Lookup and Control

Somewhere in the mbone, there is a mixer (or several), known to session directories servers around the place. Ideally these mixers will be situated close to the point where the ISDN/Unicast feed hits the Mbone. [In RTP, this "mixer" is known as a translator].

A Session Directory Server is a daemon that listens to SDP announcements and caches them. On demand, it provides the latest list to clients that query it.

When a client queries an SDS , the sessions reported include the mixer to use for the session with a unicast session address, but the media all have multicast address. The choice of mixer is dependent on the locations of the SDS and the client.

A unicast SDP client program runs on a unicast host at a site 1 or more hops removed from the mbone (i.e. one incapable of using multicast IP). On demand, the SDP client contacts an SDS to get the current list of sessions, by making a TCP call to a well known port and formulating an HTTP GET request. The returned type contains the list of all sessions. I.e. the SLP is based on HTTP over TCP.

A user then starts the media associated with a particular session by clicking on that entry in the SDP (modified) client. The modified client then sends a remote "join" request to the mixer address (either configured, or retrieved from the SLP lookup), and starts the media application on the unicast address+port of their choice. The RLJMP join message contains an SDP session description, but with added media fields ("mix-to") with the media tool's unicast address and port. RLJMP messages are text based SDP v2 messages sent in a TCP connection to the mixer on a well known port and preceded by a line containing the RLJMP command ("JOIN" or "LEAVE").

The mixer joins the group, and adds the maplet from this group to the "mix-to" site to its forwarding table. The session directory client on the monitors the receiver program, and when it exits, sends a RLJMP leave message to the mixer.