1. Peer-to-Peer Networks
Thanks to:
Vinod Muthusam, U. of Toronto
Jon Kubiatowicz, UC Berkeley
Don Towsley, U. Mass at Amherst
Mema Roussopoulos, Harvard University

2. What is P2P?
P2P is a communications model in which each party has the same capabilities and either party can initiate a communication session. Whatis.com
P2P is a class of applications that takes advantage of resources – storage, cycles, content, human presence – available at the edges of the internet. Clay Shirky, openp2p.com
A type of network in which each workstation has equivalent capabilities and responsibilities. Webopedia.com
A P2P computer network refers to any network that does not have fixed clients and servers, but a number of peer nodes that function as both clients and servers to other nodes on the network. Wikipedia.org

3. P2P is not new! Usenet: News groups first truly decentralized system
DNS: Handles huge number of clients
IP routing: Vastly decentralized, many equivalent routers

4. P2P is not new! Usenet: News groups first truly decentralized system
DNS: Handles huge number of clients
IP routing: Vastly decentralized, many equivalent routers

5. When is an application P2P?
We will do an analysis based on a decision tree developed by researchers at Harvard, Stanford, Berkeley, and HP Labs
2 P2P or Not 2 P2P (IPTPS 2004)
Exercise for end of class: critique and modify the decision tree

6. Recent Explosion of New Large Scale Applications
Applications requiring immense resources
CPU: Grid and grid computing
Files and information: Music sharing, semantic web
Bandwidth: Video streaming, content distribution
Communication: IP telephony, group collaboration
Storage: Data archives, massive storage
Thousands to millions of nodes on the edge of the Internet can participate as sources/donors and as receivers/users.

7. Large scale storage applications: Web indexing (Google)
Goal: index the entire Web
Estimate: Google has 250,000 node cluster!
Massively distributed
Crawler
Indexer
Client
Store(url, page)
Index(page, keywords)
Find(keywords)
Distributed File System
* Partial content from

8. Large scale storage applications: Web archives
Goal: make and archive a daily check point of the Web
Estimates:
Web is about 57 Tbyte, compressed HTML+img
New data per day: 580 Gbyte
128 Tbyte per year with 5 replicas
Design:
12,810 nodes: 100 Gbyte disk each
Crawler
Client
Store(url, page, date)
Get(url, date)
Distributed File System
* Partial content from

9. Large scale storage applications: File storage
OceanStore (UC Berkeley)
Untrusted Infrastructure:
The OceanStore is comprised of untrusted components
Individual hardware has finite lifetimes
All data encrypted within the infrastructure
Responsible Party:
Some organization (i.e. service provider) guarantees that your data is consistent and durable
Not trusted with content of data, merely its integrity
Mostly Well-Connected:
Data producers and consumers are connected to a high-bandwidth network most of the time
Exploit multicast for quicker consistency when possible
Promiscuous Caching:
Data may be cached anywhere, anytime

10. Utility-based Infrastructure
Pac
Bell
Sprint
IBM
AT&T
Canadian
OceanStore
Utility-based Infrastructure
Data service provided by storage federation
Cross-administrative domain
Pay for Service

11. Large scale scientific applications
and other projects at
500,000 BOINC volunteers
Featured volunteer: Work done per day 6,514 (65 GigaFLOPS)

12. GRIDs and desktop grids
GRID computing connects supercomputing labs (large parallel machines and databases), primarily for scientific computing
Desktop grids use PCs for cycle sharing (dedicated or on the edge of the Internet)
CCOF: Cluster Computing on the Fly

13. WaveGrid, Riding the wave of idle cycles

14. Peer-to-peer for large systems
Limitations of client/server architecture
Benefits of P2P
History of P2P systems
P2P architectures
P2P issues

15. Client/server architecture
Well known, powerful, reliable server is a data source
Clients request data from server
Very successful model
WWW (HTTP), FTP, Web services, etc.
Server
Client
Internet
* Figure from

16. Client/server limitations
Scalability is expensive
Presents a single point of failure
Requires administration
Unused resources at the network edge
P2P systems try to address these limitations

17. P2P vocabulary P2P application P2P architecture P2P computing
P2P network
Peer-based v. P2P
Terms are used interchangeably, sometimes sloppily but there are subtle differences in meaning.

