Fly-path: Traffic-based multi-hop routing approach for hybrid wireless data centers

Abstract

High data transfer rates achieved by 802.11ad at 60 GHz ISM band enables use of wireless communication in data centers. In this paper, we investigate the possibility of offloading traffic from wired to wireless network in hybrid data centers. By understanding the capabilities of the wireless network, we can design the hybrid data center network accordingly, to achieve better construction and operating efficiency. First, we propose a system model in which each top-of-the-rack switch is equipped with two radios, so that three non-overlapping channels of 802.11ad that are available worldwide can be assigned in an interference-free manner to any configuration of wireless links. Then, we propose multi-hop routing algorithms that assign traffic to wireless infrastructure. These algorithms consist of two families. SP family of algorithms route traffic only over shortest-paths between source and destination pairs. LP algorithms relax this restriction and assign traffic to longer paths when necessary. In order to evaluate the performance of our routing algorithms, we also propose a random data center traffic generation method, based on an analysis of a real-world data center traffic pattern. We evaluate the performance of our allocation methods in terms of different metrics for various network sizes. Results show that our methods can offload significant amount of traffic from wired to wireless network, can achieve quite high throughput, and can utilize wireless links very well.

Introduction

Unlicensed 60 GHz ISM band offers high-bandwidth line-of-sight wireless communication over short distances [1]. Line-of-sight requirement and short communication distance are handicaps for a general-purpose wireless communication protocol, but they become advantageous in a densely-packed data center network (DCN) by reducing the interference with nearby concurrent communications, therefore increasing throughput across the data center [2]. IEEE 802.11ad standardizes the use of 60 GHz band [3], [4].

There are two approaches to using wireless networking in data centers: completely wireless and hybrid. In completely wireless data centers (WDCs), all communication between servers is wireless — there is no wired communication [5], [6]. A completely wireless data center has a very different physical organization than a traditional data center. In hybrid wireless data centers, wireless communication is used to assist wired network [7], [8].

In this paper, we focus on hybrid wireless data centers because they are more applicable in short-term than completely wireless data centers. Existing data centers could be equipped with wireless networking devices with little effort. Top-of-the-rack (ToR) switches are good candidates for radio placement, so that racks can communicate wirelessly in addition the wired network.

Wireless resources are used in data centers to increase capacity at the bottlenecks of the wired network [9], [10], [11]. The traditional method of addressing bottlenecks in the wired network is to increase capacity by eliminating oversubscription to meet worst-case traffic requirements [12], [13]. Such methods require substantial capital cost because of the increased need for network equipment and the cost of wiring a larger network. Operating costs are also increased because of the power consumption and maintenance cost of a larger infrastructure. Modifying the wired network, or scaling it to increase capacity or to support more devices is also cumbersome [14]. In hybrid data centers, wireless network supports the wired network to address these problems. Wireless links can be established at the bottlenecks dynamically, increasing capacity only when and where needed. Wireless networks are also easier to modify or scale than wired networks.

Bottlenecks, also called hotspots, may occur between 5 – 10 switches in a network of 1500 servers running a Map-Reduce job [9]. Other analyses of data center traffic find that even though core switches carry a higher traffic load than edge switches, edge switches have higher link loss because of high outburst traffic [15], [16]. Alleviating hotspots can be achieved by routing the traffic to a neighboring ToR switch and forwarding it to the bottlenecked one using wireless communication, therefore increasing bandwidth only at the bottleneck [11]. An alternative is to connect two sets of hot servers directly over wireless network, using multiple radios when necessary [10].

Existing studies employ single-hop wireless communication. Multi-hop communication utilizes available network-wide wireless bandwidth better than single-hop communication. It also enables a more flexible arrangement of links that can be configured according to changing traffic requirements of the data center.

We examine the problem of offloading as much traffic as possible from wired to wireless network, so that hybrid data centers could be designed accordingly. We aim to analyze capabilities of a multi-hop wireless network by quantifying the amount of traffic carried, multi-hop path length, and throughput in a data center setting. The results can be used by data center designers to design a hybrid wired and wireless network that is more efficient to build, operate, maintain, and expand than traditional data center network designs.

A static arrangement of multi-hop wireless links cannot adapt to varying traffic needs between nodes, whereas a dynamic arrangement offers the flexibility to allocate wireless links to where they are needed. We propose multi-hop routing algorithms that assign traffic flows to wireless links in a hybrid data center. Traffic flows are evaluated in the ascending order of wireless hop distance between their source and destination. Source–destination pairs that are nearby are assigned to wireless links before the ones that are more distant to each other. The basic premise is to create longer routes between distant nodes from shorter routes that connect closer ones. The required wireless link configuration to assign a new flow may conflict with the existing configuration. In that case, the traffic flowing over the conflicting links may need to be deallocated. A cost–benefit analysis determines the result. The wireless link configuration that carries more traffic is preferred. In other words, allocation is greedy with respect to traffic amount.

Our proposed algorithms are run periodically, to assign traffic according to changing traffic needs of the data center. At the beginning of each period, an external traffic estimator outputs expected traffic exchange between ToR switches during that period. Our algorithms take this estimate as input, and output a configuration of wireless links. Reconfiguring wireless links costs bandwidth, because the traffic that has no route to its destination in the new configuration needs to be dropped. In addition, broadcasting the new configuration and making sure that all nodes have completed their configuration takes time, during which the wireless network cannot be used efficiently. Therefore, we aim to maximize the amount of traffic carried for a given configuration. The traffic that is not assigned to wireless network, flows over the wired network as usual. Our extensive simulation results show how much traffic can be offloaded to wireless network, so that data center networks could be designed accordingly.

Rest of the paper is organized as follows. Section 2 summarizes related work, discusses how our proposed methods differ, and lists our contributions. Section 3 presents the system model and its rationale. Section 4 discusses our proposed traffic allocation methods in detail. Section 5 analyzes properties of a real data center traffic and proposes a method to randomly generate traffic with those properties. Section 6 describes our simulations and discusses results. Section 7 concludes the paper.