This Recommendation defines an interface and associated protocol that introduces additional requirements on a DOCSIS CMTS and DOCSIS CM to support the configuration and transport of a class of service known as "Out-Of-Band (OOB) messaging" between a Set-top Controller (or application servers) and the customer premises equipment (CPE). In general, the CPE is intended to be a digital Set-top Device, but may include other CPE devices, suchas Residential Gateways or other electronic equipment. Figure 1-1 provides the context for this Recommendation in relation to the data-over-cable reference architecture and the other interface specifications in the DOCSIS Cable Modem series.