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# Shared Auction
Chains in the Composium opt-in to a shared auction that allows for their blocks to be bid on by builders. Once a chain opts into the shared auction, the validators of the NodeKit L1 are alerted via a WebSocket endpoint. The validators subscribe to the shared auction to receive chunks from builders. Once a chain has registered, they are always considered for shared block building and are included in the list of participating chains for the ahead-of-time (AOT) auction.
The auction sells the right to build for an epoch. An epoch is a 12 second period of time for which a builder has the right to build blocks for chains.
Auctions begin every 12 seconds (maintaining synchrony with Ethereum blocks) at epoch n-1 for block building rights of epoch n. The builder with the highest bid wins the auction and will start building blocks when their epoch starts. Auction winner and results are sent to the NodeKit L1 that is purpose-built for redistributing fees to stakers and providing a shared common ledger amongst chains.
Their are two types of blocks within the system. One is called Top of Block(ToB), and the other is called Rest of Block(RoB). ToBs are uniquely enabled by Javelin and contain cross-chain bundle. RoBs enable atomic bundles and MEV capture within individual chains. The builder who has won the auction will submit their chunks to the shared auction.
The shared auction then simulates these chunks to ensure the builder has not included a faulty bundle. Once the chunk is validated, it is then sent out to the list of subscribed validators on the L1 to be preconfirmed. Once signed by the majority of validators, it is sent back to Arcadia, where the signatures are verified.
After verification, the preconfirmed chunk is added to a list of preconfirmed chunks that chains can make GetPayload requests to retrieve.
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