Netis ST3326(ST-3302) [86/118] Introduction to qos features

Newly emerging applications demand higher service performance from IP networks. In addition

to simply delivering packets to their destinations, better network services are demanded, such as

allocating dedicated bandwidth, reducing packet loss ratio, avoiding congestion, regulating

network traffic, and setting priority of the packets. To meet those requirements, the network

should be provided with better service capability.

Traffic classification is the basis of all the above-mentioned traffic management technologies. It

identifies packets using certain rules and makes differentiated services possible. Traffic policing,

traffic shaping, congestion management, and congestion avoidance are methods for

implementing network traffic control and network resource management. They are occurrences

of differentiated services.

Introduction to QoS Features

Traffic Classification

Traffic here refers to service traffic; that is, all the packets passing the switch.

Traffic classification means identifying packets that conform to certain characteristics according

to certain rules. It is the foundation for providing differentiated services.

In traffic classification, the priority bit in the type of service (ToS) field in IP packet header can be

used to identify packets of different priorities. The network administrator can also define traffic

classification policies to identify packets by the combination of source address, destination

address, MAC address, IP protocol or the port number of an application. Normally, traffic

classification is done by checking the information carried in packet header. Packet payload is

rarely adopted for traffic classification. The identifying rule is unlimited in range. It can be a

quintuplet consisting of source address, source port number, protocol number, destination

address, and destination port number. It can also be simply a network segment.

Priority Trust Mode

Precedence types

1) IP precedence, ToS precedence, and DSCP precedence

The ToS field in an IP header contains eight bits numbered 0 through 7, among which,

 The first three bits indicate IP precedence in the range 0 to 7.

 Bit 3 to bit 6 indicate ToS precedence in the range of 0 to 15.

 In RFC2474, the ToS field in IP packet header is also known as DS field. The first six bits (bit 0

through bit 5) of the DS field indicate differentiated service codepoint (DSCP) in the range of

0 to 63,and the last two bits (bit 6 and bit 7) are reserved.

2) 802.1p priority

802.1p priority lies in Layer 2 packet headers and is applicable to occasions where the Layer 3

packet header does not need analysis but QoS must be assured at Layer 2.