SAP requires announcements to occur periodically, generally with an IP time-to-live (TTL) of 255. Announcements must be sent on Port 9875. To announce a multicast session, the session creator sends a multicast packet to a well-known multicast group using the Session Description Protocol (SDP) to describe the packets.Content Continues Below
Session Announcement Protocol is generally implemented on local area networksand requires multicast connection capabilities in Internet routers to advertise media streams over the Internet. (Multicasting is a standard feature of IPv6 but some IPv4 routers don't implement it. In those cases, the MBone, or multicast Internet backbone, can be used.)
SAP is defined in RFC 2475.