While a user would be willing to put up with a few additional seconds for a file transfer to complete, the same user would have less tolerance for similar delays when accessing a database or when running voice over an IP data network. QoS aims to ensure that your mission critical traffic has acceptable performance. In the real world where bandwidth is finite and diverse applications from videoconferencing to ERP database lookups must all vie for scarce resources, QoS becomes a vital tool to ensure that all applications can coexist and function at acceptable levels of performance. The aim of packet classification is to group packets based on predefined criteria so that the resulting groups of packets can then be subjected to specific packet treatments. The treatments might include faster forwarding by intermediate routers and switches or lesser probability of the packets being dropped due to lack of buffering resources.
Classification brings into question the right match criteria.
The simplest way of doing this is via the IP ToS field in the header of an IP datagram. The purpose behind this type of marking of packets is to ensure that downstream QoS features such as scheduling and queuing may accord the right treatment for packets thus marked. In some cases the service provider whose backbone is being used for the VPN might provide differentiated services, classification allows you to leverage these services. Once traffic has been classified the next step is to ensure that it receives special treatment in the routers. This brings into focus scheduling and queuing.