So Urbit's Layer 2 solution is a Rollup, but not an Optimistic (fraud proof) or ZK (validity proof) Rollup, but their own homespun form of Rollup that they call

So Urbit's Layer 2 solution is a Rollup, but not an Optimistic (fraud proof) or ZK (validity proof) Rollup, but their own homespun form of Rollup that they call a Naive Rollup. It's described here: https://18108973658826589741.googlegroups.com/attach/31d6ad2570085/naive-for-martians.txt?part=0.1&view=1&vt=ANaJVrEwa-sZX7Tat5LHIzfnJfvmTHvEgv5_GJnOivARN1FMnGr1O6zR5C4r2jMSvUGUUbyZ7JjHURer5hgvL9KJuZilh-KiAHg76BkMwvz4ln9ifHcToZI https://18108973658826589741.googlegroups.com/attach/31d6ad2... It appears to use Ethereum L1 solely for data availability and sorting, while the computation that ensures validity is performed inside Urbit instances. I haven't thought through the security implications of this yet. I believe the implication is that you need to run a full Naive Rollup node, that validates all Urbit transactions, in order to ensure the validity of any given transaction.