The previous section describes how NetCache makes a single connection to the streaming server for each unique live stream. This section describes the interaction between a NetCache appliance and clients when NetCache uses unicast transmission (TCP, UDP, or HTTP).
The behavior described in this section is the NetCache default behavior. NetCache support for multicast, which is described in NetCache support for live streams over multicast, must be explicitly configured on a NetCache appliance.
Unicast transmission is the NetCache default behavior. NetCache uses unicast transmission to send content to the client network under the following circumstances:
Even if you configure a NetCache appliance for multicast, if a device or network to which NetCache is to transmit the content does not support multicast, NetCache uses unicast to transmit the data to the client network. A single NetCache appliance might, therefore, send some content to the client network over unicast and send some content over multicast, depending on the devices on the network and the transmission requested by clients.
Even if NetCache sends content to clients by using unicast transmission, NetCache helps you conserve bandwidth for live streams. The reason is that NetCache conserves upstream bandwidth because it makes only a single connection to the origin server on behalf of multiple clients that request the same stream.
In the unicast streaming model, NetCache receives a single copy of the stream from the server and splits it when sending it out to clients. Therefore, NetCache conserves downstream bandwidth by replicating the stream to multiple downstream clients.
A streaming media cache handles the first request for a live media stream differently than it handles subsequent requests for the same media stream.
The following illustration shows what happens when the first client makes a request for a particular stream.
The streaming media cache intercepts the streaming request sent by the client. Because the streaming media cache is not already delivering the stream to another client, the cache connects to the streaming server to obtain the requested stream.
The following illustration shows the path for additional requests for the same unique stream that Client 1 requested.
The flow of requests and streams in the preceding illustration is as follows:
If Client 2 or 3 had requested a different unique stream than Client 1 requested, the streaming media cache would have sent that unique request to the streaming server, as it did for Client 1.
At first glance, the streaming media cache-client model resembles the server-client model illustrated in High bandwidth use with streaming media. However, in the streaming media cache-client model in the previous illustration, most of the streaming activity takes place between the client and the cache, which is closer to the clients than the streaming server. The cache connects to the streaming server only once for each unique stream, thereby conserving bandwidth. Additionally, when the stream is served from a source close to the client, quality is higher because less packet loss occurs, and packet retransmissions occur quickly because client-to-cache latency is low.
NetCache supports TCP, UDP and HTTP for unicast transmission.
NetCache supports both TCP and UDP from the client to the streaming media cache. Many streaming media administrators prefer to use UDP because it is the most efficient and provides the best stream quality. The use of UDP depends on whether the client media player is configured to use UDP and whether the firewall is configured to allow UDP ports. See Considerations for firewalls and streaming media service for more information.
Although HTTP is not usually associated with streaming media, HTTP can be used as a streaming media transport. HTTP use for streaming media is necessary in circumstances such as the following:
In these cases, NetCache uses HTTP encapsulation; that is, NetCache tunnels a streaming media protocol over HTTP. NetCache can use HTTP encapsulation in the following ways:
When the client tries to contact the streaming server using HTTP, NetCache can fetch the content from the server over either TCP or HTTP.
NetCache can use HTTP to retrieve streaming content from the server under the following circumstances:
NetCache performs server-side HTTP encapsulation for Windows Media, RealNetworks, and QuickTime clients.
Note
If you have not installed a streaming license or if streaming is disabled, NetCache will proxy streaming traffic over HTTP. NetCache does not cache or split HTTP-proxied streaming traffic.
The HTTP encapsulation protocols for Windows Media, RealSystem, and QuickTime require that multiple HTTP connections from the client are directed to the same server (or proxy-cache server). Therefore, you must configure your L4 or L7 switch's load balancing algorithm accordingly. A hash based on client IP address, server IP address, or both works but round-robin or least connections does not.
NetCache supports clients that request a change from one unique stream to another during a specific streaming media presentation, for example, a request for an upgrade from 28 kbps to 56 kbps. NetCache intercepts a request for a stream upgrade. If the streaming media cache is already delivering that upgrade stream to a client, the cache splits the stream to resolve the upgrade request. The streaming media cache does not need to open a new connection to the streaming server if the streaming media cache is already delivering the stream.
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Part No. 210-00372
File: media9.htm
Last updated: 11/13/02 18:11:36