Genuine Question - What is the point of Sovereign Roll up?

General
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So Sovereign roll up handles the actual settlement on the chain itself - this means that only DA and Consensus are posted to Celestia. In an OP roll up, the actual settlement is done on Ethereum but often times fast bridges such as Axelar or Layerzero would trust the sequencer and bridge away user assets before data blob is actually posted - taking on the risk themselves. This shows that most of the asset bridging are done before the actual transactions (the one sending assets to the bridge smart contract on L2) are settled. This weakens the security as in the event of a re-org, bridges might lose assets.

Now onto Sovereign roll up - because the settlement is done on the chain itself, and only DA and Consensus are done with Celestia - what is the actual benefit here?

Several questions:

  1. what is the actual difference between OP stack integrated with Celestia vs Sovereign roll ups? They seem to be the same to me as when OP stack is integrated with Celestia, only DA and Consensus are handled by Celestia, and the OP stack performs settlement itself.

  2. For a Sovereign roll up - aside saving $ posting data to Celestia, what other benefits there are? I do not see much. from a security perspective, because the roll up itself is handling settlement, i would argue it actually weakens the security - some insight would be helpful here.

Ultimately, I would like to understand why anyone would choose to build a sovereign roll up when they can just use OP stack to integrate with Celestia (assuming EVM roll up). In a sense, I am very confused by the the word “modular settlement layer” as in EVM, the moment you execute the transactions, the states are settled and modified, posting the altered states to Celestia for DA is part of settlement, so what part of that settlement is being broken out?

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I would say the main difference between them is that a sovereign rollup can have practically any VM - imagine Cosmos SDK, Move, or any custom VM.

For a regular rollup, the VM it can use is constrained by the settlement layer. So, in the case of an L2 settling to Ethereum, the state changes must be provable on Ethereum – which limits how its VM can be customized. Optimism had a lot of problems with their old OVM architecture because of this.

Right now, pretty much all Ethereum rollups have bridges that are upgradeable by multi-sigs – which isn’t very secure at all. I’m sure that will change in the future though, but there are a lot of nuances to consider in the tradeoff space which have varying implications on security – like is the bridge contract upgradeable (and by who) or if its immutable what do forks mean for assets locked in the original bridge.

With a sovereign rollup, you just treat it like an L1 for upgrades – if it wants to upgrade, it can just hard fork. To get a better handle on sovereign rollups, I recommend checking out this article.

And for “what part of that settlement is being broken out”, here’s what the separation roughly looks like:

  • Celestia: receives L2 transaction data
  • Ethereum: receives L2 state root + proof of data inclusion on Celestia (merkle root + proofs)
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