Peers

Peers are what make up a libp2p network. We’ll go over their key properties in this guide.

Peer ID #

A Peer Identity (often written PeerID) is a unique reference to a specific peer within the overall peer-to-peer network.

As well as serving as a unique identifier for each peer, a Peer ID is a verifiable link between a peer and its public cryptographic key.

Each libp2p peer controls a private key, which it keeps secret from all other peers. Every private key has a corresponding public key, which is shared with other peers.

Together, the public and private key (or “key pair”) allow peers to establish secure communication channels with each other.

Conceptually, a Peer ID is a cryptographic hash of a peer’s public key. When peers establish a secure channel, the hash can be used to verify that the public key used to secure the channel is the same one used to identify the peer.

The Peer ID spec goes into detail about the byte formats used for libp2p public keys and how to hash the key to produce a valid Peer ID.

How are Peer Ids represented as strings? #

Peer Ids are multihashes, which are defined as a compact binary format.

It’s very common to see multihashes encoded into base 58, using the same alphabet used by bitcoin.

Here’s an example of a Peer ID represented as a base58-encoded multihash: QmYyQSo1c1Ym7orWxLYvCrM2EmxFTANf8wXmmE7DWjhx5N

While it’s possible to represent multihashes in many textual formats (for example as hexadecimal, base64, etc), Peer Ids always use the base58 encoding, with no multibase prefix when encoded into strings.

Peer IDs in multiaddrs #

A Peer ID can be encoded into a multiaddr as a /p2p address with the Peer ID as a parameter.

If my Peer ID is QmYyQSo1c1Ym7orWxLYvCrM2EmxFTANf8wXmmE7DWjhx5N, a libp2p multiaddress for me would be:

/p2p/QmYyQSo1c1Ym7orWxLYvCrM2EmxFTANf8wXmmE7DWjhx5N

As with other multiaddrs, a /p2p address can be encapsulated into another multiaddr to compose into a new multiaddr. For example, I can combine the above with a transport address /ip4/198.51.100.0/tcp/4242 to produce this very useful address:

/ip4/198.51.100.0/tcp/4242/p2p/QmYyQSo1c1Ym7orWxLYvCrM2EmxFTANf8wXmmE7DWjhx5N

This provides enough information to dial a specific peer over a TCP/IP transport. If some other peer has taken over that IP address or port, it will be immediately obvious, since they will not have control over the key pair used to produce the Peer ID embedded in the address.

For more on addresses in libp2p, see Addressing

Peer Info #

Another common libp2p data structure related to peer identity is the PeerInfo structure.

Peer Info combines a Peer ID with a set of multiaddrs that the peer is listening on.

Peer Store #

A libp2p node will typically have a temporary store to store peer keys, addresses and associated metadata. The peer store works like a phone or address book; think of it like a universal multiaddr book that maintains the source of truth for all known peers.

Peer Discovery #

A discovery method is likely needed if no information about a peer is available in the peer store. A peer multiaddr is typically discovered with their Peer ID. Once the network successfully discovers a peer multiaddr (and able to establish a connection), the peer discovery protocol adds the Peer Info and multiaddr to the Peer Store. Learn more about how to discover un-{known, identified} peers on the peer routing guide.