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Host Migration is a feature in Call of Duty: Modern Warfare 2, Call of Duty: Black Ops, Call of Duty: Modern Warfare 3, Call of Duty: Black Ops II, Call of Duty: Ghosts, Call of Duty: Advanced Warfare designed to attempt to continue a game in the event the host player leaves.
In Black Ops II and Black Ops III, host migration can also occur in Zombies, unlike in previous installments with Zombies, as the game mode runs now on a multiplayer engine.
The matchmaking system present in the console versions (and those PC versions that do not use dedicated servers) of Call of Duty games works by gathering players into a lobby (a room players are grouped in before a match). One player is chosen as the host, who hosts the server that all other players connect to.
Before Modern Warfare 2, if the host were to have an unsavory connection or left the game entirely, the game would come to an end with the "The game has ended due to poor connection quality with the host." message and the "Host Ended Game" message. This became a problem for players who were enjoying their match. Infinity Ward addressed this problem with Modern Warfare 2's Host Migration. Host Migration occurs when the host's connection is unsavory, or if the host leaves the game.
In the event that either of these two scenarios occur, the game will come to a halt and the words "HOST MIGRATION" will appear in the upper left, along with a message saying that a certain percent of packets are sent. Then, a new host gets picked, and if the migration is successful, a 5-second countdown timer will begin to resume play. If Host Migration occurs due to an unsavory connection to the host then that player (the previous host) will be kicked once host migration is completed. The Host Migration in Black Ops is quite similar, although no percentages of packets sent are shown.
In almost all cases, a Host Migration due to the host leaving the game will be executed a lot faster than a Host Migration due to connection quality.
The Host Migration system is merely an attempt to establish a new host in order for the game to continue on. Due to the complex nature of attempting to connect to a new host, the Host Migration may or may not be able to work. Success rate is typically higher for situations where the host leaves the game (where the connection goes smoothly), but in other cases (usually when Host Migration is triggered because of lag problems), the server may simply time out. Also, the Host Migration can only work when the host selects "Leave Game" - exactly what would bring up Host Ended Game in the previous games. If the host quits any other way, i.e. going into Xbox Home, Signing Out of Xbox Live, getting Disconnected from Xbox Live, and Unplugging the console, the game server will time out every time.
Normally, a successful Host Migration takes up to approximately 15 seconds. The Host Migration typically fails after trying to connect for a longer time. If the migration occurs in a pre-game lobby, usually after 7 seconds the server times out.
In addition, any killstreaks that are called in at the time the Host Migration occurs will be affected. This is most apparent for shorter ones like the Predator Missile, where if it is being used when Host Migration occurs, it will simply hit the ground and not do any damage as it cannot be controlled and all players do not take damage during the process (including the countdown). Other killstreaks will continue to be used up though for longer ones there may be some time left for them to do damage.
In Call of Duty: Black Ops and Call of Duty: Modern Warfare 3's Theater mode, any games that were interrupted by a Host Migration will register twice on recent games - once for the section of the game before the host migrated, and one after, leading to some inconvenience for players wanting to view such games in Theater mode.
- In the Infected game mode of Call of Duty: Modern Warfare 3, a Host Migration that occurs after the 'First Infected' is selected will cause the game to reboot and choose another player as the enemy faction.
- Host Migration is absent in the Wii versions of Call of Duty: Black Ops and Call of Duty: Modern Warfare 3.