A Flow Control Algorithm for ACK-based Reliable Multicast

We introduce STBL, a flow control algorithm that uses both window and rate to regulate traffic with smoothed transmission and bounded loss, as well as to compete fairly with TCP traffic. In this paper, we evaluate how STBL behaves as part of a ACK-based reliable multicast transport protocol (TRAM), based on a simulation model. A library of standard reference scenarios is used to drive the simulation. The results are quite promising. 1.0 Introduction The primary benefit of reliable multicast is that it offers an efficient way of disseminating information to many locations (almost) simultaneously. Both error control and flow control in reliable multicast (RM) are difficult problems. The difficulty with error control is that the need to process ACKs (or NACKs) from multiple receivers can create an implosion at the sender as the number of receivers increases. There are many proposals for reliable multicast transport protocols, driven by different application requirements in the areas of: reliability, scalability and performance. These approaches have been reviewed and discussed in [SURVEY]. The problems with flow control are many-fold. Scalability is one of the problems: as the number of receivers increases, the range of suitable transmission rates diminishes and it becomes increasingly difficult to aggregate feedback signals from the receivers. Another problem is with fairness: it is strongly argued that multicast flows must be TCP-friendly which, depending on how religiously you conform, can make flow control more challenging. During periods of congestion, a multicast flow’s repair (retransmission) traffic may come from different nodes, making it harder for the sender take it into account. The following are some of the key factors affecting the design of the flow and congestion control algorithm: • If and how the sender gets feedback information from the receivers • Whether the control is window or rate based • Whether there are external knobs that are used to indicate application requirements and performance guarantees agreed to out-of-band • What the fairness goals are The answers to some of these questions depend on the algorithm used to support reliability; for example, whether it is ACK-based or NACK-based. The issue of fairness hinges on larger issues of network resource allocation in the Internet, as is discussed below. In an ACK-based protocol, there is regular feedback from the receivers to the sender for error control purposes. This feedback channel can be used to piggyback flow and congestion information as well, so it is natural to incorporate a flow control algorithm that responds to such feedback information continuously. In a 1 We assume this needs no definition. For example, TCP uses a window-based flow control scheme. Some of the transports proposed for ATM networks use rate-based flow control.