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(Towing the Bridge and the Umbrella Lady, Vincent van Gogh)
Recently, both Uniswap and Sushiswap are planning to use Arbitrum's L2 scheme, which makes Arbitrum the Ethereum L2 scheme that people pay attention to. So, how to understand Arbitrum simply?
Arbitrum is also the L2 solution of the rollups seriesThere are many Ethereum Layer 2 solutions, among which the Rollups series is the most concerned. Blue Fox Notes has introduced Optimisitc Rollups and Zk Rollups before. You can refer to the article "》。
Understanding ZK Rollup and Optimistic Rollup in one article: an important expansion direction of Ethereum
Arbitrum is also the solution of the Rollups range. These schemes basically put security on the Ethereum chain, while performing computation and storage off-chain. In simple terms, participants submit their transaction data to the Ethereum chain, and users can view the transactions, but the calculation and storage of transactions are carried out outside the chain. Arbitrum will periodically send a hash of the complete state that occurred on Arbitrum to Ethereum, and this hash will be placed on the chain as a result to achieve tamper-proof and finality.
Relatively speaking, Arbitrum and Optimistic Rollups are similar in terms of framework ideas. In terms of result verification, both belong to the category of fraud proofs, while ZK Rollups belongs to the category of encryption validity proofs.
In terms of premise assumptions, the fraud proof class Rollups assumes that the results on the submission chain are available, and anyone can extract and execute the transaction data of Layer1, and check whether the contract execution is correct or not by comparing the status. The encryption proof class Rollups adopts a mathematical method, and realizes validity through zero-knowledge proof, which cannot be faked in essence. Therefore, the encryption validity proof is an active proof, while the fraud proof is a passive proof. When people disagree with the result, disputes will arise and arbitration is required.
On the whole, ZK Rollups has higher security and faster finality, which means that its extraction time is faster, and it is more suitable for scenarios such as transfers. Both Optimistic Rollup and Arbitrum’s schemes have a challenge period, and the challenge period must be over before the final confirmation can be made and the coins can be withdrawn. This is its shortcoming. However, for ZK Rollups to generate cryptographic validity proofs, its computational cost is very high, it is very expensive, and it will take a certain amount of time to mature.
In the long run, ZK Rollpus may be the ultimate solution, but it will take longer for ZK Rollups to mature and land, and Ethereum’s expansion needs are more urgent, as can be seen from the rise of this smart chain. The landing of ZK Rollups is slower, giving opportunities to Optimistic Rollups and Arbitrum.
Therefore, in the short-to-medium term, fraud proof Rollups (Optimistic Rollups and Arbitrum Rollups) have a faster chance of landing. Fraud proofs themselves are "optimistic", presupposing that all committers are good people unless proven guilty. As long as there is a validator present, fraud proofs can be submitted and are safe. Therefore, the assumption of an honest validator becomes important. Only one verifier is required to submit an online fraud proof, and there is a time window of up to a week (or more) for fraud proof, which also leads to slow final confirmation time and long withdrawal time.
In general, Rollups of fraud proof and cryptographic validity proof have their own advantages and disadvantages. ZK Rollups are non-interactive and use mathematical methods to solve problems, while Optimistic Rollups and Arbitrum Rollups are interactive. In this sense, Optimistic Rollups and Arbitrum Rollups are similar, but there are differences in their specific paths.
Arbitrum is a multi-round interactive Rollups scheme
From the above, Arbitrum and Optimistic are both Rollups schemes in the field of fraud proof. The core problem of both schemes is: how to verify the correctness of the results? Could someone be sending malicious proof? So, both have challenge mechanics. Validators can send assertions to the chain and raise disputes. If the assertion is false, it loses its pledged margin assets.
The main difference between Arbitrum and Optimistic is the way disagreements are resolved. When a verifier submits a rollup block to L1 and someone thinks it is incorrect, what should we do at this time?
Arbitrum uses a multi-round interactive protocol to resolve disputes, subdividing large-scale disputes into small disputes until the most critical step is found, and then confirms whether it is correct through the Ethereum contract. By splitting disputes, Arbitrum attempts to achieve a more efficient resolution.
Participants in Arbitrum can stake a bond, asserting that a certain state will eventually be confirmed. If the assertion is wrong, the user's pledge deposit will be confiscated. Two users are staked on different blocks, which means only one of them can be true. In order to prevent someone from attacking, disputed assertors need to pledge margin assets to prevent them from doing evil.
In its original design, the Arbitrum protocol took the approach of processing disputed assertions one at a time. A dispute assertion is made by a participant, and there is a challenge period for the dispute assertion, during which anyone can challenge the dispute assertion. If not challenged, then the disputed assertion will be deemed valid.
However, there is a bottleneck in this single-line propulsion mode. Activating one dispute assertion at a time is inefficient and limits the VM process. Meanwhile, perpetrators can intentionally slow down the VM by challenging disputed assertions. Even if pledged funds need to be paid, it can be delayed.
Therefore, in the new design, Arbitrum can process multiple disputed assertions at the same time, not linearly. It is more difficult for perpetrators to slow down the process. At present, Arbitrum adopts the method of processing multiple dispute assertions at the same time, one staker asserts one at a time, and different stakers can process multiple assertions at the same time.
