Metis PoS "Sorter Pool": the first practitioners of decentralized Rollup

DBA co-founder Jon Charbonnea once said: All Rollups now are not real Rollups.

A very important reason for such an extreme view is that almost all Rollup networks currently adopt a centralized single sequencer model. For a network that uses Optimistic Rollup, although the project itself will most likely not choose to do evil on the sequencer, once the sequencer is attacked or controlled by others, it may have catastrophic consequences. Even leaving aside security issues, a single sequencer is completely incapable of censorship resistance.

There have been discussions and initiatives on sorter decentralization in the industry for a long time, and recently Metis took the lead in giving an answer to this problem.

Why is decentralization of sorters so important?

Before explaining Metis’ decentralized sorter solution in detail, we first need to briefly understand two questions: What is a sorter? Why is sorter decentralization important?

The Layer 2 network based on Rollup on Ethereum is essentially composed of an independent chain and the “Rollup contract” on Ethereum. At the same time, the Layer 2 network also needs an off-chain sequencer to receive transactions that occur on Layer 2, determine the execution order of the transactions, and then package the transactions into Batch and transmit them to the “Rollup Contract” on Ethereum. When the “Rollup Contract” on Ethereum packages the transmitted Batch into a block, the transaction will be finally confirmed.

In essence, the real “confirmation” of transactions on the Layer 2 network is still on Ethereum, and the sequencer plays the role of information transmission in the middle.

Metis adopts the Optimistic Rollup scheme. As the name suggests, the characteristic of this Rollup scheme is that it assumes that all transactions in the Batch are real and valid, without checking each transaction. It is precisely because of this, and because transactions are centralized and then confirmed, that Layer 2 network transaction fees based on Optimistic Rollup are very low.

The current Layer 2 network is almost all run by the project team. In order not to “smash their own brand”, the transactions submitted in the sequencer are rarely invalid. Even so, centralized structures still have the possibility of being attacked and forced to eliminate certain transactions. This is obviously unacceptable in the cryptocurrency world where “permissionless” and “trustless” are regarded as the supreme principles. Decentralization is the path that must be taken.

Metis’ road to decentralization

As one of the first Layer 2 networks to implement decentralized orderers, Metis provides a template for the way forward. This template not only realizes the decentralization of the sorter, but also provides a “decentralized Optimistic Rollup” solution. In this template, there are three main roles to achieve the decentralization of the sorter: the administrator (Admin), the sorter, and a consensus layer based on PoS.

Admin): The role of Admin is to manage the decentralized system. Its main responsibilities include adding sequencers that meet the requirements to the whitelist, setting the pledge limit of a single node, and setting the release rate of block rewards.

Sorter pool: The sorter pool consists of multiple sorters that have been added to the whitelist. In Metis, the sequencer includes L2 Geth, which is responsible for transaction sorting and block formation on Metis; the adapter module, which is responsible for interacting with the PoS consensus layer and other external modules mentioned below, and the one responsible for establishing Batch and sorting on multiple nodes. After signing, submit it to the Proposer of L1.

PoS consensus layer: The PoS consensus layer designed by Metis exists independently of Metis, and its main responsibility is to manage MPC signatures. When there are multiple sequencers, at least 2/3 of the sequencer signatures are required to submit the packed Batch to L1, and the consensus layer is responsible for resharding and allocating private keys when sequencers in the sequencer pool join or exit. Waiting for work.

Mechanism design

With the cooperation of the above three roles, Metis’ decentralized Optimistic Rollup has begun to take shape. Among them, the “administrator” will set important parameters of the overall network and manage the qualifications of the sequencers added to the pool, so that the agreement parties no longer have absolute control over such matters, but will hand over specific proposals after review. Administrator executes.

For the Rollup network, the most difficult point in the decentralization process is the decentralization of the sorter. This is because the management of the sorter itself must also be carried out in a decentralized manner, and it needs to be as efficient and effective as possible. convenient.

