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PEER_DISCOVERY.md

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Peer Discovery and Auto Dial

Synopsis:

  • All peers discovered are emitted via peer:discovery so applications can take any desired action.
  • Libp2p defaults to automatically connecting to new peers, when under the ConnectionManager low watermark (minimum peers).
    • Applications can disable this via the peerDiscovery.autoDial config property, and handle connections themselves.
    • Applications who have not disabled this should never connect on peer discovery. Applications should use the peer:connect event if they wish to take a specific action on new peers.

Scenarios

In any scenario, if a peer is discovered it should be added to the PeerBook. This ensures that even if we don't dial to a node when we discover it, we know about it in the event that it becomes known as a provider for something we need. The scenarios listed below detail what actions the auto dialer will take when peers are discovered.

1. Joining the network

The node is new and needs to join the network. It currently has 0 peers. Discovery Mechanisms: Ambient Discovery

Action to take

Connect to discovered peers. This should have some degree of concurrency limiting. While the case should be low, if we immediately discover more peers than our high watermark we should avoid dialing them all.

2. Connected to some

The node is connected to other nodes. The current number of connections is less than the desired low watermark. Discovery Mechanisms: Ambient Discovery and Active Discovery

Action to take

Connect to discovered peers. This should have some degree of concurrency limiting. The concurrency may need to be modified to reflect the current number of peers connected. The more peers we have, the lower the concurrency may need to be.

3. Connected to enough

Discovery Mechanisms: Ambient Discovery and Active Discovery

Action to take

None. If we are connected to enough peers, the low watermark, we should not connect to discovered peers. As other peers discover us, they may connect to us based on their current scenario.

For example, a long running node with adequate peers is on an MDNS network. A new peer joins the network and both become aware of each other. The new peer should be the peer that dials, as it has too few peers. The existing node has no reason to dial the new peer, but should keep a record of it in case it later becomes an important node due to its contents/capabilities.

Avoiding dials above the low watermark also allows for a pool of connections to be reserved for application specific actions, such as connecting to a specific content provider via a DHT query to find that content (ipfs-bitswap).

4. Connected to too many

The node has more connections than it wants. The current number of connections is greater than the high watermark.

WIP Connection Manager v2 spec Discovery Mechanisms: Ambient Discovery and Active Discovery

Action to take

None, the ConnectionManager will automatically prune connections.

Discovery Mechanisms

Means of which a libp2p node discovers other peers.

Active Discovery

Through active use of the libp2p network, a node may discovery peers.

  • Content/Peer routing (DHT, delegated, etc) provider and peer queries
  • DHT random walk
  • Rendezvous servers

Ambient Discovery

Leveraging known addresses, or network discovery mechanisms, a node may discover peers outside of the bounds of the libp2p network.

  • Bootstrap
  • MDNS
  • proximity based (bluetooth, sound, etc)