THE THETA BLOCKCHAIN

Transcription

1 THE THETA BLOCKCHAIN Theta is a decentalized video steaming netwok, poweed by a new blockchain and token. By Theta Labs, Inc. Last Updated: Nov 21, 2017 esion 1.0 1

2 OUTLINE Motivation Reputation Dependent Mining Theta is a decentalized video delivey Reputation Scoe Calculation netwok, poweed by uses and an innovative Global Reputation Consensus new blockchain. Suppot fo Pooled Mining Futue Wok 2

3 MOTIATION ideo steaming challenges Last-mile delivey poblem: Content delivey netwok (CDN) seves too fa CDN fom end viewes, especially in developing counties. High CDN bandwidth cost: CDN cost fo popula steaming sites could be tens of millions of dollas pe yea POP Poposal: Encouage viewes to shae thei edundant bandwidth esouces to elay video steam to thei neighboing viewes Caching Nodes (mine) Caching nodes: viewes that shae thei computes to elay video steams iewes ae close to each othe geogaphically, which help addess the last-mile steam delivey poblem iewes Could educe CDN bandwidth cost significantly if most of the end viewes pull steam fom pee nodes iewes 3

4 MOTIATION The Theta potocol: a blockchain based incentive mechanism to encouage viewes to shae thei bandwidth CDN Incentive mechanism: taditional model Tempting to follow taditional model whee viewes send Theta tokens to POP caching nodes fo thei video elay sevice Howeve, this model is impactical and not sustainable in the long-un since today viewes ae aleady used to fee videos on Youtube/Twitch, etc. Caching Nodes (mine) Incentive mechanism: eputation dependent mining Caching nodes play the ole of blockchain mines They obtain mining ewads fo assembling new blocks and elaying video steams iewes But unlike Bitcoin, the block ewad is not a constant A caching node can gain eputation by elaying video steams With a highe eputation scoe, caching nodes can claim highe block ewads iewes 4

5 REPUTATION DEPENDENT MINING Reputation dependent block ewad ewad = (1 + ) ewad base CDN is the eputation scoe of the caching node that mined the new block, ewad base is a constant shaed among all mines POP High eputation scoe leads to high block ewad Reputation scoe A eal numbe between 0 and 1 which measues the amount of video elaying Caching Nodes (mine) wok a caching node pefomed duing a ecent time window Moe video elaying wok leads to a highe eputation scoe Recent time window: e.g. the past 24 hous, we account fo ecency so a caching node cannot ely on wok done in the distant past to claim high eputation scoe iewes Key design elements follow How Theta calculates the eputation scoe When a new block is mined, how Theta poves its eputation scoe to othe iewes mines and fom a global consensus 5

6 REPUTATION DEPENDENT MINING Sevice Cetificate Block 100 Block 101 Block 102 A sevice cetificate is a solution to the sevice cetificate PoW puzzle, defined as a minimization poblem in the ight figue Requies hashing powe to compute the solution, hence difficult to fake without spending hashing powe Assume * is the best solution viewe j found. The coesponding hash value H* eflects the time viewe j spent to find the solution, which is equal to the time caching node i seved viewe j H* = HASH(pki pkj block_hash k *) tansactions tansactions tansactions The expected amount of time node i seved viewe j E[t H*] = M / (s H*), whee s is the equied numbe of hashes pe unit time The HASH function needs to be ASIC esistant to educe the advantage of using special-pupose mining hadwae block_hash 100 block_hash 101 block_hash 102 Sevice cetificate PoW puzzle Sevice cetificate * = agmin HASH(pk i pk j block_hash k ) (pk i, pk j, block_hash k, )* 6

7 REPUTATION DEPENDENT MINING Sevice Cetificate Block 100 Block 101 Block 102 The caching node sends a new sevice cetificate PoW puzzle to its downsteam viewes wheneve a new block is added to the longest chain Each puzzle in effect points to a specific block though the block_hash k paamete Thus we can use the coesponding block to infe the geneation time of the sevice cetificate tansactions tansactions tansactions block_hash 100 block_hash 101 block_hash 102 Sevice cetificate PoW puzzle Sevice cetificate * = agmin HASH(pk i pk j block_hash k ) (pk i, pk j, block_hash k, )* 7

