nim-libp2p/examples/tutorial_2_customproto.md

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In the [previous tutorial](tutorial_1_connect.md), we've looked at how to create a simple ping program using the `nim-libp2p`.
We'll now look at how to create a custom protocol inside the libp2p
# Custom protocol in libp2p
Let's create a `part2.nim`, and import our dependencies:
```nim
import chronos
import stew/byteutils
import libp2p
```
This is similar to the first tutorial, except we don't need to import the `Ping` protocol.
Next, we'll declare our custom protocol
```nim
const TestCodec = "/test/proto/1.0.0"
type TestProto = ref object of LPProtocol
```
We've set a [protocol ID](https://docs.libp2p.io/concepts/protocols/#protocol-ids), and created a custom `LPProtocol`. In a more complex protocol, we could use this structure to store interesting variables.
A protocol generally has two part: and handling/server part, and a dialing/client part.
Theses two parts can be identical, but in our trivial protocol, the server will wait for a message from the client, and the client will send a message, so we have to handle the two cases separately.
Let's start with the server part:
```nim
proc new(T: typedesc[TestProto]): T =
# every incoming connections will in be handled in this closure
proc handle(conn: Connection, proto: string) {.async, gcsafe.} =
echo "Got from remote - ", string.fromBytes(await conn.readLp(1024))
# We must close the connections ourselves when we're done with it
await conn.close()
return T(codecs: @[TestCodec], handler: handle)
```
This is a constructor for our `TestProto`, that will specify our `codecs` and a `handler`, which will be called for each incoming peer asking for this protocol.
In our handle, we simply read a message from the connection and `echo` it.
We can now create our client part:
```nim
proc hello(p: TestProto, conn: Connection) {.async.} =
await conn.writeLp("Hello p2p!")
```
Again, pretty straight-forward, we just send a message on the connection.
We can now create our main procedure:
```nim
proc main() {.async, gcsafe.} =
let
rng = newRng()
testProto = TestProto.new()
switch1 = newStandardSwitch(rng=rng)
switch2 = newStandardSwitch(rng=rng)
switch1.mount(testProto)
await switch1.start()
await switch2.start()
let conn = await switch2.dial(switch1.peerInfo.peerId, switch1.peerInfo.addrs, TestCodec)
await testProto.hello(conn)
# We must close the connection ourselves when we're done with it
await conn.close()
await allFutures(switch1.stop(), switch2.stop()) # close connections and shutdown all transports
```
This is very similar to the first tutorial's `main`, the only noteworthy difference is that we use `newStandardSwitch`, which is similar to `createSwitch` but is bundled directly in libp2p
We can now wrap our program by calling our main proc:
```nim
waitFor(main())
```
And that's it!