Node Discovery Protocol v5
Draft of April 2019
Welcome to the Node Discovery Protocol v5 specification!
Note that this specification is a work in progress and may change incompatibly without prior notice.
Node Discovery is a system for finding other participants in a peer-to-peer network. The system can be used by any node, for any purpose, at no cost other than running the network protocol and storing a limited number of other nodes' records. Any node can be used as the an entry point into the network.
The system's design is loosely inspired by the Kademlia DHT, but unlike most DHTs no arbitrary keys and values are stored. Instead, the DHT stores and relays 'node records', which are signed documents providing information about nodes in the network. Node Discovery acts as a database of all live nodes in the network and performs three basic functions:
- Sampling the set of all live participants: by walking the DHT, the network can be enumerated.
- Searching for participants providing a certain service: Node Discovery v5 includes a scalable facility for registering 'topic advertisements'. These advertisements can be queried and nodes advertising a topic found.
- Authoritative resolution of node records: if a node's ID is known, the most recent version of its record can be retrieved.
The specification has three parts:
- discv5-wire.md defines the wire protocol.
- discv5-theory.md describes the algorithms and data structures.
- discv5-rationale.md (outdated) contains a design rationale and communication examples.
Comparison With Other Discovery Mechanisms
Systems such as MDNS/Bonjour allow finding hosts in a local-area network. The Node Discovery Protocol is designed to work on the Internet and is most useful for applications with a large number of participants spread across the Internet.
Systems using a rendezvous server: these systems are commonly used by desktop applications or cloud services to connect participants to each other. While undoubtedly efficient, this requires trust in the operator of the rendezvous server and these systems are prone to censorship. Compared to a rendezvous server, The Node Discovery Protocol doesn't rely on a single operator and places a small amount of trust in every participant. It becomes more resistant to censorship as the size of the network increases and participants of multiple distinct peer-to-peer networks can share the discovery network to further increase its resilience.
The Achilles heel of the Node Discovery Protocol is the process of joining the network: while any other node may be used as an entry point, such a node must first be located through some other mechanism. Several approaches including scalable listing of initial entry points in DNS or discovery of participants in the local network can be used for reasonable secure entry into the network.
Comparison With Node Discovery v4
- Topic advertisement was added.
- Arbitrary node metadata can be stored/relayed.
- Node identity crypto is extensible, use of secp256k1 keys isn't strictly required.
- The protocol no longer relies on the system clock.
- Communication is encrypted, protecting topic searches and record lookups against passive observers.