Frame Relay Traffic Policing and Shaping

Policing and shaping are two types of traffic regulation used in frame relay. In this penultimate segment from Cisco Press' Network Consultants Handbook, you'll learn about the rate-limiting and the other methods used to ensure proper traffic flow.

By Cisco Press | Posted Feb 11, 2002
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Network Consultants Handbook - Frame Relay
by Matthew Castelli

Network Consultants Handbook -- Click here to go to publisher's site

Cisco IOS QoS offers two types of traffic regulation mechanisms: policing and shaping.

The rate-limiting features of committed access rate (CAR) and the traffic policing feature provide the functionality for policing traffic.

The features of GTS, class-based shaping, DTS, and FRTS provide the functionality for shaping traffic.

These features can be deployed throughout the network to ensure that a frame, packet, or other data source adheres to a stipulated contract. These features can also be used to determine the QoS with which to render the packet. Both policing and shaping mechanisms use the traffic descriptor for a packet -- indicated by the classification of the packet -- to ensure adherence and service. Traffic policers and shapers identify traffic descriptor violations in an identical manner. These policers and shapers differ in how they respond to violations, for example:

  • A policer drops traffic. For example, the CAR rate-limiting policer will either drop the packet or rewrite its IP precedence, resetting the type of service bits in the packet header.
  • A shaper delays excess traffic using a buffer, or queuing mechanism, to hold packets and shape the flow when the data rate of the source is higher than expected. For example, GTS and class-based shaping use a weighted fair queue to delay packets to shape the flow. DTS and FRTS use either a priority queue, a custom queue, or a FIFO queue for the same, depending on how the queue is configured.

Traffic shaping and policing can work in tandem. For example, a good traffic-shaping scheme should make it easy for nodes that are inside the network to detect misbehaving flows. This activity is sometimes called "policing the traffic of the flow."

Because policing and shaping each use the token bucket mechanism, token buckets will be discussed in the next section.

Token Bucket
A token bucket is a formal definition of a rate of transfer with three components: burst size, mean rate, and a time interval (TC). The mean rate is generally represented as bits per second, and any two values can be derived from the third, as shown by the following formula:

  mean rate = burst size / time interval
where:
  • Mean rate -- Also called the CIR. The mean rate specifies how much data can be sent or forwarded per unit time on average.
  • Burst size -- Also called the committed burst (Bc) size. The burst size specifies in bits (or bytes) per burst how much traffic can be sent within a given unit of time without creating scheduling concerns. For a shaper, such as GTS, burst size specifies bits per burst; for a policer, such as CAR, burst size specifies bytes per burst.
  • Time interval (TC) -- Also called the measurement interval. The time interval specifies the time in seconds per burst. By definition, over any integral multiple of the interval, the bit rate of the interface will not exceed the mean rate. The bit rate, however, might be arbitrarily fast within the interval.

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