Policing and shaping within QoS

Learn about the policing and shaping methods within QoS that allow IT professionals to smooth traffic flows for VoIP transmission.

As mentioned in a previous tip, policing and shaping are QoS components used to limit traffic flow. Policing drops or remarks traffic that exceeds limits, but shaping regulates the traffic back to a defined rate by delaying or queuing the traffic. Due to the special network performance requirements that are essential to transmitting voice traffic across a data infrastructure, policing and shaping techniques enforce rate compliance as traffic enters the network. Therefore, bursty, nonconforming traffic can be shaped into regular traffic patterns so that the delay-sensitive voice traffic streams can be prioritized and transmitted down a smooth, predictable medium. This tip provides the details of the policing and shaping methods available for today's networks in the world of Quality of Service (QoS).


Committed Access Rate (CAR)
CAR is a policing mechanism that controls the maximum transmission rate of traffic arriving or leaving an interface. Committed Access Rate additionally provides traffic policing on all traffic over an interface or on individual traffic flows over an interface. In order to be granular, CAR utilizes QoS classification parameters, such as IP Precedence values, DSCP values, or MAC address. Lower priority traffic typically will have lower transmission rate limits.

Class-Based Policing
Class Based Policing conforms to the Differentiated Services QoS model by policing traffic within classifications of traffic. It uses drop precedence bits based on the committed and the excess burst traffic rate limit. Traffic flows within a class are colored with a lower drop probability when traffic is below the committed rate, a higher drop probability when traffic is between the committed and excess rate, and the highest drop probability when traffic is above the excess rate. As with CAR, class-based policing utilizes QoS classification parameters, such as IP Precedence values, DSCP values, or MAC address.


More about QoS policing and shaping

VoIP marking schemes

QoS classification and marking for VoIP

QoS congestion avoidance techniques

QoS classification mapping techniques

Generic Traffic Shaping (GTS)
GTS restrains outbound traffic to a specified flow rate using a token bucket policy, which utilizes committed burst size (Bc), the timing interval (Tc), and the committed information rate (CIR) variables. GTS provides traffic shaping on all traffic over an outbound router interface and utilizes access lists to identify the traffic to be shaped.

Class-based Traffic Shaping
Class-based traffic shaping has the ability to apply Generic Traffic Shaping (GTS) to classifications of traffic using the appropriate access-lists, protocols, or inbound interfaces as "classifiers". In addition, class-based shaping allows for higher burst rates than GTS to potentially utilize more available bandwidth (peak rate shaping). The danger, however, is the likelihood of traffic being dropped during times of congestion. If the business applications have minimal tolerance for packet loss, it is recommended to use average rate shaping.

Distributed Traffic Shaping (DTS)
Distributed traffic shaping also has the ability to apply Generic Traffic Shaping (GTS) to classifications of traffic using the same classifiers as class-based shaping along with IP Precedence and DSCP values. Distributed shaping also supports the advantages and disadvantages of peak rate shaping. The two big differentiators of DTS are its ability to offload the shaping processing from the route switch to the versatile interface processor (VIP) and the ability to assign shaping parameters at the sub-interface level.

About the author: Richard Parsons (CCIE#5719) is a Manager of Professional Services for Callisma Inc., a wholly owned subsidiary of SBC. He has built a solid foundation in networking concepts, advanced troubleshooting, and monitoring in areas such as optical, ATM, VoIP, routed, routing, and storage infrastructures. Rich resides in Atlanta GA, and is a graduate of Clemson University. His background includes senior and principal consulting positions at International Network Services, Lucent, and Callisma.


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