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© 2006 Cisco Systems, Inc. All rights reserved. Module 4: Implement the DiffServ QoS Model Lesson 4.7: Introducing Traffic Policing and Shaping
© 2006 Cisco Systems, Inc. All rights reserved. Objectives Explain the purpose and function of traffic conditioning methods: policing and shaping. Compare and contrast traffic policing and traffic shaping. Give examples of how policing and shaping are used in typical network topologies. Describe the purpose and function of a token bucket.
© 2006 Cisco Systems, Inc. All rights reserved. Traffic Conditioners Policing Limits bandwidth by discarding traffic. Can re-mark excess traffic and attempt to send. Should be used on higher-speed interfaces. Can be applied inbound or outbound. Shaping Limits excess traffic by buffering. Buffering can lead to a delay. Recommended for slower-speed interfaces. Cannot re-mark traffic. Can only be applied in the outbound direction.
© 2006 Cisco Systems, Inc. All rights reserved. Traffic Policing and Shaping Overview These mechanisms must classify packets before policing or shaping the traffic rate. Traffic policing typically drops or marks excess traffic to stay within a traffic rate limit. Traffic shaping queues excess packets to stay within the desired traffic rate.
© 2006 Cisco Systems, Inc. All rights reserved. Why Use Policing?Why Use Shaping? To limit access to resources when high-speed access is used but not desired (subrate access) To limit the traffic rate of certain applications or traffic classes To mark down (recolor) exceeding traffic at Layer 2 or Layer 3 To prevent and manage congestion in ATM, Frame Relay, and Metro Ethernet networks, where asymmetric bandwidths are used along the traffic path To regulate the sending traffic rate to match the subscribed (committed) rate in ATM, Frame Relay, or Metro Ethernet networks To implement shaping at the network edge
© 2006 Cisco Systems, Inc. All rights reserved. Policing Versus Shaping Incoming and outgoing directions. Out-of-profile packets are dropped. Dropping causes TCP retransmits. Policing supports packet marking or re-marking. 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.
© 2006 Cisco Systems, Inc. All rights reserved. Traffic Policing Example Do not rate-limit traffic from mission-critical server. Rate-limit file-sharing application traffic to 56 kbps.
© 2006 Cisco Systems, Inc. All rights reserved. Traffic Policing and Shaping Example Central to remote site speed mismatch Remote to central site oversubscription Both situations result in buffering and in delayed or dropped packets.
© 2006 Cisco Systems, Inc. All rights reserved. Token Bucket Mathematical model used by routers and switches to regulate traffic flow. Tokens represent permission to send a number of bits into the network. Tokens are put into the bucket at a certain rate by IOS. Token bucket holds tokens. Tokens are removed from the bucket when packets are forwarded. If there are not enough tokens in the bucket to send the packet, traffic conditioning is invoked (shaping or policing).
© 2006 Cisco Systems, Inc. All rights reserved. Single Token Bucket If sufficient tokens are available (conform action): Tokens equivalent to the packet size are removed from the bucket. The packet is transmitted.
© 2006 Cisco Systems, Inc. All rights reserved. Single Token Bucket Exceed Action If sufficient tokens are not available (exceed action): Drop (or mark) the packet.
© 2006 Cisco Systems, Inc. All rights reserved. Single Token Bucket Class-Based Policing Bc is normal burst size. Tc is the time interval. CIR is the committed information rate. CIR = Bc / Tc
© 2006 Cisco Systems, Inc. All rights reserved. Cisco IOS Traffic-Policing Mechanism Class-Based Policing Enable methodEnabled in policy map Conditions Actions Conform, exceed, violate Drop, set, transmit Implementations Single or dual token bucket, single- or dual- rate policing, multiactions
© 2006 Cisco Systems, Inc. All rights reserved. Cisco IOS Traffic-Shaping Mechanisms Class-Based ShapingFRTS Restriction Shaper for any subinterface Shaper for Frame Relay only ClassificationClass-based Per DLCI or subinterface Link fragmentation and interleaving No support for FRF.12 Supports FRF.12 Frame Relay Support Understands BECN and FECN ConfigurationSupported via MQC
© 2006 Cisco Systems, Inc. All rights reserved. Applying Rate Limiting
© 2006 Cisco Systems, Inc. All rights reserved. Self Check 1.Which traffic conditioning mechanism adds delay? Why? 2.What are the recommendations for conditioning mechanisms with regard to link speed? 3.Which mechanism has the ability to re-mark packets that exceed the identified rate? 4.Does conform and exceed indicate with regard to the token bucket?
© 2006 Cisco Systems, Inc. All rights reserved. Summary Traffic conditioners are QoS mechanisms that limit bandwidth, and include policing and shaping. Both of these approaches limit bandwidth, but each has different characteristics. Policing typically limits bandwidth by discarding traffic that exceeds a specified rate. Shaping limits excess traffic, not by dropping it but by buffering it. The token bucket is used to determine if traffic flow exceeds or conforms to predetermined rates.
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