# Stratosphere: A Fast Virtual Circuit Switch Layer for Data Center Network
## 1 Introduction  
Cloud computing applications and data center networks have many tussles, including (1) Application latency vs. network bandwidth. Cloud computing applications expect to transmit data quickly, and the underlying network devices take the way of storage-forward, so the transmission control protocol usually determines the bottleneck link according to whether the packet is lost or not. At this time, the latency of packet queues has already increased greatly. (2) The application asymmetry vs. network symmetry. The roles of compute nodes in the process of applications are not equivalent, and the bandwidth and volume are also different. However, the data center network always provides a symmetrical forwarding path. (3) The cross-border of application vs. hierarchical level of the network. A scalable data center network is built in a hierarchical way based on network equipment. However, the applications, which are distributed in different racks, communicate across multiple levels of networks. As cloud computing applications continue to enrich, data center network traffic is growing and unpredictable, which brings greater challenges to the design of network topology and transmission control. The extension of the data center network has a variety of methods:  
* Isomorphic extension. To support the continuous expansion of the data center scale, VL2 [1], Portland [2], Fat-tree [3], and other network topologies are proposed, mainly with hierarchical stacking of the same type of network equipment to build high-performance networks. The network has the determined convergence ratio of the upper and downward link in the design, which is scalable and extensible. * Heterogeneous expansion. A variety of communication technologies are used to construct heterogeneous network topology, and minimal changes to the existing network infrastructure are made through the auxiliary links to optimize the network. Due to the dynamics of the network hotspot, a new link is established reactively. ProjectToR [4], Helios [5], Modria, and other methods mainly use free space opticalcommunication. Three-dimensional Beamforming, diamond, and other methods mainly use wireless technology. 3. Reconfiguration. To make changes to the network infrastructure as small as possible, dynamically reconfiguring the original links of the data center network is proposed to meet the requirements of applications, such as Microsoft's Larry architecture [6], in which a circuit switching chip (ASIC) is adopted to connect Tor switches in the adjacent rack and converged switches. By controlling the connection of circuit switch ports, the Tor switch could take the uplink of other Tor switches to realize the uplink sharing between adjacent racks, which relieves network congestion. The scale of cloud data center networks and the number of virtual machine nodes are continuing to increase, and east-west traffic is also growing rapidly. By the isomorphic extended mode, the layered architecture of the network inevitably makes hotspots at aggregation links. In addition, the application is more diversified, and the tidal phenomenon of hot spots in the network is becoming more and more difficult to predict. In heterogeneous expansion mode, the network topology changes dynamically, which delays the convergence of data center network routing. In the reconfiguration mode, the circuit switching is used to reconnect Tor switches with convergent switches, which also changes the network topology, brings the routing oscillation, and causes the routing convergence. Facing the hotspot problem in data center networks, we propose a stratospheric network to alleviate network congestion. It dynamically establishes a VCS module between multiple Tor switches and provides a direct path for Tor communication, which can exchange packets directly and share uplinks to accelerate the transmission of east-west traffic.