## 2 Background
### Load Balancing  
The key of data center network traffic load balancing is to study how to distribute traffic to these equivalent links as evenly as possible. Most of the research work used link utilization as the optimization goal. However, this goal is not appropriate in data center networks. There are four reasons:  
1. Link utilization is a statistical average value, which requires continuity of network traffic on a time scale. However, data center network traffic characteristics do not meet such needs. Devices such as servers and switches in the data center network are usually only distributed in a small physical area, so the transmission delay is lower than the Internet. Therefore, the switch buffer pool size is typically much larger than the number of packets being transmitted on the link. 2. In data center networks, many-to-one or many-to-many communication modes are widely found in the parallel computing systems (MapReduce, Dryad and CIEL) and distributed file storage systems (GFS and HDFS). In order to adapt to these special communication modes, the data center transmission protocol has also changed compared to the Internet backbone. Even so, there are still TCP Incast and TCP Outcast problems [15]. TCP Incast is an uncontrolled TCP behavior that causes considerable link underutilization in many-to-one communication paradigms [16]. TCP outcast is that when traffic that consists of connections of many incoming flows and few flows at two ports switch (or router) and compete to one common output port, throughput the few flows is affected implicitly [17]. The main reason for these two problems is competing port resources. These will not happen on the Internet backbone. 3. The data center network contains a large number of short burst flows. The related measurement work [18] used Micro-Burst to describe these short burst flows. The results showed that most of the bursts have a duration of less than 10 s. Other measurements [19] showed that 90% of burst durations do not exceed 2 s. These traffic bursts are strong, difficult to predict, and have a short duration. Internet backbone network traffic engineering does not consider this situation. The traffic in the backbone network is generally regular and relatively stable. This is the main difference between a backbone network and a data center network. Many previous studies have used the backbone network approach to the data center. What we want to say is that the  
Figure 1: Data center network with multi-tier Clos topology (Fat-Tree). characteristics of these two networks are different, so different research ideas need to be used to solve load balance problems. 4. The communication between data center servers is widely used in the TCP protocol, accounting for 95% of the total traffic [20]. TCP is designed for low-bandwidth, low-latency WANs, and is not suitable for high-bandwidth, low-latency data center networks. Even though there is a variety of TCP modified transport protocols for data center networks, congestion is inevitable in these protocols, which do not exist on the Internet backbone. Therefore, we propose minimizing network packet loss rate instead of balancing link utilization optimization factor of data center network traffic load balancing. The loss of packets is a systemic problem of the network [21]. There are five main reasons for packet loss in the data center network:  
1. Unreachable Route This paper does not consider packet loss due to an unreachable route. 2. Equipment Failure This paper does not consider packet loss due to equipment failure. 3. Poor Link Quality Packet loss caused by poor link quality can be solved by improving link quality. Traditional data center internal networks often use lossy links, but many companies have tried to deploy lossless network in data centers [22]. 4. Link Congestion Link congestion refers to the random packet loss caused by the data that is not forwarded in time when the amount of data to be forwarded on one link exceeds the link capacity.