Computer-implemented method for data transmission between a first network having a high security requirement and a second network having a low security requirement, wherein a) a data transmission from the first network to the second network involves a first session being set up between the first network and the second network and a first control data packet being transmitted from a transmitting unit in the first network via a first one-way communication unit, arranged between the first network and the second network, to a receiving unit in the second network, and b) a data transmission from the second network to the first network involves a second session being set up and a second control data packet being transmitted from a transmitting unit in the second network via a second one-way communication unit to a validation unit arranged between the first network and the second network, - the second control data packet is validated in the validation unit on the basis of a prescribed rule, and - positive validation of the second control data packet results in a third session being set up and the second control data packet being transmitted from the validation unit to a receiving unit in the first network, wherein - the first control data packet is checked in the second network for acceptance, - wherein, if the first control data packet is accepted, the second control data packet is transmitted from the second network to the validation unit, validated there and, in the event of positive validation, transmitted to the first network, at least one payload data packet is transmitted from the first network to the second network, and a third control data packet is transmitted from the first network to the second network, wherein the first and third control data packets are used to check a completeness of the at least one transmitted payload data packet in the second network, and a fourth control data packet comprising the result of the check is transmitted to the first network. a) a data transmission from the first network to the second network involves a first session being set up between the first network and the second network and a first control data packet being transmitted from a transmitting unit in the first network via a first one-way communication unit, arranged between the first network and the second network, to a receiving unit in the second network, and b) a data transmission from the second network to the first network involves a second session being set up and a second control data packet being transmitted from a transmitting unit in the second network via a second one-way communication unit to a validation unit arranged between the first network and the second network, - the second control data packet is validated in the validation unit on the basis of a prescribed rule, and - positive validation of the second control data packet results in a third session being set up and the second control data packet being transmitted from the validation unit to a receiving unit in the first network, wherein - the first control data packet is checked in the second network for acceptance, - wherein, if the first control data packet is accepted, the second control data packet is transmitted from the second network to the validation unit, validated there and, in the event of positive validation, transmitted to the first network, at least one payload data packet is transmitted from the first network to the second network, and a third control data packet is transmitted from the first network to the second network, - the second control data packet is validated in the validation unit on the basis of a prescribed rule, and - positive validation of the second control data packet results in a third session being set up and the second control data packet being transmitted from the validation unit to a receiving unit in the first network, wherein - the first control data packet is checked in the second network for acceptance, - wherein, if the first control data packet is accepted, the second control data packet is transmitted from the second network to the validation unit, validated there and, in the event of positive validation, transmitted to the first network, at least one payload data packet is transmitted from the first network to the second network, and a third control data packet is transmitted from the first network to the second network,