Comparing Multilateral and Bilateral Exchange Models for Content Distribution

Users of peer-to-peer systems are often incentivized to contribute their upload capacity in a bilateral manner: downloading is possible in return for uploading to the same peer (e.g., BitTorrent). An alternative is to use multilateral exchange to match user demand for content to available supply at other peers in the system. Multilateral exchange can be enabled through prices and a virtual currency. Monetary incentives have been previously proposed to incentivize uploading in P2P systems [1], [2], [3], [4]. We provide a formal comparison of P2P system designs based on bilateral exchange with those that enable multilateral exchange via a price-based market mechanism to match supply and demand. This work surveys and generalizes [5] and [6]. We start with a fundamental abstraction of content exchange in systems with bilateral barter: exchange ratios. The exchange ratio from one peer to another gives the download rate received per unit upload rate. We show that exchange ratios can be used to model filesharing systems such as BitTorrent and variants like BitTyrant. Moreover, exchange ratios are a useful formal tool that directly allows us to compare bilateral P2P systems with price-based multilateral P2P systems. We compare the allocations that arise at equilibria of bilateral P2P systems using exchange ratios, with those that arise at equilibria of price-based multilateral P2P systems. We consider a set of peers that shares a set of files F. Peer i has a subset of the files Fi ⊆ F, and is interested in downloading files in Ti ⊆ F − Fi. We use rijf to denote the rate at which user i uploads file f to user j. We then let dif = ∑ j rjif be the total rate at which user i downloads file f. We measure the desirability of a download vector to peer i by a utility function Vi(di) that is nondecreasing in every dif for f ∈ Ti. Peer i incurs a cost ci(yi) for uploading at rate yi. Given a vector of exchange ratios γ, where γij is the ratio at which peer i can exchange with peer j, peer i solves the following optimization problem. maximize Vi(di) − ci(yi) subject to dif = ∑ rjif, ∀f j rkjf = 0, if f ̸ ∈ Fk j,