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Network World - This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.
Software-defined networking (SDN) is emerging as an alternative to proprietary data center networks, a way to separate the control plane from the data plane in data center switches and routers. With SDN, network control is implemented in software and can be executed from a server, which reduces network complexity and provides a common interface that is an alternative to vendors' proprietary and expensive options.
There are several developments of SDN solutions, with OpenFlow (in its early stages) and OpenSM (in production today) being the two leading options. OpenSM is the SDN solution for InfiniBand, a fast interconnect technology that was built as an SDN from its basic architecture level and used in the most demanding production sites in existence today.
SDN promises a high-level "virtual" representation of the network, a standard means to control the network physical elements, a scalable architecture that provides high-performance connectivity even for large flat networks, and the quick addition of new network features via open, industry-standard interfaces. In order to achieve this, a layered abstraction of the network control plane and separation of the control and data planes are required.
In his excellent presentation titled "The Future of Networking, and the Past of Protocols," Scott Shenker described SDN as a three-layer abstraction for the network control plane. The SDN abstraction for the network control plane, as he presented it, is shown in Figure 1.
The network equipment is represented at the bottom of the diagram and the data links are shown in blue to complete the network. The control plane is depicted as arrows connecting the equipment to a software layer named the network operating system (NOS). These connections are the ones standardized by Ethernet SDN, also known as OpenFlow. On the one hand, the task of the NOS is to represent a global network view, such as the connected graph of the data links to the upper layers. On the other hand, the NOS takes directives about the configuration to be applied to each system and performs the actual setting.
A control program, sometimes called compiler, takes the directives provided as end-to-end behavior targets and converts them into specific-system NOS settings on top of the global network view graph.
A user interface program that converts the network-manager intent into features of a high-level virtual topology is at the top of the abstraction. This program is named virtual network controller.
SDN is more than just the standard for device configuration. SDN promises a whole new range of network flexibility and scalability.
InfiniBand was first specified in 1999, and has evolved and matured over time into a much richer specification. From the start, it embodied concepts similar to those of software-defined networking. Datapath mechanisms such as forwarding were defined with a clear control plane interface. Every InfiniBand device must have an embedded control plane agent named subnet management agent (SMA). The protocol and message format for in-band communication with the SMAs is well-defined. Subnet management packets (SMPs) are used for control plane communication via software, thus SMP and their protocols are the equivalent to OpenFlow. An InfiniBand-based SDN equivalent is described in Figure 2.