A mobility management system comprising a user equipment, a first base station, a second base station, a first gateway, a second gateway, a communication peer, and a mobility management entity configured to manage the second base station and the second gateway, wherein the first base station is connected to the first gateway, and the second base station is connected to the second gateway, • wherein the user equipment is configured to establish a first substream with the communication peer, wherein the first substream is a substream established by the user equipment with the communication peer by using a first Internet Protocol, IP, address, and the first IP address is an IP address allocated by the first gateway to the user equipment; • when the user equipment moves from the first base station to the second base station, the first base station is further configured to: ∘ obtain an identity of the second base station; ∘ determine whether the identity of the second base station belongs to a base station identity list stored in the first base station; ∘ when determining that the identity of the second base station does not belong to the base station identity list stored in the first base station, the first base station is configured to send a determining indication message to the mobility management entity, wherein the base station identity list comprises identities of base stations connected to the first gateway, and the determining indication message is used to indicate that the second gateway and the first gateway are not the same gateway; • after receiving the determining indication message, the mobility management entity is configured to request the second gateway to allocate a second IP address to the user equipment; • the second gateway is configured to allocate the second IP address to the user equipment and is configured to send the second IP address to the mobility management entity; • the mobility management entity is configured to send the second IP address received from the second gateway to the user equipment; • the user equipment is further configured to establish a second substream with the communication peer after the user equipment has received the second IP address, wherein the second substream is a substream established by the user equipment with the communication peer by using the second IP address; and • the user equipment is configured to remove the first substream after establishing the second substream. • wherein the user equipment is configured to establish a first substream with the communication peer, wherein the first substream is a substream established by the user equipment with the communication peer by using a first Internet Protocol, IP, address, and the first IP address is an IP address allocated by the first gateway to the user equipment; • when the user equipment moves from the first base station to the second base station, the first base station is further configured to: ∘ obtain an identity of the second base station; ∘ determine whether the identity of the second base station belongs to a base station identity list stored in the first base station; ∘ when determining that the identity of the second base station does not belong to the base station identity list stored in the first base station, the first base station is configured to send a determining indication message to the mobility management entity, wherein the base station identity list comprises identities of base stations connected to the first gateway, and the determining indication message is used to indicate that the second gateway and the first gateway are not the same gateway; ∘ obtain an identity of the second base station; ∘ determine whether the identity of the second base station belongs to a base station identity list stored in the first base station; ∘ when determining that the identity of the second base station does not belong to the base station identity list stored in the first base station, the first base station is configured to send a determining indication message to the mobility management entity, wherein the base station identity list comprises identities of base stations connected to the first gateway, and the determining indication message is used to indicate that the second gateway and the first gateway are not the same gateway; • after receiving the determining indication message, the mobility management entity is configured to request the second gateway to allocate a second IP address to the user equipment; • the second gateway is configured to allocate the second IP address to the user equipment and is configured to send the second IP address to the mobility management entity; • the mobility management entity is configured to send the second IP address received from the second gateway to the user equipment; • the user equipment is further configured to establish a second substream with the communication peer after the user equipment has received the second IP address, wherein the second substream is a substream established by the user equipment with the communication peer by using the second IP address; and • the user equipment is configured to remove the first substream after establishing the second substream.