Based Rollup
A based rollup delegates transaction sequencing to the base layer's validators rather than using a centralized sequencer.
Key Takeaways
- A based rollup (also called an L1-sequenced rollup) delegates transaction ordering to Layer 1 validators instead of relying on a centralized sequencer. This means the rollup inherits the base layer's decentralization and liveness guarantees directly.
- Based rollups gain strong censorship resistance and eliminate single points of failure, but face slower confirmation times tied to L1 block intervals unless supplemented by preconfirmation mechanisms.
- The concept, coined by Ethereum Foundation researcher Justin Drake in 2023, represents a fundamental design choice for any Layer 2 system: should sequencing be centralized for speed, or decentralized for security? Bitcoin L2s face this same question.
What Is a Based Rollup?
A based rollup is a type of rollup that uses the underlying Layer 1 blockchain's validators to sequence (order) its transactions, rather than operating a dedicated centralized sequencer. The term was introduced by Justin Drake of the Ethereum Foundation in March 2023. His definition: "A rollup is said to be based, or L1-sequenced, when its sequencing is driven by the base L1."
In most conventional rollups, a single operator (the sequencer) collects Layer 2 transactions, determines their order, and submits them in batches to the base chain. This centralized sequencer can offer near-instant "soft confirmations" because it has unilateral control over ordering. However, it also introduces a single point of failure: if the sequencer goes down, users must fall back to slow escape-hatch mechanisms to get their transactions included.
Based rollups take a fundamentally different approach. They remove the dedicated sequencer entirely and let L1 block proposers include rollup transactions as part of the normal block-building process. Any L1 validator can permissionlessly sequence the rollup, making the system as decentralized as the base layer itself.
How It Works
A based rollup reuses the existing L1 block production pipeline. On Ethereum, this means leveraging Proposer-Builder Separation (PBS), where different actors handle transaction searching, block building, and block proposing.
- Users submit Layer 2 transactions to a public mempool or directly to block builders and searchers
- L2 searchers identify profitable transaction bundles, similar to how MEV extraction works on L1
- L1 builders construct blocks that include both L1 transactions and L2 rollup blocks
- L1 proposers select and propose blocks containing the L2 data via the standard validator duties
- The rollup smart contract on L1 processes the included L2 block as the canonical next state transition
Because the rollup piggybacks on the existing L1 infrastructure, there is no need to bootstrap a separate validator set, design a new consensus mechanism, or issue a staking token for sequencer coordination.
Comparison with Centralized Sequencers
The architectural difference between a based rollup and a traditional rollup comes down to who controls transaction ordering:
| Property | Centralized Sequencer | Based Rollup |
|---|---|---|
| Sequencing | Single operator | L1 validator set |
| Confirmation speed | Milliseconds (soft confirmation) | L1 block time (e.g., 12 seconds on Ethereum) |
| Censorship resistance | Relies on escape hatches | Inherits L1 guarantees |
| Liveness | Degrades if sequencer fails | Same as L1 |
| MEV revenue | Captured by rollup operator | Flows to L1 proposers |
| Complexity | Requires sequencer infrastructure | Reuses existing L1 infrastructure |
| Decentralization | Must be bootstrapped separately | Inherited from L1 |
Based Preconfirmations
The primary UX limitation of based rollups is confirmation latency. To address this, Justin Drake proposed "based preconfirmations" in November 2023: a mechanism where a subset of L1 validators opt in as "preconfers" by accepting additional slashing conditions.
When a user wants a fast confirmation, they send a request to a preconfer who is scheduled to propose a block soon. The preconfer evaluates the transaction and returns a signed cryptographic promise guaranteeing inclusion and a specific execution result. If the preconfer fails to honor the promise, they are slashed.
This approach can achieve preconfirmation latencies on the order of 100 milliseconds using direct point-to-point connections, closing the gap with centralized sequencers while preserving decentralization. Taiko, the first based rollup on Ethereum, launched Phase 1 preconfirmations on mainnet in August 2025 with approximately 2-second finality.
Advantages of Based Rollups
Based rollups offer several properties that centralized sequencer designs cannot easily replicate:
- Liveness: the rollup is as live as the base layer. There is no sequencer that can go offline and halt L2 block production. As long as L1 validators are producing blocks, the rollup continues functioning.
- Censorship resistance: the only way to censor a based rollup transaction is to censor it at the L1 level, which requires compromising the entire validator set. This is a far higher bar than censoring a single centralized sequencer.