In order to achieve trustless features, in Arbitrum's design, any honest participant can promote VM correctness and progress. If a participant always stakes on the correct branch, TA wins all dispute assertions. If others disagree, they will only lose their staked deposit. Only honest participants can avoid punishment.
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(Alice and Bob stake on different branches, via Ed Felten of Arbitrum)
If Alice's assertion in the above figure proves to be correct, then Bob's pledged margin assets will be lost, while Alice will benefit.
In single-round interactive Rollups, the assertion includes the result of each call, and the challenger points to the specific call in the assertion that has the wrong result. The on-chain contract simulates the challenged call and checks for errors. If it is wrong, the entire assertion is canceled and its assertor loses the security deposit staked. If the challenge window expires without a successful challenge, the assertion is accepted and made final.
In Rollups with multiple rounds of interaction, there is also a challenge time window. There are multiple rounds of interaction between the asserter and the challenger, and the on-chain contract acts as a referee to decide who is wrong. The main consideration in the design of multi-round interactions is to minimize the on-chain work of resolving disputes. On-chain work is minimized through multiple rounds of interaction between challengers and assertors.
Single-round interaction and multi-round interaction are essentially a balance between on-chain costs and dispute resolution time. Both single-round and multi-round interactive Rollups need to write all calls to the contract and their data on-chain, the difference is what they need to commit to the chain as part of the assertion. The single-round interaction mode requires simulating a complete call on the chain, and it will be more expensive in terms of cost. However, multiple rounds of interactions can be subdivided to narrow the scope of disputes, and relatively less data is written to the chain, making its execution cost on the chain lower. However, it is not without its disadvantages, its determination increases the number of rounds and increases the time.
In addition, Arbitrum's multi-round interaction mode may face delay attacks. Because the system does not need to be trusted. Malicious attackers can delay the process, but of course, they do so at the cost of forfeiting their pledged deposits. It is also important to note that malicious actors cannot prevent honest participants from continuing to build honest forks, nor can they prevent honest participants from obtaining final confirmation on honest forks. What they can do is by staking a bond on the wrong branch and delaying the confirmation of the honest branch on the chain. They can make multiple dispute assertions which, if there are enough honest participants, they can simultaneously address and capture the attacker's stake deposit. That is to say, the more honest participants there are, the more pledged assets the attacker needs to delay the attack, and the more difficult the attack will be.
In addition, in Rollups with multiple rounds of interaction, it is possible to achieve a valid "assertion" that becomes "trustless finality", and anyone can be sure that its final confirmation is inevitable. To ensure their own finality, users can participate in the agreement to defend the assertion, even acting alone, can promote its finality.
If there is no challenge, the confirmation speed of multi-round and single-round interaction is similar. If there is a challenge, the multi-round dispute will take longer. In this regard, single-turn interactions have an advantage. A single round of interaction will not allow confirmation delay attacks, and the benefit of multiple rounds of interaction lies in smaller on-chain data, which can handle contracts (computation and storage) that exceed the Ethereum gas limit.
There is a certain period of time for the final confirmation of Rollups of the fraud proof class. Therefore, there is no way for transfers via Arbitrum to achieve fast finality like the ZK Rollps scheme. Its possible solution is through the intervention of a third party. The third party can transfer their funds to the user instantly, but the user also pays an early withdrawal fee. Generally speaking, the third party believes that the undetermined transfer will definitely be realized, and the third party will make a deposit pledge on the honest result. Since anyone can transfer funds to users who need to withdraw quickly. Then, there is also a market in this way, which will lead to an equilibrium in the fee, which will not be too high or too low.
Arbitrum EVM Compatibility
Arbitrum tries to be fully compatible with Ethereum, compatible with EVM, without rewriting the program. Fully compatible with Ethereum, we have seen the power of this on the smart chain. This is the most important measure for it to be able to smoothly embed into the current ecology of Ethereum.
dApp developers use the Solidity language to write their smart contracts, and these smart contracts are compiled into the Arbitrum VM, which can run on Arbitrum Rollups.
Progress in Arbitrum
Arbitrum Technologies is currently launching Arbitrum One. The Arbitrum contract is deployed on the Ethereum mainnet and has begun to accept projects. It is currently in the mainnet testing phase. According to Arbitrum, there are currently more than 250 teams applying to enter, and it is open to all developers. In the next few weeks, we can see that more and more DeFi projects will enter the Arbitrum ecosystem.
At present, some important participants in the Ethereum ecosystem, such as Etherscan, are also supporting the Arbitrum One chain.
The Landing of Ethereum and L2
Interestingly, Arbitrum has no native tokens, uses ETH as fuel, and uses ETH to pay for all expenses. This is also a good thing for ETH, let alone how much demand it will have for ETH in the future.
For now, the scalability of Ethereum requires a combination of long-term and short-term strategies to meet its rapid growth needs, especially the explosive growth of DeFi and NFT. In the long run, L2 may be the winner of ZK Rollups, but due to its high cost and lag in landing, it may take a long time to digest. However, Arbitrum and Optimistic Rollups may land earlier. Although their extraction time is long, they are not unsolvable. In addition, growing up in practice may also give birth to new inspiration.
