IMPLEMENTATION_NOTES.md

EDIT: This document is outdated and here only for historical purposes

NOTE: This document is structured in an if-then/else[if]-then manner, each line is a precondition for following lines with a higher number of indentation

Example:

• if there are apples
  • eat them
• if not, check for pears
  • then eat them
• if not, check for cherries
  • then eat them

Or,

• if there are apples
  • eat them
• if not
  • check for pears
    • then eat them
• if not
  • check for cherries
    • then eat them

In order to minimize nesting, the first example is preferred

Relay flow

Relay transport (dialer/listener)

• Dial over a relay

  • See if there is a relay that's already connected to the destination peer, if not
    • Ask all the peer's known relays to dial the destination peer until an active relay (one that can dial on behalf of other peers), or a relay that may have recently acquired a connection to the destination peer is successful.
      • If successful
        • Write the /ipfs/relay/circuit/1.0.0 header to the relay, followed by the destination address
          • e.g. /ipfs/relay/circuit/1.0.0\n/p2p-circuit/ipfs/QmDest.
      • If no relays could connect, fail the same way a regular transport would
  • Once the connection has been established, the swarm should treat it as a regular connection,
    • i.e. muxing, encrypt, etc should all be performed on the relayed connection

• Listen for relayed connections

  • Peer mounts the /ipfs/relay/circuit/1.0.0 proto and listens for relayed connections
    • A connection arrives
      • read the address of the source peer from the incoming connection stream
        • if valid, create a PeerInfo object for that peer and add the incoming address to its multiaddresses list
        • pass the connection to protocolMuxer(swarm.protocols, conn) to have it go through the regular muxing/encryption flow

• Relay discovery and static relay addresses in swarm config

  • Relay address in swarm config

    • A peer has relay addresses in its swarm config section
      • On node startup, connect to the relays in swarm config
        • if successful add address to swarms PeerInfo's multiaddresses
          • identify should take care of announcing that the peer is reachable over the listed relays

  • Passive relay discovery

    • A peer that can dial over /ipfs/relay/circuit/1.0.0 listens for the peer-mux-established swarm event, every time a new muxed connection arrives, it checks if the incoming peer is a relay. (How would this work? Some way of discovering if its a relay is required.)
      • Useful in cases when the peer/node doesn't know of any relays on startup and also, to learn of as many additional relays in the network as possible
      • Useful during startup, when connecting to bootstrap nodes. It allows us to implicitly learn if its a relay without having to explicitly add /p2p-circuit addresses to the bootstrap list
      • Also useful if the relay communicates its capabilities upon connecting to it, as to avoid additional unnecessary requests/queries. I.e. if it supports weather its able to forward connections and weather it supports the ls or other commands.
      • Should it be possible to disable passive relay discovery?
        • This could be useful when the peer wants to be reachable only over the listed relays
    • If the incoming peer is a relay, send an ls and record its peers

Relay Nodes

• Passive relay node

  • A passive relay does not explicitly dial into any requested peer, only those that it's swarm already has connections to.
    • When the relay gets a request, read the the destination peer's multiaddr from the connection stream and if its a valid address and peer id
      • check its swarm's peerbook(?) see if its a known peer, if it is
        • use the swarms existing connection and
          • send the multistream header and the source peer address to the dest peer
            • e.g. /ipfs/relay/circuit/1.0.0\n/p2p-circuit/ipfs/QmSource
          • circuit the source and dest connections
        • if couldn't dial, or the connection/stream to the dest peer closed prematurelly
          • close the src stream

• Active relay node

  • An active relay node can dial other peers even if its swarm doesnt know about those peers
    • When the relay gets a request, read the the destination peer's multiaddr from the connection stream and if its a valid address and peer id
      • use the swarm to dial to the dest node
        • send the multistream header and the source peer address to the dest peer
          • e.g. /ipfs/relay/circuit/1.0.0\n/p2p-circuit/ipfs/QmSource
        • circuit the source and dest connections
      • if couldn't dial, or the connection/stream to the dest peer closed prematurely
        • close the src stream

• ls command

  • A relay node can allow the peers known to it's swarm to be listed
    • this should be possible to enable/disable from the config
      • when a relay gets the ls request
        • if enabled, get its swarm's peerbook's known peers and return their ids and multiaddrs
          • e.g [{id: /ipfs/QmPeerId, addrs: ['ma1', 'ma2', 'ma3']}, ...]
        • if disabled, respond with na

Relay Implementation notes

• Relay transport

  • Currently I've implemented the dialer and listener parts of the relay as a transport, meaning that it tries to implement the interface-transport interface as closely as possible. This seems to work pretty well and it's makes the dialer/listener parts really easy to plug in into the swarm. I think this is the cleanest solution.

• circuit-relay

  • This is implemented as a separate piece (not a transport), and it can be enabled/disabled with a config. The transport listener however, will do the initial parsing of the incoming header and figure out weather it's a connection that's needs to be handled by the circuit-relay, or its a connection that is being relayed from a circuit-relay.

Relay swarm integration

• The relay transport is mounted explicitly by calling the swarm.connection.relay(config.relay) from libp2p

  • Swarm will register the dialer and listener using the swarm transport.add and transport.listen methods

  • Listener

    • the listener registers itself as a multistream handler on the /ipfs/relay/circuit/1.0.0 proto
      • if circuit-relay is enabled, the listener will delegate connections to it if appropriate
    • when the listener receives a connection, it will read the multiaddr and determine if its a connection that needs to be relayed, or its a connection that is being relayed

  • Dialer

    • When the swarm attempts to dial to a peer, it will filter the protocols that the peer can be reached on
      • The relay will be used in two cases
      • If the peer has an explicit relay address that it can be reached on
        • no other transport is available
          • The relay will attempt to dial the peer over that relay
      • If no explicit relay address is provided
        • no other transport is available
          • A generic circuit address will be added to the peers multiaddr list
            • i.e. /p2p-circuit/ipfs/QmDest
      • If another transport is available, then use that instead of the relay