[BCP119]   Best Current Practice 119,
              <https://www.rfc-editor.org/info/bcp119>.
              At the time of writing, this BCP comprises the following:

              Johnston, A. and O. Levin, "Session Initiation Protocol
              (SIP) Call Control - Conferencing for User Agents",
              BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006,
              <https://www.rfc-editor.org/info/rfc4579>.