3Com 7757 Configuration Manual page 658

658 C 61: Q S C
HAPTER O ONFIGURATION
Precedence
1 IP precedence, ToS precedence and differentiated services code point (DSCP)
precedence
Figure 170 DS fields and TOS bytes
Bits: 0 1 2
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 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.
■
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 first three bits in DSCP precedence are class selector
codepoints, bit 4 and bit 5 indicate drop precedence, and bit 6 is zero
indicating that the device sets the service class with the DS model.
The last two bits (bit 6 and bit 7) are reserved bits.
■
The precedence values of the IP packet indicate 8 different service classes.
Table 528 Description on IP Precedence
IP Precedence (decimal)
0
1
2
3
4
5
6
7
The Diff-Serv network defines four traffic classes:
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 variation 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
3 4 5 6 7
IPv4 ToS
CU
Currently
Unused
IP Precedence (binary)
000
001
010
011
100
101
110
111
Bits: 0 1 2 3 4 5
Preced Type of
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