Quality of Service
Quality of Service |
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On each host, traffic is marked and transmitted as priority QoS traffic by the process of packet classification and scheduling.
Packet classification provides a means by which packets generated by an application can be classified and subsequently prioritized before being sent across a network. The Generic Packet Classifier is the mechanism by which traffic control determines the flow for any packet, and therefore, the treatment that the packet receives. Once a packet has been classified as belonging to a particular flow, the QoS Packet Scheduler is able to give it treatment in accordance with that flow's parameters.
Packet scheduling is the means by which data (packet) transmission management (a key function of Quality of Service) is achieved. The QoS Packet Scheduler enforces QoS parameters for a particular flow. Traffic shaping (smoothing bursts and peaks in traffic to an even flow) relies on the packet classifier to assign packets into queues based upon their QoS parameters. The QoS Packet Scheduler retrieves the packets from the queues and transmits them according to the QoS parameters, which generally include a scheduled rate and some indication of priority. The scheduled rate is used to pace the transmission of packets to the network. The priority is used to determine the order in which packets need to be submitted to the network when congestion occurs. This smoothes bursts or peaks of traffic over a period of time, thereby effecting a steadier use of the network and maintaining resource integrity.
The QoS Packet Scheduler can be installed on any computer on which you want to have traffic control services. The QoS Packet Scheduler must be installed on all end-systems that make reservations on subnets where you are running an QoS Admission Control Service, such as any host that sends data to other hosts (for example, a multimedia server or inventory control server).
To ensure quality of service, packets must be marked in such a way that network devices along the data path can properly provide the required QoS, or at least attempt to if they are not RSVP-enabled. The QoS Packet Scheduler provides 802.1p marking while Differentiated Class of Service marking is done by TCP/IP. The QoS Packet Scheduler cannot be installed on Microsoft® Windows® 98–based computers; therefore, 802.1p functionality is not available on them. Diff-serv Code Point (DSCP) marking is available on Windows 98–based computers using the RSVP SP. Windows 2000 defines a default mapping to 802.1p and Differentiated Class of Services in the registry. This is discussed in more detail later in this chapter. Network devices can override this default mapping by inserting a special object into RSVP signaling messages. This is done at the determination of a QoS policy server that might map a user's traffic to a lower or higher priority than the default mapping. A policy server may insert a traffic class (T class) object to override the 802.1p class and a D class object to override the Diff-serv class.