TheSession Announcement Protocol (SAP) is an experimentalprotocol for advertisingmulticast session information. SAP typically usesSession Description Protocol (SDP) as the format forReal-time Transport Protocol (RTP) session descriptions. Announcement data is sent usingIP multicast and theUser Datagram Protocol (UDP).
Under SAP, senders periodically transmit SDP descriptions to a well-knownmulticast address andport number (9875).[1] A listening application constructs a guide of all advertised multicast sessions.
SAP was published by theIETF as RFC 2974.[2]
The announcement interval is cooperatively modulated such that all SAP announcements in the multicast delivery scope, by default, consume 4000 bits per second. Regardless, the maximum announce interval is 300 seconds (5 minutes). Announcements automatically expire after 10 times the announcement interval or one hour, whichever is greater. Announcements may also be explicitly withdrawn by the original issuer.
SAP features separate methods forauthenticating andencrypting announcements. Use of encryption is not recommended. Authentication prevents unauthorized modification and other DoS attacks. Authentication is optional. Two authentication schemes are supported:
The message body may optionally becompressed using thezlib format as defined in RFC 1950.
VLC media player monitors SAP announcements and presents the user a list of available streams.[3]
SAP is one of the optional discovery and connection management techniques described in theAES67 audio-over-Ethernet interoperability standard.[4]