上海海事大学计算机网络技术-data center networking

Data Center Networking

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Data Center Networking

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Abstract In recent years, Internet companies such as Google, Microsoft, Facebook, and Amazon have built massive data centers, each housing tens to hundreds of thousands of hosts, and concurrently supporting many distinct cloud applications. Each data center has its own data center network that interconnects its hosts with each other and interconnects the data center with the Internet. In this paper, I provide a brief introduction to data center networking. First of all, I introduce the Data center with brief and vivid expression, then the working principle of traditional data center working is the main part I describe. The third part of this paper is one of the improved data center networking architectures-Highly-interconnected Data Center networking. Ultimately I point out the current trend in development of Data Center Networking.

Keywords: Data center network; Internet; Architectures

Contents

Chapter1 Introduction of Data Center (3)

1.1 what is the Data center (3)

1.2 What is the role Data Center plays (3)

Chapter2 Traditional Data Center Networking (4)

2.1 The architecture of traditional data center working (4)

2.2 Core of the working principle-Load Balancer (5)

2.3 Drawbacks of traditional data center working (5)

2.3.1 Imbalance of working load (5)

2.3.2 Limited host-to-host capacity (6)

2.3.3 The high cost (6)

Chapter 3 Improved Data Center Networking (7)

3.1 The architecture of Highly-interconnected Data Center (7)

3.2 The strengths of Highly-interconnected Data Center (7)

Chapter4 Trend of Data Center networking (8)

Chapter1 Introduction of Data Center

1.1 what is the Data center

In professional words, Data Center is featured as a complete set of complicated facilities. It contains not only computer systems and corresponding devices such as communication and storage systems but also redundancy data communicating links, environmental control equipments, monitoring equipments and variety of safety devices. Allowing for that the abstract description of Data Center above might not give us a easy access to what the Data Center is , I find another way to make us understand the Data Center comprehensively. In the book 《The Datacenter as a Computer 》published by Google ,Data Center is described as a multifunctional construction containing several servers and communication devices .These devices are put together because their same requirements of environment and physical security and the convenience to maintain, rather than a gather of some servers.

As its name implies ,Data Center Networking is the interrelation among these devices.

1.2 What is the role Data Center plays

In the past several decades, Internet has exerted revolutionary influences on people's daily life. For instance, instead of transferring message by pigeon posting hundreds of years ago, we now are able to contact from our families or friends to anyone we want to via WeChat , microbolg or any applications we prefer to use. The convenience to communicating and doing other things nowadays no doubt owes to the rapid development of Internet. Those applications provide unprecedented high-quality services, at the same time ,they store a great deal of information about us they need to serve us. How do they manage the multiple complex data clearly? The answer of this question is the core of this section-The role of the Data Center .

In the processing of every applications in Internet , the transferring and storage of information are crucial issues. With the management of Data center ,huge number of traffic can transfer from one place to another clearly and be stored safely. Hence, the significance of the role of data center is as well as that of information ,the basic element of the Internet dominating the world today

Chapter2 Traditional Data Center Networking

2.1 The architecture of traditional data center working

With the development of Internet in the past several years, many structures of data center has been designed to support diversity of applications. Among those structures ,there is a basic design on which scientists have designed many improved structures base. We call that traditional Data Center Networking.

As is shown in the figure 1, traditional Data Center is comprised of Border router, Access router, Load balancer, Tie-1 switches ,Tie-2 switches , TOR switches and Server racks.

The hosts stacked in racks serve content such as Web pages and videos, store emails and documents, and collectively perform massively distributed computations, for example, distributed index computations for search engines. The hosts in data centers, called blades and resembling pizza boxes, are generally commodity hosts that include CPU, memory, and disk storage. At the top of each rack there is a switch, apt- ly named the Top of Rack (TOR) switch, that interconnects the hosts in the rack with each other and with other switches in the data center. Specifically, each host in the ra- ck has a network interface card that connects to its TOR switch, and each TOR switch has additional ports that can be connected to other switches.

On the purpose of dealing with the two types of traffic flowing in the data center external clients and internal hosts, the data center network includes one or more border routers, connecting the data center network to the public Internet. Therefore, the data center network are capable of interconnecting the racks with each other and connecting the racks to the border routers.

At the top of the architecture, the border router connects to several access routers. Below each access router there are three tiers of switches.Each access router connects to a top-tier switch, and each top-tier switch connects to multiple second-tier switches and a load balancer. Each second-tier switch in turn connects to multiple racks via the racks' TOR switches. All links typically use Ethernet for their link-layer and physical-layer protocols, with a mix of copper and fiber cabling. With such a design, it is possible to scale a data center to hundreds of thousands of hosts.

2.2 Core of the working principle-Load Balancer

A data center is capable of providing many applications simultaneously such as e-mail, video applications. In order to address the requests from the external clients in the Internet , each application the data center supports is associated with a publicly visible IP address for the purpose of that external clients can send their requests to the address and receive the response from it. Inside the data center, the external requests are first directed to a load balancer whose job it is to distribute requests to the hosts, balancing the load across the hosts as a function of their current load. Actually, several load balancers serves in one data center generally, and each of them is dedicated to a set of specific applications. Load balancers make decisions based on the destination port number as well as destination IP address in the packet.

