Let's look at the costs (in terms of computation) and rewards (in tokens) of the main parties involved in a rollup (both zk and optimistic).
The prover (or as I will refer to it in the case of optimistic rollups, proposer) posts some updated state to the blockchain (L1) after applying some update . In a zk-rollup, also posts a SNARK that .
[zk only]
Computing [optimistic only]
Collateral to back the claim that is correct[optimistic only]
Engaging with any interactive challengers (i.e., bisection game)The verifier runs on the L1 and determines whether or not to accept . In an optimistic rollup, this means arbitrating the dispute if a challenge is submitted; in a zk-rollup, this means running the SNARK verifier.
[zk only]
Computing [optimistic only]
Resolving dispute (e.g., a step of the evaluation of as determined by the bisection game)N/A
The challenger monitors the rollup for any faulty submissions to challenge. In the case of a zk-rollup, there is no challenger role.
[optimistic only]
Collateral to back the claim that is incorrect (i.e., to back the challenge)[optimistic only]
Engaging in an interactive challenge with (i.e., bisection game)[optimistic only]
's collateral if is rejectedThese constraints determine lower bounds for the values of , , and . The final lower bounds are bolded. The funding for (and V1) comes from transaction fees paid by users of the rollup service. This accounts for all the costs/rewards in the system.