First make a point. I recently participated in the latest round of financing from Offchain Labs (Arbitrum development company), and Mechanism Capital also participated. Although it is futile to pretend that our views in this article are objective, I hope this article will help readers understand some of the key differences between these two projects, even though I may be biased.
All Rollup solutions follow a similar basic architecture and internal logic. Nonetheless, as we saw in the first part of this series, the single difference between Optimistic Rollup and ZK Rollup—how the respective “review process” works—produces safety, usability, and EVM compatibility Many downstream differences.
There are similar situations in each Rollup category. Although the two leading Optimistic Rollups, Arbitrum and Optimism, have a lot in common, the difference between the two is not just camp loyalty. In particular, the differences in their respective dispute resolution methods have created some important performance trade-offs. Given that both platforms are designed to provide complete extensions to Ethereum in the coming months, these trade-offs are worth discussing.
First of all, it is necessary to introduce some brief historical background of each project. As it happens, both Arbitrum and Optimism have some unique origin stories.
Six and a half years ago, on a cold morning in Princeton, a group of undergraduates working with Professor Ed Felten gave a speech on the project they signed to create: a blockchain-based arbitration system. The goal is to circumvent some of the expected expansion challenges of the smart contract platform. The plan is to design a blockchain that relies on the challenge and dispute resolution system to reduce the computational workload of traditional miners. This system is called “Arbitrum”, and if two ambitious doctoral students Steven Goldfeder and Harry Kalodner approached Felten a few years later, the system would suffer the same fate as most other promising academic computer science projects. Build a powerful layer 2 solution based on the initial concept. Soon thereafter, Felten, Goldfeder, and Kalodner co-founded Offchain Labs and transformed Arbitrum from an abstract idea into a concrete reality.
Optimism also has a history that predates its current form. In mid-2017, Vitalik Buterin and Joseph Poon co-wrote a paper and proposed Plasma, an early scaling solution for Ethereum. A core group of Ethereum researchers took over the idea and formed a non-profit research group to build the vision. As some of Plasma’s key design limitations became apparent, development stalled at the end of 2019. The three lead researchers of Plasma-Karl Floersch, Jinglan Wang and Ben Jones were not deterred and decided to turn to Optimistic Rollup, which seems to be Plasma’s natural successor. They established the Optimism PBC team in early 2020.
Dispute resolution: a very brief (re)introduction
Recall that Optimistic Rollups adopted an “innocent unless proven guilty” approach to transaction validity. Optimistic Rollups process transactions and feed the results back to Ethereum for inclusion in the basic chain. The dispute period ensures that anyone monitoring the status of Rollup can submit an inquiry when the Rollup sequencer processes invalid transactions. This challenge immediately triggered the dispute resolution process. The difference between Arbitrum and Optimism lies in how the dispute resolution process works-including how much it takes and how long it takes.
A preliminary comparison between Arbitrum and Optimism in dispute resolution
The easiest way to describe the difference is that Optimism’s dispute resolution is more dependent on the Ethereum Virtual Machine (EVM) than Arbitrum. When someone submits a challenge about Optimism, the entire problematic transaction is run through EVM. In contrast, Arbitrum uses an off-chain dispute resolution process to reduce disputes to one step in a transaction. Then, the protocol sends this one-step assertion (rather than the entire transaction) to the EVM for final verification. Therefore, conceptually, the dispute resolution process of Optimism is much simpler than that of Arbitrum.
As far as Arbitrum is concerned, the off-chain component of its dispute resolution process uses a recursive dichotomy algorithm. This sounds complicated, but in reality, the algorithm just forces the “assertor” (the party that handles the transaction) and the “challenger” (the party that submits the challenge) back and forth to narrow the points of dispute, in the way shown in the figure below. Interestingly, this process of recursive dichotomy to solve back and forth was part of the original Arbitrum concept in 2015.
Source: OffChain Labs Development Center
Optimism’s method of resolving disputes—that is, running the entire transaction through the EVM—is not only conceptually simpler: it is also faster. There is no “multiple rounds” of processing back and forth like the Arbitrum process. In fact, for this reason, Optimism Rollup is often called “single round”, while Arbitrum Rollup is “multi round”. In practice, this means that in the case of a disputed transaction, in the case of Arbitrum, the final confirmation of Ethereum is delayed longer than in the case of Optimism. As we discussed in the first part of this series, the speed of dispute resolution is important because it determines the time it takes for users to exchange tokens from Rollup back to Ethereum.
On the other hand, the advantage of Arbitrum dispute resolution is that it is cheaper in terms of on-chain (ie Ethereum) transaction costs. After the round-trip dispute resolution process is completed, the small piece of code finally processed by the EVM requires much less gas than the gas cost required to reprocess the entire chain transaction (in most cases).
