Nov 15, 2007

Police vs Shape

Policing can be applied to either the inbound or outbound direction, while shaping can be applied only 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 shaping ensure that traffic does not exceed a bandwidth limit, but each mechanism has different impacts on the traffic:

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

2. Shaping adds variable delay to traffic, possibly causing jitter. Shaping queues excess traffic by holding packets in a shaping queue.

Traffic shaping is used to shape the outbound traffic flow when the outbound traffic rate is higher than a configured 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.
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