Force Inclusion

Force Inclusion is a protocol-enforced right that allows a user or sequencer to compel the publication of a transaction's data to the underlying Layer 1 (L1) blockchain after a predefined delay period. Its primary purpose is to guarantee data availability and censorship resistance, preventing a malicious sequencer from indefinitely withholding transaction data to freeze user funds or halt chain progress.

The mechanism operates on a timer. When a user submits a transaction to an L2 sequencer, a force inclusion window (e.g., 24 hours) begins. If the sequencer fails to include the transaction in a batch submitted to L1 before this window expires, the user can submit a force inclusion transaction directly to a special contract on L1. This contract verifies the delay has passed and forces the data to be recorded on-chain, ensuring the L2 state can be advanced.

This is the definitive defense against a data withholding attack. In optimistic rollups, if a sequencer produces a state root but withholds the corresponding transaction data, verifiers cannot reconstruct the state to challenge invalid transitions. Force Inclusion breaks this deadlock by allowing the data to be published independently, enabling the fraud proof process to proceed and protecting users from having their assets locked.

Force Inclusion is implemented as a smart contract on the L1, often called the Inbox or Canonical Transaction Chain. Key implementations include:

• Arbitrum's Delayed Inbox: Users can force-include messages after a ~24h delay.
• Optimism's protocol also incorporates similar delayed submission mechanisms. The design involves careful economic incentives to prevent spam, often requiring the user to pay L1 gas costs for the forced publication.

For users, force inclusion acts as a guaranteed escape hatch. If a sequencer is malicious or offline, users are not permanently trapped. By paying the L1 gas fee, they can ensure their withdrawal or critical transaction is eventually processed. This makes trust in the sequencer temporary and conditional, fundamentally aligning with blockchain's trust-minimization principles.

Force Inclusion is often confused with but distinct from other L2 safety features:

• Fraud Proofs: Challenge invalid state transitions; require data to be available (which force inclusion ensures).
• Validity Proofs (ZK-Rollups): Provide immediate cryptographic proof of correctness; data availability is still required but censorship is mitigated through different means.
• Sequencer Decentralization: A complementary approach to reduce reliance on any single actor, but force inclusion provides a cryptographic guarantee even against a fully malicious sequencer.

Force inclusion is a protocol-level feature, most notably implemented in Optimistic Rollups like Arbitrum, that allows a sequencer to be compelled to include a transaction in the L2 chain. It works by submitting the transaction directly to a special inbox contract on the parent L1 (e.g., Ethereum). The key guarantee is liveness, not latency—the transaction will be included eventually, but with no guarantee of when, making it unsuitable for time-sensitive operations.

The dominant security application is to prevent a malicious or malfunctioning sequencer from censoring user transactions. If a sequencer refuses to process a valid transaction, a user can bypass it by forcing the transaction via the L1 contract. This ensures the network cannot be halted for a specific user or application, upholding a foundational blockchain property. It acts as a circuit breaker against centralized points of failure.

Force inclusion is inherently slow and expensive, creating significant practical limitations.

• Latency: The transaction must wait for L1 block confirmations and the L2's dispute window or batch posting schedule, leading to delays of hours or even days.
• Cost: The user must pay the full L1 gas fee, which is typically orders of magnitude higher than standard L2 fees. This makes it economically prohibitive for routine use.

The mechanism's effectiveness depends on economic incentives and protocol rules.

• Bond Slashing: Sequencers usually post a bond that can be slashed for malfeasance, including unjustified censorship. Force inclusion provides the proof needed to trigger slashing.
• Rate Limiting: Protocols often impose limits (e.g., one forced transaction per block) to prevent spam and denial-of-service attacks on the sequencer.
• Fee Payment: The forced transaction must still pay a fee, but the fee can be paid from a contract's balance, not solely the sender's EOA.

Not all Layer 2s implement force inclusion identically, affecting its security properties.

• Optimistic Rollups (Arbitrum, Optimism): Have explicit forceFeed or similar functions in their L1 inbox contracts.
• ZK-Rollups: Often have different censorship-resistance models, sometimes relying on proof submission rights rather than a direct force inclusion path.
• Validiums & Volitions: May not offer force inclusion at all if data availability is off-chain, representing a trade-off for higher throughput.

Relying on force inclusion requires careful user and developer planning.

• Fallback Logic: DApps should monitor transaction receipt and have clear, automated fallback paths to the force inclusion method.
• Gas Budgeting: Users must ensure their contract or wallet has sufficient ETH on L1 to cover the potentially high gas cost.
• Not for Speed: It is critical to educate users that this is an emergency exit or liveness tool, not a way to expedite a transaction. Understanding these constraints is vital for realistic security planning.

The act of a block builder or validator intentionally excluding valid transactions from a block, often for regulatory or political reasons. Force inclusion is the primary protocol-level defense against this.

• Key Target: Often affects transactions interacting with sanctioned smart contracts.
• Systemic Risk: Can undermine blockchain neutrality and permissionless access.

The profit validators or block builders can extract by reordering, including, or excluding transactions within a block. Force inclusion interacts with MEV strategies.

• Builder Influence: Builders may censor transactions to preserve exclusive MEV opportunities.
• Counter-Measure: Force inclusion protocols can be used to break up such monopolistic bundles.

A cryptographic technique used to submit sensitive data (like a bid or transaction) without revealing it immediately. It's an alternative privacy solution that can circumvent censorship.

• How it works: A user commits a hash of their transaction. Later, they reveal the actual data, which builders must include to prove validity.
• Use Case: Protects against frontrunning and targeted exclusion in environments without force inclusion.

A transaction type on Ethereum that carries large data 'blobs' separate from execution. These have unique inclusion guarantees.

• Propagation Rules: The protocol mandates that validators must propagate blobs, creating a weak form of forced data availability.
• Contrast: While not forcing execution, it ensures censored data is still available to the network.

A market design separating the roles of block proposer (validator) and block builder. This is the architectural context where force inclusion is most relevant.

• Censorship Vector: Builders in a PBS system have the power to censor. Force inclusion is a rule applied to them.
• Enforcement: Implemented via a list of transactions that the next proposer must include, bypassing a malicious builder.

A specific implementation of force inclusion, often associated with Ethereum's PBS roadmap. It is a protocol-enforced list of transactions that must be included in a block.

• Mechanism: Users submit transactions to an inclusion list managed by validators. The next block builder must include these transactions or face penalties.
• Goal: Ensures a credible commitment to transaction inclusion without relying on builder goodwill.