Virtual Host Discovery Service (VHDS) is used to dynamically update the list of virtual hosts for
a given RouteConfiguration. If VHDS is configured a virtual host list update will be triggered
during the processing of an HTTP request if a route for the request cannot be resolved. The
:ref:`resource_names_subscribe <envoy_api_field_service.discovery.v3.DeltaDiscoveryRequest.resource_names_subscribe>`
field contains a list of virtual host names or aliases to track. The contents of an alias would
be the contents of a *host* or *authority* header used to make an http request. An xDS server
will match an alias to a virtual host based on the content of :ref:`domains'
<envoy_api_field_config.route.v3.VirtualHost.domains>` field. The *resource_names_unsubscribe* field
contains a list of virtual host names that have been :ref:`unsubscribed
<xds_protocol_unsubscribe>` from the routing table associated with the RouteConfiguration.