Multicast conferencing tools have been written and improved continuously by their developers over the past three or four years, and new tools are still appearing.
As described in chapter five, the media tools send media data inside RTP packets on one multicast address, and session message information inside RTCP packets on another multicast address. For example, the audio tool used by each conference participant sends audio data in small chunks of about 20 ms duration. Each chunk of audio data is preceded by an RTP header; the RTP header and data are in turn contained in a UDP packet. The RTP header indicates what type of audio encoding, such as PCM, ADPCM or GSM, is contained in each packet so that senders can change the encoding during a conference, for example, to accommodate a new participant or from indications of network congestion. Similarly, the session message data which uses the RTCP format is also contained in a UDP packet.
As all media data and session messages are sent within UDP packets there is no guarantee of packet delivery to the receivers. Due to the nature of busy packet-switched networks, some packets get seriously delayed and others are lost in transit. To accommodate this most media tools have a playout mechanism that uses and adaptive playout buffer [#!lws!#] in an attempt to hide these effects from the end user. In some circumstances the media tool cannot cope and the media is presented to the user with loss.
Next: Reliable Multicast Up: Recording Previous: Using IP Multicast Jon CROWCROFT