Performance of Shared-Memory Switches Under Multicast Bursty Traffic

In this paper, we study shared-memory switches under multicast bursty traffic and characterize the relation between their performance and the multicast distribution that defines the mix of multicast traffic arriving at the switches. We consider two schemes that have been used in practical realizations of these switches to replicate multicast cells: 1) replication-at-receiving (RAR), where multiple copies of a multicast cell are stored in the buffer and served independently, and 2) replication-at-sending (RAS), where a single instance of a multicast cell is stored in the buffer, and the cell is replicated as it is transmitted to the output ports. For each scheme, we study two configurations: 1) the shared-memory-only (SMO) configuration, where the bandwidth of the replication mechanism is sufficient to accommodate even the worst-case replication requirements, and 2) the shared-memory-with-replication-first-infirst-out (SM+RFIFO) configuration, where the bandwidth of the replication mechanism is lower than what required by the worst case, and thus an additional buffer is used in front of the shared memory to temporarily store cells while they are replicated. For all cases, using simulation, we find upper bounds for the buffer requirements to achieve a desired cell-loss rate. We show that these upper bounds are significantly larger than the buffer requirements under unicast traffic and are approached even for very small volumes of multicast traffic; thus, these upper bounds are what should be used in practice to size the buffers to achieve desired performance under traffic with general multicast distributions. We also study shared-memory switches with output demultiplexers and characterize and compare the different multicasting schemes that are used in these switches.