System Administration Guide: IP Services

How Traffic Flows Through the IPQoS Modules

The next figure shows a path that incoming traffic might take through some of the IPQoS modules.

Figure 32–1 Traffic Flow Through the IPQoS Implementation of the Diffserv Model

The context follows the graphic, which is a flow diagram.

This figure illustrates a common traffic flow sequence on an IPQoS-enabled machine:

  1. The classifier selects from the packet stream all packets that match the filtering criteria in the system's QoS policy.

  2. The selected packets are then evaluated for the next action to be taken.

  3. The classifier sends to the marker any traffic that does not require flow control.

  4. Traffic to be flow-controlled is sent to the meter.

  5. The meter enforces the configured rate. Then, the meter assigns a traffic conformance value to the flow-controlled packets.

  6. The flow-controlled packets are then evaluated to determine if any packets require accounting.

  7. The meter sends to the marker any traffic that does not require flow accounting.

  8. The flow-accounting module gathers statistics on received packets. The module then sends the packets to the marker.

  9. The marker assigns a DS codepoint to the packet header. This DSCP indicates the per-hop behavior that a Diffserv-aware system must apply to the packet.