Packet Precedences - HP V1910 Switch Series User Manual

To provide differentiated services, traffic classes must be associated with certain traffic control actions or
resource allocation actions. What traffic control actions to adopt depends on the current phase and the
resources of the network. For example, CAR is adopted to police packets when they enter the network;
GTS is performed on packets when they flow out of the node; queue scheduling is performed when
congestion happens; congestion avoidance measures are taken when the congestion deteriorates.

Packet precedences

This section introduces IP precedence, ToS precedence, differentiated services codepoint (DSCP) values,
and 802.1p precedence.
IP precedence, ToS precedence, and DSCP values
1.
Figure 389 DS field and ToS bytes
As shown in
represent IP precedence from 0 to 7; the subsequent four bits (3 to 6) represent a ToS value from 0 to 15.
According to RFC 2474, the ToS field of the IP header is redefined as the differentiated services (DS) field,
where a DSCP value is represented by the first six bits (0 to 5) and is in the range 0 to 63. The remaining
two bits (6 and 7) are reserved.
Table 153 Description on IP Precedence
IP Precedence (decimal)
0
1
2
3
4
5
6
7
In a network in the Diff-Serve model, traffic is grouped into the following four classes, and packets are
processed according to their DSCP values.
Expedited Forwarding (EF) class: In this class, packets are forwarded regardless of link share of

other traffic. The class is suitable for preferential services requiring low delay, low packet loss, low
jitter, and high bandwidth.
Assured forwarding (AF) class: This class is divided into four subclasses (AF 1 to AF 4), each

containing three drop priorities for more granular classification. The QoS level of the AF class is
lower than that of the EF class.
Figure 389, the ToS field of the IP header contains eight bits: the first three bits (0 to 2)
IP Precedence (binary)
000
001
010
011
100
101
110
111
416
Description
Routine
priority
immediate
flash
flash-override
critical
internet
network