Alternate PBS: A PBS Proposal for Based Rollups

[CTra1n]

@jsky

My understanding is that this design allows for multiple L2 blocks in a given L1 block, but doesn’t provide pre confirmations on those blocks, is this correct?

You’re right. The multiple blocks decision was more to keep it inline with the current Taiko design, as opposed to a hard and fast rule. The notion of preconfirmations is still possible (I briefly mentioned this to @JustinDrake), although this would require an even more complex backdoorFee() function. One possibility would be to drop backdoorFee() to 0 for designated preconfers. This would have other implications, but all preconf hope is not lost!

@jsky

Are there more details on how proposing a block via the backdoorFee is done? My assumption is that the rollup’s fork choice is such that a trusted proposer block is used unless a backdoorFee greater than the floor is present in the L1 block for a given height. In the event there are multiple backdoorFees the highest paying one is chosen.

So there is only one value for backdoorFee() in any given block. If multiple builders want to pay it, we expect the highest bribe to get ordered first in the L1 block; just as it is done in Taiko today without the fee.

@jsky

payment of a backdoorFee to include a block essentially becomes a MEV auction above a certain price tier

This is exactly right.

@CTra1n

We envision a reactive _backdoorFee(), which drops quickly when L2 blocks have not been proposed, but not so quickly so as to incentivize concurrent-block L1 strategies from L1 builders (e.g. censor L2 blocks in the current L1 block to drop _backdoorFee() to 0 in the next L1 block, then extracting L2 value “for free” in that block).

The third issue is something that we are also concerned about. Ideally there is “big trust” in the trusted set (high unreactive backdoorFee()) to mitigate “Profitable Censorship MEV”. There are other censorship vectors, but a hope would be the cost to censor corresponds almost directly with reduced auction bids.