8 REPUTATION DEPENDENT MINING The solution Caching mine node i elays video steam to viewe node j as a sevice In etun, viewe j needs to spend its hashing powe compute sevice cetificates and sends back to caching node i egulaly (e.g. evey minute) If viewe j stops sending the sevice cetificates, caching mine node i can teminate the video steam Sevice Cetificate ideo Steam Wheneve caching node i mines a new block, it gathes all the sevice cetificates eceived in the defined time window (e.g. the past 24 hous) The caching mine node can then calculate its eputation scoe using these sevice cetificates The caching mine node then pesents these sevice cetificates to othe mines to justify its eputation scoe, and thus its block ewad Sevice cetificate PoW puzzle * = agmin HASH(pk i pk j block_hash k ) Sevice cetificate (pk i, pk j, block_hash k, )* 8

9 REPUTATION SCORE CALCULATION Reputation scoe calculation Given a sevice cetificate sc = (pk i, pk j, block_hash k, *), the expected amount of time node i seved node j can be calculated with T sc = min( E[t H*], T b ) = min(m / (s H*), T b ), whee T b is the expected amount of time to mine a new block Thus, the expected total amount of time node i seved its downsteam nodes can be calculated as the sum of T sc fo each sevice eceipt it collected T total = SUM(T sc ) The eputation scoe = f (T total ) is a monotonically inceasing function of T total which maps T total to a eal numbe in the ange of [0, 1] as shown in the ight figue 9

10 REPUTATION SCORE CALCULATION Reputation scoe calculation Can we design = f (T total ) such that a ational mine would not ty to foge fake sevice cetificates? The answe is yes. Intuitively, a mine needs to spend hashingpowe to make fake sevice cetificates. Thus if it ties to make fake eceipts, its chance of mining the next block educes. It is possible to design = f (T total ) such that the expected mining ewad pe unit time stictly educes if the mine splits its hashing powe to make fake sevice cetificates. See the Appendix in the whitepape fo futhe analysis. 10

11 GLOBAL REPUTATION CONSENSUS Extend the blockchain data stuctue to facilitate global eputation consensus So that othe mines can validate the eputation scoe of the mine that mined the new block Block k Block heade additional fields Reputation scoe of the mine that mined the block H sc H sc, the hash of the concatenation of all sevice cetificates Note that in the following mining PoW puzzle, the block_heade needs to contain and H sc to pevent potential tampeing tansactions HASH(block_heade ) < R Block body Adding the sevice cetificates to the block body. Typical size is 100K+ bytes pe block fo individual caching mines sevice cetificates 11

12 GLOBAL REPUTATION CONSENSUS The blockchain potocol Once a mine solves the PoW mining puzzle, it assembles a new block with the additional fields descibed in the pevious slide including its eputation scoe calculated with the sevice cetificates it collected The mine sends the new block though the netwok Block k Othe mines validate the PoW mining puzzle solution, the Theta token tansactions, and veify the eputation scoe using the sevice cetificates embedded in the block H sc tansactions sevice cetificates 12

13 SUPPORT FOR POOLED MINING Pooled Mining Pooled mining is a popula appoach to educe mining ewad vaiance On the Theta blockchain, mines can also fom pools. Simila to Bitcoin mining pools, mines solve the PoW mining puzzle on behalf of the maste. In addition, thei downsteam viewes geneate the sevice cetificates fo the maste, by using the public key of the maste as pki in the sevice cetificate PoW puzzle New pooled mining potocols: ewad split fo a caching node is not just based on the numbe of nea-miss solutions found fo the mining puzzles, but also on the numbe of sevice cetificates it collected Block k H sc tansactions Challenges with Pooled Mining The size of sevice cetificates fo each block could potentially be lage, since a pool can seve many viewes. This inceases both bandwidth and stoage ovehead sevice cetificates 13

14 SUPPORT FOR POOLED MINING In the next couple slides, we pesent seveal techniques to addess the two challenges mentioned ealie Technique #1: Limit the total numbe of sevice cetificates pe block Mines need to select the best subset of solutions to the sevice cetificate PoW puzzles This puts a limit of the size of the sevices cetificates fo each block 14

15 SUPPORT FOR POOLED MINING Technique #2: Less fequent sevice cetificate PoW puzzle change No need to change the block_hash in the sevice cetificate PoW puzzle each time a new block is added We can use the same puzzle fo d consecutive blocks. In the example below, d = 3 This educes the total numbe of cetificates given the same amount of sevice time H oot H oot H oot H oot H oot H oot block_hash 100 block_hash 101 block_hash 102 block_hash 103 block_hash 104 block_hash 105 * = agmin HASH(pk i pk j block_hash 100 ) * = agmin HASH(pk i pk j block_hash 103 ) 15

16 SUPPORT FOR POOLED MINING Technique #3: Randomized eputation scoe validation potocol The sevice cetificates ae stoed off-chain (eithe on the mine o decentalized stoage systems) The sevice cetificates ae split into small sets (each 100K+ bytes), and oganized into a Mekle tee. The heade of each set contains the set index On-Chain Off-Chain T 1... T n T 1... T n T 1... T n Hoot Hoot H oot The block heade contains the T 1, T 2,, T n, the expected total viewing time fo each set of sevice cetificates 16

17 T 1... T n Hoot T 1... T n Hoot T 1... T n On-Chain H oot Off-Chain H oot H 1234 H 12 H 34 H n-1, n H 1 H 2 H 3 H 4 H n-1 H n heade 1 T 1 2 T 2 3 T 3 4 T 4 n - 1 T n-1 n T n sevice cetificates Set 1 Set 2 Set 3 Set 4 Set n-1 Set n 17

18 SUPPORT FOR POOLED MINING Technique #3: Randomized eputation scoe validation potocol Once a validato mine eceives a new block, it andomly selects and downloads k sets of sevice cetificates and thei Mekle banches, and veifies the viewing time of each of these set matches with the coesponding watching time in the heade If a validato mine finds a mismatch, it popagates the set of cetificates thoughout the netwok. Othe mines can then validate this set and decide whethe to eject the block On-Chain Off-Chain T 1... T n T 1... T n T 1... T n Hoot Hoot H oot The pobability that the majoity of mines accept an invalid block appoaches zeo as moe mines join the netwok 18

19 FUTURE WORK Compess the sevice cetificates futhe to educe bandwidth and stoage ovehead May be possible to geneate a succinct non-inteactive poof of the sevice cetificates fo the mining pools Incopoate Poof-of-Stake to educe enegy waste Sevice cetificate computation equies hashing powe, and could lead to enegy waste. We can design the block ewad to be depending on the stake the caching node owns. Thus, instead of sevice cetificate, a viewe can send token guaantees to the upsteam caching node duing the steaming session. The caching node etuns the same amount of tokens to the viewe afte the steaming session (enfoced by a smat contact). Since the block ewad depends on the caching node s stake, the viewe still gets the video steam fo fee, but the expected mining ewad of the upsteam caching node inceases. This only equies a slight modification of the block ewad fomula ewad = (1 + + g(stake)) ewad base g(stake) is a monotonically inceasing function of the stake (i.e. the total amount of tokens) the caching node owns 19

20 FUTURE WORK Heteogeneous mining pool Achitectue Maste manages the ewad split. The maste can be just a smat contact (simila to smatpool.io). maste Mines solve mining puzzles and assemble blocks. Thei pimay job is to secue the system. They do not elay video steams. Caching nodes elay video steam and collect sevice cetificates and token guaantees. They don t need to assemble the blocks. iewes can submit eithe sevice cetificates o token guaantees in exchange fo video steams. token guaantees / sevice cetificates mining puzzle solutions Mining ewad split A mine gets the mining ewad split based on how many nea-miss solutions it found. A caching node gets the split based on how many sevice cetificates and token guaantees it collected sevice cetificates token guaantees mines Benefits The caching nodes can get mining ewad split without computing hashes to solve the mining puzzle. They ae ewaded puely fo the elay sevice. If a viewe chooses to send token guaantees instead of sevice cetificates, he doesn t need to pefom exta computation. This allows low-end devices (e.g. mobile phones) to benefit fom the elay sevice. 20