In data communications, Quality of Service (QoS) is a network's ability to provide differentiated service
guarantees for diversified traffic in terms of bandwidth, delay, jitter, and drop rate.
Network resources are scarce. The contention for resources requires that QoS prioritize important traffic
flows over trivial ones. For example, in the case of fixed bandwidth, if a traffic flow gets more bandwidth,
the other traffic flows will get less bandwidth and may be affected. When making a QoS scheme, you
must consider the characteristics of various applications to balance the interests of diversified users and
to utilize network resources.
The following section describes some typical QoS service models and widely used, mature QoS
QoS service models
Best-effort service model
The best-effort model is a single-service model and also the simplest service model. In this service model,
the network does its best to deliver packets, but does not guarantee delivery or control delay.
The best-effort service model is the default model in the Internet and applies to most network applications.
It uses the first in first out (FIFO) queuing mechanism.
The integrated service (IntServ) model is a multiple-service model that can accommodate diverse QoS
requirements. This service model provides the most granularly differentiated QoS by identifying and
guaranteeing definite QoS for each data flow.
In the IntServ model, an application must request service from the network before it sends data. IntServ
signals the service request with the Resource Reservation Protocol (RSVP). All nodes receiving the request
reserve resources as requested and maintain state information for the application flow.
The IntServ model demands high storage and processing capabilities because it requires all nodes along
the transmission path to maintain resource state information for each flow. This model is suitable for
small-sized or edge networks, but not large-sized networks, for example, the core layer of the Internet,
where billions of flows are present.
The differentiated service (DiffServ) model is a multiple-service model that can satisfy diverse QoS
requirements. It is easy to implement and extend. DiffServ does not signal the network to reserve
resources before sending data, as IntServ does.
All QoS techniques in this document are based on the DiffServ model.