In today's datacenters handling big data and for exascale computers of tomorrow, there is a pressing need for high-radix switches to economically and efficiently unify the computing and storage resources that are dispersed across multiple racks. In this paper, we present SCOC, a switch architecture suitable for economical IC implementation that can efficiently replace crossbars for high-radix switch nodes. SCOC is a multi-stage bufferless network with O(N2/m) cost, where m is a design parameter, practically ranging between 4-16. We identify and resolve more than five fairness violations that are pertinent to hierarchical scheduling. Effectively, from a performance perspective, SCOC is indistinguishable from efficient flat crossbars. Computer simulations show that it competes well or even outperforms flat crossbars and hierarchical switches. We report data from our ASIC implementation at 32 nm of a SCOC 136×136 switch, with shallow buffers, connecting 25 Gb/s links. In this first incarnation, SCOC is used at the spines of a server-rack, fat-tree network. Internally, it runs at 9.9 Tb/s, thus offering a speedup of 1.45 ×, and provides a fall-through latency of just 61 ns.

Flexible datacenters rely on high-bandwidth server-rack fabrics to allocate their distributed computing and storage resources anywhere,anyhow, and anytime demanded. We describe the multicast architecture of a distributed server-rack fabric, which is arranged around a spine-leaf topology and connects 640 Ethernet ports running at 100G. To cope with the immense fabric speed, we resort to hierarchical, tree-based replication, facilitated by specially commissioned fabric-end ports. At each (port-to-port) leg of the tree, a frame copy is forwarded after a request-grant admission phase and is ACKed by the receiver. To save on bandwidth, we use a packet cache in our input-queued switching-nodes, which replicates asynchronously forwarded frames thus tolerating the variable-delay in the admission phase. Because the cache has limited size, we loosely synchronize the multicast subflows to protect the cache from thrashing. We describe our policies for lossy classes, which segregate and provide fair treatment to multicast subflows. Finally, we show that industry-standard Level2 congestion control does not adapt well to one-to-many flows, and demonstrate that the methods that we implement achieve the best performance.