ICMP Time Exceeded messages are usually sent to inform the sender that the IPv4 datagram packets are discarded for two reasons listed below.
- When the IPv4 datagram’s Time to Live (TTL) field is decremented to zero by network routers, before the packet reaches the destination network: When an IPv4 datagram is sent from a sender, a value in the Time to Live (TTL) field of the IPv4 datagram header is set. Each router on IPv4 datagram’s path to the destination network will decrement TTL value by 1. If the TTL value in IPv4 datagram had reached zero before the IPv4 datagram header had reached the destination network, the packet will be discarded. This can happen if a routing loop is present in the network, or TTL value field of IPv4 datagram header is set very low at the sending computer. An ICMP Time Exceeded message is sent to the sender by the router where the IPv4 datagram’s value is decremented to zero. Click following link to learn more about IPv4 datagram header and it’s Time to Live (TTL) field.
- The IPv4 datagram was fragmented and some of the fragmented datagram haven’t reached the destination computer before its reassembly timer timed out. If any fragmented IPv4 datagrams are not received at the reassembling computer before its timer times out, all the fragmented IPv4 datagrams are discarded and the Time Exceeded message is sent to the source node by the destination node. Click following link to learn more about IPv4 datagram fragmentaion.
ICMP Time Exceeded message Type number is 11. The possible ICMP Code values related with ICMP Time Exceeded messages are explained below. Please click next link to know more about Type and Code fields in ICMP header
|0||Time to live (TTL) value of IPv4 datagram header exceeded|
|1||IPv4 datagram fragment reassembly timer timed out|