Headers-First Sync

Headers-First Sync is the initial-block-download strategy Bitcoin Core has used since version 0.10.0. Rather than walking the chain one full block at a time, a starting node first requests and validates the chain of compact 80-byte block headers. Once it knows the most-work header chain, it downloads the corresponding full blocks afterward, in parallel from many peers, then fully validates them in order.

Why headers come first

Each header commits to its predecessor and carries a proof-of-work target, so a node can cheaply establish the shape and total work of the best chain before spending bandwidth on multi-megabyte blocks. This sidesteps a weakness of the older blocks-first approach, where a dishonest peer could feed a long but invalid alternative chain and waste a syncing node's bandwidth and CPU on data it would ultimately discard.

Performance characteristics

Because the full header chain is known in advance, block bodies can be fetched concurrently from all available peers instead of serially from one, sharply improving initial-block-download throughput. The header chain is small relative to the full blockchain, so validating it is fast even on modest hardware. Headers-first is foundational to how lightweight clients also operate: they may keep only headers and rely on filters or proofs to learn which full blocks they actually need.

For how light clients then identify relevant blocks without downloading everything, see Compact Block Filters (BIP158). For how full blocks propagate efficiently once known, see Compact Block Relay (BIP152).

See current chain height in the live network vitals.