peer-to-peer is not always decentralized …when centralization is good nelson minar nelson

Peer-to-Peer is Not Always Decentralized…when Centralization is Good

Nelson Minar

<[email protected]>http://www.media.mit.edu/~nelson/

2001-11-06Slide 2/23

Network Topologies: Nelson Minar

Talk Overview

• Where I come from

• Topologies of distributed systems

• Strengths and weaknesses

• Conclusions

Warning: Broad generalizations ahead

2001-11-06Slide 3/23


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

2001-11-06Slide 4/23


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

2001-11-06Slide 5/23


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?

2001-11-06Slide 6/23


Centralized

• Client/server

• Web servers

• Databases

• Napster search

• Instant Messaging

• Popular Power

2001-11-06Slide 7/23


Ring

• Fail-over clusters

• Simple load balancing

• Assumption– Single owner

2001-11-06Slide 8/23


Hierarchical

• DNS

• NTP

• Usenet (sort of)

2001-11-06Slide 9/23


Decentralized

• Gnutella

• Freenet

• Hive

• Internet routing

2001-11-06Slide 10/23


2001-11-06Slide 11/23


Centralized + Centralized

• N-tier apps

• Database heavy systems

• Web services gateways

• Grand Central

2001-11-06Slide 12/23


Centralized + Ring

• Serious web applications

• High availability servers

2001-11-06Slide 13/23


Centralized + Decentralized

• Clip2 Gnutella Reflector

• FastTrack / KaZaA– Morpheus

• Email

2001-11-06Slide 14/23


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?

2001-11-06Slide 15/23


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

2001-11-06Slide 16/23


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?

2001-11-06Slide 17/23


Centralized

Manageable

Coherent

Extensible

Fault Tolerant

Secure

Lawsuit-proof

Scalable

System is all in one place All information is in one place

X No one can add on to system

X Single point of failure Simply secure one host

X Easy to shut down

? One machine. But in practice?

2001-11-06Slide 18/23


Ring

Manageable

Coherent

Extensible

Fault Tolerant

Secure

Lawsuit-proof

Scalable

Simple rules for relationships Easy logic for state

X Only ring owner can add Fail-over to next host As long as ring has one owner

X Shut down owner Just add more hosts

2001-11-06Slide 19/23


Hierarchical

Manageable

Coherent

Extensible

Fault Tolerant

Secure

Lawsuit-proof

Scalable

½ Chain of authority

½ Cache consistency

½ Add more leaves, rebalance

½ Root is vulnerable

X Too easy to spoof links

X Just shut down the root Hugely scalable – DNS

2001-11-06Slide 20/23


Decentralized

Manageable

Coherent

Extensible

Fault Tolerant

Secure

Lawsuit-proof

Scalable

X Very difficult, many owners

X Difficult, unreliable peers Anyone can join in! Redundancy

X Difficult, open research No one to sue! (…but follow $)

? Theory – yes : Practice – no

2001-11-06Slide 21/23


Centralized + Ring

Manageable

Coherent

Extensible

Fault Tolerant

Secure

Lawsuit-proof

Scalable

Just manage the ring As coherent as ring

X No more than ring Ring is a huge win As secure as ring

X Still single place to shut down Ring is a huge win

Common architecture for web applications

2001-11-06Slide 22/23


Centralized + Decentralized

Manageable

Coherent

Extensible

Fault Tolerant

Secure

Lawsuit-proof

Scalable

X Same as decentralized

½ Better than decentralized Anyone can still join! Plenty of redundancy

X Same as decentralized Still no one to sue

? Looking very hopeful

Best architecture for P2P networks?

2001-11-06Slide 23/23


Centralized vs. Decentralized

• Centralized is pretty good!– Manageable– Coherent– Security

• Decentralized is exciting– Extensible– Massive fault tolerance– Lawsuit-proof

• Scalability is the big question

2001-11-06Slide 24/23


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

Peer-to-Peer is Not Always Decentralized…when Centralization is Good

Nelson Minar

<[email protected]>http://www.media.mit.edu/~nelson/

Thanks to Marc Hedlund, Raffi Krikorian, Tony White

peer-to-peer is not always decentralized …when centralization is good nelson minar nelson

Documents

centralized slide

network topologies

high level slide

distributed systems

good nelson minar http

p2p network of fail

decentralized systems

distributed agents