Crate sc_network_gossip

Polite gossiping.

This crate provides gossiping capabilities on top of a network.

Gossip messages are separated by two categories: “topics” and consensus engine ID. The consensus engine ID is sent over the wire with the message, while the topic is not, with the expectation that the topic can be derived implicitly from the content of the message, assuming it is valid.

Topics are a single 32-byte tag associated with a message, used to group those messages in an opaque way. Consensus code can invoke ValidatorContext::broadcast_topic to attempt to send all messages under a single topic to all peers who don’t have them yet, and ValidatorContext::send_topic to send all messages under a single topic to a specific peer.

Usage

• Implement the Network trait, representing the low-level networking primitives. It is already implemented on sc_network::NetworkService.
• Implement the Validator trait. See the section below.
• Decide on a protocol name. Each gossiping protocol should have a different one.
• Build a GossipEngine using these three elements.
• Use the methods of the GossipEngine in order to send out messages and receive incoming messages.

The GossipEngine will automatically use Network::add_set_reserved and [NetworkPeers::remove_peers_from_reserved_set] to maintain a set of peers equal to the set of peers the node is syncing from. See the documentation of sc-network for more explanations about the concepts of peer sets.

What is a validator?

The primary role of a Validator is to process incoming messages from peers, and decide whether to discard them or process them. It also decides whether to re-broadcast the message.

The secondary role of the Validator is to check if a message is allowed to be sent to a given peer. All messages, before being sent, will be checked against this filter. This enables the validator to use information it’s aware of about connected peers to decide whether to send messages to them at any given moment in time - In particular, to wait until peers can accept and process the message before sending it.

Lastly, the fact that gossip validators can decide not to rebroadcast messages opens the door for neighbor status packets to be baked into the gossip protocol. These status packets will typically contain light pieces of information used to inform peers of a current view of protocol state.

Structs

• DiscardAll
  A gossip message validator that discards all messages.
• GossipEngine
  Wraps around an implementation of the Network trait and provides gossiping capabilities on top of it.
• TopicNotification
  Topic stream message with sender.

Enums

• MessageIntent
  The reason for sending out the message.
• ValidationResult
  Message validation result.

Traits

• Network
  Abstraction over a network.
• Syncing
  Abstraction over the syncing subsystem.
• Validator
  Validates consensus messages.
• ValidatorContext
  Validation context. Allows reacting to incoming messages by sending out further messages.