Policing vs Shaping

Traffic Rate

Traffic Rate

• Incoming and outgoing directions

POLICING

• Incoming and outgoing directions

• Out-of-profile packets are dropped

• Dropping causes TCP retransmits

• Supports packet marking or re-marking

• Less buffer usage (shaping requires an additional shaping queuing system)

Traffic Rate

Traffic Rate

-Time

SHAPING

-Time

SHAPING

• Outgoing direction only

• Out-of-profile packets are queued until a buffer gets full

• Buffering minimizes TCP retransmits

• Marking or re-marking not supported

• Shaping supports interaction with Frame Relay congestion indication

Shaping queues excess traffic by holding packets inside a shaping queue. Use traffic shaping to shape the outbound traffic flow when the outbound traffic rate is higher than a configured shape rate. Traffic shaping smoothes traffic by storing traffic above the configured rate in a shaping queue. Therefore, shaping increases buffer utilization on a router and causes unpredictable packet delays. Traffic shaping can also interact with a Frame Relay network, adapting to indications of Layer 2 congestion in the WAN. For example, if the backward explicit congestion notification (BECN) bit is received, the router can lower the rate limit to help reduce congestion in the Frame Relay network.

You can apply policing to either the inbound or outbound direction, while you can only apply shaping in the outbound direction. Policing drops nonconforming traffic instead of queuing the traffic like shaping. Policing also supports marking of traffic. Traffic policing is more efficient in terms of memory utilization than traffic shaping because no additional queuing of packets is needed.

Both traffic policing and traffic shaping ensure that traffic does not exceed a bandwidth limit, but each mechanism has different impacts on the traffic:

■ Policing drops packets more often, generally causing more retransmissions of connection-oriented protocols such as TCP.

■ Shaping adds variable delay to traffic, possibly causing jitter.

This topic describes how a token bucket can be used by network devices to measure traffic rates.

Single Token Bucket

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