BIP 152 (Compact Blocks)

BIP-152 defines compact block relay, the protocol Bitcoin nodes use to propagate newly-mined blocks efficiently. Activated in Bitcoin Core in 2016, it dramatically reduced the bandwidth and latency of block propagation.

The problem it solves: when a new block is found, every node on the network needs to receive it as quickly as possible. Slow propagation means stale-block rates go up, which hurts miners and makes the network less efficient overall. Naively sending the full block to every peer is wasteful - if those peers already have most of the transactions in their mempools, you'd be sending data they already have.

How compact blocks work:

1. New block arrives. A miner publishes a new block.
2. The relay sends a compact summary first. Instead of the full block, peers receive the block header plus a list of short transaction IDs (6-byte hashes) of every transaction in the block.
3. Each peer checks its mempool. For every short ID, it tries to find the matching transaction in its own mempool. Hits are filled in locally.
4. Peer requests the misses. Any transactions the peer didn't already have are requested explicitly.
5. Block reconstruction. With most transactions already in mempool and the few missing ones now fetched, the peer reconstructs the full block and validates it.

Typical bandwidth savings: ~90%+ for well-connected nodes whose mempools are mostly synced with the network's. The block-propagation time drops from seconds to hundreds of milliseconds, which keeps stale block rates low and the network healthy.

Compact block relay is silently load-bearing for Bitcoin's network performance. Most users never know it exists.