorrent

• However, not free from controversy


BitTorrent - introduction

• Peers run the BitTorrent client which implements the BitTorrent protocol

• To share, the peer creates a metadata file called the torrent

• The torrent file is shared with the BitTorrent tracker, a server which assists


How BitTorrent works

• For distributing a data file– The peer treats the file as a number of identically-sized pieces. – Creates a checksum for each piece (using the SHA1 hashing algorithm) and records

it in the torrent file.– Peers that provide a complete file are called seeders

• For sharing files:– Users download and open a torrent of interest with a BitTorrent client. – The client connects to the tracker(s) specified in the torrent file and receives a list

of peers currently transferring pieces of the file(s) – The client connects to those peers to obtain the various pieces. Such a group of

peers connected to each other to share a torrent is called a swarm. • For efficiency:

– Download speed is controlled by Torrent tracking servers, who monitor all swarm users. I

– Swarm users who share are rewarded by increasing the alotted swarm bandwidth – Those who leech and limit sharing, tracking servers are choked– To help newcomers, where the client reserves a portion of its available bandwidth

for sending pieces to random peers Check sums ensure non corruption


BitTorrent: How it differs from HTTP

BitTorrent HTTP

Makes many small data requests over different TCP sockets

Typically a single HTTP GET request over a single TCP socket.

Downloads in a random or in a "rarest-first" approach

Downloads in a sequential manner.

Downloads can take time to rise to full speed because it may take time for enough peer connections to be established, and it takes time for a node to receive sufficient data to become an effective uploader

Rises to full speed very quickly and maintains this speed throughout.


Summary

• Fascinating History

• Untapped potential

• The story’s not over yet.


Date post:	18-Jan-2015
Category:	Technology
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Peerto Peer Networks

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