Classification & Marking

Classification and marking is the process of identifying, then grouping traffic. Traffic descriptors are methods of identifying traffic to be put into classes, the following are methods of traffic identification:-

Ingress (incoming interface)
Source/destination IP address
CoS (class of service) layer 2 marking on ISL/802.1p frame
IP Precedence/DSCP layer 3 marking on IP packet
MPLS EXP value on MPLS header
Application type

Common practise nowadays is to mark the packet using deep-inspection as close to the edge as possible, then as the packet traverses the network only the packet marking needs to be checked.