* ABSTRACT
Network virtualization is the key to the current and future success of cloud computing. In this survey, we explain key reasons for virtualization and virtualization in computing. We explain software defined networking SDN, which is the key to network programmability, and standardized APIs, the Impact of Software Defined Network and its future.
* INTRODUCTION
Network virtualization is the procedure to merge the available resources and network functionality into a single software-based administration structure. Every subscriber shared the access to all resources on the network from a single computer. The Internet has resulted in virtualization of all parts of our life. Today, our workplaces are virtual, and we shop virtually, we get virtual education, entertainment is virtual, and of course, much of our computing is virtual. The key enabler for all virtualizations is the Internet and various computer network technologies. It turns out that computer network itself must be virtualized.
WHY VIRTUALIZE?
There are many reasons why we need to virtualize resources. The five most common reasons are:
1. Sharing: When a resource is too big for a single user, it is best to divide it into multiple virtual pieces, as is the case with today’s multi-core processors. Each processor can run many virtual machines (VMs), and each machine can used by a different user. The same applied to high speed links and large capacity disks.
2. Isolation: Multiple users sharing a resource may not trust each other, so it is important to provide isolation among users. Users using one virtual component should not be able to monitor the activities or interfere with the activities of other users. This may apply even if different users belong to the same organization since different departments of the organization (e.g., finance and engineering) may have data that is confidential to the department.
3. Aggregation: If the resource is too small, it is possible to build large virtual resource that act like a large resource. This case with storage, where many inexpensive unreliable disks can used to make up large reliable storage.
4. Dynamics: overwhelming resource requirements change fast due to user mobility, and a way to reallocate the resources quickly is required. This easier with virtual resources than with physical resources.
5. Ease of management: Last but probably the most important reason for virtualization is the ease of management. Virtual devices easier to manage because they are software-based and expose a uniform interface through standard abstracts.
VIRTUALIZATION IN COMPUTING
Virtualization is not a new concept to computer scientists. Memory was the first among the computer components to be virtualized. Memory was an expensive part of the original computers, so virtual memory concepts were developed in the 1970s. Study and comparison of various page replacement algorithms was a popular research topic then. Today’s computers have very sophisticated and multiple levels of caching for memory. Storage virtualization was a natural next step with virtual disks, virtual compact disk (CD)drives, leading to cloud storage today. Virtualization of desktops performed in thin clients, eventually leads to virtualization of servers and cloud computing. Computer networking is the plumbing of computing, and like plumbing in all beautiful buildings, networking is the key to many of the features offered by new computing architectures. Virtual channel in telecommunication networks and all subsequent networks allow multiple users to share a large physical channel. Virtual local area networks (VLANs) allow multiple departments of a company to share a physical LAN with isolation. Similarly, virtual private networks (VPNs) allow companies and employees to use public networks with the same level of security they enjoy in their private networks. However, there has been significant renewed interest in network virtualization fueled primarily by cloud computing. Several new standards have been developed and are being developed. Software defined networking (SDN) also help in network virtualization. These recent standards and SDN are the topics of this article. We discuss several recent network virtualization technologies. Software defined networking is discussed in detail.
NETWORK FUNCTION VIRTUALIZATION
Standard multi-core processors are now so fast that it is possible to design network devices using software modules that run on standard processors. By combining many different functional modules, any networking device – switch, router, application delivery controller, and so on – can be composed cost effectively and with acceptable performance. The Network Function Virtualization (NFV) group of the European Telecommunications Standards Institute (ETSI) is working on developing standards to enable this [9].
SOFTWARE DEFINED NETWORKING
Software defined network is the latest revolution in network innovations. All components of the network industry, including network equipment vendors, Internet service providers, cloud service providers, and users, are working on or looking forward to various aspects of SDN. This section provides an overview of SDN.
SDN consists of four innovations:
1. Separation of the control and data planes.
2. Centralization of the control plane.
3. Programmability of the control plane.
4. Standardization of application programming interfaces (APIs) Each of these innovations is explained briefly below.
SEPARATION OF CONTROL AND DATA PLANE
Network protocols are often arranged in three planes: data plan, control plan, and management. The data plane consists of all messages that are generated by users. To transport these messages, the network needs to do some management work, such as finding the shortest path using layer 3 routing protocols such as Open Shortest Path First (OSPF) or layer 2 forwarding protocols such as Spanning Tree. The messages used for this purpose are called control messages and are essential for network operation. In addition, the network manager may want to keep track of traffic statistics and the state of various networking equipment. This is done via network management. Management, although important, is different from control in that it is optional and is often not done for small networks such as home networks. One of the key innovations of SDN is that the control should be separated from the data plane. The data plane consists of forwarding the packets using the forwarding tables prepared by the control plane. The control logic is separated and implemented in a controller that prepares the forwarding table. The switches implement data plane (forwarding) logic that is greatly simplified. This reduces the complexity and cost of the switches significantly.
