Patent ID: 7436833

Claim:
A communication system, comprising: a server connected to a first network; and a router connected to the first network, wherein the server includes an option setting unit that grants to a response packet that is a response to a packet which is destined for an anycast address and which is sent from a communication apparatus connected to a second network different from the first network, anycast address option information which indicates that an anycast address is allocated to a sender of the response packet; a field putting unit that puts a signature field for a setting of an electronic signature in the response packet; and a transmitting unit that transmits to the communication apparatus the response packet in which the anycast address option information has been set and the signature field has been put; and the router includes a receiving unit that receives the response packet sent from the server to the communication apparatus; a sender validating unit that validates whether the response packet is sent from a legitimate server based on the anycast address option information if the response packet includes the anycast address option information; a validation information setting unit that sets an electronic signature based on information related with the response packet in the signature field put in the response packet, when the response packet is validated to be sent from the legitimate server; and a transferring unit that transmits the response packet in which the electronic signature has been set.