That depends on the scope and size of the attack. If the attack is small, which the vast majority of attacks are, (either in bandwidth, packets per second, or

That depends on the scope and size of the attack. If the attack is small, which the vast majority of attacks are, (either in bandwidth, packets per second, or both) then the responsibility is on the dedicated server owner to fix it via a software fix. (iptables with perl scripts to ban offending ips, etc..). If you have a managed host, they will obviously help with that. If the attack is larger than that, but still not epic (say, 2gbps attack) then the responsibility is on the host/datacenter, who will most likely null route your server. What that means, is they will tell their upstream providers not to send any more traffic to your IP address and NOBODY will be able to access your site. This is done until the attack ends. If they dont null route your server, they will attempt to filter the traffic coming in themselves through a in house solution. If the attack is of epic scale, it becomes less of a issue for the datacenter you are hosted in and more of a issue for the upstream providers to filter it on their end. A average datacenter can only do so much when 50gbps is coming in when they normally only see 10.