Peer-to-Peer Computing Ding Choon Hoong Grid Computing and Distributed Systems (GRIDS) Lab. The University of Melbourne Melbourne, Australia WW Grid An Introduction

Outline What is Peer-to-Peer Computing? P2P Topologies Example P2P Applications Some key issues Conclusion

What is peer-to-peer (P2P) computing? Webster definition Peer: one that is of equal standing with another Computing between equals

Resource Sharing Exploit idle resources available in the edges E.g. CPU idle cycles, unused storage space, spare network bandwidth,… Exploit plentiful resources among network edges E.g. network bandwidth Federated cooperation among companies Sharing unavailable resources (e.g. databases) ……

Client-Server vs. P2P Client-Server paradigm The client is a dumb device The server performs all computation, stores data, and handle the control Simple architecture, but introduces:- Performance bottlenecks, single point of failure, etc. Each peer in P2P can be Client Server Intermediate: relaying requests/responses

History of P2P Origin of P2P dates back to ARPANET Early P2P applications/servers is Usenet and DNS 1990s: Shift in paradigm to client-server 1999: Napster => explosion of P2P usage 2000s: Gnutella, Kazaa, Audiogalaxy, etc.

Cluster, Grid, P2P: Characteristics CharacteristicClusterGridP2P PopulationCommodity Computers High-end computersEdge of network (desktop PC) OwnershipSingleMultiple DiscoveryMembership Services Centralised Index & Decentralised Info Decentralized User ManagementCentralisedDecentralised Resource managementCentralizedDistributed Allocation/SchedulingCentralisedDecentralised Inter-OperabilityVIA based?No standards yetNo standards Single System ImageYesNo Scalability100s1000?Millions? CapacityGuaranteedVaries, but highVaries ThroughputMediumHighVery High Speed(Lat. Bandwidth)Low, highHigh, Low

Types of P2P applications Instant messaging Managing and sharing information Collaboration Distributed Services …more to come?

Generic P2P Topologies Centralized Topology

Generic P2P Topologies (cont) Ring Topology

Generic P2P Topologies (cont) Hierarchical Topology

Generic P2P Topologies (cont) Decentralized Topology

Generic P2P Topologies (cont) Hybrid Topology Centralized and Ring Topology

Generic P2P Topologies (cont) Hybrid Topology Centralized and Centralized Topology

Generic P2P Topologies (cont) Hybrid Topology Centralized and Decentralized Topology

Example P2P Applications Napster Gnutella FastTrack

uses the National Astronomy and Ionospheric Center's 305 meter telescope at Arecibo, Puerto Rico. A screenshot of the client program. 2.4 mil volunteers as of Oct. 2000

Napster Centralized MP3 file sharing Clients/Peers hold the files Servers holds catalog and broker relationships Clients upload IP address, music file shared, and requests Clients request locations where requests can be met File transfer is P2P – proprietary protocol

Napster (cont)

Gnutella Completely decentralized – no servers with catalogs Shares any files Gnutella node ---- SERVENT Issue the query and view search result Accept the query from other SERVENTs and check the match against its database and response with corresponding result

Gnutella (cont) Joining the network:- The new node connects to a well-known SERVENT Then sends a PING message to discover other nodes PONG message are sent in reply from hosts offering connections with the new node Direct connection are then made

Gnutella (cont) Searching a file:- A node broadcasts its QUERY to all its peers who in turn broadcasts to their peers Nodes route back QUERYHITS along the QUERY path back to the sender containing the location detail To download the files a direct connection is made using details of the host in the QUERYHIT message

Gnutella (cont) Gnutella broadcasts its messages. To prevent flooding -TTL is introduced. To prevent forwarding same mesg. twice - each servent maintains a list of recently seen mesg.

Gnutella (cont) GnuCache A User A connects to the GnuCache to get the list of available servents already connected in the network GnuCache sends back the list to the user A User A sends the request message GNUTELLA CONNECT to the user B User B replies with the GNUTELLA OK message granting user A to join the network B D C F G E H J I (1)(2) (3) (4) (1) (2) (3) (2)

Gnutella (cont) Typical query scenario:- A sends a query message to its neighbor, B B first checks that the message is not an old one Then checks for a match with its local data If there is a match, it sends the queryHit message back to user A B then decrements TTL by 1 and forwards the query message to users C, D, and E C, D, and E performs the same steps as user B and forwards the query message further to users F, G, H, and I

Gnutella (cont) Problems Broadcast mesg. congests the network Lost of reply packets (dynamic environment)

FastTrack Hybrid between centralized and decentralized Has 2 tiers of control:- Ordinary nodes that connect to super nodes in a centralized fashion Super nodes that connect to each other in a decentralized manner

FastTrack (cont)

Joining the network? - Bootstrapping node Querying? Problems (Like Gnutella) Broadcast mesg. between Super Nodes Lost of reply packets

Some key issues Scalability Networks can grow to millions of nodes Challenge in achieving efficient peer and resource discovery High amount of query/response traffic Availability Potential for commercial content provision Such services require high availability and accessibility Anonymity What is the right level of anonymity?

Some key issues (cont) Security Due to open nature, have to assume environment is hostile Concerns include: Privacy and anonymity File authenticity Threats like worms and virus Fault Resilience The system must still be able to function even though several important nodes goes off-line.

Some key issues (cont) Standards and Interoperability Lack of standards lead to poor interoperability between applications Can be improved by using common protocols Copyright / Access Control Classic case of Napster being shut down Other applications have learned to get around the law Possibility of paid access in future

Some key issues (cont) Quality of Service (QoS) Metrics to be used is not clearly defined Tradeoff between achieving QoS and costs Complexity of Queries Must be able to support query languages of varying degree of expressiveness Simple keywords to SQL-like searches Search Mechanism Different search algorithms are used to reduced search time and maximize search space

Load Balancing existence of hot-spots (overloaded nodes) due to: uneven node distribution throughout logical space uneven object distribution among nodes uneven demand distribution among objects query and routing hot-spots Self-organization Ability to adapt itself to the dynamic nature of the Internet Depends on the architecture of the system

Conclusion Different P2P network topologies Examples of different P2P applications Key issues related to P2P Further reading:- ing.pdf