module: topology
  +--rw topology
     +--rw node* [node-id]
     |  +--rw node-id    string
     |  +--rw port* [port-id]
     |     +--rw port-id                string
     |     +--rw layer-protocol-name?   layer-protocol-name
     +--rw link* [link-id]
        +--rw link-id        string
        +--rw source-node?   -> /topology/node/node-id
        +--rw target-node?   -> /topology/node/node-id
        +--rw source-port?   -> /topology/node/port/port-id
        +--rw target-port?   -> /topology/node/port/port-id