Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

Peer to Peer NETWORKING

David Tacconi

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

OutlineWhat is P2P:

Definitions and basicHistory

Principal P2P applicationsP2P goes mobile

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

What is P2P The term P2P refers to "peer-to-peer"

networkingA peer-to-peer network allows computer

hardware and software to function without the need for special server device

"P2P" as "people-to-people“:help individuals on the Internet meet each other

and share common interestssocial networking technology is an example of

this concept

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

Definition of P2P A P2P network needs these 7 characteristics

(according to Dave Winer UserLand):1. The user interface runs outsides of a Web browser

2. computers in the system can act as both clients and servers

3. the software is easy to use and well-integrated

4. the application includes tools to support users wanting to create content or add functionality

5. the application makes connections with other users

6. the application does something new or exciting

7. the software supports "cross-network" protocols like SOAP or XML-RPC

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

Definition of P2P (Ross&Rubenstein)Autonomy from central serversExploits resources at the edges of the

Internetstorage and contentCPU cycleshuman presence

Resources at edge have intermittent connectivity, being added & removed

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

P2P basic: Overlay Networks

Virtual edge TCP connection or simply a pointer to

an IP address Overlay maintenance

Periodically ping to make sure neighbor is alive

verify liveness while messaging

If neighbor goes down, may want to establish new edge

New node needs to bootstrap

Overlay Edge

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

P2P basic: typical problems Scalability

Search queries Download and uploads Graph theory can help…

Free riding Nodes consumes resources without sharing

Several works analyzing factors affecting the degree of free-riding finding incentive mechanisms to encourage user cooperation Defining challenges in the design of incentive mechanisms for P2P

systems (economic models) Security issues

Distributed systems => easy to be attacked Copyright issues

Mp3 and videos shared without control

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

P2P Applications

P2P file sharingNapster, Gnutella, Kazaa, BitTorrent,…

Instant messaging / voice over IPSkype and many others…

P2P distributed computingSeti Project

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

File Sharing Roberto runs P2P client application on his

notebook computer Intermittently connects to Internetgets new IP address for each connectionRegisters her content in P2P system

Asks for “Non amarmi” di A. Baldi Application displays other peers that have copy of

“Non amarmi”Roberto chooses one of the peers, FranzFile is copied from Franz’s PC to Roberto’s laptop: P2P

While Roberto downloads, other users uploading from Roberto

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

History of file sharing File sharing and legacy is the main problem Napster:

First P2P software started in 90s for mp3 file sharing Big internet breakthrough in 2000 Sold for 8 milions $ in June 2002 Legal controversy; closed in September 2002

When Napster was shut down: Napster users migrated to the Kazaa and Kazaa Lite FastTrack grew to become even larger than the original Napster

network. Kazaa suffered from legal troubles Other systems, like eDonkey/ Overnet, have continued the

legacy of free P2P file sharing software.

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

Napster: how did it workApplication-level, client-server protocol over

point-to-point TCPCentralized directory serverSteps:

connect to Napster serverupload your list of files to server.give server keywords to search the full list with.select “best” of correct answers. (pings)

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

Napster: the cycle1 2

3 4

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

Gnutelladecentralized method of searching for files

central directory server no longer the bottleneck

more difficult to “pull plug”

each application instance serves to:store selected filesroute queries from and to its neighboring peersrespond to queries if file stored locallyserve files

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

Gnutella: how it works Searching by (controlled) flooding:

if you don’t have the file you want, query 7 of your neighbors.

if they don’t have it, they contact 7 of their neighbors, for a maximum hop count of 10.

reverse path forwarding for responses (not files)

Node join:New node uses bootstrap node to get IP addresses of

existing Gnutella nodesNew node establishes neighboring relations by sending

join messages

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

KaZaaMore than 3 million up peers sharing over 3,000 terabytes of content

more popular than Napster ever was more than 50% of Internet traffic MP3s & entire albums, videos, games

optional parallel downloading of filesautomatically switches to new download server when current server becomes unavailableprovides estimated download timesUser can configure max number of simultaneous uploads and max number of simultaneous downloadsFrom user’s perspective, service resembles Google, but provides links to MP3s and videos rather than Web pages

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

KaZaa architecture Each peer is either a supernode or is assigned to

a supernode Nodes that have more connection bandwidth and

are more available are designated as supernodes Each supernode has TCP connections with 30-50

supernodes Each supernode acts as a mini-Napster hub,

tracking the content and IP addresses of its descendants

A KaZaA SN track only the content of its children

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

KaZaa overlay maintanceList of potential supernodes included within

software downloadNew peer goes through list until it finds

operational supernodeConnects, obtains more up-to-date list, with

200 entriesNodes in list are “close” to ONNode then pings 5 nodes on list and connects

with the oneIf supernode goes down, node obtains

updated list and chooses new supernode

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

KaZaa QueriesNode first sends query to supernodeSupernode responds with matches

If x matches found, done.

