# How PBS works and why it's important to Ethereum
## Recap: hard-to-discover MEV
* Internal Uniswap arbitrage (A -> B, B -> C, C -> A)
* Uniswap vs Sushiswap arbitrage (A -> B on UNI, B -> A on SUSHI)
* Uniswap vs offchain exchange arbitrage (A -> B on UNI, B -> A on Binance)
* Liquidations (eg. of MakerDAO CDPs)
* NFT purchases
## Why is hard-to-discover MEV a problem?
* MEV discovery requires specialized software that needs to be constantly updated
* Only specialized actors can effectively optimize MEV revenue
* Large pools can hire them -> creates pressure for stakers to join large pools
## Proposer / builder separation
* Goal: separate out the task with strong centralizing pressures (selection of block contents) from the rest of block validation
* Idea: block proposers sell the right to decide the contents of their block to the highest bidder, on-chain
## Current proposed design
## Security properties of the current design
* Payment to the proposer is unconditional (so proposers do not need to trust builders)
* Builder has to reveal on time. If they do not, they pay their bid to the proposer but do not collect fees from the block
* No risk of "MEV stealing" in the normal case: builder only reveals once block that confirms winning bid is "safe"
* Censorship of builder requires 51% attack on committee
## Challenges with the current design
* Involves multiple steps per slot, requiring increased block times or stronger latency assumptions
* If one builder consistently wins, can they censor?