The session is not authenticated

I am trying to communicate with vcenter server using webservices SDK.

I send Login XML and is successful.

I tried RetrieveServiceContent method and is successful and able to get all details.

I tried FetchDvPorts method and is resulting in error (The session is not authenticated).

<soapenv:Envelope xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/" xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

   <soapenv:Body>

      <soapenv:Fault>

         <faultcode>ServerFaultCode</faultcode>

         <faultstring>The session is not authenticated.</faultstring>

         <detail>

            <NotAuthenticatedFault xsi:type="NotAuthenticated" xmlns="urn:vim25">

               <object type="ManagedEntity">DistributedVirtualSwitch</object>

               <privilegeId>System.View</privilegeId>

            </NotAuthenticatedFault>

         </detail>

      </soapenv:Fault>

   </soapenv:Body>

</soapenv:Envelope>

I guess this error is due to privilege difference.(RetrieveServiceContent  privilege is "System.Anonymous"

  and privilege for FetchDvPorts is "System.Read" System.Anonymous )

What should I do to get complete privilege to access all methods successfully.