sping/README.md

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:

+-------------------------------+
| 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.

make build

Run

Run the build and launch the program, you must have the superuser rights to run it (raw socket access).

make run

By default, packets are sent to google.com, if you want to change the destination update HOST variable:

make run HOST=anotherhost.com