Fragmentation is done at routers which make them complex to implement
IP fragmentation occurs when IP datagrams are broken apart into small packets, then transmitted across a network, and finally reassembled into the original datagram as part of normal communications. This process is necessary to meet size limits that each network can handle. Such a limit is described as a maximum transmission unit (MTU).
when routers take too much time to fragment and defragment packets this may lead to DOS attack to other packets .
The attacker can employ IP fragmentation to target communications systems, as well as security components. ICMP-based fragmentation attacks typically submit fake fragments that cannot be defragmented. This in turn causes the fragments to be placed in temporary storage, taking up memory and in some cases exhausting all available memory resources.
IP/ICMP fragmentation attacks, like many other Dos attack will overwhelm the destination resources due to the massive traffic volumes. However, this attack will also force the destination to use resources to attempt to reassemble the packets which will often result in network devices and servers crashing