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Rollups and Ethereum Security

Rollups scale Ethereum by offloading execution overhead to a different sequencer while inheriting Ethereum's security. They achieve this by:

  1. Tracking Ethereum's Timeline:
    • Each rollup block associates with a corresponding Ethereum L1 block (known as L1Origin).
    • The L1Origin can be at various depths on Ethereum's timeline, even on a fork.
      • This is also how users bridge funds to the rollup via the native bridge, entering the off-chain environment, or force include transactions. image (22) In this image, Base has already seen the L1 block Arbitrum is using as its L1Origin since it is further in depth. However, Base is unaware of Optimism as it is too early in depth.
  2. Sequencer Posting Transactions to L1 (DA):
    • The sequencer batches transactions (ordered) and posts them on L1 for data availability.
    • Ethereum takes it own time to finalizes this batch transaction (containing the ordering), ensuring the sequencer cannot alter transaction ordering in future, reducing the need for trust.
  3. Proposing the Rollup State Root to L1:
    • The proposer recomputes the state root by executing transactions from the posted data batches, and then updates it on Ethereum.
    • This update is followed by a challenge window (typically 7 days) begins, allowing anyone to submit a fault proof.
    • Users experience a 7-day exit delay due to this challenge window.

Rollup's Relationship to Ethereum Timeline​

Every rollup anchors to Ethereum via its L1Origin, defining its position on the timeline. The depth at which a rollup follows Ethereum matters: image (23)

  • Shallow Depth implies a higher risk of encountering Ethereum forks.
  • Greater depth provides more confirmations, thereby reducing the risk of following a forked timeline.

For instance, Optimism and Base mainnets are configured with confirmation depths of 11 and 16, respectively, while some newer rollups operate at shallower depths, ranging from 4 to 6 Ethereum block confirmations.

Consequences of Tracking the Wrong Timeline​

image (24)

If a rollup follows an incorrect Ethereum fork, the L2 blocks are derived from an alternate chain that will eventually be pruned.

  • The rollup must switch to the correct timeline upon Ethereum's finalization.
  • Transactions executed on the wrong timeline revert to the mempool.
  • The sequencer replays these transactions; failure to catch up may lead to state reversion i.e., a reorg as seen in the 2-week reorg on Degen Chain.