Otherwise, supernode forwards query to subset of supernodesIf total of x matches found, done.

Otherwise, query further forwardedProbably by original supernode rather than

recursively

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

KaZaa finallyKaZaa-lite is a freeware version of KaZaaLesson learned from KaZaa

KaZaA provides powerful file search and transfer service without server infrastructure

Exploit heterogeneityProvide automatic recovery for interrupted

downloadsPowerful, intuitive user interface

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

BitTorrent BitTorrent terms:

Leeches - People who download files but do not share files on their own computer with others

Seed or seeder - A computer with a complete copy of a BitTorrent file (At least one seed computer is necessary for a BitTorrent download to operate.)

Swarm - A group of computers simultaneously sending (uploading) or receiving (downloading) the same file

.torrent - A pointer file that directs your computer to the file you want to download

Tracker - A server that manages the BitTorrent file-transfer process A node gets file.torrent info and then file.torrent info:

length name hash url of tracker

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

BitTorrent: file broken in pieces File is broken into pieces

Typically piece is 256 KBytesUpload pieces while downloading pieces

Piece selectionSelect rarest pieceExcept at beginning, select random pieces

Tit-for-tatBit-torrent uploads to at most four peersAmong the uploaders, upload to the four that are

downloading to you at the highest ratesA little randomness too, for probing

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

BitTorrent cycle Click on a link for the file you want communicates with a tracker to find

other computers running BitTorrent that have the complete file (seed computers) and those with a portion of the file

The tracker identifies the swarm, which is the connected computers that have all of or a portion of the file and are in the process of sending or receiving it

Trading process: your computer receives multiple

pieces of the file simultaneously. Keep BitTorrent client software

alive: your future download rates improve

because you are ranked higher in the "tit-for-tat" system

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

Distributed P2P: DHTDHT service and issuesCARPConsistent HashingChordCANPastry/TapestryHierarchical lookup servicesTopology-centric lookup service

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

DHT in a nutshell

CAN

Pastry

Chord

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

P2P 4 VoIP: SkypeHierarchical architecture

Central login serversupernodes

Any node with a public IP address having sufficient CPU, memory and network bandwidth is a candidate to become a super nodeAn ordinary host must connect to a super node and must register itself with the Skype login server

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

Skype: some details… Skype search

Skype uses Global Index technology to search for a user Skype claims that search is distributed and is guaranteed to find a

user if it exists and has logged in during last 72 hours Search results are observed to be cached at intermediate nodes

Call establishment and teardown Call signaling is always carried over TCP For user not present in buddy list, call placement is equal to user

search plus call signaling If caller is behind port-restricted NAT and callee is on public IP,

signaling and media flow through an onlineSkype node which forwards signaling to callee over TCP and routes media over UDP

If both users are behind port-restricted NAT and UDP restricted firewall, both caller and callee SCs exchange signaling over TCP with another online Skype node, which also forwards media between caller and callee

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

Distributed computing seti@home Search for ET intelligence Central site collects radio telescope data Data is divided into work chunks of 300 Kbytes User obtains client, which runs in background Peer sets up TCP connection to central computer,

downloads chunk Peer does FFT on chunk, uploads results, gets

new chunks Not peer to peer, but exploits resources at

network edge

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

P2P goes mobile 1/3 JXTA for ad-hoc networks

Introduced by Sun Microsystems is a set of open, generalized peer-to-peer protocols that allows any connected device (cell phone to PDA, PC to server) on the network to communicate and collaborate

Open source projectSeveral attempts to bring it on ad hoc networksDifficult to port it on real mobile devices

Peer2MEA j2me framework for developing mobile P2P

applicationsSimilar to U-Hopper in the basic ideas Include real file sharing

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

P2P goes mobile 2/3BEDD

Started in 2001 as commercial BT P2P

Now BT + WiFi + GPRSMore than 250000 BEDD

buddiesDownloads, chatting,

socializingU-Hopper

Well known middleware for P2P applications…

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

P2P goes mobile 3/3Difficult to export the P2P classic model to

mobile decentralized environmentDifferent operating systemsDifferent user interfaces

Incentives for file exchange needs to be found

Economic models for classical P2P can work?

Few successfull examples can be found

Create-Net, 18th March 2008Pervasive Computing and Communication Environment Group

ConclusionsHow can we use the lesson learned from

classical P2P systems in extremely mobile systems?From a technological prospective…From an economical point of view…

How can we contribute in the P2P research community?