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Add documentation at the root of substrate-network (#2902)
* Add documentation at the root of substrate-network * Apply suggestions from code review Co-Authored-By: Toralf Wittner <tw@dtex.org> * Grmlblbl * Adjustments * Link to Yamux specs
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Pierre Krieger
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DemiMarie-parity
DemiMarie-parity
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@@ -17,11 +17,149 @@
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#![warn(unused_extern_crates)]
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#![warn(missing_docs)]
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//! Substrate-specific P2P networking: synchronizing blocks, propagating BFT messages.
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//! Allows attachment of an optional subprotocol for chain-specific requests.
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//! Substrate-specific P2P networking.
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//!
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//! **Important**: This crate is unstable and the API and usage may change.
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//!
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//! # Node identities and addresses
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//!
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//! In a decentralized network, each node possesses a network private key and a network public key.
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//! In Substrate, the keys are based on the ed25519 curve. As of the writing of this documentation,
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//! the secp256k1 curve can also be used, but is deprecated. Our local node's keypair must be
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//! passed as part of the network configuration.
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//!
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//! From a node's public key, we can derive its *identity*. In Substrate and libp2p, a node's
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//! identity is represented with the [`PeerId`] struct. All network communications between nodes on
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//! the network use encryption derived from both sides's keys, which means that **identities cannot
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//! be faked**.
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//!
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//! A node's identity uniquely identifies a machine on the network. If you start two or more
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//! clients using the same network key, large interferences will happen.
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//!
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//! # Substrate's network protocol
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//!
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//! Substrate's networking protocol is based upon libp2p. It is at the moment not possible and not
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//! planned to permit using something else than the libp2p network stack and the rust-libp2p
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//! library. However the libp2p framework is very flexible and the rust-libp2p library could be
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//! extended to support a wider range of protocols than what is offered by libp2p.
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//!
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//! ## Discovery mechanisms
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//!
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//! In order for our node to join a peer-to-peer network, it has to know a list of nodes that are
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//! part of said network. This includes nodes identities and their address (how to reach them).
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//! Building such a list is called the **discovery** mechanism. There are three mechanisms that
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//! Substrate uses:
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//!
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//! - Bootstrap nodes. These are hard-coded node identities and addresses passed alongside with
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//! the network configuration.
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//! - mDNS. We perform a UDP broadcast on the local network. Nodes that listen may respond with
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//! their identity. More info [here](https://github.com/libp2p/specs/blob/master/discovery/mdns.md).
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//! mDNS can be disabled in the network configuration.
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//! - Kademlia random walk. Once connected, we perform random Kademlia `FIND_NODE` requests in
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//! order for nodes to propagate to us their view of the network. More information about Kademlia
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//! can be found [on Wikipedia](https://en.wikipedia.org/wiki/Kademlia).
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//!
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//! ## Connection establishment
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//!
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//! When node Alice knows node Bob's identity and address, it can establish a connection with Bob.
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//! All connections must always use encryption and multiplexing. While some node addresses (eg.
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//! addresses using `/quic`) already imply which encryption and/or multiplexing to use, for others
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//! the **multistream-select** protocol is used in order to negotiate an encryption layer and/or a
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//! multiplexing layer.
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//!
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//! The connection establishment mechanism is called the **transport**.
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//!
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//! As of the writing of this documentation, the following base-layer protocols are supported by
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//! Substrate:
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//!
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//! - TCP/IP for addresses of the form `/ip4/1.2.3.4/tcp/5`. Once the TCP connection is open, an
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//! encryption and a multiplexing layer are negotiated on top.
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//! - WebSockets for addresses of the form `/ip4/1.2.3.4/tcp/5/ws`. A TCP/IP connection is open and
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//! the WebSockets protocol is negotiated on top. Communications then happen inside WebSockets data
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//! frames. Encryption and multiplexing are additionally negotiated again inside this channel.
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//! - DNS for addresses of the form `/dns4/example.com/tcp/5` or `/dns4/example.com/tcp/5/ws`. A
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//! node's address can contain a domain name.
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//!
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//! The following encryption protocols are supported:
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//!
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//! - [Secio](https://github.com/libp2p/specs/tree/master/secio). A TLS-1.2-like protocol but
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//! without certificates. Support for secio will likely be deprecated in the far future.
