Hi, I realised today that I never included a definition for URL references in the topology definition. We have had a discussion about this in the NML group, and it was decided that this is a control-plan construct and would not be added to NML. So, for the NSI extension I am proposing to add an attribute to Network Object "isReference" which has a URI as value which points to a topology file that contains more information on that object. This attribute would be defined in the NSI extension schema. Now, if everyone agrees, I have a practical question: how do I express this in the XML Schema? The schemas are available here: https://github.com/jeroenh/NSI-Topology/tree/master/schemas Jeroen.