Google has built a dozen data centers around the world. To avoid data loss and offer high availability, Google replicates a massive amount of data through its own WAN network connecting all its sites with terabits/s bandwidth. But these kinds of networks are *very* expensive, and are usually over-provisioned to face traffic variance. Google B4 optimizes the utilization of its network via sophisticated software-defined networking on handcrafted hardware. Let’s scratch together the surface of this large-scale real-world SDN implementation.