Internet Control Message Protocol (RFC 792)

• 0Echo Reply
• 3Destination Unreachable
• 4Source Quench
• 5Redirect
• 11 Time Exceeded
• 15 Information Request

traceroute

• Trace the route taken by IP packets from source to destination
• Send packets with small ttl (time to live) and listen for ICMP "time exceeded" reply from a gateway
• Start with ttl of one and keep increasing until ICMP "port unreachable"