117 lines
6.3 KiB
Markdown
117 lines
6.3 KiB
Markdown
---
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eip: 3436
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title: Expanded Clique Block Choice Rule
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author: Danno Ferrin (@shemnon)
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discussions-to: https://ethereum-magicians.org/t/eip-3436-expanded-clique-block-choice-rule/5809
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status: Stagnant
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type: Standards Track
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category: Core
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created: 2021-03-25
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requires: 225
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---
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## Simple Summary
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Add a four step block rule to Clique that should reduce block production deadlocks
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## Abstract
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The current specification of Clique allows for multiple competing blocks from producers but does not
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provide any strategies to pick blocks aside from the current "highest total difficulty" rule. This
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EIP proposes a four step choice rule of highest total difficulty, shortest chain, most recently
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in-turn, and lowest hash. This would prevent deadlocks that have occurred in production systems.
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## Motivation
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There has been more than one deadlock in the Goerli multi-client Clique network. The number of
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active validators was greater than 1/2 of the available validators so a chain halt should not have
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occurred. The halt was resolved by an inactive validator coming back on line. The state of the chain
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was in one of two configurations of 8 validators that can result in a chain halt. Three of the four
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clients observed a choice sequence of lowest total difficulty followed by first observed block. Geth
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added one extra rule of preferring the shortest chain before preferring the first observed block.
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This fork would have resolved itself with Geth's rule, but there is still a configuration where the
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chain can halt with a shortest chain rule.
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## Specification
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When a Clique validator is arbitrating the canonical status between two different chain head blocks,
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they should choose the canonical block with the following ordered priorities.
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1. Choose the block with the most total difficulty.
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2. Then choose the block with the lowest block number.
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3. Then choose the block whose validator had the least recent in-turn block assignment.
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4. Then choose the block with the lowest hash.
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When resolving rule 3 clients should use the following formula, where `validator_index` is the integer
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index of the validator that signed the block when sorted as per epoch checkpointing,
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`header_number` is the number of the header, and `validator_count` is the count of the current
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validators. Clients should choose the block with the **largest** value. Note that an in-turn block
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is considered to be the most recent in-turn block.
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```
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(header_number - validator_index) % validator_count
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```
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When resolving rule 4 the hash should be converted into an unsigned 256 bit integer.
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## Rationale
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Two scenarios of a halted chain are known based on the current total difficulty then first observed
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rule. One of the scenarios is also resistant to the shortest chain rule.
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For the first scenario where chains of different lengths can halt consider a block with 8
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validators, whose addresses sort to the same order as their designation in this example. A fully
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in-order chain exists and validator number 8 has just produced an in-turn block and then validators
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5, 7 and 8 go offline, leaving validators 1 to 6 to produce blocks. Two forks form, one with an
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in-order block from validator 1 and then an out of order block from validator 3. The second fork
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forms from validators 2, 4, and 6 in order. Both have a net total difficulty of 3 more than the
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common ancestor. So in this case if both forks become aware of the other fork then both are
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considered equally viable and neither set of validators should switch to the newly observed fork. In
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this case, adding a shortest chain rule would break the deadlock as the even numbered validators
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would adopt the shorter chain.
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For the second scenario with the same validator set and in-order chain with validator 7 having just
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produced an in order block, then validators 7 and 8 go offline. Two forks form, 1,3,5 on one side
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and 2,4,6 on the other. Both forks become aware of the other fork after producing their third block.
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In this case both forks have equal total difficulty and equal length. So Geth's rule would not break
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the tie and only the arrival of one of the missing validators fix the chain. In a worst case
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scenario the odd and even chains would produce a block for 7 and 8 respectively, and chain halt
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would result with no validators that have not chosen a fork. Only a manual rollback would fix this.
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One consideration when formulating the rules is that the block choice should be chosen so that it
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would encourage the maximum amount of in-order blocks. Selecting a chain based on shortest chain
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implicitly prefers the chain with more in-order blocks. When selecting between competing out of
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order chains the validator who is closest to producing an in-order block in the future should have
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their chain declined so that they are available to produce an in-order block sooner.
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At least one client has been observed producing multiple blocks at the same height with the same
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difficulty, so a final catch-all standard of lowest block hash should break any remaining ties.
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## Backwards Compatibility
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The current block choice rules are a mix of most total difficulty and most total difficulty plus
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shortest chain.
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As long as the majority of the active validators implement the block choice rules then a client who
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only implements the existing difficulty based rule will eventually align with the chain preferred by
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these rules. If less than a majority implement these rules then deadlocks can still occur, and
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depend on the first observation of problematic blocks, which is no worse than the current situation.
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If clients only partially implement the rule as long as every higher ranked rule is also implemented
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then the situation will be no worse than today.
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## Security Considerations
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Malicious and motivated attackers who are participating in the network can force the chain to halt
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with well crafted block production. With a fully deterministic choice rule the opportunity to halt
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is diminished. Attackers still have the same opportunities to flood the network with multiple blocks
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at the same height. A deterministic rule based on the lowest hash reduces the impact of such a
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flooding attack. A malicious validator could exploit this deterministic rule to produce a
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replacement block. Such an attack exists in current implementations but a deterministic hash rule
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makes such replacements more likely. However the impact of such an attack seems low to trivial at
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this time.
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## Copyright
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Copyright and related rights waived via [CC0](../LICENSE.md).
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