A service mesh, such as Anthos Service Mesh, is specifically designed to manage and secure communication between microservices (VM-to-VM traffic). Its core security feature is the ability to enforce mutual TLS (mTLS) for all traffic within the mesh. This is achieved by deploying a sidecar proxy alongside each application instance, which automatically encrypts and decrypts all incoming and outgoing traffic. This provides strong, identity-based authentication and encryption for all internal service communication, directly fulfilling the CSO's mandate without requiring changes to the application code. This aligns with a zero-trust security posture where internal traffic is not implicitly trusted.

A. Private Google Access allows VMs with only internal IP addresses to reach Google APIs and services; it does not manage or encrypt VM-to-VM traffic within the VPC.

C. VPC Firewall Rules operate at the network layer (Layer 4). While they can restrict traffic to a specific port like 443, they cannot inspect the payload to enforce that the traffic is actually encrypted with TLS.

D. A Global HTTP(S) Load Balancer manages north-south traffic (client-to-service). It does not enforce encryption for direct east-west (VM-to-VM) communication that bypasses the load balancer.

1. Anthos Service Mesh Documentation: "Anthos Service Mesh helps you secure your services by providing a way to manage authentication, authorization, and encryption... Anthos Service Mesh automatically encrypts traffic between services with mutual TLS (mTLS)."

Source: Google Cloud Documentation, "Anthos Service Mesh security", Section: "Encryption in transit".

2. BeyondProd Whitepaper: This Google whitepaper outlines the principles of zero-trust security, which includes encrypting all network traffic, even internal service-to-service communication. A service mesh is a key technology for implementing this. "All network traffic between services... must be encrypted and authenticated via mTLS."

Source: Google Cloud Whitepaper, "BeyondProd: A new approach to cloud-native security", Page 6, Section: "Transport Security".

3. VPC Firewall Rules Documentation: "VPC firewall rules let you allow or deny traffic to and from your virtual machine (VM) instances... Rules are defined at the VPC network level and are specific to the project and network." The documentation specifies that rules are based on protocol and port, not payload content.

Source: Google Cloud Documentation, "VPC firewall rules overview".

4. Private Google Access Documentation: "With Private Google Access, VM instances that do not have external IP addresses can reach the external IP addresses of Google APIs and services." This confirms its purpose is for accessing Google services, not for internal VM-to-VM security.

Source: Google Cloud Documentation, "Private Google Access overview".