A method of transmitting data in a media stream to a receiving device, the media stream being in accordance with a media streaming protocol which defines a data structure comprising a type identifier field for specifying a type of data contained within the data structure, wherein the type identifier field can be set to a first type identifier or a second type identifier, wherein the second type identifier is different to the first type identifier and is unspecified in the protocol, characterised in that the method comprises: determining whether or not the receiving device is capable of processing data in a data structure having the type identifier field set to the second type identifier; wherein if it is determined that the receiving device is capable of processing data in a data structure having the type identifier field set to the second type identifier, the method further comprises: generating quality data, the quality data being capable of being used to improve the media quality at the receiving device or to optimise the playback of the media stream at the receiving device and associated with the first type identifier in accordance with the protocol; encapsulating the generated quality data in a data structure defined by the protocol; setting the type identifier field of the data structure to the second type identifier; forming a packet comprising the data structure; and transmitting the packet in the media stream; and if it is determined that the receiving device is not capable of processing data in a data structure having the type identifier field set to the second type identifier, disabling the transmission of quality data. determining whether or not the receiving device is capable of processing data in a data structure having the type identifier field set to the second type identifier; wherein if it is determined that the receiving device is capable of processing data in a data structure having the type identifier field set to the second type identifier, the method further comprises: generating quality data, the quality data being capable of being used to improve the media quality at the receiving device or to optimise the playback of the media stream at the receiving device and associated with the first type identifier in accordance with the protocol; encapsulating the generated quality data in a data structure defined by the protocol; setting the type identifier field of the data structure to the second type identifier; forming a packet comprising the data structure; and transmitting the packet in the media stream; and generating quality data, the quality data being capable of being used to improve the media quality at the receiving device or to optimise the playback of the media stream at the receiving device and associated with the first type identifier in accordance with the protocol; encapsulating the generated quality data in a data structure defined by the protocol; setting the type identifier field of the data structure to the second type identifier; forming a packet comprising the data structure; and transmitting the packet in the media stream; and if it is determined that the receiving device is not capable of processing data in a data structure having the type identifier field set to the second type identifier, disabling the transmission of quality data.