Mitigate or Stop the Effects of DDoS Attacks

Load Balancing. 

For network providers, there are a number of techniques used to mitigate the effects of a DDoS attack.  Providers can increase bandwidth on critical connections to prevent them from going down in the event of an attack.  Replicating servers can help provide additional failsafe protection in the event some go down during a DDoS attack.  Balancing the load to each server in a multiple-server architecture can improve both normal performance as well as mitigate the effect of a DDoS attack.

Throttling.  

One proposed method to prevent servers from going down is to use Max-min Fair server-centric router throttles. This method sets up routers that access a server with logic to adjust (throttle) incoming traffic to levels that will be safe for the server to process.  This will prevent flood damage to servers.  Additionally, this method can be extended to throttle DDoS attacking traffic versus legitimate user traffic for better results. This method is still in the experimental stage, however similar techniques to throttling are being implemented by network operators.  The difficulty with implementing throttling is that it is still hard to decipher legitimate traffic from malicious traffic.  In the process of throttling, legitimate traffic may sometimes be dropped or delayed and malicious traffic may be allowed to pass to the servers.

Drop Requests.  

Another method is to simply drop requests when the load increases.  This can be done by the router or the server.  Alternatively, the requester may be induced to drop the request by making the requester system solve a hard puzzle that takes a lot of compute power or memory space, before continuing with the request.  This causes the users of zombie systems to detect performance degradation, and could possibly stop their participation in sending DDoS attack traffic.