Bundle Submission

Bundle submission is a process where a specialized network participant, known as a searcher or builder, groups multiple transactions into a single bundle and submits it to a block builder or directly to a validator. The core guarantee is atomicity: either all transactions in the bundle are included in a block and executed successfully, or none are. This is distinct from standard transaction submission, where individual transactions are queued and processed independently, with no guarantee of shared success or ordering.

The primary use case for bundle submission is enabling complex, multi-step DeFi strategies that would be risky or impossible otherwise. For example, an arbitrage opportunity might require a sequence of swaps across multiple decentralized exchanges (DEXs) to capture a price discrepancy. Submitting these as separate transactions exposes the searcher to maximal extractable value (MEV) risks like sandwich attacks or front-running. By bundling them, the entire profitable sequence is protected, ensuring the later steps aren't executed if the initial swap fails or is intercepted.

Technically, bundles are facilitated by infrastructure like the mev-geth client on Ethereum or dedicated relays in proposer-builder separation (PBS) architectures. Searchers often pay a premium, known as a priority fee or direct payment to the builder, to ensure their bundle is included. This creates a competitive marketplace for block space. The process is fundamental to the MEV supply chain, connecting searchers who identify opportunities with builders who construct profitable blocks and validators who propose them.

While powerful, bundle submission raises considerations around network fairness and centralization. The ability to pay for guaranteed, atomic execution can favor well-capitalized players. Furthermore, the infrastructure for submitting and processing bundles is often operated by a small set of professional entities. Protocols like Flashbots aim to democratize access through transparent, open-source systems like the Flashbots Relay and SUAVE, seeking to mitigate negative externalities of MEV while preserving the utility of complex transaction bundling.

A bundle is a set of transactions submitted with the explicit guarantee of atomic execution: either all transactions in the bundle succeed and are included in a block, or the entire bundle is discarded. This is distinct from the standard mempool, where transactions are processed individually. Bundles are typically submitted via a dedicated RPC endpoint (e.g., eth_sendBundle) to specialized actors like block builders or relays in a Proposer-Builder Separation (PBS) architecture. The submitter often attaches a bid or priority fee to incentivize the block producer to include their bundle.

The submission process involves several key participants. A searcher (e.g., an arbitrage bot or liquidator) constructs a bundle containing a profitable sequence of transactions. They send this bundle, often via a private relay, to a block builder. The builder aggregates multiple bundles and pending mempool transactions to construct the most profitable block possible. Finally, the builder's block proposal is delivered to the validator (or proposer) for signing and inclusion on-chain. This flow is designed to be permissionless and competitive, though it often occurs in private channels to prevent front-running.

Bundles enable sophisticated DeFi strategies that would be impossible with single transactions. Common use cases include arbitrage (exploiting price differences across DEXs), liquidations (repaying undercollateralized loans for a reward), and NFT sweeping (purchasing multiple items in one atomic operation). For example, a searcher might bundle a flash loan, a series of swaps on Uniswap and SushiSwap to capture an arbitrage spread, and a final repayment—all conditional on the entire sequence being profitable.

The technical specification for a bundle includes critical parameters beyond the raw transaction data. These include the blockNumber it targets, a minTimestamp and maxTimestamp for validity windows, and a revertingTxHashes list to specify which transactions are allowed to fail without invalidating the entire bundle (a feature known as partial bundle revert protection). Builders evaluate bundles based on their total effective gas price, which includes the base fee, priority fee, and any potential MEV (Maximal Extractable Value) they extract, such as captured arbitrage profits.

Bundle submission exists within a broader MEV supply chain and interacts with related concepts like Flashbots MEV-Share, which allows users to share potential MEV with searchers, and Order Flow Auctions (OFAs), which create a market for transaction ordering rights. The ecosystem is evolving with standards like MEV-Boost for Ethereum, which formalizes the relay-and-builder market, and SUAVE (Single Unifying Auction for Value Expression), a proposed decentralized block-building network intended to democratize access to this critical infrastructure.

The bundle submission flow is the end-to-end process by which a searcher's transaction bundle is delivered to a block builder and ultimately included in a finalized block on a blockchain. It begins when a searcher, having identified a profitable opportunity through MEV (Maximal Extractable Value), constructs a bundle containing a sequence of transactions designed to capture that value. This bundle is then submitted to a relay, a trusted intermediary that acts as a communication layer between searchers and builders, ensuring the integrity and order of the submitted bundles.

Upon receiving a bundle, the relay performs critical validation checks. These include verifying the bundle's signature, ensuring it does not contain invalid or conflicting transactions, and checking that it adheres to the network's current rules and state. The relay then forwards valid bundles to connected block builders. Builders aggregate multiple bundles and individual transactions to construct the most profitable block possible, optimizing for the total bid value offered by searchers. The builder's final block candidate, which may include one or more bundles, is sent back to the relay.

The relay's final role is to present the most valuable valid block candidate from all builders to a validator (or proposer) for the current slot. The validator, incentivized by the block's attached bid, signs and proposes this block to the network. A successful flow concludes when the block is propagated, added to the chain, and achieves finality, at which point the searcher's transactions are executed and any profits are realized. This entire pipeline—from searcher to relay to builder to validator—forms the core infrastructure for permissionless MEV extraction on networks like Ethereum.