Event ID 349 — Unicast Streaming
Applies To: Windows Server 2008 R2
You can configure Unicast Streaming plug-ins in Windows Media Services to enable the distribution of content using unicast streaming, the default method by which a Windows Media server delivers content. A unicast stream is a one-to-one connection between the server and a client, which means that each client receives a distinct stream and only those clients that request the stream receive it. It offers the benefits of interactivity between a player and server, easier setup, and multiple-bit-rate (MBR) streaming capability. However, the number of users that are able to receive unicast streams is limited by content bit rate and the speed of the server network. For more information, see Delivering content as a unicast stream.
|Product:||Windows Media Services|
|Message:||The number of negative acknowledgements (NACKs) received by the Windows Media server exceeded the NACK warning limit. The server received %1 NACK requests in %2 milliseconds. The server will stop receiving NACK requests for %3 milliseconds so that it can recover.|
Protect against denial-of-service attacks
Ordinarily, negative acknowledgement (NACK) requests occur when the server or network is overloaded and packets cannot be sent through the User Datagram Protocol (UDP) reliably. Clients must request that packets be resent. This NACK warning limit in Windows Media Services is set so that only a very high number of NACK requests, a condition that indicates a denial-of-service attack, cause this issue.
First, confirm that bandwidth bottleneck issues on the network are not causing the problem. For more information, see Bandwidth. If the network does not appear to be at fault, review the server log files to determine whether clients are instigating a denial-of-service attack by flooding the network with content requests so that the server cannot respond adequately to legitimate client requests for content. If your system is subjected to a denial-of-service attack, the log files can help you determine which clients are being used in the attack. For more information about the fields used in Windows Media server log files, see Logging Model for Windows Media Services.
Note: A streaming media network that has been correctly planned and configured will improve response time, data throughput, content availability, and reduce the data error rate. To estimate the server requirements that are necessary to ensure that your content can reach all your clients without delays or interruptions, see Capacity planning. To test the capacity of your Windows Media server, you can simulate client requests for unicast streams from the server by using Microsoft Windows Media Load Simulator.
To verify that the unicast stream can be delivered to clients, test the stream by using Windows Media Player:
- If you want to test the stream by using Windows Media Player on the computer that is running Windows Media Services, you must install Desktop Experience. For more information, see Installing Desktop Experience.
- On the Windows Media server, open Windows Media Services. To open Windows Media Services, click Start, point to Administrative Tools, and then click Windows Media Services.
- In the console tree, click the publishing point that hosts the stream that you want to test.
- In the details pane, click the Announce tab, and then, in Connect to a unicast stream, note the value of the URL that a client can use to access the content.
- Start Windows Media Player on a computer that can access the stream, and enter the URL that you noted in the previous step.
- Using the Player controls, test the control functionality of the content stream. Broadcast streams can use the Start and Stop commands. On-demand streams can use the Start, Stop, and Pause commands, and the Seek bar.
- Test all the available streaming protocols. A unicast stream will try to connect by using the MMS protocol, but will switch to the RTSP protocol if network conditions or the Player version requires it. The HTTP protocol is not active unless the WMS HTTP Server Control Protocol plug-in is enabled. For more information, see About data transfer protocols.
- Allow the stream to play for a representative period of time and check that the stream quality is sufficient for the type of content and the capabilities of the equipment.
Note: If some members of your expected audience will access the stream from outside your network firewall, your testing scenario should include that condition. For more information about the firewall configuration for Windows Media Services, see Firewall Information for Windows Media Services.
Note: To ensure that your content can reach all your clients without delays or interruptions, perform network load tests by using Microsoft Windows Media Load Simulator to determine the maximum capacity of your server, and then make the appropriate adjustments to the Limits properties in Windows Media Services that specify the Windows Media server performance boundaries. A streaming media network that has been correctly planned and configured will improve response time, data throughput, content availability, and reduce the data error rate. To estimate the server requirements that are necessary to ensure that your transmission does not exceed the capabilities of your server, network, or audience, see Capacity planning.