A communication apparatus includes an acquisition unit that acquires a size of free space provided in each of a plurality of data transmission units of a first layer; and an allocation unit that allocates to each of a plurality of pieces of data of a second layer a contiguous sequence of free space from among the free space whose size is acquired by the acquisition unit, the contiguous sequence of free space being equal in size to each piece of data and provided in one of the data transmission units among the plurality of data transmission units. The apparatus also includes a multiplexing unit that multiplexes the plurality of pieces of data of the second layer into the plurality of data transmission units of the first layer according to the allocation of the free space by the allocation unit.