Transcript P2P Networks new - Information Technology Pakistan

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ADVANCED COMPUTER NETWORKS
Peer-Peer (P2P) Networks
What is P2P?
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
“the sharing of computer resources and services by
direct exchange of information”
What is P2P?
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
Various definitions seem to agree on
 sharing
of resources
 direct communication between equals (peers)
 no centralized control
What is a peer?
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
“…an entity with capabilities similar to other
entities in the system.”
Client/Server Architecture
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Well known, powerful,
reliable server is a data
source
Clients request data
from server
Server
Very successful model

WWW (HTTP), FTP, Web
services, etc.
Client
Client
Internet
Client
Client
Client/Server Limitations
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
Scalability is hard to achieve
Presents a single point of failure
Requires administration
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P2P systems try to address these limitations
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P2P Architecture
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All nodes are both clients
and servers
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
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Provide and consume data
Any node can initiate a
connection
No centralized data source
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
“The ultimate form of
democracy on the Internet”
“The ultimate threat to copyright protection on the
Internet”
Node
Node
Node
Internet
Node
Node
P2P Network Characteristics
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Clients are also servers and routers
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Nodes contribute content, storage, memory, CPU
Nodes are autonomous (no administrative authority)
Network is dynamic: nodes enter and leave the network
“frequently”
Nodes collaborate directly with each other (not through wellknown servers)
P2P Applications
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File sharing (BitTorrent, Napster, Kazaa)
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Multiplayer games (Counter Strike, Unreal Tournament, DOOM)
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Collaborative applications (ICQ, shared whiteboard)
Popular file sharing P2P Systems
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BitTorrent
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Large scale sharing of files.
 User
A makes files (music, video, etc.) on their computer
available to others
 User B connects to the network, searches for files and
downloads files directly from user A
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Issues of copyright violation
Approaches
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Structured
Unstructured
Structured
original “Napster” design
- when peer connects, it informs
central server:
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
IP address
content
2) User B searches for file
3) User B requests file from
User A
User A
centralized
directory server
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peers
1
3
1
2
1
User B
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Centralized model
A
B
file transfer is
decentralized, but
locating content is
highly centralized
C
D
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Napster: how does it work
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Four 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.
Napster
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1. File list is
uploaded
napster.com
users
Napster
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2. User
requests
search at
server.
napster.com
Request
and
results
user
Napster
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3.
napster.com
User pings
hosts that
apparently
have data.
pings
user
Napster
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napster.com
4. User retrieves
file
Retrieves
file
user
Unstructured
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Fully distributed
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no central server
Used by KaZaa
Each peer indexes the files it makes available for sharing
(and no other files)
KaZaa: Ping flooding
File transfer:
HTTP
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 Ping message
sent over existing TCP
connections
Query
QueryHit
 peers forward
ping message
Pong
sent over
reverse
path
Query
QueryHit
KaZaa
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file divided into 256KB chunks.
while downloading, peer uploads chunks to other peers.
peers may come and go
once peer has entire file, it may leave or remain
P2P : Skype
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Skype clients (SC)
Supernode
(SN)
Issues with P2P
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Free Riding
 Downloading
 On
but not sharing any data
BitTorrent
 15%
of users contribute 94% of content
 63% of users never responded to a query
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No ranking: what is a trusted source?