Transmission method and device for keywork compacting and grouping

A sender converts a non-compressed packet, which is to be transmitted, into a full-header packet including a full header or a header-compressed packet including a compressed header, and sends the converted packet to a receiver. The receiver receives the packet transmitted from the sender, and converts the received packet into a decompressed packet. In cases the full-header packet or header-compressed packet is lost between the sender and receiver, the receiver keeps header-compressed packers received during an interval from the packet loss to the next earliest reception of a full-header packet, and decompresses the compressed headers of the kept header-compressed packets on the basis of contents of the full header of the full-header packet.