VL-89                             October 2013

Software Defined Networks Address
Provisioning Scale and High Availability

By William Flanagan, Publisher 

Issue 1 of the ViewsLetter in July 2002 stated a concern for the difficulty of provisioning data circuits. At that time the demand for data connectivity was growing faster than carriers could hope to match with traditional manual methods of building circuits on wire distribution frames. Digital cross connects, while faster and more flexible, couldn't handle the rapidly rising demand for bandwidth.

With some automation and a migration to IP network services, the provisioning problem faded from view. Carrier equipment such as routers found paths on their own. Operations support systems moved to monitoring and troubleshooting rather than just provisioning. As a result, the ViewsLetter shifted its focus to High Availability. As that feature became recognized as a necessity for IP networks, the VL shifted to Voice over IP.

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Now a solution to those two concerns seems at hand. The Software Defined Network (SDN) offers simplified provisioning via automation for data centers and WANs. A logical extension of that function provides automated recovery from faults, raising availability.

In practice, SDN adds a standard configuration API (application programming interface) to each switch and router. This control interface lets an external server calculate paths and update the forwarding tables in each device. In the extreme, switches and routers become packet forwarders, some with different abilities to examine and modify packets. The packet forwarding devices no longer need to run routing protocols (e.g., EIGP, OSPF, BGP) or worry about path loops (prevented by the Spanning Tree Protocol on Ethernet LANs).

SDN takes the idea of automated provisioning much further than a tool for telco employees. A "northbound" API on the central control server lets a customer application request a circuit, automatically, on demand. Manual provisioning by an operator or technician isn't needed (although it remains available at a console).

Just as Generalized MPLS extends Label Switched Paths outside the packet domain, SDN can include optical wavelengths, radio links, and other transmission facilities between two packet forwarders. There's no reason to exclude circuit switched services such as T-1s and PRIs (other than carriers will discontinue such services where they can).

Coming along at the right time to help the SDN of packet forwarders are more flexible optical nodes. They move wavelength connections from inbound fibers to outbound or add and drop wavelengths locally. If an arriving wavelength is in use on the desired outbound fiber, some optical nodes can regenerate the stream on a different wavelength to prevent blocking the path.

Is SDN a big deal? Worked correctly, SDN will have as much impact as the moves from analog to digital and from circuit to packet. Hang on tight.

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