How 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 module. The meter determines the rate at which traffic is released onto the network. For an introduction to the meter, 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 to identify customers and the type of service that is guaranteed to each customer.

   Because the SLA guarantees a certain level of service to a customer, 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 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.

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   Note –

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

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4. Refine your list of classes to be metered by determining which filters in the class select traffic that needs flow control.

   Classes that have more than one filter might require metering for only one filter. Suppose you define filters for incoming and outgoing traffic of a certain class. You might conclude that only traffic in one of the directions 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 organizational 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 sufficient to meter a particular class, you can define only the committed rate and 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 this action is to mark the packet header with a per-hop behavior, but not always. 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.

The next table shows meter entries for a class of email traffic. The network on which the IPQoS system is located has a total bandwidth of 100 Mbps, or 100000000 bits per second. The QoS policy assigns a low priority and best-effort forwarding behavior for the email class.

Table 2–5 Example QoS Policy With Meters Defined

Class	Priority	Filters	Selectors	Rate
email	8	mail_in	daddr 10.50.50.5 dport imap direction LOCAL_IN
		mail_out	saddr 10.50.50.5 sport imap direction LOCAL_OUT	meter=tokenmt committed rate=5000000  committed burst =5000000  peak rate =10000000  peak burst=1000000  green precedence=continue processing  yellow precedence=mark yellow PHB  red precedence=drop

Where to Go From Here

Task	For Information
Define forwarding behaviors for flows as they return to the network stream	How to Plan Forwarding Behavior
Plan for flow accounting of certain types of traffic	How to Plan for Flow Accounting
Add more classes to the QoS policy	How to Define the Classes for Your QoS Policy
Add more filters to the QoS policy	How to Define Filters in the QoS Policy
Define another flow-control scheme	How to Plan Flow Control
Create an IPQoS configuration file	How to Begin the IPQoS Configuration File and Define Traffic Classes