nim-eth/doc/p2p.md

p2p

Introduction

This library implements the DevP2P family of networking protocols used in the Ethereum world.

Connecting to the Ethereum network

A connection to the Ethereum network can be created by instantiating the EthereumNode type:

proc newEthereumNode*(keys: KeyPair,
                      listeningAddress: Address,
                      networkId: uint,
                      chain: AbstractChainDB,
                      clientId = "nim-eth-p2p",
                      addAllCapabilities = true): EthereumNode =

Parameters:

keys: A pair of public and private keys used to authenticate the node on the network and to determine its node ID. See the keys library for utilities that will help you generate and manage such keys.

listeningAddress: The network interface and port where your client will be accepting incoming connections.

networkId: The Ethereum network ID. The client will disconnect immediately from any peers who don't use the same network.

chain: An abstract instance of the Ethereum blockchain associated with the node. This library allows you to plug any instance conforming to the abstract interface defined in the eth_common package.

clientId: A name used to identify the software package connecting to the network (i.e. similar to the User-Agent string in a browser).

addAllCapabilities: By default, the node will support all RPLx protocols imported in your project. You can specify false if you prefer to create a node with a more limited set of protocols. Use one or more calls to node.addCapability to specify the desired set:

node.addCapability(eth)

Each supplied protocol identifier is a name of a protocol introduced by the p2pProtocol macro discussed later in this document.

Instantiating an EthereumNode does not immediately connect you to the network. To start the connection process, call node.connectToNetwork:

proc connectToNetwork*(node: var EthereumNode,
                       bootstrapNodes: openArray[ENode],
                       startListening = true,
                       enableDiscovery = true)

The EthereumNode will automatically find and maintain a pool of peers using the Ethereum node discovery protocol. You can access the pool as node.peers.

Communicating with Peers using RLPx

RLPx is the high-level protocol for exchanging messages between peers in the Ethereum network. Most of the client code of this library should not be concerned with the implementation details of the underlying protocols and should use the high-level APIs described in this section.

The RLPx protocols are defined as a collection of strongly-typed messages, which are grouped into sub-protocols multiplexed over the same TCP connection.

This library represents each such message as a regular Nim function call over the Peer object. Certain messages act only as notifications, while others fit the request/response pattern.

To understand more about how messages are defined and used, let's look at the definition of a RLPx protocol:

RLPx sub-protocols

The sub-protocols are defined with the p2pProtocol macro. It will accept a short identifier for the protocol and the current protocol version:

Here is how the DevP2P wire protocol might look like:

p2pProtocol DevP2P(version = 0, rlpxName = "p2p"):
  proc hello(peer: Peer,
             version: uint,
             clientId: string,
             capabilities: openArray[Capability],
             listenPort: uint,
             nodeId: P2PNodeId) =
    peer.id = nodeId

  proc disconnect(peer: Peer, reason: DisconnectionReason)

  proc ping(peer: Peer) =
    await peer.pong()

  proc pong(peer: Peer) =
    echo "received pong from ", peer.id

As seen in the example above, a protocol definition determines both the available messages that can be sent to another peer (e.g. as in peer.pong()) and the asynchronous code responsible for handling the incoming messages.

Protocol state

The protocol implementations are expected to maintain a state and to act like a state machine handling the incoming messages. You are allowed to define an arbitrary state type that can be specified in the peerState protocol option. Later, instances of the state object can be obtained though the state pseudo-field of the Peer object:

type AbcPeerState = object
  receivedMsgsCount: int

p2pProtocol abc(version = 1,
                peerState = AbcPeerState):

  proc incomingMessage(p: Peer) =
    p.state.receivedMsgsCount += 1

Besides the per-peer state demonstrated above, there is also support for maintaining a network-wide state. It's enabled by specifying the networkState option of the protocol and the state object can be obtained through accessor of the same name.

