IP fragmentation attack

Process

• To set the IP packet size equal or smaller than the directly attached medium and delegate all further fragmentation of packets to routers, meaning that routers decide if the current packet should be re-fragmented or not. This offloads a lot of work on to routers, and can also result in packets being segmented by several IP routers one after another, resulting in very peculiar fragmentation.
• To preview all links between source and destination and select the smallest MTU in this route, assuming there is a unique route. This way we make sure that the fragmentation is done by the sender, using a packet-size smaller than the selected MTU, and there is no further fragmentation en route. This solution, called Path MTU Discovery, allows a sender to fragment/segment a long Internet packet, rather than relying on routers to perform IP-level fragmentation. This is more efficient and more scalable. It is therefore the recommended method in the current Internet. The problem with this approach is that each packet is routed independently; they may well typically follow the same route, but they may not, and so a probe packet to determine fragmentation may follow a path different from paths taken by later packets.

• In some datalink protocols such as Ethernet, only the first fragment contains the full upper layer header, meaning that other fragments look like beheaded packets.
• Additional overhead imposed over network because all fragments contains their own IP header. Additional overhead = * ;

Exploits

Fragmentation for evasion