Designing QoS © 2004 Cisco Systems, Inc. All rights reserved. Identifying QoS Mechanisms ARCH v1.27-1

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Enterprise Network Requirements for QoS

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Network Reliability Problem Areas We need a way to manage problem areas on an application basis.

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Solution: QoS-Enabled Infrastructure Predicts response times for end-to-end network services Manages jitter-sensitive applications, such as audio and video playbacks Manages delay-sensitive traffic, such as real-time voice Controls loss in times of inevitable bursty congestion Sets traffic priorities across the network Supports dedicated bandwidth Avoids and manages network congestion

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IntServ Architecture Manages the traffic on a per-flow basis Provides customized services per traffic stream Results in greater network costs Severely limits network scalability

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v DiffServ Architecture Manages traffic on a type-of-traffic basis Provides a lower implementation cost Does not provide individual stream visibility Implemented through 6-bit DSCP field definitions DSCP field is in IP header in the CoS field

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v QoS Service Levels

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Classification Tools: Trust Boundaries A device is trusted if it correctly classifies packets. For scalability, implement classification as close to the endpoint as possible. The outermost trusted devices represent the trust boundary. 1 and 2 are optimal, 3 is acceptable (if the Building Access switch cannot perform classification). 123

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Classification and Marking

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Congestion Avoidance Identifies traffic to drop at a given queue depth Not designed for voice or video Only used with TCP traffic

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Congestion Management Congestion can occur at any point in the network where there are speed mismatches. CBWFQ is used to guarantee bandwidth to data applications. LLQ is used for the highest-priority traffic (voice/video).

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Traffic Conditioning (Policing and Shaping) Policers typically tag or drop traffic, depending on the mechanism, protocol, and severity of offense. Shaping is typically on egress ports and uses a token bucket mechanism and buffers excess traffic. Policing, historically in ATM, is on ingress ports and uses a leaky bucket mechanism.

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Signaling Provides signaling between QoS neighbors Coordinates traffic-handling techniques Supports end-to-end communication

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Link Efficiency Mechanisms LFI –Reduces serialization delay –Works as a Layer 2 mechanism –Used on links of less than 768 kbps –Creates additional CPU load cRTP –Compresses RTP, UDP, IP headers –Uses a 40-byte header down to 2 to 4 bytes –Increases CPU load –Enabled on both ends

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Summary of Key Cisco IOS Software QoS Categories and Features

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Summary QoS tools manage bandwidth and minimize loss, delay, and delay variation between enterprise sites and within the campus. Between sites, bandwidth availability is most pressing. Within the campus, buffer management issues dominate. The three QoS architectures used in IP networks are best-effort, IntServ, and DiffServ. Classification tools mark packets with a value used to determine how the packet should be treated throughout the network. Congestion avoidance techniques monitor network traffic loads to anticipate and reduce the impact of congestion at common network and internetwork bottlenecks before they pose a significant problem.

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Summary (Cont.) