POLKADOT: VISION FOR A HETEROGENEOUS MULTI-CHAIN FRAMEWORK DRAFT 1 3 2.2.2. Heterogeneous Chain Systems. Side-chains [3] is a remains unseen as to how Casper will iterate in the future proposed addition to the Bitcoin protocol which would al- and what it will look like should it finally be deployed. low trustless interaction between the main Bitcoin chain While Casper and Polkadot both represent interest- and additional side-chains. There is no provision for any ing new protocols and, in some sense, augmentations of degree of ‘rich’ interaction between side-chains: the in- Ethereum, there are substantial differences between their teraction would be limited to allowing side-chains to be ultimate goals and paths to deployment. Casper is an custodians of each other’s assets, effecting—in the local EthereumFoundation-centeredprojectoriginallydesigned jargon—a two-way peg3. The end vision is for a frame- to be a PoS alteration to the protocol with no desire to work where the Bitcoin currency could be provided with create a fundamentally scalable blockchain. Crucially, it is additional, if peripheral, functionality through pegging it designed to be a hard-fork, rather than anything more ex- onto some other chains with more exotic state transition pansive and thus all Ethereum clients and users would be systems than the Bitcoin protocol allows. In this sense, required to upgrade or remain on a fork of uncertain adop- side-chains addresses extensibility rather than scalability. tion. As such, deployment is made substantially more dif- Indeed, there is fundamentally no provision for the va- ficult as is inherent in a decentralised project where tight lidity of side-chains; tokens from one chain (e.g. Bitcoin) coordination is necessary. held on behalf of a side-chain are secured only by the Polkadot differs in several ways; first and foremost, side-chain’s ability to incentivise miners to canonicalise Polkadot is designed to be a fully extensible and scalable valid transitions. The security of the Bitcoin network blockchain development, deployment and interaction test cannot easily be transitioned to work on behalf of other bed. It is built to be a largely future-proof harness able to blockchains. Furthermore, a protocol for ensuring Bitcoin assimilate new blockchain technology as it becomes avail- miners merge-mine (that is duplicate their canonicalisa- able without over-complicated decentralised coordination tion power onto that of the side-chain) and, more impor- or hard forks. We already envision several use cases such tantly, validate the side-chain’s transitions is outside the as encrypted consortium chains and high-frequency chains scope of this proposal. with very low block times that are unrealistic to do in Cosmos [10] is a proposed multi-chain system in the any future version of Ethereum currently envisioned. Fi- same vein as side-chains, swapping the Nakamoto PoW nally, the coupling between it and Ethereum is extremely consensus method for Jae Kwon’s Tendermint algorithm. loose; no action on the part of Ethereum is necessary to Essentially, it describes multiple chains (operating in enable trustless transaction forwarding between the two zones) each using individual instances of Tendermint, to- networks. gether with a means for trust-free communication via a In short, while Casper/Ethereum 2.0 and Polkadot master hub chain. This interchain communication is lim- share some fleeting similarities we believe their end goal ited to the transfer of digital assets (“specifically about to- is substantially different and that rather than competing, kens”) rather than arbitrary information, however such in- the two protocols are likely to ultimately co-exist under a terchain communication does have a return path for data, mutually beneficial relationship for the foreseeable future. e.g. to report to the sender on the status of the transfer. Validator sets for the zoned chains, and in particular the means of incentivising them, are, like side-chains, left 3. Summary as an unsolved problem. The general assumption is that each zoned chain will itself hold a token of value whose in- Polkadot is a scalable heterogeneous multi-chain. This flation is used to pay for validators. Still in the early stages means that unlike previous blockchain implementations of design, at present the proposal lacks comprehensive de- which have focused on providing a single chain of varying tails over the economic means of achieving the scalable degrees of generality over potential applications, Polka- certainty over global validity. However, the loose coher- dot itself is designed to provide no inherent applica- ence required between the zones and the hub will allow tion functionality at all. Rather, Polkadot provides the for additional flexibility over the parameters of the zoned bedrock “relay-chain” upon which a large number of val- chains compared to that of a system enforcing stronger idatable, globally-coherent dynamic data-structures may coherence. be hosted. We call these data-structures “parallelised” chains or parachains, though there is no specific need for them to be blockchain in nature. 2.2.3. Casper. As yet no comprehensive review or side- In other words, Polkadot may be considered equiva- by-side comparison between Casper [7] and Polkadot lent to a set of independent chains (e.g. the set containing have been made, though one can make a fairly sweeping Ethereum, Ethereum Classic, Namecoin and Bitcoin) ex- (and accordingly inaccurate) characterisation of the two. cept for two very important points: Casper is a reimagining of how a PoS consensus algorithm • Pooled security; could be based around participants betting on which fork • trust-free interchain transactability. would ultimately become canonical. Substantial consider- ation was given to ensuring that it be robust to network These points are why we consider Polkadot to be “scal- forks, even when prolonged, and have some additional de- able”. In principle, a problem to be deployed on Polka- gree of scalability on top of the basic Ethereum model. As dot may be substantially parallelised—scaled out—over such, Casper to date has tended to be a substantially more a large number of parachains. Since all aspects of each complex protocol than Polkadot and its forebears, and a parachain may be conducted in parallel by a different seg- substantial deviation from the basic blockchain format. It ment of the network, the system has some ability to scale. 3as opposed to a one-way peg which is essentially the action of destroying tokens in one chain to create tokens in another without the mechanism to do the converse in order to recover the original tokens