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//! - [Noise](https://noiseprotocol.org/). Support for noise is very experimental. The details are
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//! very blurry and may change at any moment.
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//!
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//! The following multiplexing protocols are supported:
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//!
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//! - [Mplex](https://github.com/libp2p/specs/tree/master/mplex). Support for mplex will likely
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//! be deprecated in the future.
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//! - [Yamux](https://github.com/hashicorp/yamux/blob/master/spec.md).
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//!
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//! ## Substreams
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//!
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//! Once a connection has been established and uses multiplexing, substreams can be opened. When
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//! a substream is open, the **multistream-select** protocol is used to negotiate which protocol to
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//! use on that given substream. In practice, Substrate opens the following substreams:
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//!
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//! - We periodically open an ephemeral substream in order to ping the remote and check whether the
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//! connection is still alive. Failure for the remote to reply leads to a disconnection. This uses
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//! the libp2p ping protocol.
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//! - We periodically open an ephemeral substream in order to ask information from the remote. This
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//! is called [the `identify` protocol](https://github.com/libp2p/specs/tree/master/identify).
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//! - We periodically open ephemeral substreams for Kademlia random walk queries. Each Kademlia
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//! query is done in a new separate substream. This uses the
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//! [standard libp2p Kademlia protocol](https://github.com/libp2p/specs/pull/108).
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//! - We optionally keep a substream alive for all Substrate-based communications. The name of the
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//! protocol negotiated is based on the *protocol ID* passed as part of the network configuration.
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//! This protocol ID should be unique for each chain and prevents nodes from different chains from
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//! connecting to each other. More information below.
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//!
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//! ## The Substrate substream
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//!
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//! Substrate uses a component named the **peerset manager (PSM)**. Through the discovery
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//! mechanism, the PSM is aware of the nodes that are part of the network and decides which nodes
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//! we should perform Substrate-based communications with. For these nodes, we open a connection
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//! if necessary and open a unique substream for Substrate-based communications. If the PSM decides
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//! that we should disconnect a node, then that substream is closed.
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//!
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//! For more information about the PSM, see the *substrate-peerset* crate.
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//!
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//! Note that at the moment there is no mechanism in place to solve the issues that arise where the
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//! two sides of a connection open the unique substream simultaneously. In order to not run into
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//! issues, only the dialer of a connection is allowed to open the unique substream. When the
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//! substream is closed, the entire connection is closed as well. This is a bug, and should be
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//! fixed by improving the protocol.
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//!
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//! Within the unique Substrate substream, messages encoded using
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//! [*parity-scale-codec*](https://github.com/paritytech/parity-scale-codec) are exchanged.
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//! The detail of theses messages is not totally in place, but they can be found in the
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//! `message.rs` file.
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//!
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//! Once the substream is open, the first step is an exchange of a *status* message from both
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//! sides, containing information such as the chain root hash, head of chain, and so on.
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//!
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//! Communications within this substream include:
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//!
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//! - Syncing. Blocks are announced and requested from other nodes.
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//! - Light-client requests. When a light client requires information, a random node we have a
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//! substream open with is chosen, and the information is requested from it.
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//! - Gossiping. Used for example by grandpa.
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//! - Network specialization. The network protocol can be specialized through a template parameter
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//! of the network service. This specialization is free to send and receive messages with the
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//! remote. This is meant to be used by the chain that is being built on top of Substrate
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//! (eg. Polkadot).
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//!
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//! It is intended that in the future each of these components gets more isolated, so that they
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//! are free to open and close their own substreams, and so that syncing and light client requests
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//! are able to communicate with nodes outside of the range of the PSM.
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//!
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//! # Usage
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//!
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//! Using the `substrate-network` crate is done through the [`NetworkWorker`] struct. Create this
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//! struct by passing a [`config::Params`], then poll it as if it was a `Future`. You can extract an
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//! `Arc<NetworkService>` from the `NetworkWorker`, which can be shared amongst multiple places
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//! in order to give orders to the networking.
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//!
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//! More precise usage details are still being worked on and will likely change in the future.
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//!
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mod behaviour;
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mod chain;
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