libp2p doesn't make assumptions for you, instead, it enables you as the developer of the application to pick the modules you need to run your application, which can vary depending on the runtime you are executing. A libp2p node can use one or more Transports to dial and listen for Connections. These transports are modules that offer a clean interface for dialing and listening, defined by the interface-transport specification. Some examples of possible transports are: TCP, UTP, WebRTC, QUIC, HTTP, Pigeon and so on.

A more complete definition of what is a transport can be found on the interface-transport specification. A way to recognize a candidate transport is through the badge:

1. Creating a libp2p Bundle with TCP

When using libp2p, you always want to create your own libp2p Bundle, that is, pick your set of modules and create your network stack with the properties you need. In this example, we will create a bundle with TCP. You can find the complete solution on the file 1.js.

You will need 5 deps total, so go ahead and install all of them with:

> npm install libp2p libp2p-tcp peer-info async @nodeutils/defaults-deep

Then, on your favorite text editor create a file with the .js extension. I've called mine 1.js.

First thing is to create our own bundle! Insert:

'use strict'

const libp2p = require('libp2p')
const TCP = require('libp2p-tcp')
const PeerInfo = require('peer-info')
const waterfall = require('async/waterfall')
const defaultsDeep = require('@nodeutils/defaults-deep')

// This MyBundle class is your libp2p bundle packed with TCP
class MyBundle extends libp2p {
  constructor (_options) {
    const defaults = {
      // modules is a JS object that will describe the components
      // we want for our libp2p bundle
      modules: {
        transport: [
          TCP
        ]
      }
    }

    super(defaultsDeep(_options, defaults))
  }
}

Now that we have our own MyBundle class that extends libp2p, let's create a node with it. We will use async/waterfall just for code structure, but you don't need to. Append to the same file:

let node

waterfall([
  // First we create a PeerInfo object, which will pack the
  // info about our peer. Creating a PeerInfo is an async
  // operation because we use the WebCrypto API
  // (yeei Universal JS)
  (cb) => PeerInfo.create(cb),
  (peerInfo, cb) => {
    // To signall the addresses we want to be available, we use
    // the multiaddr format, a self describable address
    peerInfo.multiaddrs.add('/ip4/0.0.0.0/tcp/0')
    // Now we can create a node with that PeerInfo object
    node = new MyBundle({ peerInfo: peerInfo })
    // Last, we start the node!
    node.start(cb)
  }
], (err) => {
  if (err) { throw err }

  // At this point the node has started
  console.log('node has started (true/false):', node.isStarted())
  // And we can print the now listening addresses.
  // If you are familiar with TCP, you might have noticed
  // that we specified the node to listen in 0.0.0.0 and port
  // 0, which means "listen in any network interface and pick
  // a port for me
  console.log('listening on:')
  node.peerInfo.multiaddrs.forEach((ma) => console.log(ma.toString()))
})

Running this should result in something like:

> node 1.js
node has started (true/false): true
listening on:
/ip4/127.0.0.1/tcp/61329/ipfs/QmW2cKTakTYqbQkUzBTEGXgWYFj1YEPeUndE1YWs6CBzDQ
/ip4/192.168.2.156/tcp/61329/ipfs/QmW2cKTakTYqbQkUzBTEGXgWYFj1YEPeUndE1YWs6CBzDQ

That QmW2cKTakTYqbQkUzBTEGXgWYFj1YEPeUndE1YWs6CBzDQ is the PeerId that was created during the PeerInfo generation.

2. Dialing from one node to another node

Now that we have our bundle, let's create two nodes and make them dial to each other! You can find the complete solution at 2.js.

For this step, we will need one more dependency.

