UPC-BarcelonaTech Huawei This draft introduces the concept of a Network Digital Twin (NDT), including the architecture as well as the interfaces. Then two specific instances of the NDT are introduced, the first one for packet networks. This produces performance estimates (delay, jitter, loss) for a packet network with a specified topology, traffic demand, and routing and scheduling configuration. Second, a NDT for optical networks, this produces transmission performance estimates of an optical network with specified optical service topologies and network equipment types, topology and status.