This paper presents the design, implementation, and evaluation of B4, a private Software Defined WAN (SDN) connecting Google's data centers globally. B4 is characterized by massive bandwidth requirements, elastic traffic demand, and full control over edge servers and networks. The SDN architecture uses OpenFlow to control simple switches built from merchant silicon, enabling centralized traffic engineering (TE) to drive links to near 100% utilization and split application flows among multiple paths to balance capacity and application priority. The paper discusses the experience with three years of production deployment, including lessons learned and areas for future work. Key features include: - **Design and Implementation**: B4's SDN architecture is layered into switch hardware, site controller, and global layers. The site controller layer includes Network Control Servers (NCS) hosting OpenFlow controllers (OFC) and Network Control Applications (NCAs). The global layer consists of centralized applications like an SDN Gateway and a central TE server. - **Traffic Engineering (TE)**: B4's TE system aims to maximize average bandwidth utilization while balancing application priorities. It uses a centralized TE server to adjudicate among competing demands, multipath forwarding, and dynamic bandwidth reallocation in response to failures or changing demands. - **Evaluation**: B4 has achieved high utilization, with links running at near 100% utilization and all links sustaining full utilization over long periods. The system has also demonstrated resilience to failures, with minimal impact on traffic during outages. The paper discusses the impact of failures, the performance of the TE algorithm, and the effectiveness of load balancing across links. - **Experience from an Outage**: A planned maintenance operation led to a substantial outage due to a configuration error. The outage highlighted the importance of managing large WANs and the challenges of SDN in such environments. The paper concludes by summarizing the benefits and challenges of B4, emphasizing its efficiency, scalability, and adaptability to changing application demands.This paper presents the design, implementation, and evaluation of B4, a private Software Defined WAN (SDN) connecting Google's data centers globally. B4 is characterized by massive bandwidth requirements, elastic traffic demand, and full control over edge servers and networks. The SDN architecture uses OpenFlow to control simple switches built from merchant silicon, enabling centralized traffic engineering (TE) to drive links to near 100% utilization and split application flows among multiple paths to balance capacity and application priority. The paper discusses the experience with three years of production deployment, including lessons learned and areas for future work. Key features include: - **Design and Implementation**: B4's SDN architecture is layered into switch hardware, site controller, and global layers. The site controller layer includes Network Control Servers (NCS) hosting OpenFlow controllers (OFC) and Network Control Applications (NCAs). The global layer consists of centralized applications like an SDN Gateway and a central TE server. - **Traffic Engineering (TE)**: B4's TE system aims to maximize average bandwidth utilization while balancing application priorities. It uses a centralized TE server to adjudicate among competing demands, multipath forwarding, and dynamic bandwidth reallocation in response to failures or changing demands. - **Evaluation**: B4 has achieved high utilization, with links running at near 100% utilization and all links sustaining full utilization over long periods. The system has also demonstrated resilience to failures, with minimal impact on traffic during outages. The paper discusses the impact of failures, the performance of the TE algorithm, and the effectiveness of load balancing across links. - **Experience from an Outage**: A planned maintenance operation led to a substantial outage due to a configuration error. The outage highlighted the importance of managing large WANs and the challenges of SDN in such environments. The paper concludes by summarizing the benefits and challenges of B4, emphasizing its efficiency, scalability, and adaptability to changing application demands.