NetCache support for live streams over multicast

About this section

This section describes NetCache support for multicast. You must explicitly configure a NetCache appliance for multicast. Unicast support, as described in NetCache support for live streams over unicast, is the NetCache default behavior.

NetCache support for multicast

NetCache supports both multicast in and multicast out. You can configure the same NetCache appliance for both multicast in and multicast out, or for only one of these settings. The following table describes multicast in and multicast out.

Support for...	Description
multicast in (or input)	NetCache receives a live stream through the multicast transport from a streaming server that is configured to support multicast streaming.
multicast out (or output)	NetCache sends the live stream to clients through the multicast transport, regardless of how the stream was received from the streaming server. NetCache uses multicast when the next device on the hop to the client supports multicast and the client requesting multicast transmission supports multicast. A single NetCache appliance might, therefore, send some content to a client network over unicast and some content over multicast, depending on the devices on the network and the transmission requested by clients.

Support for...

Description

multicast in (or input)

NetCache receives a live stream through the multicast transport from a streaming server that is configured to support multicast streaming.

multicast out (or output)

NetCache sends the live stream to clients through the multicast transport, regardless of how the stream was received from the streaming server.

NetCache uses multicast when the next device on the hop to the client supports multicast and the client requesting multicast transmission supports multicast. A single NetCache appliance might, therefore, send some content to a client network over unicast and some content over multicast, depending on the devices on the network and the transmission requested by clients.

A stream does not have to travel over multicast the entire distance from the origin server to the client. NetCache can

Receive a stream from the server over unicast (default behavior) and send the stream to the client network over multicast
In contrast, multicast-enabled routers cannot rout unicast transmissions.
Receive a stream from the server over multicast and send the stream to the client network over unicast
For RealPlayer connections, receive a stream from the server over multicast and send it to the client network over multicast

NetCache behavior depends on how you have configured the appliance and whether origin servers, client networks, and clients support multicast.

The following illustration shows a simplified concept of stream transmission with multicast in a deployment with a streaming media cache.

Transmission from the media server to the NetCache appliance

If the streaming media server sends the content over multicast, a NetCache appliance configured for multicast input joins the multicast channel to receive the stream. By default, a NetCache appliance can receive unicast transmissions.

Transmission from the NetCache appliance to clients

If the NetCache appliance is configured for multicast output, NetCache sends one copy of the stream to each multicast-enabled network on which the clients reside. Each router on the network that listens for multicast transmissions determines whether to split the copy or to pass the single copy further on the network. Therefore, multicast-enabled routers are responsible for splitting at appropriate points in the routing tree, not the NetCache appliance. Clients configured for multicast also listen on the multicast channel.

As the previous illustration shows, the multicast router sends only one copy of the data to the network of multicast-enabled clients. However, the multicast router must send a separate copy of the data to each client whose media player is not set for multicast and to each network that is not multicast-enabled.

If the network on which the clients reside does not support multicast, NetCache splits the stream, as described in NetCache support for live streams over unicast.

Type of multicast that NetCache supports

NetCache supports the types of multicast shown in the following table.

For..	NetCache supports...
RealNetworks multicast	Back-channel multicast. Back-channel multicast uses a control data connection between the server and the client in order to track information, such as client setup and client logging. NetCache does not support scalable multicast for RealNetworks.
QuickTime multicast	Back-channel multicast. NetCache does not support scalable multicast for QuickTime.
MMS multicast	Scalable multicast. Scalable multicast does not use a control connection. Media player clients receive streaming parameters through a nonstreaming protocol, such as HTTP.

For..

NetCache supports...

RealNetworks multicast

Back-channel multicast.

Back-channel multicast uses a control data connection between the server and the client in order to track information, such as client setup and client logging.

NetCache does not support scalable multicast for RealNetworks.

QuickTime multicast

Back-channel multicast.

NetCache does not support scalable multicast for QuickTime.

MMS multicast

Scalable multicast.

Scalable multicast does not use a control connection. Media player clients receive streaming parameters through a nonstreaming protocol, such as HTTP.