on an unreliable protocol. An extremely common example is the layering of Transmission Control Protocol on the Internet Protocol, a combination known as TCP/IP. Strong reliability properties are offered by group communication systems (GCSs) such as IS-IS, Appia framework, Spread, JGroups or QuickSilver Scalable Multicast. The QuickSilver Properties Framework is a flexible platform that allows strong reliability properties to be expressed in a purely declarative manner, using a simple rule-based language, and automatically translated into a hierarchical protocol. One protocol that implements reliable messaging is WS-ReliableMessaging, which handles reliable delivery of SOAP messages. The ATM Service-Specific Coordination Function provides for transparent assured delivery with AAL5. IEEE 802.11 attempts to provide reliable service for all traffic. The sending station will resend a frame if the sending station doesn't receive an ACK frame within a predetermined period of time. Real-time systems There is, however, a problem with the definition of reliability as "delivery or notification of failure" in real-time computing. In such systems, failure to deliver the real-time data will adversely affect the performance of the systems, and some systems, e.g. safety-critical, safety-involved, and some secure mission-critical systems, must be proved to perform at some specified minimum level. This, in turn, requires that a specified minimum reliability for the delivery of the critical data be met. Therefore, in these cases, it is only the delivery that matters; notification of the failure to deliver does ameliorate the failure. In hard real-time systems, all data must be delivered by the deadline or it is considered a system failure. In firm real-time systems, late data is still valueless but the system can tolerate some amount of late or missing data. There are a number of protocols that are capable of addressing real-time requirements for reliable delivery and timeliness: MIL-STD-1553B and STANAG 3910 are well-known examples of such timely and reliable protocols for avionic data buses. MIL-1553 uses a 1 Mbit/s shared media for the transmission of data and the control of these transmissions, and is widely used in federated military avionics systems. It uses a bus controller (BC) to command the connected remote terminals (RTs) to receive or transmit this data. The BC can, therefore, ensure that there will be no congestion, and transfers are always timely. The MIL-1553 protocol also allows for automatic retries that can still ensure timely delivery and increase the reliability above that of the physical layer. STANAG 3910, also known