P2P Seminar

Peer to Peer NETWORKING David Tacconi

Outline
What is P2P:
Definitions and basic
History
Principal P2P applications
P2P goes mobile

What is P2P
The term P2P refers to "peer-to-peer" networking
A 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 interests
social networking technology is an example of this concept

Definition of P2P
A P2P network needs these 7 characteristics (according to Dave Winer UserLand):
The user interface runs outsides of a Web browser
computers in the system can act as both clients and servers
the software is easy to use and well-integrated
the application includes tools to support users wanting to create content or add functionality
the application makes connections with other users
the application does something new or exciting
the software supports "cross-network" protocols like SOAP or XML-RPC

Definition of P2P (Ross&Rubenstein)
Autonomy from central servers
Exploits resources at the edges of the Internet
storage and content
CPU cycles
human presence
Resources at edge have intermittent connectivity, being added & removed

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

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

P2P Applications
P2P file sharing
Napster, Gnutella, Kazaa, BitTorrent,…
Instant messaging / voice over IP
Skype and many others…
P2P distributed computing
Seti Project

File Sharing
Roberto runs P2P client application on his notebook computer
Intermittently connects to Internet
gets new IP address for each connection
Registers 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, Franz
File is copied from Franz’s PC to Roberto’s laptop: P2P
While Roberto downloads, other users uploading from Roberto

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.

Napster: how did it work
Application-level, client-server protocol over point-to-point TCP
Centralized directory server
Steps:
connect to Napster server
upload your list of files to server.
give server keywords to search the full list with.
select “best” of correct answers. (pings)

Napster: the cycle 1 2 3 4

Gnutella
decentralized method of searching for files
central directory server no longer the bottleneck
more difficult to “pull plug”
each application instance serves to:
store selected files
route queries from and to its neighboring peers
respond to queries if file stored locally
serve files

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 nodes
New node establishes neighboring relations by sending join messages

KaZaa
More 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 files
automatically switches to new download server when current server becomes unavailable
provides estimated download times
User can configure max number of simultaneous uploads and max number of simultaneous downloads
From user’s perspective, service resembles Google, but provides links to MP3s and videos rather than Web pages

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

KaZaa overlay maintance
List of potential supernodes included within software download
New peer goes through list until it finds operational supernode
Connects, obtains more up-to-date list, with 200 entries
Nodes in list are “close” to ON
Node then pings 5 nodes on list and connects with the one
If supernode goes down, node obtains updated list and chooses new supernode

KaZaa Queries
Node first sends query to supernode
Supernode responds with matches
If x matches found, done.
Otherwise, supernode forwards query to subset of supernodes
If total of x matches found, done.
Otherwise, query further forwarded
Probably by original supernode rather than recursively

KaZaa finally
KaZaa-lite is a freeware version of KaZaa
Lesson learned from KaZaa
KaZaA provides powerful file search and transfer service without server infrastructure
Exploit heterogeneity
Provide automatic recovery for interrupted downloads
Powerful, intuitive user interface

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

BitTorrent: file broken in pieces
File is broken into pieces
Typically piece is 256 KBytes
Upload pieces while downloading pieces
Piece selection
Select rarest piece
Except at beginning, select random pieces
Tit-for-tat
Bit-torrent uploads to at most four peers
Among the uploaders, upload to the four that are downloading to you at the highest rates
A little randomness too, for probing

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

Distributed P2P: DHT
DHT service and issues
CARP
Consistent Hashing
Chord
CAN
Pastry/Tapestry
Hierarchical lookup services
Topology-centric lookup service

DHT in a nutshell CAN Pastry Chord

P2P 4 VoIP: Skype
Hierarchical architecture
Central login server
supernodes
Any node with a public IP address having sufficient CPU, memory and network bandwidth is a candidate to become a super node
An ordinary host must connect to a super node and must register itself with the Skype login server

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

Distributed computing
[email_address]
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

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 project
Several attempts to bring it on ad hoc networks
Difficult to port it on real mobile devices
Peer2ME
A j2me framework for developing mobile P2P applications
Similar to U-Hopper in the basic ideas
Include real file sharing

P2P goes mobile 2/3
BEDD
Started in 2001 as commercial BT P2P
Now BT + WiFi + GPRS
More than 250000 BEDD buddies
Downloads, chatting, socializing
U-Hopper
Well known middleware for P2P applications…

P2P goes mobile 3/3
Difficult to export the P2P classic model to mobile decentralized environment
Different operating systems
Different user interfaces
Incentives for file exchange needs to be found
Economic models for classical P2P can work?
Few successfull examples can be found

Conclusions
How 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?

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