What router will do when it sees IP datagram with TTL=0?

 When a router encounters an IP datagram with a Time-to-Live (TTL) value of 0, it takes specific actions as defined by the Internet Protocol (IP) specifications. The TTL field in the IP header is used to limit the time or "hops" a packet can take in the network. Here's what happens when a router sees an IP datagram with TTL=0:

 

1. **Drop the Datagram:**

   - The router immediately drops the IP datagram with TTL=0.

   - This is a standard behavior, and the router does not forward the packet.

 

2. **Generate an ICMP Time Exceeded Message:**

   - The router generates an ICMP (Internet Control Message Protocol) "Time Exceeded" message.

   - The ICMP Time Exceeded message informs the source host that the time allowed for the packet to live (TTL) has expired while the packet was in transit through the network.

   - There are two types of ICMP Time Exceeded messages: "Time to Live Exceeded in Transit" and "Fragment Reassembly Time Exceeded."

 

3. **Send ICMP Time Exceeded Message to the Source:**

   - The router sends the ICMP Time Exceeded message back to the source IP address mentioned in the IP header of the original datagram.

   - The ICMP Time Exceeded message includes a portion of the original IP header and data to assist in identifying the source of the packet.

 

4. **Additional Information in ICMP Message:**

   - The ICMP Time Exceeded message may include information such as the router's IP address, allowing the source host to identify where the TTL was exceeded.

 

The purpose of dropping the packet and sending an ICMP Time Exceeded message is to prevent the IP datagram from circulating indefinitely in the network due to routing loops or other issues. The TTL field serves as a mechanism to limit the time a packet can spend in the network and helps prevent packets from endlessly circulating.

 

The ICMP Time Exceeded message assists network administrators in diagnosing potential issues, as it provides information about where the packet's TTL expired. This information can be valuable in troubleshooting network problems and identifying the source of routing anomalies.