This Recommendation defines the signalling interworking between the Bearer Independent Call Control (BICC) or ISDN User Part (ISUP) protocols and Session Initiation Protocol (SIP) with its associated Session Description Protocol (SDP) at an Interworking Unit (IWU). ISUP is defined in accordance with ITU-T Recs Q.761 to Q.764 and BICC is defined in accordance with ITU-T Recs Q.1902.1 to Q.1902.4. SIP and SDP are defined by the IETF. The capabilities of SIP and SDP that are needed to interwork with BICC or ISUP are defined in Annex C.