NetFence, discussed in class, is a proposed solution to the problem of DoS attacks. It departs from previous work in that it places the 'middle' in the first line of defense against these types of attacks, rather than the 'ends'.
We have already learned that the 'middle' has access to important information that can be very difficult for the 'ends' to infer. For example, the total number of flows at a bottleneck link and the capacity of that link. This type of information is easily accessible at the bottleneck router, but can be very inaccessible to the affected senders. Such information can make it possible for senders to avoid congestion.
Security is yet another concern which demonstrates the need for smart 'middles'. This is all interesting in light of the end-to-end principle. This principle might be interpreted as stating that the 'middle' should not be replicating work which is better done at the 'ends'. However, there is significant work that the 'ends' are ill-equipped to do.
If, as discussed in NetFence, a sender and receiver collude to overwhelm a link, the 'ends' are both malicious and the target of the attack is the 'middle' itself. Certainly in such a case, initiative is needed in the 'middle'.