862 CHAPTER 66: TRAFFIC CLASSIFICATION, TP, AND LR CONFIGURATION
occurs, queue scheduling is performed on the packets; when congestion get
worse, congestion avoidance is performed on the packets.
Priority The following describes several types of precedence:
1 IP precedence, ToS precedence, and DSCP precedence
Figure 256 DS field and ToS field
The ToS field in an IP header contains eight bits, which are described as follows:
■ 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.
■ RFC2474 re-defines the ToS field in the IP packet header, which is called the DS
field. The first six (bit 0 to bit 5) bits of the DS field indicate DSCP precedence in
the range of 0 to 63. The last two bits (bit 6 and bit 7) are reserved bits.
In a network providing differentiated services, traffics are grouped into the
following four classes, and packets are processed according to their DSCP values.
■ Expedited Forwarding (EF) class: In this class, packets can be forwarded
regardless of link share of other traffic. The class is suitable for preferential
services with low delay, low packet loss ratio, low jitter, and assured bandwidth
(such as virtual leased line);
■ Assured forwarding (AF) class: This class is further divided into four subclasses
(AF1/2/3/4) and a subclass is further divided into three drop priorities, so the AF
service level can be segmented. The QoS rank of the AF class is lower than that
of the EF class;
Table 65 Description on IP Precedence
IP Precedence (decimal) IP Precedence (binary) Description
0 000 Routine
1 001 priority
2 010 immediate
3 011 flash
4 100 flash-override
5 101 critical
6 110 internet
7 111 network
076
DSCP
M
B
Z
Class Selector
codepoints
Differentiated Services
Codepoint ( DSCP)
RFC 2474
Currently
Unused
RFC 1122
IP Type of Service (ToS)
RFC 791
Must
Be
Zero
RFC 1349
DS-Field
(for IPv4,ToS
octet,and for
IPv6,Traffic
Class octet )
IPv4 ToS
byte
15432Bits:
07615432Bits:
Preced
ence
Type of
Service
CU
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