Send some sort of ICMP message upstream that indicates your maximum capacity for handling traffic. It's a DOS vector in itself, but you could minimize it.
Umm... No. Any such form of congestion notification, if respected by upstream parties, would certainly reduce traffic to you. The obvious problem, however, is that it will reduce NASTY/BOT traffic as well as LEGITIMATE traffic. So, you send this ICMP message, and the upstreams that hear it kindly shape what's exiting their network toward you? How do they choose from the available packets they have heading toward you what to let through and what to delay/drop? If some giant number N of senders wants to swamp you, it matters little that their ISPs or your ISPs or any transports between them know that they must reduce the traffic toward you. You still have a DDoS, but now it's a self-throttled DDoS, and the upstreams are still dropping or delaying legitimate traffic that you want, only now it happens before the natural limits and instead occurs upon artificial limits. The end result is less traffic hits you, and you still go out of service to most of the world (from the end-user experience perspective), because the senders who are politely throttling can't tell which packets are evil and which packets are sent by the people you want to receive from.
Enzymes are things invented by biologists that explain things which otherwise require harder thinking. -- Jerome Lettvin