SDN and WAN optimization

Amin Vahdat described Google's SDN based wide area network traffic engineering solution at the recent Open Networking Summit. Amin stated that for existing networks, "a typical rule of thumb is to over-provision by a factor of 3." Amin further stated that moving to a logically centralized SDN based control "improves convergence times, improves failover behavior, more deterministic, more efficient, more fault tolerant."

Google's traffic engineering system is able to insert multiple non-shortest path routes depending on traffic priorities and measured demand. Using only 3 non-shortest path routes, the overall throughput can be increased by around 15%.

However, Amin stated that the big win is being able to run the backbone links at close to 100% utilization 24 x 7, a greater than a 3 times improvement over traditional WAN designs.

A key element of the Google architecture is the use of traffic prioritization. Generally over provisioning has prevailed as the technique for ensuring wide areas network quality of service, see The economics of the Internet: Utility, utilization, pricing, and Quality of Service and more recently The Concept of Quality of Service in the Internet. This seems like a contradiction - why does it make sense to use quality of service mechanisms in Google's case?

Actually there isn't a contradiction, by using SDN to accurately place traffic into just two classes (high priority and low priority), Google is effectively using over provisioning to ensure high quality of service for the high priority class (which comprises 10-20% of the link traffic). The rest of the bandwidth is filled with low priority traffic that must tollerate packet loss and lower availability, since the low priority traffic may be dropped in the case of link failure.

Figure 1: Cloud operation system (from Pragmatic software defined networking)

Pragmatic software defined networking and Multi-tenant traffic in virtualized network environments describe how the visibility and control offered by the OpenFlow and sFlow standards can be used to dynamically engineer traffic in the data center. In the data center, end-to-end control and accurate traffic classification is feasible and should have similar benefits, allowing high bandwidth background activities like data replication and migration to use all the spare capacity in the network without affecting high priority flows.

SDN and large flows describes how steering large flows can significantly increase available bandwidth. Active flow steering and traffic classification are complementary techniques that could be combined to dramatically increase the usable bandwidth in any given physical network.