> npm install pull-stream

And we also need to import the module on our .js file:

const pull = require('pull-stream')

We are going to reuse the MyBundle class from step 1, but this time to make things simpler, we will create two functions, one to create nodes and another to print the addrs to avoid duplicating code.

function createNode (callback) {
  let node

  waterfall([
    (cb) => PeerInfo.create(cb),
    (peerInfo, cb) => {
      peerInfo.multiaddrs.add('/ip4/0.0.0.0/tcp/0')
      node = new MyBundle({ peerInfo: peerInfo })
      node.start(cb)
    }
  ], (err) => callback(err, node))
}

function printAddrs (node, number) {
  console.log('node %s is listening on:', number)
  node.peerInfo.multiaddrs.forEach((ma) => console.log(ma.toString()))
}

Now we are going to use async/parallel to create two nodes, print their addresses and dial from one node to the other. We already added async as a dependency, but still need to import async/parallel:

const parallel = require('async/parallel')

Then,

parallel([
  (cb) => createNode(cb),
  (cb) => createNode(cb)
], (err, nodes) => {
  if (err) { throw err }

  const node1 = nodes[0]
  const node2 = nodes[1]

  printAddrs(node1, '1')
  printAddrs(node2, '2')

  node2.handle('/print', (protocol, conn) => {
    pull(
      conn,
      pull.map((v) => v.toString()),
      pull.log()
    )
  })

  node1.dialProtocol(node2.peerInfo, '/print', (err, conn) => {
    if (err) { throw err }

    pull(pull.values(['Hello', ' ', 'p2p', ' ', 'world', '!']), conn)
  })
})

The result should be look like:

> node 2.js
node 1 is listening on:
/ip4/127.0.0.1/tcp/62279/ipfs/QmeM4wNWv1uci7UJjUXZYfvcy9uqAbw7G9icuxdqy88Mj9
/ip4/192.168.2.156/tcp/62279/ipfs/QmeM4wNWv1uci7UJjUXZYfvcy9uqAbw7G9icuxdqy88Mj9
node 2 is listening on:
/ip4/127.0.0.1/tcp/62278/ipfs/QmWp58xJgzbouNJcyiNNTpZuqQCJU8jf6ixc7TZT9xEZhV
/ip4/192.168.2.156/tcp/62278/ipfs/QmWp58xJgzbouNJcyiNNTpZuqQCJU8jf6ixc7TZT9xEZhV
Hello p2p world!

3. Using multiple transports

Next, we want to be available in multiple transports to increase our chances of having common transports in the network. A simple scenario, a node running in the browser only has access to HTTP, WebSockets and WebRTC since the browser doesn't let you open any other kind of transport, for this node to dial to some other node, that other node needs to share a common transport.

What we are going to do in this step is to create 3 nodes, one with TCP, another with TCP+WebSockets and another one with just WebSockets. The full solution can be found on 3.js.

In this example, we will need to also install libp2p-websockets, go ahead and install:

> npm install libp2p-websockets

We want to create 3 nodes, one with TCP, one with TCP+WebSockets and one with just WebSockets. We need to update our MyBundle class to contemplate WebSockets as well:

const WebSockets = require('libp2p-websockets')
// ...

class MyBundle extends libp2p {
  constructor (_options) {
    const defaults = {
      modules: {
        transport: [
          TCP,
          WebSockets
        ]
      }
    }

    super(defaultsDeep(_options, defaults))
  }
}

Now that we have our bundle ready, let's upgrade our createNode function to enable us to pick the addrs in which a node will start a listener.

function createNode (addrs, callback) {
  if (!Array.isArray(addrs)) {
    addrs = [addrs]
  }

  let node

  waterfall([
    (cb) => PeerInfo.create(cb),
    (peerInfo, cb) => {
      addrs.forEach((addr) => peerInfo.multiaddrs.add(addr))
      node = new MyBundle({ peerInfo: peerInfo })
      node.start(cb)
    }
  ], (err) => callback(err, node))
}

As a rule, a libp2p node will only be capable of using a transport if: a) it has the module for it and b) it was given a multiaddr to listen on. The only exception to this rule is WebSockets in the browser, where a node can dial out, but unfortunately cannot open a socket.

