peer interactions between similar types of network controllers may be needed. A transport flow between two customers, regions, etc. may involve two separate network controller clusters depending on the outcome of the data sharding. The network controller on one side of the flow may communicate requests to its peer network controller on the other side to dynamically change the flow between the two domains. For example, a customer could run their own network controller with visibility of both the cam- pus and WAN network and send requests to the AT&T network controller to make changes to the flows on the WAN under the direction of the customer controller. 6.4.6 ABSTRACTION MODELING AT&T's service portfolio includes network connectivity services, which functionally encompasses L0 through L3 and L4 service solutions and geographically can span over Data-Center LANs, MANs and WAN, and complex L4+ services such as VoIP. To meet the service and network man- agement needs of this portfolio, the information modeling approach in the SDN architecture is based on a novel three-tier abstraction design in lieu of the traditional single-tier abstraction design commonly used for multivendor network management. The network service abstraction model describes the network service as presented to the customer application-independent of the particular network protocol architecture and technology solution chosen to implement the network service for a given customer service Software-Defined Networking instance. Individual service models can be concatenated ("chained together") to create a richer end-to-end chain of respective individual services. Valid service chains are pre- scribed by means of a service chain graph. The network service implementation abstraction model describes a network protocol archi- tecture and technology design solution used to implement the given network service- independent of vendor-specific platform used in the implementation. The network device abstraction model describes the set of functional networking bu