About Epochs

Introduction

Epochs are the fundamental time unit in OP_NET's consensus mechanism. They function as the heartbeat of the system, providing regular intervals that group transactions, finalize state, and create immutable checkpoints of everything that occurred during each period.

Unlike traditional blockchains that create new blocks through mining, OP_NET uses Bitcoin's existing blocks and organizes them into epochs. This architectural choice enables OP_NET to operate directly on Bitcoin Layer 1 without requiring any modifications to Bitcoin's protocol.

What Are Epochs and Why Do They Matter?

An epoch consists of exactly five consecutive Bitcoin blocks, taking approximately 50 minutes to complete. This serves as the fundamental time unit in OP_NET's consensus mechanism. Every transaction confirmed in Bitcoin during those five blocks becomes part of that epoch's state and is processed and finalized together.

Why Epochs Are Critical

Epochs serve a critical architectural purpose beyond simple time division. They address fundamental challenges required to make OP_NET possible:

Enable Smart Contracts on Bitcoin

By grouping transactions into epochs and creating checkpoints, OP_NET can maintain complex state on top of Bitcoin without modifying Bitcoin's protocol. The epoch system provides the temporal structure needed for Turing complete computation.

Achieve Mathematical Consensus

Epochs enable deterministic consensus through mathematical proofs rather than social agreement or voting. Every node independently processes the same epochs and arrives at provably identical results.

Make Forks Mathematically Impossible

The epoch structure, combined with time delayed attestations, makes forks impossible. All decisions reference immutable Bitcoin history (21+ blocks deep) that all nodes must agree on.

Create Sustainable Economic Incentives

Epoch mining provides a decentralized way to incentivize network operation without requiring a separate token. Miners earn future Bitcoin transaction fees, aligning incentives with long-term network health.

Detailed Epoch Flow

The epoch process spans from transaction processing through mining competition, winner selection, and reward distribution, demonstrating how OP_NET achieves deterministic consensus.

Five Bitcoin Blocks

The selection of five Bitcoin blocks per epoch balances several competing factors:

Transaction Batching

Five blocks provides enough time to accumulate meaningful transaction volume while keeping finalization intervals reasonable. Too few blocks would create overhead; too many would delay finality.

Finality Timing

~50 minutes gives users predictable finalization windows. Combined with the attestation delay, transactions achieve practical irreversibility within 3.5 hours (~21 blocks).

Mining Economics

The interval provides sufficient time for miners to compute SHA-1 collisions while ensuring regular reward opportunities. This creates a competitive mining market without excessive computational waste.

State Management

Five blocks allows for manageable state checkpoints. Each epoch's state root represents a reasonable amount of state changes without creating excessively large Merkle trees.

Transaction Ordering

Transaction ordering within each epoch follows a deterministic algorithm rather than simple Bitcoin block sequence. Transactions are grouped by dependencies and sorted to guarantee identical processing order across all nodes.

• Dependency Grouping
  Transactions spending the same UTXOs stay together, maintaining topological order (parent before child).
• Fee-Based Ranking
  Groups compete based on economic incentives. Higher priority fees and lower gas usage = better rank = earlier execution.
• Mathematical Tiebreakers
  When ranks are equal, concatenated transaction hashes provide deterministic ordering. No ambiguity, no voting.
• Coinbase Priority
  Block rewards process first because they have no dependencies, ensuring they're always available for subsequent transactions.

State Finalization at Epoch End

When the last block of an epoch is mined, several critical things happen:

1. Transaction Processing Completes
   All transactions from blocks of this epoch have been executed through OP_VM in deterministic order.
2. State Root Generation
   The system generates a checksum root; a cryptographic hash (SHA-256) of the entire state after all transactions have executed. This includes:
   • All contract storage values.
   • All token balances.
   • All contract variables.
   • The complete transaction set and their results.
3. Mining Competition Begins
   Miners can now compete to create the winning solution for the epoch, which involves finding a SHA-1 near-collision with this checksum root.

Mining and Submission Window

After an epoch ends, miners have the duration of the next epoch to submit their solutions.

OP_NET uses SHA-1 for epoch mining instead of SHA-256. While this might seem surprising, it's a deliberate choice that provides Bitcoin compatibility, decentralization benefits, and secure proof-of-work for state finalization.

Finality Stages

OP_NET transactions progress through several stages of finality, each providing stronger guarantees than the previous.

After Stage 4, transactions are protected not by probabilistic security but by mathematical impossibility. Reversal would require reorganizing 21 or more Bitcoin blocks, which becomes economically irrational after a few hours and physically impossible after a few days.

Epochs vs Bitcoin Blocks

Epochs are fundamentally different from Bitcoin blocks, even though they are built on top of them.