Carried by the first 3 bits in the Tag Control Information of 802.1Q frame header, which
contains the priority information of one of the 8 categories. These three bits are
generally called User Priority bits.
Carried by the first 3 bits of the TOS field for IPv4 packet header or Traffic Class field for
IPv6 packet header, called IPprecedence value; or carried by the first 6 bits of the
TOS field for IPv4 packet header or Traffic Class field for IPv6 packet header, called
Differentiated Services Code Point (DSCP) value.
In a DiffServ-compliant network, every device has the same transmission service policy for
the packets with the same classification information, and vice versa. The class information in
the packet can be assigned by all the systems along the way, such as hosts, devices, or
other network devices. It‘s based on a policy set by a manager, or contents in the packet, or
both. The assignment of class information in order to identify packets usually consumes
enormous resources of the network device. To reduce the processing overhead on the
backbone network, such assignment is often used on the network edge. Based on the class
information, the devices can provide different priorities for different traffic, or limit the amount
of resources allocated per traffic class, or appropriately discard the packets of less important,
or perform other operations. This behavior of these independent devices is called per-hop
behavior in the DiffServ architecture.
If all devices in the network are providing consistent per-hop behavior, this network forms the
end-to-end QoSsolution for the DiffServ architecture.
45.1.2 QOS Processing Flow
45.1.2.1 Classifying
The process of classifying involves putting the packets to the dataflow indicated with CoS
value according to the trust policy or the analysis of the packet contents. As a result, the core
task of classifying is to determine the CoS value of a packet. It happens when the port is
receiving the inbound packets. When a port is associated with a policy-map that represents
a QoS policy, the classification will take effect and be applied on all the packets input through
that port.
For general non-IP packets, the switch classifies the packets according to the following
criteria:
If the packet itself does not contain any QoS information, which means the layer-2
packet header has no User Priority bits, it gets the QoS information of the packet by
using the default CoS value of the packet input port. Like the User Priority bits of the
packet, the default CoS value of the port ranges 0~7.
If the packet itself contains QoS information, which means the layer-2 packet header
has User Priority bits, it gets the CoS information directly from the packet.
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