Explanation/Reference:
The following steps describe the CoS process:
1. A logical interface has one or more classifiers of different types applied to it.
2. The classifier assigns the packet to a forwarding class and a loss priority
3. Each forwarding class is assigned to a queue
4. Input (and output) policers meter traffic and might change the forwarding class and loss priority if a traffic flow exceeds its service level.
5. The physical or logical interface has a scheduler map applied to it
6. The scheduler defines how traffic is treated in the output queue-for example, the transmit rate, buffer size, priority, and drop profile
7. The scheduler map assigns a scheduler to each forwarding class (
8. The drop-profile defines how aggressively to drop packets that are using a particular scheduler
9. The rewrite rule takes effect as the packet leaves a logical interface that has a rewrite rule configured.
The rewrite rule writes information to the packet (for example, EXP or DSCP bits) according to the forwarding class and loss priority of the packet.
References: http://www.juniper.net/documentation/en_US/junos16.1/topics/concept/packet-flow-cos- process-cos-config-guide.html