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At present, due to the rapid advancement of technology and the importance of data exchange, computer networks are the key to the survival of organizations and the consistency of their growth and development path.
Virtualization can be defined as the creation of a virtual version of a server, operating system, storage device, or network resources.
Network virtualization can also be defined by the capability to create logical, virtual networks that are used for decoupling of functions by dividing the ISPs (internet service providers) into InPs (infrastructure providers) and SPs (service providers). The InPs “manages the physical infrastructure” , while the SPs “create virtual networks by aggregating resources from multiple infrastructure providers and offer end-to-end network services.” If multiple virtual networks coexisting on the same substrate is allowed then the networking environments support the virtualization. A virtual network is a group of virtual links and nodes and it is a subset of the physical network resources. It is the next step after storage and server virtualization. Different applications to run one next to other over the same physical network is allowed.
. In information technology, there are three basic bases for virtualization:
1- Virtualization of the network
2. Virtualization of space
3. Server virtualization
Additionally, the virtualization of the operating system is such that an application allows the partition or part of the drive to perform several activities simultaneously and simultaneously. The concept of virtualization of the network is not something new and with the passage of time, it is becoming more modern and state-of-the-art. This technology has been used in large computers for decades.
The use of virtualization is one of the most commonly used ways in the field of information technology. The virtual machine effectively makes it easier to manage and reduce the IT budget as it allows multiple networks to run within the same machine. In this paper, we will be reviewing some literature on network virtualization to study in depth.
Fig 1: Network virtualization 
In recent years, Network Virtualization has become an important technology in the field of information technology. Experts in this field believe that this issue will change the processes in the field of information technology.
However, the concept of virtualization is not a very recent one. In the 1970s, the idea of virtual machines was introduced, which made it possible to use several different computing environments on a single physical environment, the idea of which was to increase the time sharing that was introduced in the 1950s. In the 1990s, telecom companies made it possible to access virtualized communications. Instead of creating an independent physical structure for each user, it was possible to share physical infrastructure for a wide range of users.
- Network Virtualization Techniques:
The virtualization of the network can be dependent upon – 
Node virtualization: it is established “on isolation and partitioning of hardware resources”. As per a lot of prerequisites each segment which is separated by the physical resource of a substrate node is designated to a virtual node. The latest improvement in OS virtualization has allowed a noteworthy advance as far as fairness and execution.
- Link virtualization: the transport of various separate virtual links is permitted over a shared physical link. the link is regularly recognized explicitly with a tag; however, it can be distinguished implicitly with a time slot or a wavelength.
The network virtualization components are:
- device virtualization: it virtualizes the devices in the network.
- data path virtualization: the communication between the network access points is virtualized
- hop to hop: single hop data path which is applied by virtualization is considered here.
- -hop data path is permitted by the virtualization tunnels is considered.
- the types of protocols that are used to approach the virtual data path
- 802.1Q: the data path virtualization implements hop to hop.
- MPLS: the router and switch layer virtualization are implemented by the multiprotocol label.
- GRE (General Routing Encapsulation): among different networks with tunneling technique, the generic routing encapsulation is implemented in virtualization.
4. Types of network virtualization:
There are two types of network virtualization:
Internal network: Internal network virtualization: A virtualized internal network is a network that is restricted to one machine only. Hence it is also called “network in a box.” It is essential to improve the efficiency of the network as communication over a network interface – which is also virtual – is allowed for communication.
External network: In the external network, at least one local network is subdivided or joined into the virtual network. To improve the effectiveness of big corporate networks or data center is the goal. The principle of an external network is the virtual local area network and the switch. By utilizing these, systems that are physically attached to a similar local network into various virtual networks can be configured by the administrator. Virtual local area network permits the administrator to join systems on another local network into spanning the portions of a big network. whereas in the internal network, to create a “network in a box” “hypervisor control programs or pseudo interfaces” with containers are joined to configure a single system.
- Virtualization in the concept of coexisting logical networks:
There are four main classes in the concept of multiple coexisting logical networks: VLAN, VPN, active and programmable and lastly overlay networks. 
VLAN: The VLAN is a set of hosts that has a typical interest which brings them together paying little respect to their physical connectivity under a single broadcast domain. As Virtual local area networks are logical entities, as far as network administration, management and reconfiguration they are flexible. “In addition, they raise the level of trust, security, and isolation, and are cost-effective.” Despite the fact that implementation in various layers does exist, traditional VLANs is basically layer 2.” All frames in a VLAN bear a typical ID in their MAC header” and both MAC address and VLAN ID are used by VLAN switches to forward packets. This is called as frame coloring. using trunking, various VLANS on numerous switches are connected, this permits data from various VLANS to be done over a single link between the switches.
