# sping

A simple **ping** implementation to briefly details how it works under the hood.

## Overview

The **Internet Control Message Protocol (ICMP)** is a network layer protocol in the IP suite used for diagnostics, error reporting, and network management. It supports tools like `ping` by sending messages to test connectivity and measure round-trip time.

How ICMP Works ?
* **Purpose**: Facilitates error reporting (e.g., "destination unreachable") and diagnostic queries (e.g., Echo Request/Reply for ping).
* **Operation**: ICMP packets are encapsulated in IP packets (IPv4/IPv6) and sent between hosts or routers. No TCP/UDP is used.
* **Key Messages**:
    * Echo Request (Type 8): Sent by ping to test if a host is reachable.
    * Echo Reply (Type 0): Response from the target confirming reachability.
    * Other types: Destination Unreachable (Type 3), Time Exceeded (Type 11).

Below, the ICMP packet structure:
```ascii
+-------------------------------+
| Ethernet Frame (optional)     |
|-------------------------------|
| Destination MAC (6 bytes)     |
| Source MAC (6 bytes)          |
| EtherType (2 bytes) = 0x0800  |
+-------------------------------+
| IPv4 Header (20 bytes)        |
|-------------------------------|
| Version (4b) = 4              |
| Header Len (4b) = 5           |
| Type of Service (8b) = 0      |
| Total Length (16b) = 84       |
| Identification (16b)          |
| Flags/Offset (16b) = 0        |
| TTL (8b) = e.g., 64           |
| Protocol (8b) = 1 (ICMP)      |
| Checksum (16b)                |
| Source IP (32b)               |
| Destination IP (32b)          |
+-------------------------------+
| ICMP Packet (64 bytes def)    |
|-------------------------------|
| Type (8b) = 8 / 0             |
| Code (8b) = 0                 |
| Checksum (16b)                |
| Identifier (16b)              |
| Sequence Number (16b)         |
| Payload (56 bytes, e.g., 0s)  |
+-------------------------------+
| Ethernet FCS (4 bytes)        |
+-------------------------------+
```


## Build
Of course a C compiler must be installed.
```bash
make build
```

## Run
Run the build and launch the program, you must have the superuser rights to run it (raw socket access).
```bash
make run
```
By default, packets are sent to [google.com](google.com), if you want to change the destination update **HOST** variable:
```bash
make run HOST=anotherhost.com
```