Upon receiving a request for a particular application, the load balancer forwards it to one of the hosts that handles the application. A host may then invoke the services of other hosts to help process the request. When the host finishes processing the request, it sends its response back to the load balancer, which in turn relays the response back to the external client.

The load balancer not only balances the work load across hosts, but also provides a NAT-like function, translating the public external IP address to the internal IP address of the appropriate host, and then translating back for packets traveling in the reverse direction back to the clients. This prevents clients from contacting hosts directly, which has the security benefit of hiding the internal network structure and preventing clients from directly interacting with the hosts.

2.3 Drawbacks of traditional data center working

Although the conventional architecture of data center just described solves the

problem of scale, it suffers from many shortcomings emerging during the rapid development of Internet.

2.3.1 Imbalance of working load

On account of the hierarchical architecture of traditional data center, which looks like a tree, the working load among these devices in the data center is not distributed

on average. This unreasonable design may lead up to the waste of resource, which means financially loss of money. To be specific, the border router at the top of the structure is greatly likely to work at high load, yet some of the access routers and other devices under that don't or even do not work when the number of external requests go up to a specific level. The border router may shut down if the situation deteriorates constantly, which is worst consequence we must ward off .

2.3.2 Limited host-to-host capacity

To understand this limitation, consider Figure 1, and suppose each host connects to its TOR switch with a 1 Gbps link, whereas the links between switches are 10 Gbps Ethernet links. Two hosts in the same rack can always communicate at a full 1 Gbps, limited only by the rate of the hosts' network interface cards. However, if there are many simultaneous flows in the data center network, the maximum rate between two hosts in different racks can be much less. To gain insight into this issue, consider a traffic pattern consisting of 40 simultaneous flows between 40 pairs of hosts in different rack. Specifically, suppose each of 10 hosts in rack 1 in Figure 1sends a flow to a corresponding host in rack 5. Similarly, there are ten simultaneous flows between pairs of hosts in racks 2 and 6, ten simultaneous flows between racks 3 and 7, and ten simultaneous flows between racks 4 and 8. If each flow evenly shares a link's capacity with other flows traversing that link, then the 40 flows crossing the 10 Gbps A-to-B link will each only receive 10 Gbps / 40 = 250 Mbps, which is significantly less than the 1 Gbps network interface card rate.

2.3.3 The high cost

The cost of a large data center is huge, exceeding $12 million per month for a 100,000 host data center. Of these costs, about 45 percent can be attributed to the hosts themselves; 25 percent to infrastructure, including transformers, uninterruptable power supplies systems, generators for long-term outages, and cooling systems; 15 percent for electric utility costs for the power draw; and 15 percent for networking, including network devices such as switches, routers and load balancers, external links, and transit traffic costs. In these percentages, costs for equipment are amortized so that a common cost metric is applied for one-time purchases and ongoing expenses such as power. While networking is not the largest cost, networking innovation is the key to reducing overall cost and maximizing performance.

Chapter 3 Improved Data Center Networking

In order to overcome the drawbacks listed in the chapter 2(in fact, there are several shortcomings beyond these, but they are not discussed above) and at the same time improve the delay and throughput performance, people have figured out several ways to design high-efficiency structures. One of them is the approach to replace the hierarchy of switches and routers with a fully connected topology. Such topology is shown in the figure 2 and discussed in the ensuing sections.

3.1 The architecture of Highly-interconnected Data Center

In the design of Highly-interconnected Data Center, as shown in figure 2, instead of that every tie-1 switch connects to two tie-2 switches only(virtually, there are several tie-2 switches but not all connected to the corresponding tie-1 switch, but not shown in figure 2), each tier-1 switch connects to all of the tier-2 switches so that host-to-host traffic never has to rise above the switch tiers, and with n tier-1 switches, between any two tier-2 switches there are n disjoint paths.

3.2 The strengths of Highly-interconnected Data Center

With the improved structure, many disadvantages generated in the traditional structure can be alleviated or even eliminated. Firstly, attribute to the highly-interconnected architecture applied, working load have been effectively allocated to every devices, which alleviates the burden of the border router discussed before. This kind of improvement weighs heavily for taking full advantages for the limited resources we have and prolonging the endurance of the devices.

For another thing, Such a design can significantly improve the host-to-host

capacity. To see this, consider again our example of 40 flows. The topology in Figure 2 can handle such a flow pattern since there are four distinct paths between the first tier-2 switch and the second tier-2 switch, together providing an aggregate capacity of

40 Gbps between the first two tier-2 switches. Such a design not only mollify the host-to-host capacity limitation, but also creates a more flexible computation and

service environment in which communication between any two racks not connected to the same switch is logically equivalent, irrespective of their locations in the data center.

Chapter4 Trend of Data Center networking

In view of the irreplaceable role in the Internet development, innovation in data center networking is unlikely to stop practically. In order to support the ever-increasing demands of the Internet, data center networking will be more and more complicated and high-efficiency, and research personnel are liable to innovate and create many new structures such as distributed system of data center networking to work for it. Currently, many trends such as virtualization, energy-saving mechanism, optical switching and wireless transmission and so forth are becoming more and more dominant in the development of data center design.

I believe more innovations in data center design are likely to come and interested researches are encouraged to read the many recent papers on data center network design so that they can design more efficient data center networks.