Extensibility means that you can relatively easily add individual elements (users, computers, applications, services), increasing the length of network segments and replacing existing equipment more powerful. In this case, it is essential that the ease of expanding the system can sometimes be provided in certain very limited extent. For example, LAN Ethernet, built on the basis of one segment of thick coaxial cable, and has good scalability, meaning that you can easily connect the new station. However, the network has a limit on the number of stations - their number should not exceed 30-40. Although the network allows for a physical connection to the segment and the large number of stations (up to 100), but most sharply reduced network performance. The presence of such limitations and is a sign of poor scalability with good scalability.
Scalability (scalability) means that the network can increase the number of nodes and the length of links in a very wide range, with the performance of the network is not affected. To ensure network scalability necessary to use additional communications equipment and specially structured network. For example, good scalability has multisegment network, built using switches and routers and has a hierarchical structure of relationships. Such a network may include several thousand computers while providing each user the right network service quality.
The traditional network architectures, as we know, were designed to control, routing and network signaling, were part of a management structure static, hierarchical and dependent on the network infrastructure. One of his greatest challenges remains the inability to make changes dynamically, depending on traffic requirements or services. For example, a change in control of the network to implement routines, change routing or implement a new service means scheduling routines to intervene in each of the devices and endpoints involved in the service(Fares, 2010, pp.96-105).
In an effort spearheaded by the Open Networking Foundation (ONF), a nonprofit multi-enterprise, has tried to develop a flexible network model that fits the realities of unpredictable data traffic. Thus was born the network architecture SDN (Software Defined Networking, for its acronym in English) or software defined networks, which are expected to change the architecture of the networks as we know them. Among the members of the ONF are Google, Verizon, T-Mobile, Alcatel-Lucent, Facebook, Yahoo, Microsoft, Cisco, IBM, HP (Casado, 2008, pp.45-75)..
The ONF has led the development of the SDN and standardization of critical elements of architecture, including the OpenFlow protocol developed to manage the control software of the network and its interconnection with other network devices, establishing communication mechanisms between control structures of the network, the data flow and the signaling plane.
OpenFlow is a standard interface specifications SDN, is designed to interface to ensure connectivity and performance of all devices manufactured by different vendors that connect to the network. OpenFlow is being deployed in a variety of network devices and software components, both for companies and operators, in order to cover and ensure the following:
-Centralized control multiple networked devices(Dobrescu, 2009, ...