A method of transmitting data between first and second network entities in a communications network, the method including, transmitting from the first network entity an establishment message providing a first indication which indicates whether the first network entity is capable of handling extended messages, receiving the message at the second network entity and returning to the first network entity an establishment response which provides the first indication in respect of the second network entity, and transmitting a data message from the first network entity to the second network entity, the data message selectively being an extended message in dependence on the first indication received in the response message, the data message further providing a second indication which indicates whether or not the data message is an extended message.