Available with Network Analyst license.
When authoring a geoprocessing tool that will be published as a geoprocessing service for solving network analysis layers, you need to start by asking yourself one yes-or-no question: Will the geoprocessing tool solve vehicle routing problems, generate service areas, or find closest facilities?
If the answer is yes, you can plan to add one of three geoprocessing tools to your model or script: Solve Vehicle Routing Problem, Generate Service Areas, or Find Closest Facilities. Read Geoprocessing service example: Drive-time polygons if you plan to publish one of these three kinds of tools.
Geoprocessing service example: Drive-time polygons and its companion tutorial data demonstrate how to create a tool for generating service areas on ArcGIS Server. The tutorial media also provides sample data for setting up services to solve vehicle routing problems and find closest facilities. However, tutorial topics specific to VRP and find closest facilities aren't provided because the workflows for configuring these two types of services are similar to setting up a service area service. So read Geoprocessing service example: Drive-time polygons even if you intend to set up a vehicle routing problem or closest facility service.
If the answer is no, this means you are publishing a geoprocessing tool based on one of the other ArcGIS Network Analyst extension solvers (Route, OD Cost Matrix, or Location-Allocation), and you will need to chain a series of tools together in your model or script.
Geoprocessing service example: Shortest route on a street network and its companion tutorial data demonstrate how to create a model tool for finding best routes. Read this topic and look at its associated tutorial data if you plan to publish a routing model tool. If you plan to publish a service based on OD cost matrix or location-allocation, read this topic, but follow along using the provided tutorial data for these other solvers.