On Internet Multicast backbone (Mbone) a session directory tool is used to advertise multimedia conferences and communicate the conference addresses and conference tool-specific information required for participation and this is done by SDP. SDP describes multimedia sessions for the purpose of session announcement, session invitation and other forms of multimedia session initiation.[para] The SDP communicates the existence of a session and conveys sufficient information to enable participation in the session and such messages are sent by periodically multicasting an announcement packet to a well-known multicast address and port using SAP (session announcement protocol). The messages are sent in the form of UDP packets with a SAP header and a text payload which is the SDP session description.