SDN website - WordPress.com
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<li>It is an approach to computer networking which evolved from work done at UC Berkeley and
Standford University around 2008</li>
<li>SDN allows network administrators to manage network services through abstraction of lower level functionality</li>
<li>SDN requires control plane to communicate with the data plane</li>
<li>OpenFlow is often misunderstood to equivalent to SDN but mechanisms could also fit into the concept</li>
<li>Open Networking Foundation was founded to promote SDN and OpenFlow, marketing the use of the term cloud computing before it became popular</li>
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<h1><center>Background</center></h1>
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<li>Internet Protocol (IP) based networks were initially built based on the notion of Autonomous
Systems (AS)</li>
<li>The AS principle does not allow the designated destinations to move without changing their identity as far as the packet delivery service is concerned</li>
<li>The topological location of destinations, which is the network interface they are attached to, dictates their identity</li>
<li>Complementary standards by the Internet Engineering Task Force (IETF) were put in place to augment identity-specific needs, standards such as virtual LANs and virtual private networks, among many others</li>
<li>As elastic cloud architectures and dynamic resource allocation evolve and as mobile computer operating systems and virtual machines usage grows, the need has arisen for an additional layer of software-defined networking (SDN)</li>
<li>It can also simplify network operations, where global definitions per identity do not have to be matched to each and every interface location</li>
<li>SDN is a step in the evolution towards programmable and active networking</li>
<li> SDN allows network administrators to have programmable central control of network traffic via a controller without requiring physical access to the network's switches</li>
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<h1><center>SDN deployment models</center></h1>
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<h2><li>Symmetric vs asymmetric</li></h2>
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<li>SDN global information is centralized as much as possible, and edge driving is distributed as much as possible</li>
<li>The considerations behind such an approach are clear, centralization makes global
consolidation a lot easier, and distribution lowers SDN traffic aggregation-encapsulation pressures</li>
<li>SDN model an effort is applied to increase global information distribution ability, and SDN aggregation performance ability so that the SDN elements are basically one type of component</li>
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<h2><li>Floodless vs flood-based</li></h2>
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<li>A significant amount of the global information sharing is achieved using well known broadcast and multicast mechanisms</li>
<li>All forwarding is based on global exact match, which is typically achieved using Distributed
Hashing and Distributed Caching of SDN lookup tables</li>
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<h2><li>Host-based vs Network-centric</li></h2>
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<li>An assumption is made regarding use of SDN in data-centers with lots of virtual machines moving to enable elasticity</li>
<li>Design a clearer demarcation is made between network edge and end points</li>
<li>There may also be dependency between these design variants, for example a HostBased implementation will typically mandate an Asymmetric centralized Lookup or Orchestration service to help organize a large distribution</li>
<li>Symmetric and FloodLess implementation model would typically mandate in-network SDN aggregation to enable lookup distribution to a reasonable amount of Edge points</li>
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<h1><center>Applications</center></h1>
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<li>SDN virtual networking combined with virtual compute (VMs) and virtual storage can emulate elastic resource allocation as if each such enterprise application was written like a
Google or Facebook application</li>
<li>The use of SDN distributed and global edge control also includes the ability to balance load on lots of links leading from the racks to the switching spine of the data-center</li>
<li>Without SDN this task is done using traditional link-state updates that update all locations upon change in any location</li>
<li>Distributed global SDN measurements may extend the cap on the scale of physical clusters</li>
<li>It has been noted that this "overlay" approach raises a high likelihood of inefficiency and low performance by ignoring the characteristics of the underlying infrastructure</li>
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<cite>http://en.wikipedia.org/wiki/Software-defined_networking</cite>
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