A communication system performs a communication after establishing a channel from an external host device in an external network to an internal host device in an internal network, where a firewall blocks communications from the external host device in the external network to the internal host device in the internal network while selectively allowing communications from the internal network to the external network. The external host device includes a transmitting unit that transmits channel maintenance data to the internal host device when the channel needs to be maintained, with the channel maintenance data being used for maintaining the channel.