How am I being DDoSed on "dedicated" servers?

Shouldn’t the existence of the dedicated server mean that these peer-to-peer connections and disruptions are to be protected?

Also, how does one report it properly, and not through the worthless reporting system on Xbox?

And I know I got DDoSed. Same person, 3 successive games. 1 opposing player drops within seconds of starting the game - twice being me. My entire internet is knocked offline momentarily, both times.