1. Peer-to-Peer is Not Always Decentralized …when Centralization is Good Nelson Minar http://www.media.mit.edu/~nelson/

2. Network Topologies: Nelson Minar 2001-11-06 Slide 1/23 Talk Overview Where I come from Topologies of distributed systems Strengths and weaknesses Conclusions Warning: Broad generalizations ahead

3. Network Topologies: Nelson Minar 2001-11-06 Slide 2/23 Systems I’ve Designed MIT Media Lab: Hive –Distributed Agents for Networking Things –Distributed objects –Mobile agents –Fully decentralized (cheating a bit) Popular Power –Give your computer something to dream about –SOAP-like client/server system –Mobile code –Fully centralized

4. Network Topologies: Nelson Minar 2001-11-06 Slide 3/23 What is P2P Anyway? Decentralized Systems: no –Popular Power fails test –Napster fails test –Most Instant Messaging fails test –Confuses topology with function Edge Resources: yes –Small computers on edges contribute back –All peers are active participants

5. Network Topologies: Nelson Minar 2001-11-06 Slide 4/23 Distributed Systems Topologies Get away from fundamentalism –“Pure P2P”, “True P2P”, etc Focus instead on system architecture –How do the pieces fit together? Concentrate on connection topology Which topology for which problem?

6. Network Topologies: Nelson Minar 2001-11-06 Slide 5/23 Centralized Client/server Web servers Databases Napster search Instant Messaging Popular Power

7. Network Topologies: Nelson Minar 2001-11-06 Slide 6/23 Ring Fail-over clusters Simple load balancing Assumption –Single owner

8. Network Topologies: Nelson Minar 2001-11-06 Slide 7/23 Hierarchical DNS NTP Usenet (sort of)

9. Network Topologies: Nelson Minar 2001-11-06 Slide 8/23 Decentralized Gnutella Freenet Hive Internet routing

10. Network Topologies: Nelson Minar 2001-11-06 Slide 9/23

11. Network Topologies: Nelson Minar 2001-11-06 Slide 10/23 Centralized + Centralized N-tier apps Database heavy systems Web services gateways Grand Central

12. Network Topologies: Nelson Minar 2001-11-06 Slide 11/23 Centralized + Ring Serious web applications High availability servers

13. Network Topologies: Nelson Minar 2001-11-06 Slide 12/23 Centralized + Decentralized Clip2 Gnutella Reflector FastTrack / KaZaA –Morpheus Email

14. Network Topologies: Nelson Minar 2001-11-06 Slide 13/23 What about other topologies? Centralized + Hierarchical? –Back end tree of information –Caching architectures Decentralized + Ring? –P2P network of fail-over clusters Decentralized + Hierarchical? Decentralized + Centralized?

15. Network Topologies: Nelson Minar 2001-11-06 Slide 14/23 Strengths and Weaknesses Plenty of topologies to choose from What is each kind good for? Need a set of properties to measure Caution: What follows is very high level

16. Network Topologies: Nelson Minar 2001-11-06 Slide 15/23 Things to Measure Manageability –How hard is it to keep working? Information coherence –How authoritative is info? (Auditing, non-repudiation) Extensibility –How easy is it to grow? Fault tolerance –How well can it handle failures? Security –How hard is it to subvert? Resistance to legal or political intervention –How hard is it to shut down? (Can be good or bad) Scalability –How big can it grow?

17. Network Topologies: Nelson Minar 2001-11-06 Slide 16/23 Centralized Manageable Coherent Extensible Fault Tolerant Secure Lawsuit-proof Scalable System is all in one place All information is in one place XNo one can add on to system XSingle point of failure Simply secure one host XEasy to shut down ?One machine. But in practice?

18. Network Topologies: Nelson Minar 2001-11-06 Slide 17/23 Ring Manageable Coherent Extensible Fault Tolerant Secure Lawsuit-proof Scalable Simple rules for relationships Easy logic for state XOnly ring owner can add Fail-over to next host As long as ring has one owner XShut down owner Just add more hosts

19. Network Topologies: Nelson Minar 2001-11-06 Slide 18/23 Hierarchical Manageable Coherent Extensible Fault Tolerant Secure Lawsuit-proof Scalable ½Chain of authority ½Cache consistency ½Add more leaves, rebalance ½Root is vulnerable XToo easy to spoof links XJust shut down the root Hugely scalable – DNS

20. Network Topologies: Nelson Minar 2001-11-06 Slide 19/23 Decentralized Manageable Coherent Extensible Fault Tolerant Secure Lawsuit-proof Scalable XVery difficult, many owners XDifficult, unreliable peers Anyone can join in! Redundancy XDifficult, open research No one to sue! (…but follow $) ?Theory – yes : Practice – no

21. Network Topologies: Nelson Minar 2001-11-06 Slide 20/23 Centralized + Ring Manageable Coherent Extensible Fault Tolerant Secure Lawsuit-proof Scalable Just manage the ring As coherent as ring XNo more than ring Ring is a huge win As secure as ring XStill single place to shut down Ring is a huge win Common architecture for web applications

22. Network Topologies: Nelson Minar 2001-11-06 Slide 21/23 Centralized + Decentralized Manageable Coherent Extensible Fault Tolerant Secure Lawsuit-proof Scalable XSame as decentralized ½Better than decentralized Anyone can still join! Plenty of redundancy XSame as decentralized Still no one to sue ?Looking very hopeful Best architecture for P2P networks?

23. Network Topologies: Nelson Minar 2001-11-06 Slide 22/23 Centralized vs. Decentralized Centralized is pretty good! –Manageable –Coherent –Security Decentralized is exciting –Extensible –Massive fault tolerance –Lawsuit-proof Scalability is the big question

24. Network Topologies: Nelson Minar 2001-11-06 Slide 23/23 Conclusions Centralized is easy to deal with –Major architecture for distributed systems –Combines well with rings Decentralized is good, needs research –Coherence, Manageability, Security –Scalability Hierarchical is overlooked Combining architectures is powerful

25. Peer-to-Peer is Not Always Decentralized …when Centralization is Good Nelson Minar http://www.media.mit.edu/~nelson/ Thanks to Marc Hedlund, Raffi Krikorian, Tony White