3Com WX3000 operation manual Precedence, IP precedence, ToS precedence, and Dscp precedence

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.

Precedence

IP precedence, ToS precedence, and DSCP precedence

Figure 1-1DS field and ToS byte

Bits:

DS-Field

(for IPv4,ToS octet,and for IPv6,Traffic Class octet )

0	1	2	3	4	5	6	7
		DSCP				CU
Class Selector						Currently
codepoints						Unused

Bits: 0 1 2 3 4 5 6 7

IPv4 ToS	Preced	Type of	M
			B
	ence	Service
byte			Z

	RFC 1349	Must
RFC 1122		Be
		Zero

Differentiated Services

Codepoint (DSCP)

RFC 2474

IP Type of Service (ToS)

RFC 791

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

z

z

z

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.

Table 1-1Description 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

In a network providing differentiated services, traffics are grouped into the following four classes, and packets are processed according to their DSCP values.

z

z

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;

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