O S Configuration; Introduction To Qos - 3Com 4200G 12-Port Configuration Manual

27

Introduction to QoS

Traffic
Traffic Classification
Precedence
Q S C
O ONFIGURATION
QoS (Quality of Service) 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 when the service is the best and the conditions when the service still needs
improvement and then to make improvements in the specified aspects.
In 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, delay variation and packet loss ratio in the
packet delivery.
Traffic means service traffic, that is, all the packets passing the switch.
Traffic classification means to identify packets conforming to certain characters
according to certain rules.
A classification rule is a filter rule configured to meet your management requirements.
It can be very simple. For example, you can use a classification rule to identify traffic
with different priorities according to the ToS field in the IP packet header. It can be
very complicated too. For example, you can use a classification rule to identify the
packets according to the combination of link layer (Layer 2), network layer (Layer 3)
and transport layer (Layer 4) information including MAC addresses, IP protocols,
source addresses, destination addresses, the port numbers of applications and so on.
Classification is generally based on the information in the packet header and rarely
based on the packet content.
1 IP precedence, ToS precedence and DSCP precedence
Figure 64 DS fields and ToS bytes
The ToS field in an IP header contains 8 bits:
The first three bits indicate IP precedence in the range of 0 to 7.
■
Bit 3 to bit 6 indicate ToS precedence in the range of 0 to 15.
■