Polygon unpacks zkEVM outage and ‘emergency’ upgrade

Polygon’s zkEVM rollup entered an emergency state on March 23 after the chain stopped processing blocks. Last week, Polygon published an analysis of the failure that brought down his zkEVM.

polygon report Late Saturday, it was announced that its zkEVM sequencer had failed, and the cause was attributed to a realignment of the Ethereum mainnet.after death state The problem is that Polygon zkEVM’s synchronizer incorrectly handled the REORG and the chain started processing a batch of transactions with incorrect timestamps.

A blockchain reorganization (reorg) occurs when a network discards previous versions of blocks and adopts a new block chain as the accurate history of transactions. This can occur when different parts of the network temporarily disagree regarding the order or validity of recent transactions. The network eventually converges on his one version of the truth, choosing the most computationally expensive chain of blocks (or, in some consensus mechanisms, the most stakes to back it up) as the correct sequence. Reorg helps ensure network security and consistency, but it can introduce temporary uncertainty in transaction history.

read more: Polygon zkEVM prover reaches type 1 status

Polygon zkEVM sequencer. assemble It broadcasts batches of transactions to network nodes and plays an important role in the ecosystem. It is not only responsible for ordering transactions, but also assigns timestamps, which is important for maintaining a consistent history of transactions on the network. However, during the incident, Sequencer did not detect this reorganization on Ethereum’s mainnet.

This caused the transaction to be processed based on outdated information and an incorrect global exit route (Polygon zkEVM encryption method to ensure Ethereum’s Layer 1 and Polygon’s Layer 2 consensus) was added to the next transaction block. . The resulting discrepancies created inaccuracies and disrupted the network’s ability to maintain a consistent record of the network ledger.

During the disruption, the network processed a large number of transactions without any interaction, and the network was acting “as if it wasn’t there,” the post-mortem said. Synchronizing these empty transactions was part of the reason for so much network downtime.

All told, it took about 14 hours for the rollup to reopen for withdrawals, and about 4,000 transactions may have been affected by the outage, Polygon said.

The outage only affected Polygon zkEVM, which is in the so-called mainnet beta. did it According to Polygon’s support page, there are bugs and availability issues. Chains using Polygon PoS Layer 1 and Polygon’s Chain Development Kit (CDK) were not affected.

Polygon’s zkEVM, which stands for zero-knowledge Ethereum Virtual Machine, is a layer 2 that uses zero-knowledge proofs to validate transactions on a Polygon-operated sequencer before settling a batch of transactions on Ethereum. StarkWare and zkSync are developing similar products. This is all in contrast to popular Layer 2s like Arbitrum and Optimism. These act as optimistic rollups and do not use zero-knowledge proofs.

read more: StarkWare’s Zero-Knowledge Prover Stwo Comes Out of Stealth

Shortly after the outage began, Polygon’s zkEVM went into an emergency state, resulting in network outages and upgrades to network provers and verifiers, along with fixes to prevent sequencers from malfunctioning during reorganization. it was done.

Once this upgrade is complete, the emergency state has been lifted. This means that future updates to the system will be subject to a 10-day timelock before they take effect.

This event marked the first use of Polygon’s zkEVM emergency state feature, which required approval from the network’s security council. When this state is activated, certain operations, such as transaction updates and blockchain bridge functionality, are stopped to protect users’ assets.

The Council operates as a 6/8 multisig. According to Polygon, two of his council members are from his Polygon Labs, but the identities of the Security Council members have not been made public.

2023 Blog Posts Said The group is “comprised of highly reputable members of the Ethereum community” and will be dismantled once Polygon zkEVM is ready to hang up its “training wheels.”

This failure and its remediation are indicative of lingering centralization issues across Ethereum’s Layer 2 network. according to Jarrod Watts, developer relations engineer at Polygon Labs:

“The worst-case scenario is that an upgrade is implemented that steals users’ funds,” he said. Said “This kind of malicious upgrade is possible on almost any device. [layer-2] today. “

However, Watts added, “IMO over time the risk of malicious upgrades to steal users’ funds increases.” [layer-2s] It will be zero or very close to zero! ”

Polygon zkEVM has struggled to gain Total Volume Lock (TVL) and usage and has yet to benefit from Ethereum’s Dencun hard fork.The chain is It is scheduled Following the “Feijoa” upgrade, we plan to add support for EIP-4844’s “blobspace” in about a month.


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