VPN: It is a communication network of at least one enterprise that is shared over numerous sites and connected by tunnels over the public network. every site has at least one customer edge devices which are connected to at least one provider edge routers. The VPN service provider normally used to oversee and provision a VPN, this is called as PPVPN (provider provisioned VPN). Even though the implementation of VPN exists in various layers, layer 3, layer2 and layer 1 are the most well-known ones in the network stack.
Active and programmable: Here a lot of the projects are pushed forward the idea of coinciding networks through programmability. They may not be considered as an immediate case of network virtualization. they also provide separate environments to maintain a strategic distance from conflicts and network instability, so as to enable various external parties to run conflicting codes on the similar network elements. How communication hardware could be isolated from control software is examined by the programmable networks. Two different schools of thoughts rose on the most proficient method to really implement such ideas: one is IP network other is telecommunication community.
Overlay network: “It is a virtual network that creates a virtual topology over the physical topology of another network” . Through virtual links, the nodes are connected in an overlay network, which relates paths in the underlying network even though various execution at lower layers of the stack does exist. It is usually implemented in the application layer. The overlay is adaptable and versatile to changes and effectively deployable in contrast with another network and they are not geographically confined. Thus, they have for quite some time been utilized to deploy new features and fixes.
- Design Goals of a virtual network:
Flexibility: A virtual network should be flexible in every way possible, that means the service providers should be able to use any kind of topology, routing protocol or forwarding protocol regardless of the physical or the co-existing virtual networks.
Manageability: Manageability in virtual networks may be achieved by the separation of service providers from infrastructure providers. Infrastructure providers will be in charge of managing operations of physical entities while the service providers will lease resources and build virtual networks depending upon the resources available. This separation will allow for an end-to-end control of the networks and consequently will increase manageability.
Scalability: Ideally, there should be as many co-existing virtual networks as much the resources allow for. However, due to many other constraints and conflicts that may not be possible. However, scalability is one of the reasons virtual networks gained popularity.
Programmability: If a virtual network is not programmable to adjust the user’s needs and requirements, how advantageous is it really? Not much. For the implementation of customized and user-specified protocols and service, a virtual network must be programmable. However, with the option of programmability, along comes the concern of security. A balance needs to be found out to ensure both programmability and security in virtual networks.
Security, isolation and privacy requirements: It is hugely important that one virtual network stays isolated from another one in a same physical network no matter what otherwise bugs and attacks may be a common phenomenon in the networks. The breach of security can be significantly reduced by the creation of separate virtual networks in case of confidential data communication. Any bugs and failure of the system of one virtual network should not be able to hamper or reach the other ones in an ideal virtual machine.
- Naming and addressing of virtual networks:
Virtual networks offer immense flexibility in terms of deploying any kind of topology, routing or forwarding protocol and these are the best-liked qualities of a virtual network. However, despite the flexibility of each virtual network in an underlying physical structure, the overall connectivity or the universal connectivity of the network, as well as the end to end connectivity, may become a challenge. For instance, running something like a DNS server on a virtual network may become a problematic issue due to the heterogeneity and scalability of the virtual networks. In the existing IP protocol that we use for addressing, the IP address carries the location and the identification both, however, for a virtual network to operate rather smoothly, the name and address should not be coupled. The main reason behind that is, if an identity is liked to an SP (service provider), then it is not possible to change the SP with the same identity. In the virtual networks, an end user should be able to simultaneously connect to multiple service providers through infrastructure providers by “uber-homing.” Any kind of naming protocol should be feasible for supporting “uber-homing” in virtual networks.
- Handling of failure in virtual networks: Due to the complexity and co-existence of multiple heterogeneous networks, failure of any component in the underlying physical network of a virtual network can cause tricky a series of consequent errors in the virtual networks. For instance, failure of a physical link will shut down the virtual network links that pass through it. In a similar way, if a physical node is down, the re-installation of all the software relevant to the virtual networks may become a necessity for repairing. Having said that, this area of the study does require and has huge scope for improvement and future work.
- Benefits of Network Virtualization: First and foremost, virtual networks are path-breaking invention if we want to reduce the investment on hardware or cabling. Moreover, the dependency on hardware is also greatly reduced for a virtualized network. Managing the networks also become significantly easier and centralized due to centralized access control. And since the management of virtual networks is easy, the network efficiency is also bound to increase. It is also diverse with the scope of adding multiple protocols and topologies as well as scalable as well which makes the technique even more efficient compared to traditional hardware-based networks.
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