Quality of Service (QoS) Classification and Marking Notes

The first part of building a QoS policy is to identify the traffic that you need to treat preferentially (give better priority), or differentially.  This is accomplished via classification and marking.

• Classification – sorts packets into different traffic types that policies can then be applied to.
• Marking (or re-marking) – establishes a trust boundary on which scheduling tools later utilize.  The edge of the network where markings are either accepted or rejected is known as the trust-boundary.
• Classifier tools – Inspect one or more fields in a packet to identify the type of traffic that is being carried. After being identified, it is passed to the appropriate mechanism to handle that type of traffic class.
• Marking tools – actually write a field within the packet (or frame, cell, label) to preserve the classification decision.  By marking traffic at a trust boundary, subsequent nodes do not have to perform the same in-depth analysis to determine how to treat the packet.

Classification Tools

These tools can examine a number of criteria within layers 1, 2, 3, 4, and 7.

• L1 – Physical interface, subinterface, PVC, port
• L2 – MAC, 802.1Q/p CoS, VLAN, MPLS EXP, ATM Cell Loss Priority (CLP), Frame Relay DE
• L3 – IPP, DSCP, source/dest IP address
• L4 – TCP/UDP Ports
• L7 – Application signatures and URLs in packet headers or payload

Marking Tools

The primary marking tools used currently are class-based marking and marking done via class-based policing.  Legacy marking techniques include committed access rate (CAR) and policy-based routing (PBR).  Voice gateway packet marking is also an option for IPT applications.

• L2 Marking Fields – 802.1Q/p CoS, MPLS EXP, ATM CLP, Frame Relay DE
• L3 Marking Fields – IPP or DSCP

Cisco Catalyst switches perform scheduling based on L2 CoS, however DSCP is the preferred marking method for end-to-end QoS, because L2 marking is lost whenever the L2 media changes.  So it is important to ensure that L2 markings are translated to and from L3 markings consistently throughout the environment for end-to-end QoS.