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Pure go implementation of the IP routing table powered by prefix tree

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# go-iprtb [![.github/workflows/check.yml](https://github.com/moznion/go-iprtb/actions/workflows/check.yml/badge.svg)](https://github.com/moznion/go-iprtb/actions/workflows/check.yml) [![codecov](https://codecov.io/gh/moznion/go-iprtb/branch/main/graph/badge.svg?token=S3UWM0Y3LF)](https://codecov.io/gh/moznion/go-iprtb) [![GoDoc](https://godoc.org/github.com/moznion/go-iprtb?status.svg)](https://godoc.org/github.com/moznion/go-iprtb)

Pure go implementation of the IP routing table. This implementation uses a prefix tree as a backend data structure to find/match the routes.

NOTE: This library is isolated from the OS-level routing table. This intends to provide the routing table function on the user-level code.

## Synopsis

```go
import (
	"context"
	"fmt"
	"net"

"github.com/moznion/go-iprtb"
)

func main() {
	ctx := context.Background()
	rtb := iprtb.NewRouteTable()

err := rtb.AddRoute(ctx, &Route{
		Destination: &net.IPNet{
			IP:   net.IPv4(192, 0, 2, 0),
			Mask: net.IPv4Mask(255, 255, 255, 0),
		},
		Gateway:          net.IPv4(192, 0, 2, 1),
		NetworkInterface: "ifb0",
		Metric:           1,
	})
	if err != nil {
		panic(err)
	}

err = rtb.AddRoute(ctx, &Route{
		Destination: &net.IPNet{
			IP:   net.IPv4(192, 0, 2, 255),
			Mask: net.IPv4Mask(255, 255, 255, 255),
		},
		Gateway:          net.IPv4(192, 0, 2, 255),
		NetworkInterface: "ifb0",
		Metric:           1,
	})
	if err != nil {
		panic(err)
	}

maybeRoute, err := rtb.MatchRoute(ctx, net.IPv4(192, 0, 2, 100))
	if err != nil {
		panic(err)
	}
	fmt.Println(maybeRoute.IsSome()) // => true
	fmt.Println(maybeRoute.Unwrap().String()) // => 192.0.2.0/24	192.0.2.1	ifb0	1

maybeRoute, err = rtb.MatchRoute(ctx, net.IPv4(192, 0, 2, 254))
	if err != nil {
		panic(err)
	}
	fmt.Println(maybeRoute.IsSome()) // => true
	fmt.Println(maybeRoute.Unwrap().String()) // => 192.0.2.0/24	192.0.2.1	ifb0	1

// longest match
	maybeRoute, err = rtb.MatchRoute(ctx, net.IPv4(192, 0, 2, 255))
	if err != nil {
		panic(err)
	}
	fmt.Println(maybeRoute.IsSome()) // => true
	fmt.Println(maybeRoute.Unwrap().String()) // => 192.0.2.255/32	192.0.2.255	ifb0	1

// not routes
	maybeRoute, err = rtb.MatchRoute(ctx, net.IPv4(198, 51, 100, 123))
	if err != nil {
		panic(err)
	}
	fmt.Println(maybeRoute.IsSome()) // => false
}
```

And please see also [examples_test.go](./examples_test.go).

## Docs

[![GoDoc](https://godoc.org/github.com/moznion/go-iprtb?status.svg)](https://godoc.org/github.com/moznion/go-iprtb)

### Longest matching by prefix tree

In the scenario that the routing table has the following three routes;

| Route          | Route Binary                            | Gateway |
|----------------|-----------------------------------------|---------|
| 10.0.0.0/8     | **00001010** 00000000 00000000 00000000 | GW1     |
| 192.0.0.0/8    | **11000000** 00000000 00000000 00000000 | GW2     |
| 192.128.0.0/9  | **11000000 1**0000000 00000000 00000000 | GW3     |

This route table can transform into the following prefix tree:

```
                 R
                / \
              /     \
            0         1
           /           \
          0             1
         /             /
        0             0
       /             /
      0             0
       \           /
        1         0
       /         /
      0         0
       \       /
        1     0
       /     /
 GW1 [0]   [0] GW2
             \
             [1] GW3

† R: Root Node
†† [n]: Terminal Node
```

Then the target IP address only has to traverse this tree as much as longer to look up a route. It derives the result like the following.

| Target IP     | Target IP Binary                      | Found Gateway |
|---------------|---------------------------------------|---------------|
| 10.10.10.10   | 0000101[0] 00001010 00001010 00001010 | GW1           |
| 192.10.10.10  | 1100000[0] 00001010 00001010 00001010 | GW2           |
| 192.192.10.10 | 11000000 [1]1000000 00001010 00001010 | GW3           |
| 127.0.0.1     | 01111111 00000000 00000000 00000001   | N/A           |

This tree implementation contributes to good performance for route matching. This describes the result of the benchmark against the linear search based simple implementation (that comes from [e9a01d251b4388d77f27d3e7ff51d217cecaf7d9](https://github.com/moznion/go-iprtb/tree/e9a01d251b4388d77f27d3e7ff51d217cecaf7d9)):

```
$ go test -bench=. -benchmem
goos: linux
goarch: amd64
pkg: github.com/moznion/go-iprtb
cpu: 12th Gen Intel(R) Core(TM) i7-1280P
Benchmark_PrefixTreeAlgorithm-20        18535410                81.68 ns/op           64 B/op          1 allocs/op
Benchmark_LinearSearch-20                6148155               192.1 ns/op            96 B/op          1 allocs/op
PASS
ok      github.com/moznion/go-iprtb     2.968s
```

The benchmark code is here: https://gist.github.com/moznion/eb867c8dd2a708f0acd184ee8d5c758b

### Label support

This library provides "label" support on `AddRouteWithLabel()`, `UpdateRouteByLabel()`, and `RemoveRouteByLabel()`.

`AddRouteWithLabel()` function registers a route with a label and that label can be used to update and remove the route
by `UpdateRouteByLabel()` and `RemoveRouteByLabel()` instead of passing the actual destination information.

If there is no associated label, those updating functions with the label do nothing.

## Author

moznion (<moznion@mail.moznion.net>)

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