Stages of network QoS
 
|
|||
 | |||
|
|
|
|||
| |||
|
Classification: Recognizing packets moving across a network as part of an application or flow. This is done by inspecting the packets as they cross the network or by accepting "marks" for traffic that have already been classified. Classification is the most important part of quality of service and in some environments can be the most difficult step. As applications advance, classification becomes more difficult.
A classic example is H.323, used for videoconferencing, which must be examined all the way to the application layer to ensure it is properly classified.
Marking: Using information learned in the classification stage to tell other network equipment how to handle individual packets. This is usually done by inserting tags such as 802.1p, Differentiated Services Code Point or IP Precedence into the packet (see story "QoS defined," page 54).
In an end-to-end QoS environment, classification and marking are performed at the network edge, even though traffic may be remarked as it moves across the network.
Enforcement: Also known as policing. The information learned in the classification stage is used along with current network conditions, such as congestion level, to determine how to handle the packets. Policing can involve many techniques, including packet drop, various flavors of queuing, traffic shaping and TCP rate shaping. However, the heavy queuing of real-time traffic such as video or voice over IP can make things worse because of increased latency and jitter. Fortunately, some implementations let you control the queue depth. The purpose of enforcement is threefold -- to prioritize one type of traffic over another, to avoid congestion by preventing or delaying the transmission of low-priority traffic across a network connection, and to control the rate at which individual streams and hosts are transmitting.
RELATED LINKS
