Code 0 may be received from a gateway and Code 1 from a host. If fragment zero is not available then no ICMP - time exceeded message is needed to be sent at all. If a host reassembling a fragmented datagram (or packet) cannot complete the reassembly due to missing fragments within its time limit it discards the datagram and it may send an ICMP - time exceeded message.
Let's have a look at the structure of an ICMP - Time exceeded message: Analysis of the ICMP Time Exceeded Message On the other end, the destination host will receive the fragmented pieces and put them back together to create the original large data packet which was fragmented at the source. When the data is too large to fit into one packet, it is cut into smaller pieces and sent to the destination. The term 'fragment' means to 'cut to pieces'. The same gateway may also notify the source host via the time exceeded message. The ICMP - Time exceeded message is generated when the gateway processing the datagram (or packet, depending on how you look at it) finds the Time To Live field (this field is in the IP header of all packets) is equal to zero and therefore must be discarded. Please make sure you check them out before attempting to continue! When looking at an IP header, you will see the TTL and Fragment Flag fields which play a big part in how this ICMP message works. Our readers can also visit the IP Protocol section which covers the IP protocol structure in great depth.
In order to fully understand this ICMP message, you must be familiar with the IP header within a packet. The ICMP - Time exceeded message is one which is usually created by gateways or routers.
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