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Summary

This chapter began by reviewing the QoS requirements of voice, video, and data applications.

Voice requires 150-ms one-way, end-to-end (mouth-to-ear) delay; 30 ms of one-way jitter; and no more than 1 percent packet loss. Voice should receive strict-priority servicing, and the amount of priority bandwidth assigned for it should take into account the VoIP codec; the packetization rate; IP, UDP, and RTP headers (compressed or not); and Layer 2 overhead. Additionally, provisioning QoS for IP Telephony requires that a minimal amount of guaranteed bandwidth be allocated to Call-Signaling traffic.

Video comes in two flavors: Interactive-Video and Streaming-Video. Interactive-Video has the same service-level requirements as VoIP because embedded within the video stream is a voice call. Streaming-Video has much laxer requirements because of a high amount of buffering that has been built into the applications.

Control plane requirements, such as provisioning moderate bandwidth guarantees for IP routing protocols and network-management protocols, should not be overlooked.

Data comes all shapes and sizes but generally can be classified into four main classes: Best-Effort (the default class), Bulk Data (noninteractive background flows), Transactional/Interactive (interactive, foreground flows), and Mission-Critical Data. Mission-Critical Data applications are locally defined, meaning that each organization must determine the select few Transactional Data applications that contribute the most significantly to its overall business objectives.

A less-than best-effort Scavenger class of traffic was introduced, and a strategy for using this class for DoS and worm mitigation was presented. Specifically, flows can be monitored at the campus Access-Edge, and out-of-profile flows can be marked down to the Scavenger marking (of DSCP CS1). To complement these policers, queues providing a less-than best-effort Scavenger service during periods of congestion are deployed in the LAN, WAN, and VPN.

The chapter concluded with a set of general best-practice principles relating to QoS planning, classification, marking, policing, queuing, and deployment. These best practices include the following:

  • Clearly defining the organization's business objectives to be addressed by QoS

  • Selecting an appropriate number of service classes to meet these business objectives

  • Soliciting executive endorsement, whenever possible, of the traffic classifications, especially when determining any mission-critical applications

  • Performing QoS functions in (Catalyst switch) hardware instead of (Cisco IOS router) software, whenever possible

  • Classifying traffic as close to the source as administratively feasible, preferably at Layer 3 with standards-based DSCP markings

  • Policing traffic as close to the source as possible, following standards-based rules (such as RFC 2597, "Assured Forwarding Markdown"), whenever possible

  • Provisioning at least one-quarter of a link to service Best-Effort traffic

  • Provisioning no more than one-third of a link to service real-time and strict-priority traffic

  • Provisioning a less-than best-effort Scavenger queue, which should be assigned as low of a bandwidth allocation as possible

  • Understanding and thoroughly testing desired QoS features in conjunction with features already enabled on the production network

  • Deploying end-to-end QoS in stages during scheduled network downtime, with a recommended rollback strategy

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