A Bauhaus blockchain reference manual
The distributed ledger technology underpinning all Layer-2 solutions. A blockchain is a cryptographically linked chain of blocks, each containing a batch of validated transactions, maintained by a decentralized network of nodes that achieve consensus without a central authority.
Protocols by which network participants agree on the state of the ledger. Proof-of-Work requires computational effort; Proof-of-Stake requires economic commitment. The consensus layer defines the security guarantees that Layer-2 systems inherit.
The base network layer providing physical and logical connectivity between blockchain nodes. Encompasses peer-to-peer networking protocols, data propagation mechanisms, and the communication substrate upon which Layer-1 chains operate.
Hash functions, digital signatures, and commitment schemes that provide the mathematical foundation for blockchain security. These primitives enable trustless verification, data integrity, and the construction of zero-knowledge proofs used in Layer-2 systems.
A Layer-2 scaling technique that executes transactions off-chain and posts transaction data to Layer-1, assuming all transactions are valid by default. A challenge period allows fraud proofs to be submitted if an invalid state transition is detected. Prominent implementations include Optimism and Arbitrum.
Zero-knowledge rollups bundle hundreds of transfers into a single transaction and generate a cryptographic validity proof (a SNARK or STARK) that is verified on-chain. This approach provides immediate finality without a challenge period, at the cost of more complex proof generation.
Two-party or multi-party channels that allow participants to transact off-chain with instant finality. Only the opening and closing states are recorded on Layer-1. The Lightning Network (Bitcoin) and Raiden Network (Ethereum) are canonical implementations.
A framework for creating child chains anchored to a root chain, with periodic commitment of state roots to Layer-1. Plasma chains process transactions independently and rely on fraud proofs and exit mechanisms for security, enabling significant throughput improvements.
The ability for smart contracts and protocols across Layer-1 and multiple Layer-2 systems to interoperate seamlessly. Achieving atomic composability across layers remains one of the defining challenges of the multi-rollup future.
The guarantee that transaction data required to reconstruct the Layer-2 state is accessible to all network participants. Data availability sampling, danksharding, and dedicated DA layers (Celestia, EigenDA) address the data bottleneck that constrains rollup throughput.
Succinct non-interactive arguments of knowledge that allow a prover to convince a verifier of a computational statement's truth without revealing the underlying data. SNARKs and STARKs form the cryptographic backbone of ZK-rollups, enabling scalable verification on Layer-1.
The ongoing effort to distribute the transaction ordering role in rollups across multiple independent operators. Centralized sequencers offer performance but introduce censorship risks; shared sequencer networks and based rollups propose alternative architectures.
Optimistic rollups represent one of the most significant advances in blockchain scalability. By moving computation off the main chain while retaining the security guarantees of the underlying Layer-1, they achieve a practical balance between throughput and trust minimization.
Optimistic rollups assume innocence until guilt is proven -- a philosophical inversion of the zero-knowledge approach, where validity must be demonstrated before acceptance.
An optimistic rollup operates by executing transactions on a separate chain (the Layer-2) and periodically posting compressed transaction data back to the Layer-1 as calldata. The key insight is that the Layer-1 contract does not re-execute these transactions. Instead, it optimistically assumes all posted state roots are correct.
A challenge window (typically 7 days) allows any observer to submit a fraud proof if they detect an invalid state transition. If the challenge succeeds, the invalid batch is reverted. This mechanism provides security equivalent to Layer-1 execution, with the trade-off of delayed finality.
The throughput improvement comes from two sources: compressed transaction data reduces on-chain storage costs, and off-chain execution eliminates the gas overhead of Layer-1 computation. Current implementations achieve 10-100x cost reduction compared to direct Layer-1 execution.
The security model of an optimistic rollup is only as strong as the liveness of its fraud proof submitters. At least one honest verifier must be active during every challenge window.
Optimism uses a single-round fraud proof system with an interactive verification game. Arbitrum employs a multi-round interactive fraud proof that narrows the dispute to a single instruction. Both approaches achieve the same security guarantee through different verification strategies, each with distinct trade-offs in proof cost and resolution time.