Real-time multicast with scalable reliability

This paper proposes and investigates a protocol for real-time multicast applications called MSR (Multicast with Scalable Reliability). Two essential features in MSR are its traffic shaping and scalable retransmission schemes. To minimize packet loss and delay in the network, MSR spaces out the transmission of bursty data at the source. Reliability is further enhanced with a scheme in which the receivers make use of NACK messages to request for retransmission of packets from the sender. To avoid the well-known NACK implosion problem, a mechanism that makes use of the concept of representative receivers is introduced to ensure only a single NACK is issued among a group of receivers. Unlike previous multicast protocols which attempt to achieve 100% reliability by requiring a correct copy of a packet to reach all receivers before the retransmission process stops, a key feature in our scheme is that the level of reliability can be scaled in accordance with the maximum tolerable end-to-end delay, defined as the difference between the instant at which the real-time data must be presented to the user and the instant at which it is created at the sender. MSR attempts to make optimal use of measured parameters such as delay, round-trip delay, loss rate, etc, to scale the retransmission process and provide single NACK mechanism. We adopt the framework of the standard RTP and RTCP for the implementation of MSR. This paper presents a proof to show that the time-out mechanism in MSR, which is required to effect retransmission requests, would work even if the clocks of the sender and receivers are not synchronized, a situation not uncommon in the Internet. Many multimedia streaming protocols currently used on the Web either uses a 100 % reliable protocol like TCP or unreliable protocol like UDP. The former sacrifices the "real-timeness" and the latter sacrifices the quality of the presentation. Our rough performance analysis indicates that there is perhaps a better intermediate solution using a scalable protocol like MSR.