CENTRALIZATION OF THE CONTROL PLANE
The U.S. Department of Defense funded Advanced Research Project Agency Network (ARPAnet) research in the early 1960s to counter the threat that the entire nationwide communication system could be disrupted if the telecommunication centers, which were highly centralized and owned by a single company at that time, were to be attacked. ARPAnet researchers therefore came up with a totally distributed architecture in which the communication continues and packets find the path (if one exists) even if many of the routers become nonoperational. Both the data and control planes were totally distributed. For example, each router participates in helping prepare the routing tables. Routers exchange reachability information with their neighbors and neighbors’ neighbors, and so on. This distributed control paradigm was one of the pillars of Internet design and undoubted until a few years ago. Centralization, which was considered a bad thing until a few years ago, is now considered good, and for good reason. Most organizations and teams are run using centralized control. Centralization of control makes sense and adjust the control dynamically much faster than with distributed protocols. Centralization has scaling issues but also has distributed methods. For both cases, we need to divide the network into subsets or areas that are small enough to have a common control strategy. A clear advantage of centralized control is that the state changes or policy changes propagate much faster than in a totally distributed system. Also, standby controllers can be used to take over in case of failures of the main controller. Note that the data plane is still fully distributed.
PROGRAMMABLE CONTROL PLANE
Now that the control plane is centralized in a central controller, it is easy for the network manager to implement control changes by simply changing the control program. In effect, with a suitable API, one can implement a variety of policies and change them dynamically as the system states or needs change. This programmable control plane is the most important part of the Software Defined Network. A programmable control plane in effect allows the network to be divided into several virtual networks that have very different policies and yet reside on a shared hardware infrastructure. Chane the policy will be very difficult and slow with a totally distributed control plane.
STANDARDIZED APIs
SDN consists of a centralized control plane with a southbound API for communication with the hardware infrastructure and a northbound API for communication with the network applications. The control plane can be subdivided into a hypervisor layer and a control system layer. Many controllers are already available. Floodlight [10] is one example. OpenDaylight [11] is a multi-company effort to develop an open source controller. A networking hypervisor called FlowVisor [12] that acts as a transparent proxy between forwarding hardware and multiple controllers is also available. The main southbound API is OpenFlow [13], which is being standardized by the Open Networking Foundation. Several proprietary southbound APIs also exist, such as OnePK [14] from Cisco. These later ones are especially suitable for legacy equipment from respective vendors. Some argue that a number of previously existing control and management protocols, such as Extensible Messaging and Presence Protocol (XMPP), Interface to the Routing System (I2RS), Software Driven Networking Protocol (SDNP), Active Virtual Network Management Protocol (AVNP), Simple Network Management Protocol (SNMP), Network Configuration (Net- Conf), Forwarding and Control Element Separation (ForCES), Path Computation Element (PCE), and Content Delivery Network Interconnection (CDNI), are also potential southbound APIs. However, each of these was developed for another specific application, they have limited applicability as a general-purpose southbound control API. Northbound APIs have not been standardized yet. Each controller may have a different programming interface. Until this API is standardized, development of network applications for SDN will be limited. There is also a need for an east-west API that will allow different controllers from neighbor domains or in the same domain to communicate with each other.
SDN IMPACT AND FUTURE
Networking industry has shown huge interest in SDN. SDN is expected to make the networks programmable and easily partitionable and virtualizable. These features are required for cloud computing where the network infrastructure is shared by several competing entities. Thus, SDN is expected to reduce both capital expenditure and operational expenditure for service providers, cloud service providers, and enterprise data centers that use lots of switches and routers.
Regardless of what happens to current approaches to SDN, it is certain that the networks of tomorrow will be more programmable than today. Programmability will become a common feature of all networking hardware so that many devices can be programmed simultaneously. The exact APIs that will become common will be decided by transition strategies since billions of legacy networking devices will need to be included in any orchestration. It must be pointed out that NFV (Network Function Virtualization) and SDN (Software Defined Network) are highly complementary technologies. They are not dependent on each other.
* Conclusion
The key messages of this article are:
1. Cloud computing is a result of virtualization in computing, storage, and networking.
2. Networking virtualization is still at the begining. Many standards related to network virtualization have recently been developed in the IEEE and Internet Engineering Task Force (IETF), and several are still being developed.
3. One of the key recent developments in this direction is Software Defined Networking. The key innovations of SDN are separation of the control and data planes, centralization of control, programmability, and standard southbound and northbound APIs. This will allow many devices to easily be programmed.
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