The Request/Response Header (RH) is one of three data fields of the Path Information Unit( PIU) in the Synchronous Data Link Control( SDLC). The 3- byte long RH data field is located between the Transmission Header( TH) and the Request/Response Unit( RU).
The RH data field identifies the data type of the subsequent Request Unit, it contains the relevant control information, also information about the data format and specifies the protocol of the corresponding session. The information in the Request Header is used exclusively by the Network Addressable Unit( NAU). Each request sent by a Network Addressable Unit (NAU) starts with the Request Header (RH).