System Administration Guide: IP Services

ProcedureHow to Plan Flow Control

Flow control involves measuring traffic flow for a class and then releasing packets onto the network at a defined rate. When you plan flow control, you define parameters to be used by the IPQoS metering modules. The meters determine the rate at which traffic is released onto the network. For an introduction to the metering modules, see Meter (tokenmt and tswtclmt) Overview.

The next procedure assumes that you have defined filters and selectors, as described in How to Define Filters in the QoS Policy.

  1. Determine the maximum bandwidth for your network.

  2. Review any SLAs that are supported on your network. Identify customers and the type of service that is guaranteed to each customer.

    To guarantee a certain level of service, you might need to meter certain traffic classes that are generated by the customer.

  3. Review the list of classes that you created in How to Define the Classes for Your QoS Policy.

    Determine if any classes other than those classes that are associated with SLAs need to be metered.

    Suppose the IPQoS system runs an application that generates a high level of traffic. After you classify the application's traffic, meter the flows to control the rate at which the packets of the flow return to the network.

    Note –

    Not all classes need to be metered. Remember this guideline as you review your list of classes.

  4. Determine which filters in each class select traffic that needs flow control. Then, refine your list of classes that require metering.

    Classes that have more than one filter might require metering for only one filter. Suppose that you define filters for incoming and outgoing traffic of a certain class. You might conclude that only traffic in one direction requires flow control.

  5. Choose a meter module for each class to be flow controlled.

    Add the module name to the meter column in your QoS planning table.

  6. Add the rates for each class to be metered to the organizational table.

    If you use the tokenmt module, you need to define the following rates in bits per second:

    • Committed rate

    • Peak rate

    If these rates are sufficient to meter a particular class, you can define only the committed rate and the committed burst for tokenmt.

    If needed, you can also define the following rates:

    • Committed burst

    • Peak burst

    For a complete definition of tokenmt rates, refer to Configuring tokenmt as a Two-Rate Meter. You can also find more detailed information in the tokenmt(7ipp) man page.

    If you use the tswtclmt module, you need to define the following rates in bits per second.

    • Committed rate

    • Peak rate

    You can also define the window size in milliseconds. These rates are defined in tswtclmt Metering Module and in the twstclmt(7ipp) man page.

  7. Add traffic conformance outcomes for the metered traffic.

    The outcomes for both metering modules are green, red, and yellow. Add to your QoS organizational table the traffic conformance outcomes that apply to the rates you define. Outcomes for the meters are fully explained in Meter Module.

    You need to determine what action should be taken on traffic that conforms, or does not conform, to the committed rate. Often, but not always, this action is to mark the packet header with a per-hop behavior. One acceptable action for green-level traffic could be to continue processing while traffic flows do not exceed the committed rate. Another action could be to drop packets of the class if flows exceed peak rate.

Example 33–2 Defining Meters

The next table is an example that shows meter entries for a class of email traffic. The network on which the IPQoS system is located has a total bandwidth of 100 Mbits/sec, or 10000000 bits per second. The QoS policy assigns a low priority to the email class. This class also receives best-effort forwarding behavior.









dport imap

direction LOCAL_IN





sport imap

direction LOCAL_OUT


committed rate=5000000 

committed burst =5000000 

peak rate =10000000 

peak burst=1000000 

green precedence=continue processing 

yellow precedence=mark yellow PHB 

red precedence=drop 

See Also