Question

3 A. What ICMP type is used in IPv4 to detect routing loops or slow routes? What two conditions can trigger this type of ICMP message?

Answer 1

The ICMP type that is used in IPv4 to detect routing loops or slow routes is Type 11, the "Time Exceeded" message.

Traceroute utility or software tool i.e., tracert tool or command on Windows systems, or traceroute tool or command on Linux OS machine is used to find path a network traffic traverses, knowing this is informative, important, useful, and helpful to troubleshoot network connectivity issues, such as no network connection, or slow network connection, etc.

The ICMP traceroute tool uses the Internet Control Message Protocol (ICMP) for transmitting packets to the destination with a Time to Live (TTL) value of 1 and the value increases for each hop. The traceroute tool or command uses ICMP Type 11 "Time Exceeded" messages (the type of message sent by a gateway)for tracing a network route. In IP, the Time to Live field is used for avoiding routing loops, where, a router through which a packet passes ever time, the router decrements the TTL field. Hence, as and when the TTL would reach zero, the router would drop the packet and would send an ICMP "Time Exceeded" message back to the source host, node, or machine, or the original sender. The traceroute command with ICMP is very informative in case, communications with another computer are extremely slow. The "Time Exceeded" message is the error condition. The ICMP Time Exceeded messages are used for tracing the routers along the path.

The two conditions that can trigger this type of ICMP are:

The "Time Exceeded" message is generated and sent when a router receives a datagram with a TTL of 0 or 1. IP uses the TTL field to avoid infinite routing loops. When any of the intermediate routers are not able to forward a datagram with a TTL value of 0 or 1, it trashes the datagram and sends back a "Time Exceeded" message. Hence, the two different "Time Exceeded" error codes or condition that can trigger or occur are:

1) 0 = TTL Equals 0 During Transit.
Code 0 figures out the reason for the expiration as TTL being exceeded, during datagram in transit.
2) 1 = TTL Equals 0 During Reassembly.

Hence, the ICMP "Time Exceeded" message is sent to the source when the TTL timer gets expired.