A boundary node router determines whether a received packet includes a SIP message and a REGISTER message, and, if so, adds its router identifier to a REGISTER message which includes a user identifier and a user terminal identifier, sent from a user terminal, and sends the message to a SIP server. The SIP server has a user management table which stores the user terminal identifier, the router identifier of a router which serves the user terminal, and a predetermined Quality of Service type, in association with each user identifier. The SIP server extracts a user identifier, a user terminal identifier, and a router identifier from a received REGISTER message, and updates the user terminal identifier and the router identifier corresponding to the extracted user identifier, to the extracted user terminal identifier and the extracted router identifier.