Service consumer sends specially crafted XML request over https connection and recieves responce from Plesk also in XML form.
RPC handler is located in https://<plesk_server_address>:8443/enterprise/control/agent.php
It accepts a POST of XML packets, authentification is provided by HTTP environment. 'HTTP_AUTH_PASSWD' http header should be set equal to Plesk administrator's password.
After the handler parses and processes the XML passed it will returns answer as valid XML packet. The XML schema, which may be used to verify the API-RPC input and output, provided by psa-api-rpc-protocol RPM and located (on UNIX version) at '/usr/local/psa/admin/plib/api-rpc/protocol'.