A method for use in a communication system comprising a first system entity, a second system entity and one or more third system entities, the method comprising: transmitting data in messages between the first system entity and the one or more third system entities through the second system entity by using the TCP/IP or UDP/IP transport protocol, wherein each message comprises a port number parameter indicating a TCP or UDP port number, respectively, characterized in that the method further comprises: using a modified port number parameter further indicating an identity of the third system entity, to/from which a message is transmitted, in the transmission of such a message between the first system entity and the second system entity, wherein the use of a modified port number parameter comprises: including, in the first system entity, the identity of the third system entity to which a message is transmitted in the port number parameter of the message; transmitting the message to the second system entity; and removing, in the second system entity, the identity of the third system entity from the port number parameter of the message before transmitting the message to the third system entity. transmitting data in messages between the first system entity and the one or more third system entities through the second system entity by using the TCP/IP or UDP/IP transport protocol, wherein each message comprises a port number parameter indicating a TCP or UDP port number, respectively, characterized in that the method further comprises: using a modified port number parameter further indicating an identity of the third system entity, to/from which a message is transmitted, in the transmission of such a message between the first system entity and the second system entity, wherein the use of a modified port number parameter comprises: including, in the first system entity, the identity of the third system entity to which a message is transmitted in the port number parameter of the message; transmitting the message to the second system entity; and removing, in the second system entity, the identity of the third system entity from the port number parameter of the message before transmitting the message to the third system entity. using a modified port number parameter further indicating an identity of the third system entity, to/from which a message is transmitted, in the transmission of such a message between the first system entity and the second system entity, wherein the use of a modified port number parameter comprises: including, in the first system entity, the identity of the third system entity to which a message is transmitted in the port number parameter of the message; transmitting the message to the second system entity; and removing, in the second system entity, the identity of the third system entity from the port number parameter of the message before transmitting the message to the third system entity. including, in the first system entity, the identity of the third system entity to which a message is transmitted in the port number parameter of the message; transmitting the message to the second system entity; and removing, in the second system entity, the identity of the third system entity from the port number parameter of the message before transmitting the message to the third system entity.