A method of maintaining transport address associations in a transport address translation entity connected in series between a first device and a second device is disclosed. The first device can use a first transport protocol to send a first message to cause the transport address translation entity to make a first association of a first public address with a first private address. On receiving a response to the first message from the second device, the first device can use a second transport protocol to send a second message to cause the transport address translation entity to make a second association of a second private address with a second public address and with the second transport protocol. The second message can contain correspondence information that is also contained in the first message and/or in the response to the first message.