Istio can route a service request based on HTTP headers and specific network parameters. Based on routing rules, Pilot dynamically allows the sidecar proxy to select a version of the service, defines tags based on source and destination, applies headers, assigns weights to each service, and determines the service's incremental number. Users of the microservice have no knowledge of the different versions as it doesn't disrupt their work effort. The hostname and IP address of that service will still be accessible because the sidecar proxy forwards all service requests and responses between the user and the microservice.

Istio really shines through the use of virtual services to perform traffic routing based on the following: 

• Routing traffic to a specific service (one to one)
• Routing traffic to multiple services (one to many)
• Routing traffic to multiple versions of a service (one to many)
• Adding multiple match conditions to route traffic to different services
• Routing rules to rewrite a URL
• Routing rules to set a retry policy
• Routing rules based upon HTTP headers/request cookies
• Routing rules based upon the request URI

Routing is explained with a hands-on example in Chapter 10, Exploring Istio Traffic Management Capabilities.