Let's update our flow to create nodes and see how they behave when dialing to each other:

parallel([
  (cb) => createNode('/ip4/0.0.0.0/tcp/0', cb),
  // Here we add an extra multiaddr that has a /ws at the end, this means that we want
  // to create a TCP socket, but mount it as WebSockets instead.
  (cb) => createNode(['/ip4/0.0.0.0/tcp/0', '/ip4/127.0.0.1/tcp/10000/ws'], cb),
  (cb) => createNode('/ip4/127.0.0.1/tcp/20000/ws', cb)
], (err, nodes) => {
  if (err) { throw err }

  const node1 = nodes[0]
  const node2 = nodes[1]
  const node3 = nodes[2]

  printAddrs(node1, '1')
  printAddrs(node2, '2')
  printAddrs(node3, '3')

  node1.handle('/print', print)
  node2.handle('/print', print)
  node3.handle('/print', print)

  // node 1 (TCP) dials to node 2 (TCP+WebSockets)
  node1.dialProtocol(node2.peerInfo, '/print', (err, conn) => {
    if (err) { throw err }

    pull(pull.values(['node 1 dialed to node 2 successfully']), conn)
  })

  // node 2 (TCP+WebSockets) dials to node 2 (WebSockets)
  node2.dialProtocol(node3.peerInfo, '/print', (err, conn) => {
    if (err) { throw err }

    pull(pull.values(['node 2 dialed to node 3 successfully']), conn)
  })

  // node 3 (WebSockets) attempts to dial to node 1 (TCP)
  node3.dialProtocol(node1.peerInfo, '/print', (err, conn) => {
    if (err) {
      console.log('node 3 failed to dial to node 1 with:', err.message)
    }
  })
})

print is a function created using the code from 2.js, but factored into its own function to save lines, here it is:

function print (protocol, conn) {
  pull(
    conn,
    pull.map((v) => v.toString()),
    pull.log()
  )
}

If everything was set correctly, you now should see the following after you run the script:

> node 3.js
node 1 is listening on:
/ip4/127.0.0.1/tcp/62620/ipfs/QmWpWmcVJkF6EpmAaVDauku8g1uFGuxPsGP35XZp9GYEqs
/ip4/192.168.2.156/tcp/62620/ipfs/QmWpWmcVJkF6EpmAaVDauku8g1uFGuxPsGP35XZp9GYEqs
node 2 is listening on:
/ip4/127.0.0.1/tcp/10000/ws/ipfs/QmWAQtWdzWXibgfyc7WRHhhv6MdqVKzXvyfSTnN2aAvixX
/ip4/127.0.0.1/tcp/62619/ipfs/QmWAQtWdzWXibgfyc7WRHhhv6MdqVKzXvyfSTnN2aAvixX
/ip4/192.168.2.156/tcp/62619/ipfs/QmWAQtWdzWXibgfyc7WRHhhv6MdqVKzXvyfSTnN2aAvixX
node 3 is listening on:
/ip4/127.0.0.1/tcp/20000/ws/ipfs/QmVq1PWh3VSDYdFqYMtqp4YQyXcrH27N7968tGdM1VQPj1
node 3 failed to dial to node 1 with: No available transport to dial to
node 1 dialed to node 2 successfully
node 2 dialed to node 3 successfully

As expected, we created 3 nodes, node 1 with TCP, node 2 with TCP+WebSockets and node 3 with just WebSockets. node 1 -> node 2 and node 2 -> node 3 managed to dial correctly because they shared a common transport, however, node 3 -> node 1 failed because they didn't share any.

4. How to create a new libp2p transport

Today there are already 3 transports available, one in the works and plenty to come, you can find these at interface-transport implementations list.

Adding more transports is done through the same way as you added TCP and WebSockets. Some transports might offer extra functionalities, but as far as libp2p is concerned, if it follows the interface defined at the spec it will be able to use it.

If you decide to implement a transport yourself, please consider adding to the list so that others can use it as well.

Hope this tutorial was useful. We are always looking to improve it, so contributions are welcome!