18. P2P computing
P2P computing is the sharing of computer resources and services by direct exchange between systems.
These resources and services include the exchange of information, processing cycles, cache storage, and disk storage for files.
P2P computing takes advantage of existing computing power, computer storage and networking connectivity, allowing users to leverage their collective power to the ‘benefit’ of all.
* From Publications/Peer-to-Peer_Introduction_Feb.ppt

19. P2P architecture All nodes are both clients and servers
Provide and consume data
Any node can initiate a connection
No centralized data source
“The ultimate form of democracy on the Internet”
“The ultimate threat to copy-right protection on the Internet”
Node
Internet
* Content from

20. P2P benefits Efficient use of resources Scalability
Unused bandwidth, storage, processing power at the edge of the network
Scalability
Consumers of resources also donate resources
Aggregate resources grow naturally with utilization
Organic scaling
Infrastructure-less scaling
Reliability (in aggregate)
Replicas
Geographic distribution
No single point of failure
Ease of administration
Nodes self organize
No need to deploy servers to satisfy demand (c.f. scalability)
Built-in fault tolerance, replication, and load balancing

21. P2P Challenges Efficient and fair use of resources Scalability
How to allocate? How to deliver?
How to prevent selfish behavior?
How to provide incentives?
Scalability
How to locate resources in such a large system?
How to avoid overuse of the network?
How to deal with heterogeneity?
Reliability and trustworthiness in open systems (fault tolerance and security)
How to prevent or recover from malicious behavior
Do we need or want authentication?
How to deal with churn?
Do we want to guarantee anonymity?
Ease of administration and security
What about commercial P2P?
How to deal with policy issues?

22. P2P uses Overlay Networks
Peer
Peer
Peer
Peer
IP Network
Overlay
Traditional Communication
IP Network
Tunneling Communication

23. P2P Architectures (Fig. 5.1)
Overlay Network
Unstructured
Structured
Architecture Model
Centralized
Pure P2P
Hybrid (hierarchical)
DHT

24. P2P Architectures: Tradeoffs
Centralized P2P architecture - can suffer from bottleneck at the central server, simpler design
Pure P2P - sometimes the content cannot be found, can potentially generate a lot of network traffic
Hybrid P2P - how to select and locate the supernodes?
DHT - fast routing and content discovery, more complex infrastructure and higher maintenance cost

25. Unstructured P2P Systems: File Sharing
Napster, Gnutella, Kazaa, Freenet
Large scale sharing of files.
User A makes files (music, video, etc.) on their computer available to others
User B connects to the network, searches for files and downloads files directly from user A
Issues of copyright infringement

26. P2P File sharing Traffic
Globally, P2P traffic now represents 55%-80% of Internet traffic [CacheLogic]
* Figure from

27. Napster A way to share music files with others
Users upload their list of files to Napster server
You send queries to Napster server for files of interest
Keyword search (artist, song, album, bitrate, etc.)
Napster server replies with IP address of users with matching files
You connect directly to user A to download file
* Figure from

28. Napster Central Napster server Search is centralized
Can ensure correct results
Bottleneck for scalability
Single point of failure
Susceptible to denial of service
Malicious users
Lawsuits, legislation
Search is centralized
File transfer is direct (peer-to-peer)

29. Gnutella Share any type of files (not just music)
Decentralized search unlike Napster
You ask your neighbours for files of interest
Neighbours ask their neighbours, and so on
TTL field quenches messages after a number of hops
Users with matching files reply to you
* Figure from

30. Gnutella Decentralized No single point of failure
Not as susceptible to denial of service
Cannot ensure correct results
Flooding queries
Search is now distributed but still not scalable
Good at finding popular content
Bad at finding rare content
Two level hierarchy reduces traffic
Ultrapeers
Leaf peers

31. Freenet Data flows in reverse path of query “Smart” queries
Impossible to know if a user is initiating or forwarding a query
Impossible to know if a user is consuming or forwarding data
“Smart” queries
Requests get routed to correct peer by incremental discovery
* Figure from “Protecting Freedom of Information Online with Freenet”, Ian Clarke and Scott Miller. IEEE Internet Computing, Jan/Feb 2002

