Advance Computer Networking bachelor of science in computer engineering

Networking

1. ADVANCED COMPUTER NETWORKS
Peer-Peer (P2P) Networks
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2. What is P2P?

“the sharing of computer resources and services by
direct exchange of information”
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3. What is P2P?

Various definitions seem to agree on
sharing of resources
direct communication between equals (peers)
no centralized control
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4. What is a peer?

“…an entity with capabilities similar to other
entities in the system.”
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5. Client/Server Architecture

Well known, powerful,
reliable server is a data
source

Clients request data
from server

Very successful model
WWW (HTTP), FTP, Web
services, etc.
Server
Client
Client Client
Client
Internet
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6. Client/Server Limitations

Scalability is hard to achieve

Presents a single point of failure

Requires administration

P2P systems try to address these limitations
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7. P2P Architecture

All nodes are both clients
and servers
Provide and consume data
Any node can initiate a
connection

No centralized data source
“The ultimate form of
democracy on the Internet”
“The ultimate threat to copy-
right protection on the
Internet”
Node
Node
Node Node
Node
Internet
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8. P2P Network Characteristics

Clients are also servers and routers
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 well-
known servers)
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9. P2P Applications
File sharing (BitTorrent, Napster, Kazaa)
Multiplayer games (Counter Strike, Unreal Tournament, DOOM)
Collaborative applications (ICQ, shared whiteboard)
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10. Popular file sharing P2P Systems
BitTorrent
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
Issues of copyright violation
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11. Approaches

Structured

Unstructured
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12. Structured
original “Napster” design
- when peer connects, it informs
central server:
IP address
content
2) User B searches for file
3) User B requests file from
User A
centralized
directory server
peers
User B
User A
1
1
1
1
2
3
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13. Centralized model
A B
DC
file transfer is
decentralized, but
locating content is
highly centralized
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14. Napster: how does it work
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.
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15. Napster
napster.com
users
File list is
uploaded
1.
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16. Napster
napster.com
user
Request
and
results
User
requests
search at
server.
2.
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17. Napster
napster.com
user
pings
User pings
hosts that
apparently
have data.
3.
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18. Napster
napster.com
user
Retrieves
file
User retrieves
file
4.
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19. Unstructured
Fully distributed
no central server
Used by KaZaa
Each peer indexes the files it makes available for sharing
(and no other files)
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20. KaZaa: Ping flooding
Query
QueryHit
Query
Q
u
e
r
y
QueryHit
Q
u e ry
Q
u
e
r
y
Q
ueryH
it
File transfer:
HTTP
 Ping message
sent over existing TCP
connections
 peers forward
ping message
Pong
sent over
reverse
path
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21. KaZaa

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
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22. P2P : Skype
Skype clients (SC)
Supernode
(SN)
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23. Issues with P2P
Free Riding
Downloading but not sharing any data
On BitTorrent
15% of users contribute 94% of content
63% of users never responded to a query
No ranking: what is a trusted source?
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