Chapter 1 Qos Configuration; Qos Overview - Huawei Quidway S3100 Series Operation Manual

Operation Manual – QoS
Quidway S3100 Series Ethernet Switches

Chapter 1 QoS Configuration

1.1 QoS Overview

This part explains some of the terminologies and concepts common to QoS.
I. Stream
Stream refers to traffic, including all packets that pass through a switch.
II. Precedence
This section describes the IP precedence, Type of Service(TOS) precedence,
Differentiated Services Codepoint (DSCP) precedence, and 802.1p priority.
1) IP precedence, TOS precedence and DSCP precedence
Figure 1-1 DS field and ToS byte
As shown in Figure 1-1, the TOS field of an IP header contains eight bits, among which
the first three represent the IP precedence, ranging from 0 to 7. Bits 3 to 6 (a total of 4
bits) represent the TOS precedence, ranging from 0 to 15. The TOS field of an IP
packet header is redefined as Differentiated Services (DS) field in RFC2474, among
which the first six bits (bits 0 to 5) represent DSCP precedence, ranging from 0 to 63,
and bits 6 and 7 are reserved.
2) 802.1p priority
Embedded in the header of a Layer 2 packet, 802.1p priority is intended to ensure
Layer 2 QoS rather than analysis of a Layer3 header.
Figure 1-2 An Ethernet frame with an 802.1Q tag header
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Chapter 1 QoS Configuration