# Echo client/server with libp2p

This is an example that quickly shows how to use the `go-libp2p` stack,
including Host/Basichost, Network/Swarm, Streams, Peerstores and
Multiaddresses.

This example can be started in either listen mode, or dial mode.

In listen mode, it will sit and wait for incoming connections on the
`/echo/1.0.0` protocol. Whenever it receives a stream, it will write the
message `"Hello, world!"` over the stream and close it.

In dial mode, the node will start up, connect to the given address, open a
stream to the target peer, and read a message on the protocol `/echo/1.0.0`.

## Build

From `go-libp2p` base folder:

```
> make deps
> go build ./examples/echo
```

## Usage

```
> ./echo -l 1235
2016/11/10 10:45:37 I am /ip4/127.0.0.1/tcp/1235/ipfs/QmNtX1cvrm2K6mQmMEaMxAuB4rTexhd87vpYVot4sEZzxc
2016/11/10 10:45:37 listening for connections
```

The listener libp2p host will print its `Multiaddress`, which indicates how it
can be reached (ip4+tcp) and its randomly generated ID (`QmNtX1cv...`)

Now, launch another node that talks to the listener:

```
> ./echo -d /ip4/127.0.0.1/tcp/1235/ipfs/QmNtX1cvrm2K6mQmMEaMxAuB4rTexhd87vpYVot4sEZzxc -l 1236
```

The new node with send the message `"Hello, world!"` to the
listener, which will in turn echo it over the stream and close it. The
listener logs the message, and the sender logs the response.

## Details

The `makeBasicHost()` function creates a
[go-libp2p-basichost](https://godoc.org/github.com/libp2p/go-libp2p/p2p/host/basic)
object. `basichost` objects wrap
[go-libp2 swarms](https://godoc.org/github.com/libp2p/go-libp2p-swarm#Swarm)
and should be used preferentially. A
[go-libp2p-swarm Network](https://godoc.org/github.com/libp2p/go-libp2p-swarm#Network)
is a `swarm` which complies to the
[go-libp2p-net Network interface](https://godoc.org/github.com/libp2p/go-libp2p-net#Network)
and takes care of maintaining streams, connections, multiplexing different
protocols on them, handling incoming connections etc.

In order to create the swarm (and a `basichost`), the example needs:

  * An
    [ipfs-procotol ID](https://godoc.org/github.com/libp2p/go-libp2p-peer#ID)
    like `QmNtX1cvrm2K6mQmMEaMxAuB4rTexhd87vpYVot4sEZzxc`. The example
    autogenerates this on every run. An optional key-pair to secure
    communications can be added to it. The example autogenerates them when
    using `-secio`.
  * A [Multiaddress](https://godoc.org/github.com/multiformats/go-multiaddr),
    which indicates how to reach this peer. There can be several of them
    (using different protocols or locations for example). Example:
    `/ip4/127.0.0.1/tcp/1234`.
  * A
    [go-libp2p-peerstore](https://godoc.org/github.com/libp2p/go-libp2p-peerstore),
    which is used as a address book which matches node IDs to the
    multiaddresses through which they can be contacted. This peerstore gets
    autopopulated when manually opening a connection (with
    [`Connect()`](https://godoc.org/github.com/libp2p/go-libp2p/p2p/host/basic#BasicHost.Connect). Alternatively,
    we can manually
    [`AddAddr()`](https://godoc.org/github.com/libp2p/go-libp2p-peerstore#AddrManager.AddAddr)
    as in the example.

A `basichost` can now open streams (bi-directional channel between to peers)
using
[NewStream](https://godoc.org/github.com/libp2p/go-libp2p/p2p/host/basic#BasicHost.NewStream)
and use them to send and receive data tagged with a `Protocol.ID` (a
string). The host can also listen for incoming connections for a given
`Protocol` with
[`SetStreamHandle()`](https://godoc.org/github.com/libp2p/go-libp2p/p2p/host/basic#BasicHost.SetStreamHandler).

The example makes use of all of this to enable communication between a
listener and a sender using protocol `/echo/1.0.0` (which could be any other thing).