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QoS overview
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, when bandwidth is fixed, more bandwidth for one traffic
flow means less bandwidth for the other traffic flows. When making a QoS scheme, 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
techniques.
QoS service models
Best-effort service model
The best-effort model is a single-service model and is also the simplest service model. In this service
model, the network does its best to deliver packets, but does not guarantee delay or reliability.
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.
IntServ model
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.
For more information about RSVP, see
MPLS Configuration Guide
.
DiffServ model
The differentiated service (DiffServ) model is a multiple-service model that can meet 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.
QoS techniques overview
The QoS techniques include traffic classification, traffic policing, traffic shaping, rate limit, congestion
management, and congestion avoidance. The following section briefly introduces these QoS
techniques.
