P2P Search COP5711

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P2P SearchCOP5711
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P2P Search Techniques

• Centralized P2P systems
• e.g. Napster, SETI_at_home
• Decentralized unstructured P2P systems
• e.g. Gnutella
• Hybrid - partially decentralized
• e.g., Freenet
• Structured P2P systems
• DHT
• CAN

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P2P Network

• P2P network is an overlay network built on top of
  a real physical network (e.g., Internet)
• In a P2P network, peers are network nodes
  connected by virtual or logical links
• A logical link is a path through many physical
  links in the underlying network

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Napster Publish a File

• Users upload their IP address and music titles
  they wish to share

(xyz.mp3, 192.1.2.3)
Napster server (Central Catalog)
192.1.2.3
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Napster Query for a File

• Users search for peers to download desired files

xyz.mp3 ?
192.1.2.3
192.1.2.3
Central Napster server
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Napster Transfer Requested File

• File transfer is P2P, using a proprietary protocol

xyz.mp3 ?
192.1.2.3
Central Napster server
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Disadvantage of Centralized Directory

• Performance bottleneck
• Single point of failure
• Can we do it without a directory ?

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Decentralized P2P - Gnutella

• No catalog
• Pings network to locate Gnutella peers
• File requests are broadcast to peers
• Flooding or breadth-first research
• When provider is located, the file is transferred
  via HTTP

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Gnutella Join the Network
Special peer maintained by Gnutella
Peers are Internet edges
Who are my neighbors ?
Pings network to locate peers
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Gnutella Broadcast Request to Peers
xyz.mp3 ?
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Gnutella Flood the Request (Breadth-first
research)
I have it.
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Gnutella Reply with the File(via HTTP)
I have it.
xyz.mp3
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Gnutella - Disadvantages

• Network flooding - unnecessary network traffic
• Using TTL - some files might not be found
• Alternatively,
• using ultranodes (or supernodes)
• using depth-first search, i.e., Freenet

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Morpheus, KazaaFlooding only the Supernodes
Supernode Layer
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Using Ultranodes

• Queries flood only the network of ultranodes
• Other peer nodes shielded from query traffic
• Combine the benefits of centralized and
  decentralized search
• Take advantage of the heterogeneity in peer
  capabilities

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Freenet - Depth-First Search
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Freenet File not Found
I have file X

• The requested file not found due to a poor
  routing decision made at peer D
• In this case, query backs out of the dead-end,
  and tries another peer in depth-first manner

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Using Distributed Directory

• Data objects are everywhere
• Distribute subsets of the data directory among
  peers
• If we can find the relevant sub-directory, we can
  locate the data object

Directory

Data Objects
Sub-directory
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How to Bound Search Space ?Basic Idea - Hashing
P2P Network
Publish (H(y))
Join (H(x))
Object y
Peer x
H(y)
H(x)
Peer nodes also have hash keys in the same hash
space
Objects have hash keys
y
x
Hash key
Place location information about an object at the
peer with closest hash keys (i.e., a distributed
directory)
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Viewed as a Distributed Hash Table
0
2128-1
Hash table
Peer nodes

• Each peer node is responsible for a range of the
  hash table, according to the peer hash key
• Location information about Objects are placed in
  the peer with the closest key (information
  redundancy)

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How to Find an Object ?

• Looks for a peer /w the corresponding peer hash
  key
• A peer knows its logical neighbors
• Find peer X based on multihop routing
• X knows who has the object

0
2128-1
Hash table
Peer node
X
Peer Y has the file
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Dynamic Hash Table (DHT) in action
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DHT in action
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DHT in action put()
Want to share a file
insert(K1,V1)
Operation Route message, I have the file, to
node holding key K1
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DHT in action put()
(K1,V1)
Operation take key as input route messages to
node holding key
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DHT in action get()
retrieve (K1)
Operation Retrieve message V1 at node holding
key K1
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DHT in action
Retrieve file according to V1
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Still Flooding

• Still flood the network although intermediate
  nodes do not need to search
• Can we avoid flooding ?

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CAN Content Addressable Network

• Each peer is responsible for one zone, i.e.,
  stores all (key, value) pairs of the zone
• Each peer knows the neighbors of its zone
• Random assignment of peers to zones at startup
  split zone if not empty
• Dimensional-ordered multihop routing

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CAN Object Publishing
I
node Ipublish(K,V)
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CAN Object Publishing
x a
I
node Ipublish(K,V)
(1) a hx(K)
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CAN Object Publishing
x a
I
node Ipublish(K,V)
(1) a hx(K) b hy(K)
y b
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CAN Object Publishing
I
node Ipublish(K,V)
J
(1) a hx(K) b hy(K)
(2) route (K,V) -gt J
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CAN Object Publishing
I
node Ipublish(K,V)
J
(1) a hx(K) b hy(K)
(K,V)
(2) route (K,V) -gt J (3) J stores (K,V)
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CAN Object Retrieval
node Iretrieve(K)
(1) a hx(K) b hy(K)
J
(K,V)
(2) route retrieve(K) to J that is
in charge of (a,b)
I
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Maintenance

• Inform neighbors that you are alive at discrete
  time interval t
• If your neighbor does not send alive message in
  time t, takeover its zone

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P2P Benefits

• Efficient use of resources
• Use unused bandwidth, storage, and processing
  power at the edge of the network
• Scalability
• Consumers of resources also donate resources
• Reliability
• Replicas, geographic distribution ? No single
  point of failure
• Ease of administration
• Self organized nodes
• Built-in reliability and load balancing

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Some Prototypes at UCF

• iSEE (Internet-scale Sensor Exploration
  Environement)
• Publishing real-time sensor data
• Browsing and querying real-time sensor data
• P2P Video Streaming for VoD and Live Broadcast
  Applications