ICMP errors

Understanding ICMP error generation rules in IPv4 routing

This easy networking question tests knowledge of when a router is allowed to generate ICMP error messages under standard IPv4 rules. It appears frequently in systems and networking interviews because it probes whether candidates understand the practical constraints that prevent ICMP from creating redirect loops or overwhelming networks with error traffic.

The IPv4 specification places strict limits on ICMP error generation. Routers must recognize which incoming packets should trigger error responses and which should be silently dropped or handled differently. Key restrictions include rules about which message types can trigger errors, handling of fragmented packets, and suppression of errors in error messages themselves.

• TTL expiration and Time Exceeded messages
• ICMP message types that can and cannot trigger ICMP errors
• Fragment handling and error generation
• ICMP error suppression rules