Listening and Dialing
Common transport interfaces
Transports are defined in terms of two core operations, listening and dialing.
Listening means that you can accept incoming connections from other peers,
using whatever facility is provided by the
transport implementation. For example, a TCP transport on a unix platform could
use the bind and listen system calls to have the operating system route
traffic on a given TCP port to the application.
Dialing is the process of opening an outgoing connection to a listening peer. Like listening, the specifics are determined by the implementation, but every transport in a libp2p implementation will share the same programmatic interface.
Addresses
Before you can dial up a peer and open a connection, you need to know how to
reach them. Because each transport will likely require its own address scheme,
libp2p uses a convention called a “multiaddress” or multiaddr to encode
many different addressing schemes.
The addressing doc goes into more detail, but an overview of how multiaddresses work is helpful for understanding the dial and listen interfaces.
Here’s an example of a multiaddr for a TCP/IP transport:
/ip4/198.51.100.0/tcp/6543
This is equivalent to the more familiar 198.51.100.0:6543 construction, but it
has the advantage of being explicit about the protocols that are being
described. With the multiaddr, you can see at a glance that the 198.51.100.0
address belongs to the IPv4 protocol, and the 6543 belongs to TCP.
For more complex examples, see addressing.
Both dial and listen deal with multiaddresses. When listening, you give the transport the address you’d like to listen on, and when dialing you provide the address to dial to.
When dialing a remote peer, the multiaddress should include the PeerId of the peer you’re trying to reach. This lets libp2p establish a secure communication channel and prevents impersonation.
An example multiaddress that includes a PeerId:
/ip4/192.0.2.0/tcp/4321/p2p/QmcEPrat8ShnCph8WjkREzt5CPXF2RwhYxYBALDcLC1iV6
The /p2p/QmcEPrat8ShnCph8WjkREzt5CPXF2RwhYxYBALDcLC1iV6 component uniquely
identifies the remote peer using the hash of its public key.
For more, see the peer identity content.
Supporting multiple transports
libp2p applications often need to support multiple transports at once. For example, you might want your services to be usable from long-running daemon processes via TCP, while also accepting websocket connections from peers running in a web browser.
The libp2p component responsible for managing the transports is called the switch, which also coordinates protocol negotiation, stream multiplexing, establishing secure communication and other forms of “connection upgrading”.
The switch provides a single “entry point” for dialing and listening, and frees up your application code from having to worry about the specific transports and other pieces of the “connection stack” that are used under the hood.