[discovery] Add support for AutoRelay

[jacobheun]

AutoRelay enables libp2p nodes to dynamically find and bind to relays on the network. Once binding (listening) is done, the node can and should advertise its addresses to the network, allowing any other node to dial it over its bound relay(s). While direct connections to nodes are preferable, it's not always possible to do so due to NATs.

Use Cases

Finding Relays

A connected relay

The Circuit Relay module has the ability to ask connected nodes if they support the relay HOP protocol. If they do, the libp2p node can bind to that relay as a listener. A node should record a list of relays and use only a subset of those to avoid binding to a large number of relays, as this is not considerate to those relays.

1. When a node connects to a peer that announces support for the relay protocol, /libp2p/circuit/relay/0.1.0, it may ask that node if it support HOP.
2. If the peer supports HOP, libp2p should add it to its list of known relays.
3. When a known relay is added, libp2p should check to see if it needs a/another relay (to meet some min threshold). If below that threshold, libp2p should bind to that relay and add it to its announced addresses.
   1. When adding announced addresses, libp2p SHOULD avoid adding private addresses of the relay, such as /ip4/127.0.0.1/tcp/4001.
   2. If libp2p does not have enough known relays, it should query the network if possible.
4. When libp2p updates its addresses, it should update the network.

Remote Relays

When a node with AutoRelay enabled boots, it should search the network for the relay protocol, /libp2p/relay. This search may be done via one of several options leveraging libp2p.contentRouting.findProviders(). Depending on what features are turned on this may include: the DHT, Delegate Routing, and/or the Rendezvous Protocol (WIP).

If a node does not have enough known relays connected, and is unable to connect to other known relays, it should query the network for more relays. This should be done with a backoff to avoid spamming queries.

References:

• Cant connect to /p2p-circuit peers
• go-libp2p AutoRelay

[vasco-santos]

After discussing with @jacobheun I am planning on dividing this effort in a set of PRs as follows:

Milestone	PR	State
1) Auto relay - initial implementation	#723	DONE
2) Identify push integration	#748	WIP
3) Query the network for relays	#749	WIP
4) Avoid announcing private addresses	TODO	TODO

WIP NOTES:

Milesone 1

• When a node connects to a peer that announces support for the relay protocol, /libp2p/circuit/relay/0.1.0, it may ask that node if it support HOP.
• If the peer supports HOP, libp2p should add it to its list of known relays.
• maxListeners

Milestone 2

• Identify push with multiaddrs update

Milestone 3

• If libp2p does not have enough known relays, it should query the network if possible.

Milesone 4

• Avoid announcing private addresses

[vasco-santos]

This is partially implemented from 0.30. We do need to improve the logic around finding remote relays on the network:

If a node does not have enough known relays connected, and is unable to connect to other known relays, it should query the network for more relays. This should be done with a backoff to avoid spamming queries.