3Com Switch 4800G 24-Port Configuration Manual page 862

862 C 66: T
HAPTER RAFFIC
Priority
C , TP, LR C
LASSIFICATION AND
occurs, queue scheduling is performed on the packets; when congestion get
worse, congestion avoidance is performed on the packets.
The following describes several types of precedence:
1 IP precedence, ToS precedence, and DSCP precedence
Figure 256 DS field and ToS field
Bits: 0 1
DS-Field DSCP
(for IPv4,ToS
octet,and for
IPv6,Traffic
Class octet )
Class Selector
codepoints
Differentiated Services
Codepoint ( DSCP)
RFC 2474
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.
Table 65 Description on IP Precedence
IP Precedence (decimal)
0
1
2
3
4
5
6
7
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;
ONFIGURATION
2 3 4 5 6 7
CU
Currently
Unused
IP Precedence (binary)
000
001
010
011
100
101
110
111
Bits: 0 1 2 3 4 5
Preced Type of
IPv4 ToS
ence Service
byte
RFC 1349
RFC 1122
IP Type of Service (ToS)
RFC 791
Description
Routine
priority
immediate
flash
flash-override
critical
internet
network
6 7
M
B
Z
Must
Be
Zero