Compact Blocks (BIP-152)

Compact Blocks, specified in BIP-152 and merged into Bitcoin Core in June 2016, is a peer-to-peer relay optimization that dramatically reduces the bandwidth and latency of propagating new blocks across the network. Faster, leaner block propagation matters directly to miners, because every second a block takes to reach the network raises the risk of an orphaned block.

How it saves bandwidth

By the time a block is found, most nodes have already received its transactions through normal mempool relay. Rather than resending those full transactions, the sender transmits a compact block: the 80-byte header plus a short 6-byte identifier for each transaction. A receiving node reconstructs the block from its own mempool and only requests the few transactions it is missing. A multi-megabyte block can shrink to roughly 15 KB on the wire — about a 98% reduction.

High-bandwidth vs low-bandwidth mode

In low-bandwidth mode a node announces a block and waits to be asked for the compact version. In high-bandwidth mode a peer pushes the compact block unsolicited the moment it sees a valid header, letting well-connected nodes relay most blocks in a single round trip without any follow-up request. Because compact blocks rely on a shared mempool, they help most near the chain tip, where node mempools are most similar.

This optimization complements the SegWit-era witness commitment and is part of the propagation stack every node and pool depends on. D-Central documents these internals as part of an educational mining reference.