t Figure 3-23 SAP distributes session descriptions among potential users. Evening news Today, 8 pm to 9 pm 224.2.17.12 Session description SAP distributes elaborated by the sender the session description Chapter 3 chooses a retransmission rate for the announcement of its session. Hence, the more sessions that are present, the longer the interval becomes between retransmissions. Finally, SAP announcements can be encrypted and can make use of authentication mechanisms. Encryption and authentication provide the requisite level of privacy and check the identity of the creator of a particu- lar session. Session Descriptions Although SAP multicasts session descriptions to potential receivers, it does not define the format of those descriptions. Following the IETF way, various protocols besides SAP can be used to describe sessions. (For recommended format, consult the Session Description Protocol (SDP) [RFC 2327].) SAP announcements carry all description formats, but have no way to negotiate the protocol used for describing sessions. Therefore, when a system fails to understand an SAP announcement for some reason, it has no recourse. It cannot ask to receive the same announcement with a different Session Description Protocol inside. Instead, SDP serves as the common protocol for session description, and all applications must support SDP. Session Description Protocol (SDP) Session Description Protocol (SDP) [RFC 2327] specifies how the informa- tion necessary to describe a session should be encoded. SDP does not include any transport mechanism or any kind of parameter negotiation. An SDP description is simply a chunk of information that a system can use to join a multimedia session. It includes, for instance, IP addresses, port num- bers, and times and dates when the session is active. Turning again to the program guide analogy, a session description in TV context would look like the following: "Tune in channel 5 at nine o'clock this evening to see a soccer match," or "Turn on channel 2 at nine o'clock every evening to w