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Quality of service (QoS) is a concept generally existing in occasions with service 
supply and demand. It evaluates the ability to meet the need of the customers in 
service. Generally, the evaluation is not to grade precisely. Its purpose is to analyze 
the conditions where the service is the best and the conditions where the service 
still needs improvement and then to make improvements in the specified aspects.

In an internet, QoS evaluates the ability of the network to deliver packets. The 
evaluation on QoS can be based on different aspects because the network 
provides various services. Generally speaking, QoS is the evaluation on the service 
ability to support the core requirements such as delay, jitter, and packet loss ratio 
in the packet delivery.

In traditional IP networks, packets are treated equally. That is, the FIFO (first in first 
out) policy is adopted for packet processing. Network resources required for 
packet forwarding is determined by the order in which packets arrive. All the 
packets share the resources of the network. Network resources available to the 
packets completely depend on the time they arrive. This service policy is known as 
Best-effort, which delivers the packets to their destination with the best effort, 
with no assurance and guarantee for delivery delay, jitter, packet loss ratio, 
reliability, and so on.

The traditional Best-Effort service policy is only suitable for applications insensitive 
to bandwidth and delay, such as WWW, file transfer and E-mail.

With the expansion of computer network, more and more networks become part 
of the Internet. The Internet gains rapid development in terms of scale, coverage 
and user quantities. More and more users use the Internet as a platform for their 
services and for data transmission.

Besides the traditional applications such as WWW, E-mail, and FTP, new services 
are developed on the Internet, such as tele-education, telemedicine, video 
telephone, videoconference and Video-on-Demand (VoD). Enterprise users expect 
to connect their regional branches together using VPN techniques for coping with 
daily business, for instance, accessing databases or manage remote equipments 
through Telnet.

All these new applications have one thing in common, that is, they have special 
requirements for bandwidth, delay, and jitter. For instance, bandwidth, delay, and 
jitter are critical for videoconference and VoD. As for other applications, such as 
transaction processing and Telnet, although bandwidth is not as critical, a too long