L2 & Arbitrum Orbit

This page is short. Its job is to give you just enough vocabulary to read the rest of the Learn section. The Arbitrum docs cover each of these concepts in much greater depth, and we link out throughout. If you already know what an L2, an optimistic rollup, Arbitrum Orbit, and AnyTrust are, skip ahead to How Mandala is Built.

What is an L2?

A Layer 2 (L2) is a blockchain that derives its security from a Layer 1 (Ethereum, in our case) instead of from its own consensus. Users send transactions to the L2, the L2 executes them, and the L2 periodically posts a summary of what it did back to the L1. If the L2 misbehaves, the L1 can reject its summary.

The point is throughput. Ethereum is a global settlement layer with global consensus, and that is expensive. An L2 inherits Ethereum's security guarantees while running its own execution, which means cheaper transactions and faster confirmations without asking every Ethereum validator to re-execute every transaction.

What is an optimistic rollup?

An optimistic rollup is a specific kind of L2. The "optimistic" part means it assumes the L2 sequencer is honest by default and only checks for fraud if someone challenges. The "rollup" part means it posts both transaction data and state commitments back to L1, so anyone can independently re-execute the chain.

After each L2 state assertion, there is a window (typically 7 days on Ethereum) during which validators can dispute the assertion. If no one disputes within that window, the assertion is final. If someone does dispute, the disagreement is resolved on L1 by an interactive proving game.

The challenge window is why withdrawals from optimistic rollups to Ethereum take ~7 days. It is the time the system gives validators to spot and dispute a bad state root before funds are released.

What is Arbitrum Orbit?

Arbitrum Orbit is the toolkit Arbitrum publishes for launching custom L2s and L3s on top of the Arbitrum stack. An Orbit chain runs the same Nitro execution layer that powers Arbitrum One, but with parameters of its own choosing: gas token, DA mode, settlement chain, validator set, fee distribution, and more.

Mandala is one of those Orbit chains. It runs Nitro for execution, settles to Ethereum L1, uses KPG as the gas token, and operates in AnyTrust mode for data availability.

What is AnyTrust?

AnyTrust is a data availability mode that Orbit chains can choose. In standard rollup mode, the full transaction payload goes to Ethereum calldata or blobs, where it lives forever and any Ethereum node can serve it. In AnyTrust mode, the payload goes to a Data Availability Committee (DAC) instead, and only a small header plus the DAC's signature goes to Ethereum.

The trade-off is what makes AnyTrust cheaper: instead of paying Ethereum gas for every byte of every transaction, the chain pays for a fixed-size header. The cost is a trust assumption. In AnyTrust mode, the chain assumes that at least two members of the DAC remain honest and online to serve the data when asked.

The Trust & Security Model page covers the AnyTrust assumption in detail, including failure modes and how the chain behaves if the DAC misbehaves.