Secure Incentivization for Decentralized Content Delivery Prateesh Goyal1, Ravi Netravali1,2, Mohammad Alizadeh1, Hari Balakrishnan1 MIT CSAIL1, UCLA2
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Secure Incentivization for Decentralized Content Delivery Prateesh Goyal1, Ravi Netravali1;2, Mohammad Alizadeh1, Hari Balakrishnan1 MIT CSAIL1, UCLA2 Abstract would participate. We were pleasantly surprised by this percentage, which is larger than what we expected. Those who would not Prior research has proposed technical solutions to use peer-to-peer participate were primarily concerned with device security, content (P2P) content delivery to serve Internet video, showing that it can re- liability, and impacts on device performance. To our knowledge, duce costs to content providers. Yet, such methods have not become the results we report are the first published findings on this question. widespread except for a few niche instances. An important chal- Second, how can payments for monetary incentivization be lenge is incentivization: what tangible benefits does P2P content done securely? This is challenging for several reasons. First, a delivery offer users who bring resources to the table? In this paper, content provider cannot be trusted to honor payments, and peers we ask whether monetary incentives can help attract peers in P2P may be disinclined to share payment credentials with certain content delivery systems. We commissioned a professional survey content providers. Second, existing centralized payment systems of people around the United States to answer several relevant ques- (e.g., Paypal) are not designed to support the large number of small tions. We found that 51% of the 876 respondents—substantially transactions that would be needed for P2P content delivery. This larger than our expectations—answered “yes” to whether they is particularly challenging because peers and content providers can would participate for suitable financial incentives. Encouraged by span geographic boundaries that impose foreign transaction/ex- the results of the survey, we propose Gringotts, a system to struc- change fees. Further, these issues are more pronounced for small ture incentives and securely incorporate P2P delivery into content content providers, both due to financial limitations and difficulties delivery systems. Gringotts provides a novel Proof of Delivery in convincing peers of faithful payments. mechanism that allows content providers to verify correct delivery One approach to address these challenges is to use a central au- of their files, and shows how to use cryptocurrency to pay peers thority that everyone trusts (e.g., a bank) to enforce content delivery while guarding against liars and Sybil attacks. payments. An alternative approach is to eliminate the use of a cen- tral authority (and the associated trust requirements), and to instead 1 Introduction provide the above guarantees in a completely decentralized manner. Video streams constitute over 70% of global Internet traffic [6, 28]. With either approach, content providers must be assured that deliv- Most video is delivered to users today via Content Distribution ery is happening properly. Our primary contribution is a lightweight Networks (CDNs) like Akamai and CloudFlare. Although CDN solution to this problem in which content providers, clients, and demand has nearly doubled since 2016 [6], they are too expensive peers, collectively produce a Proof of Delivery Chain (PoDC) that for many content providers [26]. serves as a proof for the delivery of a file from a set of peers to a To combat these high prices, there have been many proposals client. PoDCs are unforgeable and tamper-proof: neither the peers to have peers cache and stream videos to each other, either as nor the content providers can manipulate them to affect payments. supplements to existing CDNs [39, 38, 35], or as decentralized We present the design of Gringotts, a system that applies P2P systems [22, 29]. For example, peers within an Internet PoDC to decentralized P2P content delivery. Payments in Service Provider (ISP) in a city could stream videos directly to Gringotts are made using a cryptocurrency. Cryptocurrencies naturally address some of the aforementioned challenges, and our arXiv:1808.00826v1 [cs.NI] 2 Aug 2018 one another, avoiding expensive Internet paths. By serving content using bandwidth and storage resources that would otherwise go consumer survey revealed that 27% of users are already willing unused, such systems could significantly reduce costs for content to accept cryptocurrency payments (with 40% unsure due to providers and CDN operators. lack of familiarity). In Gringotts, peers are ensured payments by broadcasting PoDCs on a blockchain. Gringotts uses probabilistic These proposals have not seen significant adoption in practice payments to limit blockchain transaction overheads without because of a lack of sufficient participation [4, 39]. Even forms of compromising security. Gringotts is also robust to various forms incentivization like virtual tokens and in-service perks (e.g., traffic of collusion (e.g., clients and content providers, clients and peers) prioritization) have failed to attract enough peers [29, 23, 24]. We and Sybil attacks with fake clients or peers. believe that the steady increases in uplink bandwidth and storage capacity on laptops [30] motivate revisiting P2P content delivery, but with a focus on secure payments as an incentive mechanism. 