Introduction

1. P2P Streaming Protocol Pro-incentive Parametersdraft-zeng-ppsp-protocol-pro-incentive-para-00Wenjun (Kevin) ZengHuawei Technologies

2. Introduction • Lack of cooperation is one of the key problems that confront today's P2P systems • Unique set of challenges that P2P systems pose: • large populations, high turnover, asymmetry of interest, collusion, zero-cost identities, and traitors • This contribution intends to • analyze the common parameters essential for various incentive mechanisms, and • propose to incorporate these pro-incentive parameters in information exchanges in the P2P streaming protocols. • NOT intend to specify a particular incentive mechanism in the P2P streaming protocol.

3. Measurement of Peer Contribution • Rewards based on peer contribution • A typical metric: the amount of upload a peer has contributed. • For example, BitTorrent uses a bilateral mechanism: tit-for-tat. • Despite its pro-incentive approach, recent study has empirically shown that BitTorrent is vulnerable to strategic manipulation by its peers in a swarm. • For example, the Discount Parameter (DP) attack is an incentive threat that exploits the tit-for-tat based unchoking. • Honest Piece Revelation is becoming an increasing concern. • selfish BitTorrent clients can benefit from under-reporting pieces.

4. Discount Parameter Exploitation • Discount Parameter (DP) • Determines the degree to which the payoff of each move is discounted relative to the previous move • If the DP is small, players might defect and not worry about future consequences • In p2p systems, Piece Rarity is a DP • What gives Alice incentive to cheat • The rest of the pieces Bob has in not rare enough ! • She can find such pieces at lesser (L) uploads elsewhere • She can use the saved round bandwidth and L to upload to other peers with more rare pieces. (better bargain)

5. Discount Parameter Exploitation Alice Bob Connects to Bob, Request for download piece X Determines whether Alice is a good uploader ?? ALICE UNCHOKED Ok send X.. Request for rarest piece Alice has: Y Alice : “Y is a rare piece. None of the pieces Bob has is rare enough compared to X” BOB CHOKED Bob loses a round of download Alice starts looking for other peers

6. Discount Parameter Exploitation Simulating File sizes: 40 MB, 160 MB, 320 MB, 700 MB Increase in avg. download duration of fair peers = 23.8% Decrease in avg. download duration of DP Exploiters = 11.1% Tragedy of the Common

7. Starvation • All peers have all pieces, except one. • Origins: • Choices made by peers in piece and peer selection • Positive Discrimination • Use of rarest first / emptiest first approach • Can positive discrimination guarantee safe state? • Why do we have Under Reporting Peers (URPs): • Increased demand of their content in the swarm • Potential for faster completion

8. Premature Starvation due to URPs • Results show: • discrimination MUST be applied to BOTH piece and peer selection strategies to guarantee safe state. • BitTorrent currently does not use “rarity” in Unchoking i.e. unchoking = f (bandwidth). • Normally starvation begins at 0.95T • URP causes starvation at t > 0.78T (premature)

9. Piece Rarity Factor • Depends on • the global availability (in all swarms of the tracker): • the high availability of a piece outside the swarm may result in peers leaving the swarm. • local availability (in the target swarm) of a piece: • a rarer downloaded piece has more value to the swarm • number of upload slots a candidate peer has: • long term benefit can be expected from a peer with more uploading potential • the completion factor (i.e., the ratio of the number of pieces of the file that a peer has to total number of pieces of that file) of the candidate peer: • a peer with high completion factor is a good one to maintain a good upload/download relation with. • the contention in the swarm (i.e., the ratio of total number of peers to total number of seeds): • high contention implies more strategic value of a piece.

10. Pro-incentive Protocol Parameters • Important for peers to collect pro-incentive parameters to support incentive mechanisms. • no_upload_slots: a peer's upload bandwidth (i.e., number of upload slots a peer has). • bytes_uploaded: total amount of data that a peer has uploaded • bytes_downloaded: total amount of data that has been downloaded from a peer • chunk_nos: total number of chunks of a file that a peer has. • seed_nos: total number of seeds. • peer_nos: total number of peers. • chunk_copies_swarm: chunk availability, i.e., total number of copies of a chunk available in the swarm.