Metis uses TSS-based MPC signatures and a PoS network that manages the entire module to manage the signature permissions of multiple sequencers.

Metis allows any sequencer to decide a Batch and all sequencers participate in the MPC signature. If the number of signatures exceeds 2/3, the Batch is considered valid and can be submitted to the Rollup contract on L1. The MPC signature performed by the sequencer pool is managed by another contract controlled by the PoS network. When the PoS network cannot detect the MPC address, the MPC module will be called to trigger the generation of the key. The generated key will be distributed to each sequencer in the pool after sharding, and the sequencer will use the key shards it holds to perform MPC signatures. The settings of this module are mainly used to manage the life cycle of keys, including multi-signature generation, key re-sharing, application signatures, deletion signatures, etc.

TSS is used because of its high fault tolerance and high flexibility. Unlike multi-signature, which requires verification of each signature on the chain, TSS only needs to aggregate the signatures of all signers and verify them uniformly, which improves transaction confirmation. rate. In addition, PoS nodes communicate with each other through separate Tendermint channels, and the MPC runtime communication uses the libp2p protocol.

Token pledge

Similar to Optimism Rollup’s fraud proof, this decentralized model also uses economic incentives and punishment mechanisms to ensure the operation of the mechanism. Metis team members said that the initial sorter pool will contain 5 sorters, and will gradually increase in the future. Sorter operators entering the sorter pool need to pledge at least 20,000 METIS. In addition, the PoS network mentioned above will also require node operators to stake METIS. The Metis team said that this move can “further reduce the already limited METIS circulation, resulting in a very positive catalytic effect.”

Staked METIS will be deducted when the fraud proof is successfully challenged to ensure that the sequencer does not include invalid transactions in the batch submitted to L1.

Pioneer of decentralization

Metis’s decentralized Rollup solution provides a good example for Layer 2 to move to the next stage. Both in terms of mechanism design and economic mechanism design, it promotes project development on the basis of ensuring decentralization of power. effect.

Giving the sequencer entirely to the community can generate a lot of spin-off opportunities through spontaneous community promotion. On the one hand, decentralization means a certain degree of competition for participants, because sequencers participate in block production, similar to the role of miners in the Bitcoin network. In order to compete for accounting rights, all parties will continuously improve the quality of their own nodes, thus spontaneously bringing greater activity to the network. Metis said, “This will be the first chain in the Layer 2 field where the community truly participates in operations.”

In addition, due to the decentralization of the sorter, there is no “standard answer” to the sorting of transactions, which has cultivated the soil for the birth and development of the network’s MEV infrastructure and a series of applications. After MEV, METIS tokens based on sequencer pledge can also derive a series of products such as liquidity pledge, further releasing the potential of the financialization of Layer 2 infrastructure.

In terms of mechanism design, Metis does not use simple multi-signature for the sequencers operated by all parties. Instead, it uses the TSS-based MPC signature scheme that is more convenient for adding or deleting signers in real time, making the overall sequencer pool flexible. Sex is enhanced. On the other hand, the effective signature threshold of 2/3 and the “pluggable” signature scheme have made a good trade-off between security and efficiency.

Although this mechanism is designed with full consideration of security and efficiency, the addition of multiple new roles for decentralization will inevitably lead to a decrease in efficiency compared to the traditional single sorter. In addition, the increase in links may lead to overall system instability. improvement. Despite this, Metis is one of the first Layer 2 networks to provide a complete decentralized solution, and its design considerations and trade-offs are still worthy of reference. Metis also stated that it will continue to discuss the potential challenges and flaws of the decentralized solution for further development. further improvements.

[Disclaimer] There are risks in the market, so investment needs to be cautious. This article does not constitute investment advice and users should consider whether any opinions, views or conclusions contained in this article are appropriate for their particular circumstances. Invest accordingly and do so at your own risk.

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