Many people associate fragmented packets with an attack against a network. While that is quite often true, it is not always the case. There are Peer-to-Peer (P2P) protocols out there that will fragment packets, and quite a few other legitimate programs as well. When it comes to recognizing if you are actually experiencing a Denial, or Distributed Denial of Service (DoS) (DDoS), attack you should study some of the tools that will do this. Also, study why it is advantageous to fragment exploit code. Should you wish to learn more about this give this a read.