Packet transmission method and communication system

A communication system comprising a mobile terminal, a plurality of access routers, each access router forming a cell and configured to perform radio communication with a mobile terminal located in the cell, and a plurality of relay routers connected to a hierarchical structure above the access routers,

• wherein each of the relay routers comprises:
• multicasting means for, during relaying performed when a packet from a packet source is transmitted from an upper layer in the communication system toward a target mobile terminal, multicasting a router advertisement with said packet to all access routers forming cells into which the target mobile terminal may move;
• and
• delay adding means for receiving a multicast message containing delay information of a delay that the relay router should add to each path in multicasting, from the mobile terminal and adding the delay to each path in the multicasting on the basis of the delay information;
• and
• wherein the mobile terminal comprises:
• receiving means for receiving the multicast router advertisements from a plurality of neighbor access routers;
• calculating-specifying means for calculating a difference between reception times of packets from respective access routers, based on information about reception from the plurality of access routers, and specifying a relay router that should perform multicasting;
• and
• message transmitting means for transmitting a multicast message containing delay information of a delay that the relay router should add to each path in the multicasting, to the specified relay router.