Packet marking in quality of service colours a packet by modifying a field within the packet or frame header with a traffic descriptor. By adding the traffic descriptor it allows other mechanisms within quality of service to identify and apply an action to that packet. (Shaping or queuing for example)
Packets are marked as they are received and processed internally. On the egress interface the packets are unmarked when leaving the router.
The following traffic descriptors can be used for marking traffic:
- Quality of Service Groups
- Layer 2
- 802.Q/p Class of Service (CoS)
- Layer 2.5
- MPLS Experimental bits
- Layer 3
- Differentiated Services Code Points (DSCP)
- IP Precedence (IPP)
The most common descriptors are on the layers 2 and 3.