I also agree with Phil here, and want to give a bit more technical context on this response.
A DDoS falls under the broader umbrella of what's known as a resource exhaustion attack: https://owasp.org/www-project-automated-threats-to-web-applications/assets/oats/EN/OAT-015_Denial_of...
While I've seen varying descriptions, in general, any resource can fall under such an Umbrella, including network bandwidth. If someone's saturating your uplink with garbage downlink traffic, there's very little mitigating it on your network perimeter would really accomplish anyway.
In a more traditional sense, we can't mitigate against a DDoS consuming CPU or memory either; processing that many inbound packets through our IPS and inbound firewall is always going to consume both.