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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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