90 lines
4.2 KiB
Markdown
90 lines
4.2 KiB
Markdown
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# Optimistic Confirmation and Slashing
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Progress on optimistic confirmation can be tracked here
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https://github.com/solana-labs/solana/projects/52
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At the end of May, the mainnet-beta is moving to 1.1, and testnet
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is moving to 1.2. With 1.2, testnet will behave as if we have 1-block
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conf as long as no more than 4.66% of the validators are acting
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maliciously. Applications can assume that 2/3+ votes observed in
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gossip confirm a block or that at least 4.66% of the network is
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violating the protocol.
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## How does it work?
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The general idea is that validators have to continue voting, following
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their last fork, unless they can construct a proof that their fork
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may not reach finality. The way validators construct this proof is
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by collecting votes for all the other forks, excluding their own.
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If the set of valid votes represents over 1/3+X of the epoch stake
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weight, there is may not be a way for the validators current fork
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to reach 2/3+ finality. The validator hashes the proof (creates a
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witness) and submits it with their vote for the alternative fork.
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But if 2/3+ votes for the same block, it is impossible for any of
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the nodes to construct this proof, and therefore no node is able
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to switch forks and this block will be eventually finalized.
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## Tradeoffs
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The safety margin is 1/3+X, where X represents the minimum amount
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of stake that will be slashed in case the protocol is violated. The
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tradeoff is that liveness is now reduced by 2X in the worst case.
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If more than 1/3 - 2X of the network is unavailable, the network
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may stall and will resume finalizing blocks after the network
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recovers. So far, we haven’t observed a large unavailability hit
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on our mainnet, cosmos, or tezos. For our network, which is primarily
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composed of high availability systems, this seems unlikely. Currently,
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we have set the threshold percentage to 4.66%, which means that if
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23.68% have failed the network may stop finalizing blocks. For our
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network, which is primarily composed of high availability systems
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a 23.68% drop in availabilty seems unlinkely. 1:10^12 odds assuming
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five 4.7% staked nodes with 0.995 of uptime.
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## Security
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Long term average votes per slot has been 670,000,000 votes /
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12,000,000 slots, or 55 out of 64 voting validators. This includes
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missed blocks due to block producer failures. When a client sees
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55/64, or ~86% confirming a block, it can expect that ~24% or (86
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- 66.666.. + 4.666..)% of the network must be slashed for this
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block to fail full finalization.
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## Why Solana?
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This approach can be built on other networks, but the implementation
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complexity is significantly reduced on Solana because our votes
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have provable VDF-based timeouts. It’s not clear if switching proofs
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can be easily constructed in networks with weak assumptions about
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time.
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## Slashing roadmap
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Slashing is a hard problem, and it becomes harder when the goal of
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the network is to be the fastest possible implementation. The
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tradeoffs are especially apparent when optimizing for latency. For
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example, we would really like the validators to cast and propagate
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their votes before the memory has been synced to disk, which means
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that the risk of local state corruption is much higher.
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Fundamentally, our goal for slashing is to slash 100% in cases where
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the node is maliciously trying to violate safety rules and 0% during
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routine operation. How we aim to achieve that is to first implement
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slashing proofs without any automatic slashing whatsoever.
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Right now, for regular consensus, after a safety violation, the
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network will halt. We can analyze the data and figure out who was
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responsible and propose that the stake should be slashed after
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restart. A similar approach will be used with a optimistic conf.
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An optimistic conf safety violation is easily observable, but under
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normal circumstances, an optimistic confirmation safety violation
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may not halt the network. Once the violation has been observed, the
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validators will freeze the affected stake in the next epoch and
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will decide on the next upgrade if the violation requires slashing.
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In the long term, transactions should be able to recover a portion
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of the slashing collateral if the optimistic safety violation is
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proven. In that scenario, each block is effectively insured by the
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network.
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