32. Comparison of file sharing networks
Napster (centralized)
Bottleneck (scalability, failure, denial of service)
Correct search results (centralized search)
Gnutella (distributed)
No bottleneck
No guarantee on search results
Freenet
Anonymity
Less efficient data transfer

33. Anonymity Napster, Gnutella, Kazaa don’t provide anonymity Freenet
Users know who they are downloading from
Others know who sent a query
Freenet
Designed to provide anonymity among other features

34. Unstructured P2P Systems: File download (bandwidth sharing)
BitTorrent (35% of Internet traffic)
Limewire
eDonkey
Large file is divided into fixed size blocks.
Peers download missing blocks from other peers while uploading blocks they have to requesting peers (tit-for-tat)
Blocks arrive out of order, so peer must reassemble the file.
(More details later this term)

35. BitTorrent “Offline” search Bartered “Tit for Tat” download bandwidth
No search built into protocol
Bartered “Tit for Tat” download bandwidth
Download one (random) chunk from a storage peer, slowly
Subsequent chunks bartered with concurrent downloaders
As tracked by the tracker for the file
The more chunks you can upload, the more you can download
Download speed starts slow, then goes fast
Great for large files
* Content from Hellerstein’s VLDB2004 P2P Tutorial

36. Structured P2P Second generation P2P overlay networks Self-organizing
Load balanced
Fault-tolerant
Scalable guarantees on numbers of hops to answer a query
Major difference with unstructured P2P systems
Based on a distributed hash table interface

37. Distributed hash tables (DHT)
Distributed version of a hash table data structure
Store and retrieve (key, value) pairs
The key is like a filename
The value can be file contents

38. DHT applications Many services can be built on top of a DHT interface
File sharing
Archival storage
Databases
Naming, service discovery
Chat service
Rendezvous-based communication
Publish/Subscribe

39. DHT desirable properties
Keys mapped evenly to all nodes in the network
Each node maintains information about only a few other nodes
Messages can be routed to a node efficiently
Node arrival/departures only affect a few nodes

40. DHT routing protocols DHT is a generic interface
There are several implementations of this interface
Chord [MIT]
Pastry [Microsoft Research UK, Rice University]
Tapestry [UC Berkeley]
Content Addressable Network (CAN) [UC Berkeley]
SkipNet [Microsoft Research US, Univ. of Washington]
Kademlia [New York University]
Viceroy [Israel, UC Berkeley]
P-Grid [EPFL Switzerland]
Freenet [Ian Clarke]
Freenet more concerned with privacy/security, not as much on delivery guarantees.
Others: Farsite, SALAD (Douceur) are about file systems

41. P2P Challenges: Resource Discovery in Unstructured
How to find desired resources in a large scale, open and dynamic P2P network?
Think of it as a graph traversal problem
Flooding
Random walk
Expanding ring
Advertisement-based
Rendezvous-point
History-based
Many more
(Resource discovery in structured P2P networks is very different and will be covered later)

42. P2P Challenges: Incentives & Fairness
How to provide incentives for peers to participate
Goodness of their hearts (seems to work)
Fame, competitive spirit
Credit schemes
Game theory
How to ensure fairness
Prevent freeriders
Accounting mechanisms
Game theory !!
Don’t even try to??

43. P2P Challenges: Security Again Malicious Behavior
Failure to forward messages/files
Faked computational results in cycle sharing
Corrupted files/data/code
Deliberate delay of messages to gain an advantage (e.g. games)
Inconsistent information relayed to different peers
Faking work to get credit
Denial of service attacks
Collusion among several peers to do harm
Sybil attack (forging multiple identities from one peer to gain advantage)

44. P2P Challenges: routing
Peers that are close in the overlay network can be far in the physical network.
N20
N41
N80
N40
* Figure from

45. For class discussion: 1. Is a sensor network a P2P network?
2. If stores gave away free CDs and DVDs 24-7, what would happen to P2P computing and traffic?
3. Do this in pairs:
Redesign the 2 P2P or not 2 P2P decision tree to (a) include more issues such as those covered in today’s lecture, and (b) rearrange the order of the decisions in a more natural order. Optional: to be more suitable for a specific application such as gnutella