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ETH 2.0 Networking Spec - Libp2p standard protocols

Abstract

Ethereum 2.0 clients plan to use the libp2p protocol networking stack for mainnet release. This document aims to standardize the libp2p client protocols, configuration and messaging formats.

Libp2p Components

Transport

This section details the libp2p transport layer that underlies the protocols that are listed in this document.

Libp2p allows composition of multiple transports. Eth2.0 clients should support TCP/IP and optionally websockets. Websockets are useful for implementations running in the browser and therefore native clients would ideally support these implementations by supporting websockets.

An ideal libp2p transport would therefore support both TCP/IP and websockets.

Note: There is active development in libp2p to facilitate the QUIC transport, which may be adopted in the future

Encryption

Libp2p currently offers Secio which can upgrade a transport which will then encrypt all future communication. Secio generates a symmetric ephemeral key which peers use to encrypt their communication. It can support a range of ciphers and currently supports key derivation for elliptic curve-based public keys.

Current defaults are:

Key agreement: ECDH-P256 (also supports ECDH-P384)
Cipher: AES-128 (also supports AES-256, TwofishCTR)
Digests: SHA256 (also supports SHA512)

Note: Secio is being deprecated in favour of TLS 1.3. It is our intention to transition to use TLS 1.3 for encryption between nodes, rather than Secio.

Protocols

This section lists the necessary libp2p protocols required by Ethereum 2.0 running a libp2p network stack.

Multistream-select

Protocol id: `/multistream/1.0.0`

Clients running libp2p should support the multistream-select protocol which allows clients to negotiate libp2p protocols establish streams per protocol.

Multiplexing

Libp2p allows clients to compose multiple multiplexing methods. Clients should support mplex and optionally yamux (these can be composed).

Mplex protocol id: /mplex/6.7.0

Yamux protocol id: /yamux/1.0.0

Gossipsub

Protocol id: `/eth/serenity/gossipsub/1.0.0`

Note: Parameters listed here are subject to a large-scale network feasibility study

The Gossipsub protocol is used for block and attestation propagation across the network.

Configuration Parameters

Gossipsub has a number of internal configuration parameters which directly effect the network performance. Clients can implement independently, however we aim to standardize these across clients to optimize the gossip network for propagation times and message duplication. Current network-related defaults are:

(
	// The target number of peers in the overlay mesh network (D in the libp2p specs).
	mesh_size: 6
	// The minimum number of peers in the mesh network before adding more (D_lo in the libp2p specs).
	mesh_lo: 4
	// The maximum number of peers in the mesh network before removing some (D_high in the libp2p sepcs).
	mesh_high: 12
	// The number of peers to gossip to during a heartbeat (D_lazy in the libp2p sepcs).
	gossip_lazy: 6 // defaults to `mesh_size`
	// Time to live for fanout peers (seconds).
	fanout_ttl: 60
	// The number of heartbeats to gossip about.
	gossip_history: 3
	// Time between each heartbeat (seconds).
	heartbeat_interval: 1
)

Topics

The Go and Js implementations use string topics - This is likely to be updated to topic hashes in later versions - https://github.com/libp2p/rust-libp2p/issues/473

For Eth2.0 clients, topics are sent as SHA2-256 hashes of the topic string.

There are two main topics used to propagate attestations and beacon blocks to all nodes on the network.

The beacon_block topic - This topic is used solely for propagating new beacon blocks to all nodes on the networks.
The beacon_attestation topic - This topic is used to propagate aggregated attestations to subscribing nodes (typically block proposers) to be included into future blocks. Attestations are aggregated in their respective subnets before publishing on this topic.

Shards are grouped into their own subnets (defined by a shard topic). The number of shard subnets is defined via SHARD_SUBNET_COUNT and the shard shard_number % SHARD_SUBNET_COUNT is assigned to the topic: shard{shard_number % SHARD_SUBNET_COUNT}_attestation.

Messages

Note: The message format here is Eth2.0-specific

Each Gossipsub Message has a maximum size of 512KB (estimated from expected largest uncompressed block size).

The data field of a Gossipsub Message is an SSZ-encoded object. For the beacon_block topic, this is a beacon_block. For the beacon_attestation topic, this is an attestation.

Eth-2 RPC

Protocol Id: `/eth/serenity/beacon/rpc/1`

The RPC Interface is specified in this repository.

Discovery

Discovery Version 5 (discv5) will be used for discovery. This protocol uses a UDP transport and specifies its own encryption, ip-discovery and topic advertisement. Therefore, it has no need to establish streams through multistream-select, rather, act as a standalone implementation that feeds discovered peers/topics (ENR-records) as multiaddrs into the libp2p service.

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