Initial Block Download (IBD)
Initial Block Download (IBD) is the process of building the full Bitcoin blockchain from scratch. When a new node is set up and joins the network, it connects to other nodes and asks them for blocks. The new node processes these blocks and builds the blockchain until it has caught up and is in sync with the network.
Although blocks are being requested from individual nodes, the IBD process is trustless because a node can cross-check the data from different nodes and because of the nature of the blockchain’s Proof-of-Work chain. This process can take a few days or a few weeks depending on internet bandwidth and computer specifications.
When beginning IBD, a node first gathers all of the block headers from other nodes and then requests each full block. This is done for increased efficiency and to allow users to begin using their node sooner. While building the blockchain block by block, a Bitcoin node also builds the UTXO set—the comprehensive list of all valid bitcoin. This set is updated with each block as the transactions in that block destroy and create UTXOs.