Networking Specification

Exonum network consists of full nodes connected via peer-to-peer connections, and light clients. Full nodes communicate with each other using Protobuf messages over encrypted / authenticated TCP channels, and clients interact with full nodes via REST service interface and WebSockets.

Network Structure

Full Nodes

Full nodes store the entire contents of the blockchain. All the full nodes are authenticated with public-key cryptography. Full nodes are further subdivided into 2 categories:

  • Auditors replicate the entire contents of the blockchain. They can generate new transactions, but cannot choose which transactions should be committed (i.e., cannot generate new blocks)
  • Validators exchange consensus messages with each other to reach consensus and add new blocks into the blockchain. Validators receive transactions, verify them, and include into a new block. The list of the validators is restricted by network maintainers, and normally should consist of 4–16 nodes.

Note

The operability of the network with 4-16 validators has been verified by our research team. See section 7.3 of our whitepaper on the Exonum consensus algorithm. Meanwhile, the system may operate with a larger number of validators.

Light Clients

Tip

See the separate article for more details on light clients.

Light clients represent clients in the client-server paradigm; they connect to full nodes for several purposes:

  • to retrieve information they are interested in from the blockchain
  • to subscribe to events like block commits and transaction commits and be aware of new accepted blocks and transactions
  • to send transactions.

Note that this functionality may require certain services to be instantiated on the blockchain, such as the explorer service.

Exonum also provides a “proofs mechanism”, based on cryptographic commitments via Merkle / Merkle Patricia trees. This mechanism enables light clients to verify that a response from the full node has been really authorized by a supermajority of validators.

Peer-to-Peer Full Node Network

Full nodes use Protobuf over TCP to communicate with each other. Messages exchanged by full nodes include consensus messages and transactions. All network connections are encrypted using Noise Protocol. The Noise Protocol starts with a handshake message exchange. The handshake includes exchange of public keys and connect messages by the nodes.

As a result of the Diffie-Hellman key agreement, the nodes receive a shared secret key. This key is then used to send encrypted messages between these nodes.

Noise Protocol protects Exonum against a number of potential vulnerabilities, for example, traffic sniffing between nodes.

Transaction Broadcasting

A node broadcasts transactions obtained via HTTP API or created by the node itself, but does not broadcast transactions received from other nodes (via broadcasting or requests mechanism).

Consensus Messages and Requests

Validators generate and process consensus messages as specified by the consensus algorithm. Auditor nodes are set not to receive consensus messages (Propose, Prevote, Precommit) when they are broadcast by the validators.

Connect Messages

On establishing a P2P connection, nodes exchange Connect messages in which a node indicates its public key. The Connect message also contains the public IP address or the domain name of the node. Each node stores all received Connect messages in the list of known peers. As soon as a handshake is reached (the Connect message is received and successfully processed) from both sides, the nodes begin to exchange messages.

Peer Discovery

Each node regularly sends PeersRequest to a random known node with the timeout peers_timeout defined in the global configuration. In response, the addressee sends its list of known peers. Thus, it is enough to connect to one node at the start and after some time it will be possible to collect Connect messages from the entire network.

At the same time, the initial list of addresses, where other full nodes may be specified, is defined in the local configuration of the node (parameter connect_list). This list is used to discover the initial set of peers on the node start up. If some node changes its address, then through peer discovery mechanism a new address becomes known to all other nodes in some time.

The addresses in the connect_list may be specified both as host names and IP addresses.

Communication with Light Clients

Light clients interact with full nodes via service REST API endpoints and via WebSockets. Both these kinds of interfaces are defined by the services rather than the node. On its own, a node does not define any endpoints, although it does provides HTTP servers used by the Rust runtime. This design leads to greater flexibility and modularity.

Full nodes can receive transactions from light clients using the explorer service via POST requests. Transactions from light clients are authenticated with the help of signatures, which are the part of JSON serialization of transactions. Light clients can also get info from full nodes via GET endpoints defined in specific services. These requests are generally not authenticated.

Service Endpoints

Organization of service endpoints is dependent on the runtime. For example, Rust services define API endpoints via Service::wire_api hook. The Rust runtime provides two HTTP servers, a public one and a private one; the endpoints for each are separate. The idea is that public endpoints can be universally accessed, and private endpoints could be used for more delicate tasks, such as administration. Endpoints for a Rust service are prefixed with /api/services/{service_name}, where {service_name} is a string service identifier.

Note

There is no unified format for naming endpoints (e.g., passing parameters for GET endpoints via path components and/or query parameters). Thus, services need to use best practices for REST APIs.

Note

For historic reasons, the explorer service endpoints have /api/explorer prefix.