1. Determining the Peer Resource Contributions in a P2P Contract
Behrooz Khorashadi
Department of Computer Science
University of California at Davis
Joint work with D. Ghosal and X. Liu
11/19/2018
Hot-P2P 2005

2. Motivation P2P file-sharing traffic now dominates Internet traffic
The word on the street: “half of Internet traffic is BitTorrent traffic”
Limitation of BitTorrent
Some times it may take days to download an object
Methods to improve BitTorrent
11/19/2018
Hot-P2P 2005

3. Motivation (2)
Seeds (peers that have all parts of the object) may leave the system
BitTorrent like systems are robust only under flash-crowd scenarios
The system would be much more robust if seeds remained in the system
Key questions:
How long; i.e., how much resource is required from the peer?
How to incentivize seeds to remain in the system?
11/19/2018
Hot-P2P 2005

4. Talk Outline P2P Contract Key Issues
P2P Contract in a Centralized System
Relative importance of different components of the contribution primitive
P2P Contract in a Decentralized System
Methods to set contracts
Related Work
Conclusions and future work
11/19/2018
Hot-P2P 2005

5. P2P Contract A contract between a requesting client and the P2P system
Resources required from the requesting client
Quality of service guaranteed by the system
Admission control
Depending on the availability of the resources, admit clients who agree to the contract
11/19/2018
Hot-P2P 2005

6. P2P Contract in a File Sharing System
A seed gives a requesting client a list of referrals to where file can be retrieved, IF the client agrees to become a peer and
Serve up to N other requesting client
Within T seconds after retrieving file
P2P Contract
System guarantees quality of service for the peer contributions
11/19/2018
Hot-P2P 2005

7. Example System Controller Queued Requests Pool of Active Peers
All requests at sent to the seed/controller
The controller doles out the contract
Controller
Queued Requests
Pool of Active Peers
11/19/2018
Hot-P2P 2005

8. Example: N=2, T=∞ 10 11 2 4 1 3 5 6 7 8 9
11/19/2018
Hot-P2P 2005

9. Issues Performance Implementation Enforcement
Centralized vs. decentralized implementation
How should the contract be set?
Static vs. dynamic
Performance
What is the relative importance of N and T in controlling the service capacity?
Enforcement
How to enforce?
What is the impact of contract breaches?
11/19/2018
Hot-P2P 2005

10. Contract Scheme Analysis of the scheme in a centralized architecture
Show that T is a more important control parameter
Still N still plays an important role
For a decentralized architecture
Proposed and analyzed an adaptive scheme to set contracts without requiring global knowledge
11/19/2018
Hot-P2P 2005

11. Relative Importance of N and T
The number of servers in the system is a linear function of T
Impact of N is not significant for N ≥3
The N part of the contract provides an upper-bound on the bandwidth consumption
11/19/2018
Hot-P2P 2005

12. Decentralized P2P Architecture
No information about the location of seeds and active peers
Requests are flooded out with some predefined TTL value.
Each active peer/seed receiving the request issues a contract to the requesting client
If multiple contracts are received by the requesting client, the best one (i.e., smallest contract) is chosen
If no active peer/seed is reached, the query fails
11/19/2018
Hot-P2P 2005

13. Decentralized P2P Architecture
11/19/2018
Hot-P2P 2005

14. Implementation in a Decentralized System
Generation i
Generation i+1
S: Seed; AP: Active Peer; CP: Client Peer
The dark the color the higher the contract value
11/19/2018
Hot-P2P 2005

15. Decentralized P2P Architecture (2)
Key Issues
No global knowledge of the load
Setting contracts that will stabilize the system
Minimize query failure
Maximize efficiency
Number of original seeds
11/19/2018
Hot-P2P 2005

16. Decentralized Scheme System objectives
Keep query failure rate low
Keep TTL small
As in the centralized system, T is a more important parameter than N
How to set T?
Related questions
What is an acceptable failure rate?
How detrimental is it to have too many servers?
11/19/2018
Hot-P2P 2005

17. Distance Modulated Contracts
Let d be the distance from the seed/active peer to the requesting peer
The Seed/active peer sets the T part of contract proportional to d
T=d
T=d2
Distance modulated with a TTL based dampening factor
T=Kd2/TTL2 where K is some constant
11/19/2018
Hot-P2P 2005

18. Distance Modulated Contracts
11/19/2018
Hot-P2P 2005

19. Distance Modulated Contracts
11/19/2018
Hot-P2P 2005

20. Load Modulated Contracts
If the overall load (request rate l) is known
For a given value of TTL it can be shown that
To stabilize the system, i.e., the query failure rate is less than e
Where
e is some acceptable request failure rate which is set to be 0.5%
N(TTL) is the number of neighbors and is proportional to TTL2
11/19/2018
Hot-P2P 2005

21. Load Modulated Contracts
11/19/2018
Hot-P2P 2005

22. Load Modulated Contracts
11/19/2018
Hot-P2P 2005

23. Load Modulated Contracts Without Global Knowledge
In practice an active peer does not know load l
Each active peer keeps track of lN(TTL)
This information is sent to the requesting client with the contract
Each requesting client uses a simple moving average to estimate lN(TTL)
11/19/2018
Hot-P2P 2005

24. Results (1)
11/19/2018
Hot-P2P 2005

25. Results (2)
11/19/2018
Hot-P2P 2005

26. Summary P2P Contract Self-scaling - capacity scale with demand
Resource bartering and admission scheme
Self-scaling - capacity scale with demand
Centralized Implementation
Fixed value of N and T is sufficient
T is a more important control parameter
Decentralized
Load modulated contract
Distance modulated contract
11/19/2018
Hot-P2P 2005

27. Incentive Schemes BitTorrent: Tit-for-Tat type of scheme Micro-payment
Kazaa: internal token scheme
Popular Power: external currency
Gnuttella: Altruistic (good Samaritan) scheme, which isn’t really a scheme at all
70% of users are free riders
11/19/2018
Hot-P2P 2005

28. Future Work Contract enforcement Multiple classes of contracts
What is the impact of contact breaches?
What are simple methods to enforce contracts?
Multiple classes of contracts
Router contracts
Have a few servers connected by routers
Get real world results based on BitTorrent traces
Look more carefully into distance modulated contracts
11/19/2018
Hot-P2P 2005