ExpandED Schools

The specifications and performance of RAMP a reliable adaptive multicast protocol, are presented. Initially described in IETF RFC 1458 (1993), RAMP has been enhanced for use over an all-optical, circuit-switched, gigabit network under our ARPA-sponsored testbed for optical networking (TBONE) project. RAMP uses immediate, receiver-initiated, NAK-based, unicast error notification combined with originator based unicast retransmission. The approach is motivated by the loss characteristics of the TBONE network, where extremely low bit-error rates (10/sup -12/ or better) and the absence of any store-and-forward capabilities in the switches make packet losses almost entirely a result of receiver buffer overflows. As receiver losses are largely independent, use of unicast over multicast for NAKs and retransmission eliminates unnecessary receiver processing overhead associated with reading and discarding redundant packets. Use of immediate rather than delayed NAKs further improves performance by reducing both latency and the likelihood of buffer overflow. The effectiveness of this combined error control approach has been verified by other researchers, as well as through our own investigations. Interestingly, TBONE loss characteristics resemble those of switched virtual circuit ATM networks and packet-switched networks employing reservation services. As these networks provide quality of service guarantees in the switches, the likely source of packet loss is again due to receiver errors and buffer overflows. Hence, RAMP's design is also relevant for the next generation of packet switched networks.