The state objects are initialized to zero by default, but you can modify this behaviour by overriding the following procs for your state types:

proc initProtocolState*(state: MyPeerState, p: Peer)
proc initProtocolState*(state: MyNetworkState, n: EthereumNode)

Sometimes, you'll need to access the state of another protocol. To do this, specify the protocol identifier to the state accessors:

  echo "ABC protocol messages: ", peer.state(abc).receivedMsgCount

While the state machine approach may be a particularly robust way of implementing sub-protocols (it is more amenable to proving the correctness of the implementation through formal verification methods), sometimes it may be more convenient to use more imperative style of communication where the code is able to wait for a particular response after sending a particular request. The library provides two mechanisms for achieving this:

Waiting particular messages with nextMsg

The nextMsg helper proc can be used to pause the execution of an async proc until a particular incoming message from a peer arrives:

proc helloExample(peer: Peer) =
  ...
  # send a hello message
  await peer.hello(...)

  # wait for a matching hello response, might want to add a timeout here
  let response = await peer.nextMsg(p2p.hello)
  echo response.clientId # print the name of the Ethereum client
                         # used by the other peer (Geth, Parity, Nimbus, etc)

There are few things to note in the above example:

1. The p2pProtocol definition created a pseudo-variable named after the protocol holding various properties of the protocol.

2. Each message defined in the protocol received a corresponding type name, matching the message name (e.g. p2p.hello). This type will have fields matching the parameter names of the message. If the messages has openArray params, these will be remapped to seq types.

If the designated messages also has an attached handler, the future returned by nextMsg will be resolved only after the handler has been fully executed (so you can count on any side effects produced by the handler to have taken place). If there are multiple outstanding calls to nextMsg, they will complete together. Any other messages received in the meantime will still be dispatched to their respective handlers.

Please also note that the p2pProtocol macro will make this helloExample proc async. Practically see it as proc helloExample(peer: Peer) {.async.}, and thus never use waitFor, but rather await inside this proc.

For implementing protocol handshakes with nextMsg there are specific helpers which are explained below.

requestResponse pairs

p2pProtocol les(version = 2):
  ...

  requestResponse:
    proc getProofs(p: Peer, proofs: openArray[ProofRequest])
    proc proofs(p: Peer, BV: uint, proofs: openArray[Blob])

  ...

Two or more messages within the protocol may be grouped into a requestResponse block. The last message in the group is assumed to be the response while all other messages are considered requests.

When a request message is sent, the return type will be a Future that will be completed once the response is received. Please note that there is a mandatory timeout parameter, so the actual return type is Future[Option[MessageType]]. The timeout parameter can be specified for each individual call and the default value can be overridden on the level of individual message, or the entire protocol:

p2pProtocol abc(version = 1,
                useRequestIds = false,
                timeout = 5000): # value in milliseconds
  requestResponse:
    proc myReq(dataId: int, timeout = 3000)
    proc myRes(data: string)

By default, the library will take care of inserting a hidden reqId parameter as used in the LES protocol, but you can disable this behavior by overriding the protocol setting useRequestIds.

Implementing handshakes and reacting to other events

Besides message definitions and implementations, a protocol specification may also include handlers for certain important events such as newly connected peers or misbehaving or disconnecting peers:

p2pProtocol foo(version = fooVersion):
  onPeerConnected do (peer: Peer):
    let m = await peer.status(fooVersion,
                              timeout = chronos.milliseconds(5000))

    if m.protocolVersion == fooVersion:
      debug "Foo peer", peer, fooVersion
    else:
      raise newException(UselessPeerError, "Incompatible Foo version")

  onPeerDisconnected do (peer: Peer, reason: DisconnectionReason):
    debug "peer disconnected", peer

  handshake:
    proc status(peer: Peer,
                protocolVersion: uint)

For handshake messages, where the same type of message needs to be send to and received from the peer, a handshake helper is introduced, as you can see in the code example above.

Thanks to the handshake helper the status message will both be send, and be awaited for receival from the peer, with the defined timeout. In case no status message is received within the defined timeout, an error will be raised which will result in a disconnect from the peer.

Note: Be aware that if currently one of the subprotocol onPeerConnected calls fails, the client will be disconnected as UselessPeer but no onPeerDisconnect calls are run.

Checking the other peer's supported sub-protocols

Upon establishing a connection, RLPx will automatically negotiate the list of mutually supported protocols by the peers. To check whether a particular peer supports a particular sub-protocol, use the following code:

if peer.supports(les): # `les` is the identifier of the light clients sub-protocol
  peer.getReceipts(nextReqId(), neededReceipts())