Brendan from Polygon presented ‘Agg Layer’, the Polygon’s solution to solve fragmentation between zk rollups that may not share sequencers.
Bridging assets between zk rollups can result in a latency penalty of over 25 minutes (using native bridges) due to withdrawal submissions, proof generation, waiting for batch submission, and deposit transactions.
The Agg Layer, in conjunction with coordination infrastructure like shared sequencing, aims to solve this problem. It’s defined as a neutral public infrastructure that provides safety for asynchronous and synchronous cross-chain interoperability and a shared bridge.
It works by accepting blocks from L2s before they’re settled on Ethereum. It checks that cross-L2 transactions are safe and that superintents (transaction bundles between different rollups) are satisfied and included across chains by aggregating validity proofs of each zk rollup.
The Agg layer is not an L2 or a shared sequencer. It doesn’t require a specific VM, doesn’t limit the tokens that chains use, doesn’t require submission to common governance, and doesn’t require a specific or prescribed shared sequencer or even a shared sequencer at all.
The Agg Layer proves three things:
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It proves that cross-L2 transactions are valid
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It proves that the bridge invariant is respected, meaning no chain can withdraw more funds than are deposited on that chain.
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It provides an aggregated validity proof of different zk rollups to L1.
It also provides a shared bridge and shared deposit contract to enable asset fungibility for L1 and L2 native assets, avoiding the need for a wrapped synthetic on the destination chain or the need to route through L1. It tracks aggregate token balances for each chain and ensures that withdrawals do not cause a chain to go negative, which they call ‘Interchain accounting proof’.
When a chain submits a block or a batch to the Agg Layer, it looks at what is called the ‘local exit tree of messages’. These are all of the asset transfers that are coming out of that chain, and they check all of them to ensure that the withdrawals do not cause the chain to go negative.
They’re basically mirroring the same guarantees that would exist if that chain were just directly posting proofs to L1.
Agg Layer is designed to be proof-agnostic, allowing integration with various rollups regardless of their underlying proof mechanism (e.g., zk-SNARKs, STARKs).
It’s still unclear who will run the Agg Layer, it could be $POL stakers (the new Polygon’s coin), an EigenLayer’ AVS, or some other mechanism.
It’s assumed that just zk rollups can join the Agg Layer, as Optimistic rollups cannot provide fast interoperability without depending on a challenge period.
Astar zkEVM, a rollup built with Polygon CDK, is the first zk rollup connected to the Agg Layer, tapping into Polygon zkEVM’ liquidity and enabling cross-chain transactions between both rollups.
It seems Cosmos’ app-chain thesis is being validated more time over time as every ecosystem is choosing to follow its path:
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Optimism – Superchain
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zkSync – Hyperchains
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Arbitrum – Orbit L3s