
POLKADOT: VISION FOR A HETEROGENEOUS MULTI-CHAIN FRAMEWORK DRAFT 1 5
A validat or not fulfilling their duty to find consensus
under the rules of our chosen consensus algorithm is pun-
ished. For initial, unintentional failures, this is through
withholding the validator’s reward. Repeated failures re-
sult in the reduction of their security bond (throu g h burn-
ing). Provably malicious action s such as double-signing or
conspiring to provide an invalid block result in the loss of
the entire bond (whi ch is parti al ly burnt but m o st ly given
to the informant and the honest actors).
In some sense, validators are similar to the mining pools
of current PoW blockchains.
4.2. Nominators. A nominator is a stake-holding party
who contributes to the securi ty bond of a validator. The y
have no additional role except to place risk capi t al and as
such to sign al that they trust a particular validator (or
set thereof) to act respon s ib l y in their maintenance of the
network. They receive a pro-rata increase or redu c ti o n
in their deposit a c c o rd in g to the bond’s growth to which
they contribute.
Together with collators, nex t , nominators are in some
sense similar to the miners of the present-day PoW net-
works.
4.3. Collators. Transaction collators (collators for short)
are parties who a ssi st validators in producing valid
parachain blocks. They maintain a “full-node” for a par-
ticular parachain; meaning that they retain all necessary
information to be able to autho r new blocks and execute
transactions i n much t h e same way as miners do on cur-
rent PoW blockchains. Under normal circu ms ta n c es , they
will collate and execute transaction s to create an unsealed
block, and provide it, together with a zero-knowledge
proof, to one or more valida t o rs p rese ntly responsible for
proposing a parachain block.
The precise natu re of the re la t ion sh i p between colla-
tors, nomina to rs and validators will likely change over
time. Initially, we expect collato rs to work very closely
with validators, since there will be only a few (perhaps
only one) parachain(s) with little transaction volume. The
initial client implementation will include RPCs to allow a
parachain collator node to unconditionally supply a (relay-
chain) validator node with a provably valid parachain
block. As the cost of maintaining a synced version of
all such parachains increases, we expect to see additional
infrastructure in place wh i ch will help separate out the
duties to independent, economically-motivated, parties.
Eventually, we expect to see collator pools who vie to
collect the most transaction fees. Such collators may be-
come contracted to serve particular validators over a pe-
riod of time for an on-going share in the reward proceeds.
Alternatively, “freelance” collators may simply create a
market o↵ering valid parachain blocks in return for a com-
petitive share of the reward payable immediately. Simi-
larly, decentralised nominator pools would allow multiple
bonded participa nts to coordinate and share the duty of a
validator. This ability to pool ensures open participation
leading to a more decentralised system.
4.4. Fishermen. Unlike the ot h er two active parties,
fishermen are not directly related to the block-authoring
process. Rather they are independent “bounty hunters”
motivated by a large one-o↵ reward. P rec ise ly due to
the existence of fishermen, we expect events of misbe-
haviour to happen seldom, and when they do only due to
the bonded party being ca rele ss with secret key security,
rather than through malicious intent. The name comes
from the ex pected frequency of reward, the minimal re-
quirements to take part and the eventual reward size.
Fishermen get their reward through a timely proof that
at least one bonded party acted illegally. Illegal actions
include signing two blocks each with the same ratified par-
ent or, in the c a se of parachains, help i n g ratify an invalid
block. To prevent over-rewarding of the compromise and
illicit use of a session’s secret key, the base rewa rd for
providing a single validator’s illegally signed message is
minimal. This reward in cre a ses asymptotically as more
corroborating illeg a l signatures from other validators are
provided implying a genuine attack. The asymptote is set
at 66% following our base security assertion that at least
two-thirds of the validators act benevolently.
Fishermen are somewhat similar to “full nodes” in
present-day blockchain systems that the resources needed
are relatively small and the commitment of stable uptime
and bandwidth is not necessary. Fishermen di↵er in so
much as they must post a small bond. This bond prevents
sybil attacks from wasting validators’ time and compute
resources. It is immediately withdrawable, prob a b ly no
more than the equivalent of a few dollars and m ay lead
to reaping a hefty reward from spotting a misbehaving
validator.
5. Design Overview
This section is intended to g ive a brief overview of the
system as a wh o le . A more thorough exploratio n of the
system is given in the section following it.
5.1. Consensus. On the relay-chain, Polkadot achieves
low-level consensus over a set of mutually agreed valid
blocks through a modern asynchronous Byz a ntine fault-
tolerant (BFT) algorithm. The algorithm will be inspired
by th e simple Tendermint [11] and the substantially more
involved HoneyBad g erB F T [13]. The latter provides an
efficient and fault-tolerant consensus over an arbitrarily
defective network infrastructure, given a set of mostly be-
nign authorities or validators.
For a proof-of-authority (PoA) style network, this alone
would be sufficient, however Polkadot is imagined to be
also deployable as a network in a fully open and public
situation without any particular organisation or trusted
authority required to maintain it. As such we need a
means of determining a set of validators and incentivising
them to be honest. For this we utilise PoS based selection
criteria.
5.2. Proving the Stake. We assu me that the netwo rk
will have some means of measuring how much “stake”
any parti c u la r account has. For ea se of comparison to
pre-existing systems, we will call the unit of measurement
“tokens”. Unfortunately the term is less than ideal for a
number of reasons, not least that being simply a sca l a r
value associated with an acco u nt, there is no notion of
individuality.
We imagine validators be elected, infrequently ( a t most
once per day but perhaps as seldom as o n ce per quarter),
through a Nominated Proof-of-Stake (NPoS )scheme.In-
centivisation can happen through a pro-rata allocation of