[Day 5] 2. ICMP (Internet Control Message Protocol)

ICMP (Internet Control Message Protocol)
ICMP is a network protocol used by IP networks to send error messages and control information, such as status updates. Common utilities like Ping and Traceroute rely on ICMP for network troubleshooting.

• Ping: Sends ICMP Echo Request messages to a target device to check connectivity. If the target is reachable, it responds with an Echo Reply.
• Traceroute: Traces the route packets take through a network to reach a target device by sending multiple ICMP Echo Requests with varying TTL (Time-To-Live) values.

Traceroute Command

• Windows: Use
  Copy

  tracert

  in the command prompt.
• Linux/Mac: Use
  Copy

  traceroute

  in the terminal.

How Traceroute Works

1. Traceroute sends packets with incrementally increasing TTL values, starting from 1.
2. Each router (or hop) that processes the packet decrements the TTL by 1.
   • If the TTL reaches 0, the router discards the packet and sends an ICMP Time Exceeded message back to the source.
3. By analyzing the source IP address of each returned ICMP message, Traceroute identifies each hop along the path to the destination.

Additional Details

• TTL: Starts at a high value (e.g., 255) and decreases with each hop. This mechanism prevents packets from circulating indefinitely in a network.
• The final destination responds with an ICMP Echo Reply, indicating the route is fully traced.