Clique Block Choice Rule

EIPS/eip-3436.md

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+---
+eip: 3436
+title: Expanded Clique Block Choice Rule
+author: Danno Ferrin (@shemnon)
+discussions-to: https://ethereum-magicians.org/t/eip-3436-expanded-clique-block-choice-rule/5809
+status: Draft
+type: Standards Track
+category: Networking
+created: 2021-03-25
+requires: 225
+---
+
+## Simple Summary
+
+Add a four step block rule to Clique that should reduce block production deadlocks
+
+## Abstract
+
+The current specification of Clique allows for multiple competing blocks from producers but does not
+provide any strategies to pick blocks aside from the current "highest total difficulty" rule. This
+EIP proposes a four step choice rule of highest total difficulty, shortest chain, most recently
+in-turn, and lowest hash. This would prevent deadlocks that have occurred in production systems.
+
+## Motivation
+
+Recently (5 March 2021) there were a couple of deadlocks in the Goerli multi-client Clique network.
+The number of active validators was greater than 1/2 of the available validators so a chain halt
+should not have occurred. The halt was resolved by an inactive validator coming back on line. The
+state of the chain was in one of two configurations of 8 validators that can result in a chain halt.
+Three of the four clients observed a choice sequence of lowest total difficulty followed by first
+observed block. Geth added one extra rule of preferring the shortest chain before preferring the
+first observed block. This fork would have resolved itself with Geth's rule, but there is still a
+configuration where the chain can halt with a shortest chain rule.
+
+## Specification
+
+When a Click validator is choosing between two different chain head blocks to build a nre proposed
+block they should chose the blocks from the following priority list.
+
+1. Choose the block with the most total difficulty.
+2. Then choose the block with the lowest block number.
+3. Then choose the block that has had the least recent in-turn slot.
+4. Then choose the block with the lowest hash.
+
+When resolving rule 3 clients should use the following formula, where `Vi` is the integer index of
+the validator that signed the block when sorted as per epoch checkpointing, `Hn` is the number of
+the header, and `Vt` is the count of the current validators. Clients should choose the block with
+the **smallest** value.
+
+```
+(Hn - Vi) % Vt
+```
+
+When resolving rule 4 the hash should be converted into an unsigned 256 bit integer.
+
+## Rationale
+
+Two scenarios of a halted chain are known based on the current total difficulty then first observed
+rule. One of the scenarios is also resistant to the shortest chain rule.
+
+For the first scenario where chains of different lengths can halt consider a block with 8
+validators, whose addresses sort to the same order as their designation in this example. A fully
+in-order chain exists and validator number 8 has just produced an in-turn block and then validators
+5, 7 and 8 go offline, leaving validators 1 to 6 to produce blocks. Two forks form, one with an
+in-order block from validator 1 and then an out of order block from validator 3. The second fork
+forms from validators 2, 4, and 6 in order. Both have a net total difficulty of 3 more than the
+common ancestor. So in this case if both forks become aware of the other fork then both are
+considered equally viable and neither set of validators should switch to the newly observed fork. In
+this case, adding a shortest chain rule would break the deadlock as the even numbered validators
+would adopt the shorter chain.
+
+For the second scenario with the same validator set and in-order chain with validator 7 having just
+produced an in order block, then validators 7 and 8 go offline. Two forks form, 1,3,5 on one side
+and 2,4,6 on the other. Both forks become aware of the other fork after producing their third block.
+In this case both forks have equal total difficulty and equal length. So Geth's rule would not break
+the tie and only the arrival of one of the missing validators fix the chain. In a worst case
+scenario the odd and even chains would produce a block for 7 and 8 respectively, and chain halt
+would result with no validators that have not chosen a fork. Only a manual rollback would fix this.
+
+One consideration when formulating the rules is that the block choice should be chosen so that it
+would encourage the maximum amount of in-order blocks. Selecting a chain based on shortest chain
+implicitly prefers the chain with more in-order blocks. When selecting between competing out of
+order chains the validator who is closest to producing an in-order block in the future should have
+their chain declined so that they are available to produce an in-order block sooner.
+
+At least one client has been observed producing multiple blocks at the same height with the same
+difficulty, so a final catch-all standard of lowest block hash should break any remaining ties.
+
+## Backwards Compatibility
+
+The current block choice rules are a mix of most total difficulty and most total difficulty plus
+shortest chain.
+
+As long as the majority of the active validators implement the block choice rules then a client who
+only implements the existing difficulty based rule will eventually align with the chain preferred by
+these rules. If less than a majority implement these rules then deadlocks can still occur, and
+depend on the first observation of problematic blocks, which is no worse than the current situation.
+
+If clients only partially implement the rule as long as every higher ranked rule is also implemented
+then the situation will be no worse than today.
+
+## Reference Implementation
+
+Implementation in Hyperledger Besu: https://github.com/hyperledger/besu/pull/2084
+
+## Security Considerations
+
+Malicious and motivated attackers who are participating in the network can force the chain to halt
+with well crafted block production. With a fully deterministic choice rule the opportunity to halt
+is diminished. Attackers still have the same opportunities to flood the network with multiple blocks
+at the same height. A deterministic rule based on the lowest hash reduces the impact of such a
+flooding attack. A malicious validator could exploit this deterministic rule to produce a
+replacement block. Such an attack exists in current implementations but a deterministic hash rule
+makes such replacements more likely. However the impact of such an attack seems low to trivial at
+this time.
+
+## Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).