2 Consumer Survey We ask two questions. First, would users be willing to participate To understand the expectations and requirements for user participa- in such a system if they were incentivized with monetary payments? tion in a peer-to-peer content delivery service, we commissioned a To answer this question, we commissioned a consumer survey to third-party professional organization to undertake a consumer sur- understand user concerns with respect to participation in P2P con- vey. They survey garnered 876 responses from around the United tent delivery (§2). Our key finding is that 51% of the 876 users States. The respondents ranged in age, with 95% between the ages 1 100% 100% more than 100 average videos. Further, consider that the average Will you participate in the Will you accept cryptocurrency 80% service? 80% as your payment? uplink capacity in the US is 22 Mbits/s [21]. Streaming HD video 60% 60% requires an average of 5 Mbits/s throughput [20], suggesting that peers should be able to stream up to 4 HD videos in parallel. 40% 40% Experiments with an Apache web server reveal that mean and peak 20% 20% CPU utilization (single core) are 0.36% and 13%, respectively, 0% 0% Yes No Yes Maybe No when serving 4 concurrent HD videos. These numbers mildly (a) (b) increase to 1.7% and 33% for 16 concurrent video streams.2 Figure 1: Key consumer survey findings. The remaining considerations are daily participation times and earnings for peers. 65% of laptop owners in our survey stated Concern Fraction concerned that they use their computers for more than 2 hours a day. Serving Security and Privacy 82.8% Liability for Illegal Content 50.5% content for 2 hours a day on a 22 Mbits/s link amounts to 580 Performance Impact on Device 47.1% GB of data served per month. If we assume that peers earn Payment Concerns 42.1% $0.05/GB [3], they will make $29 per month, which is greater than Personal Ethics 29.4% the requirement of covering half of their monthly Internet costs (the average US plan costs $50 per month [12]). We note that these Table 1: Concerns for users who specified that they would not numbers are conservative as 66% of users would keep their laptops participate in the service. on for longer times, even when not in use, to serve (and earn) more. Concern Fraction concerned Concerns. Figure 1a shows that 49% of users would not participate Don’t Know How to Use/Sell 59.7% in the service. Table 1 lists the concerns shared by those users. Not Setup to Receive 54.7% As shown, a significant fraction of users were concerned about Volatility and Risk 49.1% the impact that the service would have on their device, both with Don’t Know What it is 21.9% respect to security and privacy, and device/network performance. Other 6.9% Thus, a practical deployment must ensure that service components running on user devices are sufficiently isolated from the rest of Table 2: Concerns for users who specified that they would not be the host device, and are capped in terms of resources consumption. willing to accept payment in cryptocurrency. Many users would not participate due to content liability concerns, stemming equally from ethical considerations, legality, and of 18-60; 52% of the participants were female; annual household in- privacy. Consequently, the majority of users were willing to serve comes were distributed between $10K-200K+, with 45% between movies/shows/news, but only 11% were willing to serve adult $25K-$100K. Each participant was asked a set of 11 questions, re- content. These preferences promote distributed content filtering, lating to payments, resource availability, and participation concerns. and the inclusion of content information to the contracts between Our key findings are: peers and content providers. • 51% of users said they would participate. The other primary user concern was with respect to payments. • Of those who would participate, 70% expect to earn no more 27% of respondents were willing to accept payments in cryptocur- than 50% of their monthly Internet bill. rency, 40% were unsure, and 33% were against it (Figure 1b). Additional questions revealed that users were predominantly • 27% of users are willing to accept payment in the form concerned with lack of familiarity with cryptocurrencies. Table 2 of cryptocurrency; 40% were unsure about cryptocurrency shows that many users did not support cryptocurrency payment payments, while 33% were unwilling. because they either did not know how to use/sell them, or they were • The largest concerns for users who would not participate were not setup to receive them. Further, a significant fraction of users security and privacy concerns (83%), liability concerns over were concerned with the volatility and risk of cryptocurrencies.