Service node, a control method, the user node, and a control method thereof

There is provided a service node that is capable of serving a first user node in a first network including a control node. The first network is capable of communicating with a second network including a second user node. The service node comprises: a request receiving unit that receives a first user identity that identifies the first user node and a second user identity that identifies the second user node; a determining unit that determines whether a received message includes the first user identity as a destination, or the message includes auxiliary information that corresponds to the first user identity; and a modifying unit that modifies the destination of the message to the second user identity and modifies the source of the message to the first user identity in accordance with the determination result by the determining unit.