1CS 6401 Peer-to-Peer Networks Outline Overview Gnutella Structured Overlays BitTorrent

2CS 6402 Peer-to-Peer Networks Overview A peer-to-peer (P2P) network allows a community of users to pool their resources –Content –Storage –CPU,… A P2P network is both decentralized and self-organizing –Just like the Internet itself! Why do we care about these networks? –It is challenging to achieve decentralization and scalability at the same time.

3CS 6403 Gnutella One of the first decentralized P2P networks for file sharing No central registry of objects. Example topology of a Gnutella P2P network Edges of the graph correspond to the relationship “A and B know each other”

4 Gnutella cont. The simple idea in Gnutella is to distribute the method for finding data –Great idea! –Lots of fun architectural possibilities! Gnutella is a distributed search protocol with a decentralized model –Clients can issue/view query results –Clients can serve/request data –Clients accept queries and respond with matches from their local data store CS 640

5 5 Gnutella Protocol Protocol defines method of client communication –Set of descriptors used for communicating data –Set of rules governing inter-client exchange of descriptors Descriptors –Ping: active discovery of hosts on a network –Pong: response to Ping includes client address and metadata –Query: Ask for an object –Query Response: response to Query includes info necessary to get data A Gnutella client connects to network by establishing a connection with another client on the network –Finding another client is not part of Gnutella spec. Host cache services are the typical way this is done

6CS 6406 Gnutella Protocol New client then creates connection to the Gnutella client and thereby becomes part of the network –Gnutella client can reject the connect request –Successful new client can then send/receive descriptors Pings/pongs are then sent to establish network –No specification as to how much/often to probe –Network data can/is cached Message routing should be well behaved –Ping/Query descriptors should be sent to all directly connected clients –Pong/Query Response descriptors should be sent back along same path –TTL is mechanism to limit distance File downloads via HTTP/1.0 protocol via direct connect

7CS 6407 Gnutella Protocol New client then creates connection to the Gnutella client and thereby becomes part of the network –Gnutella client can reject the connect request –Successful new client can then send/receive descriptors Pings/pongs are then sent to establish network –No specification as to how much/often to probe –Network data can/is cached Message routing should be well behaved –Ping/Query descriptors should be sent to all directly connected clients –Pong/Query Response descriptors should be sent back along same path –TTL is mechanism to limit distance File downloads via HTTP/1.0 protocol via direct connect

8 Gnutella’s Downside Flooding does not scale well! Alternatives: –Forward queries randomly or according to probability of success based on past results –Proactively replicate objects to make them easier to find –OR –Structured Overlays CS 640

9 Structured Overlays Designed to conform to a particular graph structure that allows reliable and efficient object location. However, with the cost of additional complexity in overlay construction and maintenance. 2 questions to answer: –How do we map objects to nodes that should serve them? –How do we find an object? CS 640

10 Mapping Objects to Nodes Hashing is used to map objects to n nodes Regular hashing: Hash(x,n){ Return x % n } Challenge: –What if a node joins the network? –What if a node leaves the network? –How do we choose the proper n? CS 640

11 Consistent Hashing Hash a set of objects x uniformly across a large ID space Each object is maintained on the node whose ID is closest Advantages: –Distributes objects fairly evenly across nodes –Only a small number of objects have to move when a node joins or leaves CS 640

12 Consistent Hashing cont. Example Bucket/Node 14 CS 640

13 How to find an object? Idea: Route the query message to the node that has the object –Each node maintains a routing table and the IP addresses of a small set of numerically larger and smaller node IDs. –Forward the query message to the node that is closer than you to the destination node –This is repeated until you get to destination CS 640

14 How to find an object: Cont. Structured Overlays provide a logarithmic bound on the number of routing hops required to locate an object However, Nodes might be arbitrarily far away from each other in the Internet! Optimizations: –Route to the physically closest node when possible –Replicate data across the nodes CS 640