3Com 8807 Configuration Manual page 182

182 C 22: Q S C
HAPTER O ONFIGURATION
Step 2: Run filtering operation (deny or permit) to the identified traffic. By default,
permit operation is selected.
Traffic policing
QoS can police traffic at the ingress port, to provide better services with the limited
network resources.
Redirection
You can re-specify forwarding direction for packets, based on QoS policy.
Traffic priority
Ethernet switches can provide priority tags, including ToS, DSCP, 802.1p, and so
on, for specific packets. These priority tags are applicable to different QoS models.
The following describes IP priority, ToS priority, DCSP priority, Exp priority and
802.1p priority.
1 IP priority, ToS priority, DSCP priority and Exp priority
Figure 42 DS field and ToS byte
As shown in Figure 42, the ToS field in the IP header contains 8 bits. The first three
bits represent IP priority, in the range of 0 to 7; bits 3-6 stand for ToS priority, in the
range of 0 to 15. RFC2474 redefines the ToS field in IP packets as DS
(differentiated services) field. The first six bits denote DSCP (differentiated services
codepoint) priority, in the range of 0 to 63, the latter two bits are reserved. The
first three bits (bit 0~2) of DSCP priority represent Exp priority, in the range of 0 to
7.
2 802.1p priority
802.1p priority is stored in the header of Layer 2 packets and is suitable for the
case where only Layer 2 QoS guarantee, not L3 header analysis, is required.
Figure 43 Ethernet frame with 802.1Q tag header
In the above figure, each host supporting 802.1Q protocol adds a 4-byte 802.1Q
tag header after the source address in Ethernet header.