Peer to peer
Also said as P2P, Mesh
Players' machines talk straight to each other, with no server in the middle. Some designs have one player's machine act as host instead, so the others connect to it.
It is cheap to run and gives the shortest route between two players, which is why two-player fighters like it. It also means nobody neutral decides, upload grows with each player, and everyone can see each other's address.
Say it
“This is a two-player game, so go peer to peer with rollback. Use a relay only when two players cannot reach each other directly, and never expose an IP address to the opponent in that case.”
Where you’ve seen it
Call of Duty: Modern Warfare 2 (2009) dropped dedicated servers on PC for its IWNet service, and players minded. A petition asking for them back passed 150,000 signatures within ten days.
Not to be confused with listen server
A listen server is one player's machine acting as the server for the rest, which gives that player an advantage. True peer to peer has no such middle.