Dandelion (BIP156)

Dandelion, specified in BIP156, is a proposed transaction-relay routing technique that makes it harder for a network observer to identify which node first broadcast a transaction. Standard Bitcoin relay uses symmetric diffusion: a transaction radiates outward from its source almost immediately, so an adversary watching many connections can often triangulate the originating node and link it to an IP address.

Stem phase and fluff phase

Dandelion splits propagation into two stages. In the stem phase, the transaction is passed serially from one node to a single randomly chosen peer along a privacy subgraph, hop by hop, without wider broadcast. After a random number of hops it enters the fluff phase, where a node releases it to all peers through normal diffusion. Because the eventual broadcaster is several hops removed from the true origin, the source is obscured. The refined Dandelion++ variant hardens the path selection and timing against more sophisticated deanonymization.

Status and practical context

Dandelion improves source privacy but was never merged into Bitcoin Core, so it is not active on the main network; it remains a documented proposal and has seen experimental use elsewhere. Its protection is strongest when combined with connection-level privacy, such as relaying over Tor or the encrypted v2 transport, so that stem hops do not leak metadata in transit.

For connection-level encryption, see BIP324 (v2 Encrypted P2P Transport). For longer onion and I2P addresses used in privacy networking, see addrv2 (BIP155).