Session Announcement Protocol (SAP) is an announcement protocol that is used by session directory clients to assist the advertisement of multicast multimedia conferences and other multicast sessions. It also is used to communicate the relevant session setup information to prospective participants.

How it Works

An SAP announcer periodically multicasts an announcement packet to a well-known multicast address and port. The announcement is multicast with the same scope as the session it is announcing, ensuring that the recipients of the announcement can also be potential recipients of the session the announcement describes (bandwidth and other such constraints permitting). This is also important for the scalability of the protocol, as it keeps local session announcements local.

A SAP listener learns of the multicast scopes it is within (for example, using the Multicast-Scope Zone Announcement Protocol) and listens on the well-known SAP address and port for those scopes. In this manner, it will eventually learn of all the sessions being announced, allowing those sessions to be joined.

Basic Usage

It is to be expected that sessions may be announced by a number of different mechanisms, not only SAP. For example, a session description may be placed on a web page, sent by email or conveyed in a session initiation protocol. To ease interoperability with these other mechanisms, application level security is employed, rather than using IPsec authentication headers.

The announcement is multicast with the same scope as the session it is announcing, ensuring that the recipients of the announcement can also be potential recipients of the session the announcement describes (bandwidth and other such constraints permitting). This is also important for the scalability of the protocol, as it keeps local session announcements local.

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