A method, template, protocol, apparatus, system, and computer software for implementing a binary protocol for session initiation in a wireless communications system. The method for creating, modifying or terminating an IP multimedia session among one or more binary session initiation protocol (B-SIP) entities includes providing a flexible template with at least one mandatory field and at least one optional field, wherein the flexible template has fixed and variable length fields, representing a message type with a template id, and maintaining session specific information in a cache. The message template for creating, modifying or terminating an IP multimedia session among one or more binary session initiation protocol (B-SIP) entities, includes at least one mandatory field and at least one optional field, at least one fixed length field and at least one variable length field, wherein each of said at least one variable length fields is preceded by a length field and each of said at least one optional fields is preceded by an identifier, wherein said at least one mandatory field precedes said at least one optional field and said at least one fixed length field precedes said at least one variable length field.