1. Bruce Hammer, Steve Wallis, Raymond Ho
Distributed Systems Concepts and Design Chapter 10: Peer-to-Peer Systems
Bruce Hammer, Steve Wallis, Raymond Ho

2. Bruce Hammer, Steve Wallis, Raymond Ho
10.1: Introduction
Peer-to-Peer Systems
Where data and computational resources are contributed by many hosts
Objective to balance network traffic and reduce the load on the primary host
Management requires knowledge of all hosts, their accessibility, (distance in number of hops), availability and performance.
They exploit existing naming, routing, data replication and security techniques in new ways
Client-Server relationships are more transaction-based than defined as permanent roles.
Peer-to-Peer elevates clients to servers – on an asynchronous basis
Subordinate hosts need to be aware that they are participating in the peer-to-peer system
Objective – to build a reliable resource sharing layer over an unreliable and untrusted collection of computers and networks
Bruce Hammer, Steve Wallis, Raymond Ho

3. Bruce Hammer, Steve Wallis, Raymond Ho
10.1: Introduction
Goal of Peer-to-Peer Systems
Sharing data and resources on a very large scale
‘Applications that exploit resources available at the edges of the Internet – storage, cycles, content, human presence’ (Shirky 2000)
Uses data and computing resources available in the personal computers and workstations
Eliminating any requirement for separately-managed servers and their associated infrastructure –
Can use peer-to-peer within one company, but this relies on each subordinate host to maintain its own infrastructure
The professor has mentioned the project, Search for Extra-Terrestrial Intelligence, which partitions a stream of digitzed radio telescope data into 107-second work units, and parcels them out to personal computers on the network. This is controlled by one single server. SETI does not involve any communication or coordination between computers while they are processing the work units. Single message to central server.
Design aims to deliver a service that is fully decentralized and self-organizing, dynamically balancing the storage and processing loades between all the participaating computers as computers join and leave the service
Increasingly attractive as the performance difference narrows between host servers and personal computers
Bruce Hammer, Steve Wallis, Raymond Ho

4. Bruce Hammer, Steve Wallis, Raymond Ho
10.1: Introduction
Characteristics of Peer-to-Peer Systems
Each computer contributes resources
All the nodes have the same functional capabilities and responsibilities
No centrally-administered system
Offers a limited degree of anonymity
Algorithm for placing and accessing the data
Balance workload, ensure availability
Without adding undue overhead
Napster legal problems need to add anonymity
Not needed in corporate peer-to-peer systems
Bruce Hammer, Steve Wallis, Raymond Ho

5. Bruce Hammer, Steve Wallis, Raymond Ho
10.1: Introduction
Evolution of Peer-to-Peer Systems
Napster – download music, return address
Freenet, Gnutella, Kazaa and BitTorrent
More sophisticated – greater scalability, anonymity and fault tolerance
Pastry, Tapestry, CAN, Chord, Kademlia
Peer-to-peer middleware
Steve will demonstrate BitTorrent
Napster and Peer-to-Peer middleware are the next two presentations
Bruce Hammer, Steve Wallis, Raymond Ho

6. Bruce Hammer, Steve Wallis, Raymond Ho
10.1: Introduction
Evolution (Continued)
Immutable Files, (music, video)
GUIDs (Globally Unique Identifiers)
Middleware to provide better routing algorithms, react to outages
Evolve to mutable files
Application within one company’s intranet
Globally Unique Identifiers, usually derived as a secure hash – from some or all of the resource’s state. (metioned in Chapter 7)
The use of a secure hash makes a resource ‘self certifying’
The use of p2p that demand a high level of availability for the objects stored requires careful application design to avoid situations in all of the replicas are unavailable. The use of randomly-distributed GUIDs assists by distributing the object replicas to randomly-located nodes in the underlying network. Spans many organizations across the globe minimizes the risk.
Bruce Hammer, Steve Wallis, Raymond Ho

