# Block Building Algorithm and Flow

1. Sub-block Construction Logic
   1. Each sub-block is built independently:
      * Sources of transactions:
        * Public mempool — sourced in real-time via Reth's local transaction pool (P2P gossip + txpool contents)
        * Private orderflow — received directly via the builder's private RPC endpoint
      * Sorting & Ranking Rules (inside each sub-block):
        1. Nonce ordering per sender — strict ascending nonce enforcement
        2. Effective gas price (priority fee + optional MEV tips / coinbase payment)
        3. Others may affect the ordering
           * Arrival timestamp (for same effective price)
2. Once a sub-block is confirmed:
   * It remains immutable — even if higher-value transactions arrive later in the same slot.
   * No retroactive replacement / reordering is allowed within already-confirmed sub-blocks.
   * This provides strong preconfirmation-like semantics for early-accepted orderflow and prevents last-moment value extraction races from destabilizing earlier commitments.
3. At the end of the slot (or when the slot deadline is reached):
   * The builder concatenates all previously confirmed sub-blocks in sequential order (respecting the exact order in which sub-blocks were sealed).
   * The complete, fully built execution payload is then submitted to one or more relays
   * Proposer then selects & signs header → Relay unblinds → Proposer publishes to network


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