Under SAP, senders periodically broadcast SDP descriptions to a well-known multicast address and port. A SAP listening application can listen to the SAP multicasts and construct a guide of all advertised multicast sessions.
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 Announcements may also be explicitly withdrawn by the original issuer.
Authentication, encryption and compression
SAP features separate methods for authenticating and encrypting announcements. Use of encryption is not recommended. Authentication prevents unauthorized modification and other DoS attacks. Authentication is optional. Two authentication schemes are supported:
- Pretty Good Privacy as defined in RFC 2440
- Cryptographic Message Syntax as defined in RFC 5652
The message body may optionally be compressed using the zlib format as defined in RFC 1950.