various ad hoc mechanisms to limit the set of receivers sending congestion signals to the source. One option is using some variant of randomized feedback, where a receiver only notifies its state with a given probability after a random waiting time (Wang et al., 1998). An alternative approach appoints a single receiver to send regular feedback on behalf of the entire group. This special receiver, known as the leader, is always the most congested receiver at any given time, so it changes during the session lifetime (Rizzo, 2000; Widmer et al., 2001). Then, the path between sender and leader can be thought of as a unicast session used to establish the transmission rate. However, it is not always easy to pick the correct leader, and frequent leader changes could generate substantial overhead traffic. Yet another possibility is to conduct a random poll on the group members, bounding the number of responses in each round (Rodríguez Pérez et al., 2005).
Scalability issues also arise related to the architec- ture of a congestion control mechanism. If congestion control is performed end-to-end, and TCP-compatibility is a requirement, it has been demonstrated that the throughput of the multicast session decreases with the logarithm of the number of receivers under very optimis- tic assumptions (homogeneous settings) (Chaintreau, Bacelli, & Diot, 2002). This result indicates that there are fundamental limits on the group size beyond which multicast with end-to-end congestion will not scale. One escape from this limitation is to use an overlay network for congestion control where transmissions between adjacent overlay nodes are decoupled and could progress at a rate determined only by the condi- tions in the path between them. A particularly appealing form of overlay is that of P2P overlays for multimedia multicast communications, either with end-to-end or with hop-by-hop congestion control.
FuturE trEndS
Despite numerous attempts and several breakthroughs, the issue of multicast congestion control has not yet been resolved in a satisfactory way. On the theoretical side, deriving conditions for stability, and character- izing fairness, in networks with unicast and multicast traffic is an essential problem. Further, modeling the interaction between multicast traffic and AQM (active queue management) algorithms may shed new light
on the dynamics of the congestion control algorithms proposed so far. Also, recently, a completely new ap- proach for multicast communications has appeared. Network coding uses an algebraic framework to exploit the full multicast capacity of any network. Packets are interpreted as symbols from a discrete alphabet and the routers perform a certain (linear) encoding of the packets they receive before forwarding them. Whether network coding has implications on the multicast congestion control problem, and whether it can be practically ap- plied, are still open problems.
For the networking practitioner, it is worth mention- ing that some single-rate TCP-compatible approaches (PGMCC (Rizzo, Iannaccone, Vicisano, & Handley, 2004) and TFMCC (Widmer & Handley, 2006)) are currently in the process of being standardized. In het- erogeneous scenarios, where single-rate protocols are less adequate, hybrid protocols are expected to provide better efficiency and smoother rate adaptation, so it is likely that they enter the track of standardization soon. As a final statement, it is still needed a large scale testbed to demonstrate that multicast, and multicast congestion control, is a robust technology. Perhaps the incentives might be driven by new bandwidth intensive multimedia applications, such as video or TV on demand.
concLuSIon
In addition to reliable transmission, congestion con- trol is a key issue to foster the adoption of multicast communications. The congestion control mechanisms must be able to scale up to large groups and, in view of the prevalence of TCP traffic, must achieve a fair share of bandwidth with TCP sessions. In homoge- neous environments, single-rate protocols guarantee TCP compatibility, but exhibit usually low bandwidth efficiency. In heterogeneous environments, many proposals are receiver-based and partition the data into multiple layers, to which receivers dynamically join. Despite the differences in the management of layers, multiple-rate protocols overload the routers due to the frequent join and leave operations and show slow convergence. Hybrid protocols leverage the existence of several multicast groups with the ability of the receivers to adapt their transmission rate within each layer. They appear the most promising approach for future multicast communications.
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Congestion Control in Multicast Networks
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