- Simplicity: eliminating the sequencer removes the need for sequencer signature verification, escape-hatch mechanisms, rotation protocols, and external consensus. The architecture is inherently simpler.
- L1 composability: because L2 transactions are included in L1 blocks, they can interact with L1 state in the same block. Multiple based rollups sharing the same L1 can achieve synchronous composability across chains.
- Economic alignment: MEV flows to L1 validators, strengthening the base layer's economic security rather than fragmenting it across separate sequencer networks.
Use Cases
Decentralized Applications Requiring Censorship Resistance
Applications where censorship resistance is a core requirement benefit directly from based rollups. The Ethereum Name Service (ENS) chose a based rollup framework (Nethermind's Surge, built on Taiko's stack) for its Namechain deployment, explicitly citing "decentralization, fast finality, Ethereum-native settlement, and censorship resistance" as deciding factors.
Cross-Rollup Composability
Because multiple based rollups share the same L1 sequencer, transactions across different L2s can be ordered atomically within the same L1 block. This enables cross-rollup interactions without relying on bridges or external messaging protocols, reducing fragmentation in the Layer 2 ecosystem.
Infrastructure-Light Rollups
Teams that want to launch a rollup without operating dedicated sequencer infrastructure can use a based rollup framework. Projects like Spire Labs offer "Based Stack" tooling that enables application-specific rollup chains (appchains) to leverage L1 sequencing out of the box, lowering the barrier to deployment.
Relevance to Bitcoin Layer 2 Design
While the based rollup concept originated in the Ethereum ecosystem, the underlying sequencing question applies directly to Bitcoin Layer 2 design. Most Bitcoin rollups and sidechains currently operate with centralized sequencers. For example, Citrea (a Bitcoin ZK-rollup) runs a single sequencer with a decentralized sequencer network on its roadmap but not yet implemented.
A direct "based rollup" on Bitcoin faces additional challenges compared to Ethereum. Bitcoin lacks Proposer-Builder Separation infrastructure, its scripting language is far more constrained for on-chain verification, and its 10-minute block time would make latency issues significantly worse. Bitcoin L2 projects like Citrea use forced-inclusion mechanisms (where users post transactions directly to Bitcoin that the rollup must include) as a censorship-resistance backstop, but this is not the same as full L1 sequencing.
Spark and other Bitcoin L2 architectures navigate this same fundamental tradeoff: inheriting sequencing from the base layer gains decentralization and censorship resistance, while running a dedicated sequencer gains speed and flexibility. The design choices made by based rollups on Ethereum offer a useful reference point for evaluating Bitcoin's second-layer scaling landscape.
Risks and Considerations
Confirmation Latency
Without preconfirmations, based rollup users must wait for the next L1 block for any confirmation. On Ethereum, this means a minimum 12-second wait. On Bitcoin, it would mean approximately 10 minutes. This is significantly slower than the sub-second soft confirmations offered by centralized sequencers, and may not be acceptable for latency-sensitive applications like high-frequency trading.
MEV Dynamics
Based rollups expose L2 transactions to L1 searchers and builders through the public mempool. This can lead to sandwich attacks and front-running that centralized sequencers can mitigate by running private mempools. Additionally, all MEV revenue flows to L1 proposers rather than the rollup operator, reducing a potential revenue stream for the rollup team.
Reduced Operator Control
The rollup team cannot implement custom transaction ordering policies (such as first-come-first-served or specialized priority auctions) because ordering is determined by the L1 block-building process. This limits the rollup's ability to offer differentiated sequencing guarantees.
Preconfirmation Trust Assumptions
Based preconfirmations reintroduce trust assumptions: users must trust that preconfers will honor their signed promises, backed by the threat of slashing. While this is cryptoeconomically enforced, it is not the same as the unconditional finality of an L1 confirmation. The preconfirmation infrastructure is still maturing, with fully decentralized implementations expected to roll out through 2026.
Notable Projects
Several projects are actively building and deploying based rollups:
- Taiko: the first based rollup on Ethereum mainnet, implementing a Type 1 ZK-EVM with full proof coverage and live preconfirmations
- Nethermind Surge: a based rollup framework built on Taiko's stack, adopted by ENS for its Namechain deployment
- Puffer UniFi: a based Layer 2 architecture targeting L1-L2 and L2-L2 atomic composability
- Spire Labs: developing Based Stack, a framework for deploying application-specific based rollup chains
This glossary entry is for informational purposes only and does not constitute financial or investment advice. Always do your own research before using any protocol or technology.