7. 10.2: Napster and its Legacy
Bruce Hammer, Steve Wallis, Raymond Ho

8. 10.2: Napster and Its Legacy
Bruce Hammer, Steve Wallis, Raymond Ho

9. 10.3: Peer-to-Peer Middleware
Bruce Hammer, Steve Wallis, Raymond Ho

10. Bruce Hammer, Steve Wallis, Raymond Ho
10.4: Routing Overlays
Routing Overlays
Sub-systems, APIs, within the peer-to-peer middleware
Responsible for locating nodes and objects
Implements a routing mechanism in the application layer
Separate from any other routing mechanisms such as IP routing
Ensures that any node can access any object by routing each request thru a sequence of nodes
Exploits knowledge at each node to locate the destination
Routing Overlay is a distributed algorithm
Bruce Hammer, Steve Wallis, Raymond Ho

11. Bruce Hammer, Steve Wallis, Raymond Ho
10.4: Routing Overlays
GUIDs
‘pure’ names or opaque identifiers
Reveal nothing about the locations of the objects
Building blocks for routing overlays
Computed from all or part of the state of the object using a function that deliver a value that is very likely to be unique. Uniqueness is then checked against all other GUIDs
Not human readable
From Section pure names are simply uninterpreted bit patterns.
Non-pure names contain information about the object that the name, e.g., the location.
Bruce Hammer, Steve Wallis, Raymond Ho

12. Bruce Hammer, Steve Wallis, Raymond Ho
10.4: Routing Overlays
Tasks of a routing overlay
Client submits a request including the object GUID, routing overlay routes the request to a node at which a replica of the object resides
A node introduces a new object by computing its GUID and announces it to the routing overlay
Clients can remove an object
Nodes may join and leave the service
Bruce Hammer, Steve Wallis, Raymond Ho

13. Bruce Hammer, Steve Wallis, Raymond Ho
10.4: Routing Overlays
Types of Routing Overlays
DHT – Distributed Hash Tables
DOLR – Distributed Object Location and Routing
DOLR is a layer over the DHT that maps GUIDs and address of nodes
DHT – GUIDs are stored based on the hash value
DOLR – GUIDs host address is notified using the Publish() operation
Bruce Hammer, Steve Wallis, Raymond Ho

14. 10.5: Overlay Case Studies: Pastry, Tapestry
Bruce Hammer, Steve Wallis, Raymond Ho

15. 10.6: Application Case Studies: Squirrel, OceanStore, Ivy Squirrel
Bruce Hammer, Steve Wallis, Raymond Ho

16. Bruce Hammer, Steve Wallis, Raymond Ho
10.7: Summary
Napster – immutable data, unsophisticated routing
Current – mutable data, routing overlays, sophisticated algorithms
Internet or company intranet support
Distributed Computing (SETI)
For the professor,
Peer-to-Peer systems seem to be an evolving technology. Have there been significant changes since the book was published?
Bruce Hammer, Steve Wallis, Raymond Ho

17. Bruce Hammer, Steve Wallis, Raymond Ho
10.7: Summary
Benefits of Peer-to-Peer Systems
Ability to exploit unused resources (storage, processing) in the host computers
Scalability to support large numbers of clients and hosts with load balancing of network links and host computer resources
Self-organizing properties of the middleware platforms reduces costs
Bruce Hammer, Steve Wallis, Raymond Ho

18. Bruce Hammer, Steve Wallis, Raymond Ho
10.7: Summary
Weaknesses of Peer-to-Peer Systems
Costly for the storage of mutable data compared to trusted, centralized service
Can not yet guarantee anonymity to hosts
Bruce Hammer, Steve Wallis, Raymond Ho

19. 10: Peer-to-Peer Systems
Questions????
Comments??
For the professor,
This fields seems to be evolving rapidly, do you have interesting updates for us?
Bruce Hammer, Steve Wallis, Raymond Ho