BIP 35 (mempool message)
Provided a way for peers to request the node's mempool transactions, though it's rarely used in modern Bitcoin Core.
BIP 35 added a mempool P2P message: a peer sends mempool and the node replies with an inv listing every txid currently in its mempool. Useful around 2012, when nascent block explorers and wallets wanted a quick way to see unconfirmed activity from any willing peer.
Then it became a footgun. Dumping the whole mempool on request lets an observer fingerprint a node's policy, time propagation, and probe for transactions other peers haven't seen yet. Bitcoin Core now gates the response behind the NODE_BLOOM service bit (or peer whitelisting), so the wider P2P network has effectively stopped serving it. In 2026, public mempool data flows through block explorers and dedicated mempool infrastructure, not opportunistic mempool queries.
Spec: BIP-35. Still defined, almost never used in the wild.
Key takeaways
- Lets peers request a node's unconfirmed tx list
- Not frequently implemented or permitted nowadays
- An early attempt to share mempool data openly