QoS (Quality of Service) means that the network can use various basic technologies to provide better service for the specified network communication. It is a security mechanism of the network and a technology to solve the problems of network delay and congestion. Under normal circumstances, if the network is only used for a specific application system and there is no time limit, it does not need QoS, such as Web applications or email settings. But it is necessary for critical applications and multimedia applications. When the network is overloaded or congested, QoS can ensure that important traffic is not delayed or discarded, and at the same time ensure the efficient operation of the network. QoS is described in RFC 3644.

Generally, QoS provides the following three service models:

Best effort service (best effort service mode)

Integrated service (integrated service)

Differentiated services (differentiated services)

1. Best effort service model is a single service model and the simplest one. For the best-effort service model, the network sends as many messages as possible. However, it does not provide any performance guarantees, such as time delay and reliability.

The best-effort service model is the default service model of the network, which is implemented by FIFO (first-in, first-out) queue. It is suitable for most network applications, such as FTP and email.

2.Int-serv service model Int-Serv is a comprehensive service model. This model uses Resource Reservation Protocol (RSVP), which runs on every device from source to destination to prevent it from consuming too many resources. The system can clearly distinguish and ensure the quality of service of each service flow, and provide the most fine-grained service quality distinction for the network.

However, Inter-Serv mode requires a large number of devices, and when the number of data streams in the network is large, the storage and processing capacity of devices will encounter great pressure. The inter-server model has poor scalability and is difficult to implement in the Internet core network.

3.Diff-serv service model Diff-Serv is a multi-service model, which can meet different QoS requirements. Unlike Int-Serv, it does not need to inform the network to reserve resources for each service. Differentiated service is simple to realize and has good expansibility.