# Chapter II: establishing TCP connections

In this chapter we explain how to measure establishing TCP connections.

We will first write a simple `main.go` file that shows how to use
this functionality. Then, we will show some runs of this file, and
we will comment the output that we see.

(This file is auto-generated. Do not edit it directly! To apply
changes you need to modify `./internal/tutorial/measurex/chapter02/main.go`.)

## main.go

We declare the package and import useful packages. The most
important package we're importing here is, of course, `internal/measurex`.

```Go
package main

import (
	"context"
	"encoding/json"
	"flag"
	"fmt"
	"time"

	"github.com/ooni/probe-cli/v3/internal/measurex"
	"github.com/ooni/probe-cli/v3/internal/runtimex"
)

func main() {
```
### Setup

This first part of `main.go` is really similar to the previous
chapter, so there is not much new to say here.

```Go
	address := flag.String("address", "8.8.4.4:443", "remote endpoint address")
	timeout := flag.Duration("timeout", 60*time.Second, "timeout to use")
	flag.Parse()
	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
	defer cancel()
```

### Creating a Measurer

We create a `Measurer` like we did in the previous chapter.

```Go
	mx := measurex.NewMeasurerWithDefaultSettings()
```

### Establishing a TCP connection

We then call `TCPConnect`, which establishes a connection
and returns the corresponding measurement.

The arguments are the context (for timeouts), and the address
of the endpoint to which we want to connect. (Here and in
most of this tutorial with "endpoint" we mean an IP address
and a port, serialized as "ADDRESS:PORT", where the
address is quoted with "[" and "]" if IPv6, e.g., `[::1]:53`.)

```Go
	m := mx.TCPConnect(ctx, *address)
```

### Printing the measurement

The rest of the main function is just like in the previous chapter.

```Go
	data, err := json.Marshal(m)
	runtimex.PanicOnError(err, "json.Marshal failed")
	fmt.Printf("%s\n", string(data))
}

```

## Running the example program

Let us run the program with default arguments first. You can do
this operation by running:

```bash
go run -race ./internal/tutorial/measurex/chapter02 | jq
```

Here is the JSON we obtain in output:

```JavaScript
{
  // These two fields identify the endpoint
  "network": "tcp",
  "address": "8.8.4.4:443",

  // This block contains the results of the connect syscall
  // using the df-008-netevents data format.
  "connect": [
    {
      "address": "8.8.4.4:443",
      "failure": null,
      "operation": "connect",
      "proto": "tcp",
      "t": 0.026879041,
      "started": 8.8625e-05,
      "oddity": ""
    }
  ]
}
```

This is what it says:

- we are connecting a "tcp" socket;

- the destination endpoint address is "8.8.4.4:443";

- connect terminated ~0.027 seconds into the program's life;

- the operation succeeded (`failure` is `nil`).

Let us now see if we can provoke some errors and timeouts.

### Measurement with connection refused

Let us start with an IP address where there's no listening socket:

```bash
go run -race ./internal/tutorial/measurex/chapter02 -address 127.0.0.1:1 | jq
```

We get this JSON:

```JSON
{
  "network": "tcp",
  "address": "127.0.0.1:1",
  "connect": [
    {
      "address": "127.0.0.1:1",
      "failure": "connection_refused",
      "operation": "connect",
      "proto": "tcp",
      "t": 0.000372167,
      "started": 8.4917e-05,
      "oddity": "tcp.connect.refused"
    }
  ]
}

```

And here's an error telling us the connection was refused and
the oddity that classifies the error.

### Measurement with timeouts

Let us now try to obtain a timeout:

```bash
go run -race ./internal/tutorial/measurex/chapter02 -address 8.8.4.4:1 | jq
```

We get this JSON:

```JSON
{
  "network": "tcp",
  "address": "8.8.4.4:1",
  "connect": [
    {
      "address": "8.8.4.4:1",
      "failure": "generic_timeout_error",
      "operation": "connect",
      "proto": "tcp",
      "t": 10.005494583,
      "started": 8.4833e-05,
      "oddity": "tcp.connect.timeout"
    }
  ]
}
```

So, we clearly see from the value of `t` that our 60 seconds
default timeout did not hit, because there is a lower watchdog
timeout (10 s). We also see again how the oddity is more
precise than just the error alone.

Let us now use a very small timeout:

```bash
go run -race ./internal/tutorial/measurex/chapter02 -address 8.8.4.4:1 -timeout 100ms | jq
```

To get this JSON:

```JSON
{
  "network": "tcp",
  "address": "8.8.4.4:1",
  "connect": [
    {
      "address": "8.8.4.4:1",
      "failure": "generic_timeout_error",
      "operation": "connect",
      "proto": "tcp",
      "t": 0.10148025,
      "started": 0.000122375,
      "oddity": "tcp.connect.timeout"
    }
  ]
}
```

We see a timeout after ~0.1s. We enforce a reasonably small
timeout for connecting, equal to 10 s, because we want to
guarantee that measurements eventually terminate. Also, since
often censorship is implemented by timing out, we don't want
to spend to much time waiting for a timeout to expire.

## Conclusions

We have seen how to measure the operation of connecting
to a specific TCP endpoint.