Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:IETF standardised VoIP is a combination of four predominate standards: RFC3261: Session Initiation Protocol (SIP) [18], RFC2327: Session Description Protocol (SDP) [12], RFC3550: Real-time Transport Protocol (RTP) [19], and later RFC3711: The Secure Real-time Transport Protocol (SRTP) [3].
Category: