Fragment reassembly in user space is the part most toy implementations skip. Handling the DF bit and ICMP Fragmentation Needed (Type 3/4) means it actually deals with path MTU discovery scenarios. Most stop at basic echo/reply.
RFC815 [0] for a nice efficient data structure and algorithm to reassemble fragments; shouldn't be much for an LLM to read and construct the data structure if inclined to do so.