POLKADOT: VISION FOR A HETEROGENEOUS MULTI-CHAIN FRAMEWORK DRAFT 1 5 A validator not fulfilling their duty to find consensus the existence of fishermen, we expect events of misbe- under the rules of our chosen consensus algorithm is pun- haviour to happen seldom, and when they do only due to ished. For initial, unintentional failures, this is through the bonded party being careless with secret key security, withholding the validator’s reward. Repeated failures re- rather than through malicious intent. The name comes sult in the reduction of their security bond (through burn- from the expected frequency of reward, the minimal re- ing). Provably malicious actions such as double-signing or quirements to take part and the eventual reward size. conspiring to provide an invalid block result in the loss of Fishermen get their reward through a timely proof that the entire bond (which is partially burnt but mostly given at least one bonded party acted illegally. Illegal actions to the informant and the honest actors). include signing two blocks each with the same ratified par- Insomesense,validatorsaresimilartotheminingpools ent or, in the case of parachains, helping ratify an invalid of current PoW blockchains. block. To prevent over-rewarding of the compromise and illicit use of a session’s secret key, the base reward for 4.2. Nominators. A nominator is a stake-holding party providing a single validator’s illegally signed message is who contributes to the security bond of a validator. They minimal. This reward increases asymptotically as more have no additional role except to place risk capital and as corroborating illegal signatures from other validators are such to signal that they trust a particular validator (or provided implying a genuine attack. The asymptote is set set thereof) to act responsibly in their maintenance of the at 66% following our base security assertion that at least network. They receive a pro-rata increase or reduction two-thirds of the validators act benevolently. in their deposit according to the bond’s growth to which Fishermen are somewhat similar to “full nodes” in they contribute. present-day blockchain systems that the resources needed Together with collators, next, nominators are in some are relatively small and the commitment of stable uptime sense similar to the miners of the present-day PoW net- and bandwidth is not necessary. Fishermen differ in so works. muchastheymustpostasmallbond. Thisbondprevents sybil attacks from wasting validators’ time and compute 4.3. Collators. Transactioncollators (collators for short) resources. It is immediately withdrawable, probably no are parties who assist validators in producing valid more than the equivalent of a few dollars and may lead parachain blocks. They maintain a “full-node” for a par- to reaping a hefty reward from spotting a misbehaving ticular parachain; meaning that they retain all necessary validator. information to be able to author new blocks and execute 5. Design Overview transactions in much the same way as miners do on cur- rent PoW blockchains. Under normal circumstances, they This section is intended to give a brief overview of the will collate and execute transactions to create an unsealed system as a whole. A more thorough exploration of the block, and provide it, together with a zero-knowledge system is given in the section following it. proof, to one or more validators presently responsible for 5.1. Consensus. On the relay-chain, Polkadot achieves proposing a parachain block. low-level consensus over a set of mutually agreed valid The precise nature of the relationship between colla- blocks through a modern asynchronous Byzantine fault- tors, nominators and validators will likely change over tolerant (BFT) algorithm. The algorithm will be inspired time. Initially, we expect collators to work very closely by the simple Tendermint [11] and the substantially more with validators, since there will be only a few (perhaps involved HoneyBadgerBFT [13]. The latter provides an only one) parachain(s) with little transaction volume. The efficient and fault-tolerant consensus over an arbitrarily initial client implementation will include RPCs to allow a defective network infrastructure, given a set of mostly be- parachaincollator node to unconditionally supply a (relay- nign authorities or validators. chain) validator node with a provably valid parachain Foraproof-of-authority (PoA)style network, this alone block. As the cost of maintaining a synced version of would be sufficient, however Polkadot is imagined to be all such parachains increases, we expect to see additional also deployable as a network in a fully open and public infrastructure in place which will help separate out the situation without any particular organisation or trusted duties to independent, economically-motivated, parties. authority required to maintain it. As such we need a Eventually, we expect to see collator pools who vie to means of determining a set of validators and incentivising collect the most transaction fees. Such collators may be- them to be honest. For this we utilise PoS based selection come contracted to serve particular validators over a pe- criteria. riod of time for an on-going share in the reward proceeds. Alternatively, “freelance” collators may simply create a 5.2. Proving the Stake. We assume that the network market offering valid parachain blocks in return for a com- will have some means of measuring how much “stake” petitive share of the reward payable immediately. Simi- any particular account has. For ease of comparison to larly, decentralised nominator pools would allow multiple pre-existing systems, we will call the unit of measurement bonded participants to coordinate and share the duty of a “tokens”. Unfortunately the term is less than ideal for a validator. This ability to pool ensures open participation number of reasons, not least that being simply a scalar leading to a more decentralised system. value associated with an account, there is no notion of individuality. 4.4. Fishermen. Unlike the other two active parties, Weimaginevalidators be elected, infrequently (at most fishermen are not directly related to the block-authoring once per day but perhaps as seldom as once per quarter), process. Rather they are independent “bounty hunters” through a Nominated Proof-of-Stake (NPoS) scheme. In- motivated by a large one-off reward. Precisely due to centivisation can happen through a pro-rata allocation of