Rebuild the comparison
The basic trade-off between the two dispute resolution designs seems to be between speed and on-chain cost. But in fact, this is a bit too simple, because few people think that the emergence of controversy is due to the following two reasons:
- Transaction processors on Arbitrum and Optimism have no economic incentive to process fraudulent transactions. They are forced to invest in collateral/bonds in advance, and the collateral will be reduced in the event of fraudulent transactions.
- The parties monitoring the status of Rollup are reluctant to submit false fraud proofs-in Optimism, because the challenger must pay the gas fee for the fraud proof on the chain, and in Arbitrum, because the challenger must provide the confiscated certificate when the dispute fails Margin.
So, if disputes are expected to be few and far apart, then why is the structure of the dispute resolution process important?
Although disputes rarely occur, the design of Rollup must be able to deal with disputes that may occur at any time. Therefore, the design of “controversial” situations will affect the structure of “non-controversial” situations.
Since Optimism must be able to run every transaction through the EVM in the event of a dispute, it cannot process transactions that exceed the Ethereum gas limit because these transactions cannot be correctly verified on the chain. In contrast, Arbitrum can execute arbitrarily large transactions, even if they exceed the gas limit of Ethereum, because transactions are never run in batches through the EVM, but are first broken down into tiny “step assertions”.
It is not yet clear how much the gas limit of Optimism will impose practical restrictions on the application. However, another and possibly more important implication of the design differences in dispute resolution is that Arbitrum can save gas by reducing the frequency of on-chain checkpoints (updated “state roots”). More specifically, Arbitrum can allocate a large number of off-chain calculations for one update, because the state root update can theoretically include (minimal) single-step fraud proofs for all transactions contained in it. On the other hand, Optimism must check points on the chain after each transaction, thereby significantly increasing its footprint on the chain.
All in all, Arbitrum should be less gas-efficient than Optimism-and therefore cheaper for users-not only in rare controversial situations, but also in the main “non-controversial” situations.
Dispute resolution and potential attack vectors
The final point about these different dispute resolution processes is worth discussing: how resistant each design is to potential attacks. Above, we talked about economic incentives to prevent spam attacks. More specifically, both Optimism and Arbitrum verifiers are reluctant to submit unnecessary challenges.
But what about malicious attackers who don’t mind bearing the economic cost of spam Rollup? In other words, what happens if a person or entity is so committed to slowing down the Optimistic Rollup that they are willing to do so, even if it means repeatedly paying for false challenges?
As mentioned above, Optimism’s dispute resolution process is simpler and faster than Arbitrum because it only provides disputed transactions through EVM. This speed is the advantage of Optimism here, because disputes can be resolved quickly and will not hinder the future progress of the rollup chain.
What people worry about is the “multi-round” dispute resolution process, such as the one used by Arbitrum. At least in theory, spammers can stop Rollup’s progress by launching a series of consecutive challenges, each of which takes a considerable amount of time to resolve. In fact, this is a problem that plagued Arbitrum’s previous iterations.
However, Arbitrum’s updated protocol is suitable for this problem, an elegant solution called “Pipelining”. The pipeline allows the network validator to continue processing transactions for final approval, even if the previously processed transaction is disputed. This creates a “pipeline” of recently processed but unfinished transactions, rather than a bottleneck that prevents the sequencer from processing transactions and submitting challenges from all parties on the network.
The pipeline is possible because anyone monitoring the network can know whether the dispute is valid or invalid immediately before the dispute resolution process is completed. Essentially, the validator can operate as if the disputed transaction has already been completed and continue to build the chain (that is, process the transaction) based on the correct result or “branch”. This process, as shown in the figure below , weakens the power of any possible spam attacks.
Source: OffChain Labs Development Center
In addition to the design of dispute resolution procedures, there are other significant differences between Arbitrum and Optimism, especially
- Their code base architecture, and
- Their method of extractable value (MEV) for miners
To summarize these differences very briefly: Optimism’s code base is relatively simple, while Arbitrum’s code base is more complex and ambitious; Optimism has said in the past that it prefers the MEV auction method, and Arbitrum plans to implement a fair sequencing service (FSS). Naturally, these two comparison points are worthy of a separate article to discuss in detail. MEV in particular is a matter of philosophical debate between the two projects-although at least in the early days after launch, for simplicity, it is expected that both will use the trusted sequencer model.
In the end, take a step back from the nuances of the protocol level (although they are important) and distinguish the two heavyweights with “soft” things: guiding strategy, incentive design, and community spirit, to name a few. In fact, if they are to succeed in the long run, Optimistic Rollups will have to become their own world, not just an appendage of Ethereum. Therefore, capacity expansion is not so much an arms race as it is a multi-line war. It may have one winner; it may have more than one. It may last for many years; it may end sooner or later. This will definitely have a major impact on the future of cryptocurrencies.
Posted by:CoinYuppie，Reprinted with attribution to:https://coinyuppie.com/understand-l2-strong-players-arbitrum-and-optimism-in-one-article-with-the-same-clan-from-the-same-source-but-different-paths/
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