P2P Gossip Propagation

Bitcoin and most blockchains use a gossip protocol to spread blocks and transactions across the network. Each node maintains ~8 outbound peers + up to ~125 inbound. When a node learns of a new tx or block, it:

1. Sends a tiny INV message to each peer (just an item id).
2. Peer responds GETDATA if they don't have it yet.
3. Original node sends the full item.

This INV-GETDATA dance saves bandwidth: peers don't re-request what they already have.

Propagation time

For Bitcoin's real network, a typical block reaches >50% of nodes within ~5 seconds and >99% within ~10 seconds (Bitnodes measurements). The variance matters: miners on the well-connected backbone get blocks first and have a head start on the next block — a small selfish-mining advantage.

Eclipse attacks

If an attacker controls all of your peer connections, they control your view of the chain. Mitigations: random peer selection, address gossip diversity, prefer high-uptime peers, manual peer pinning for important nodes.

Compact block relay (BIP-152)

To reduce block propagation latency:

• Instead of sending the full block, send a compact block sketch: 6-byte short txid per tx.
• Receiver reconstructs the block from its mempool.
• Missing txs requested separately.

This reduced block propagation from ~5s to ~1s on average.

Erlay (proposed)

An even smarter relay using set reconciliation instead of full INV gossip. Saves bandwidth at the cost of CPU.

This exercise simulates basic gossip on a peer graph: announce an item, tick the clock, watch it spread one hop per tick.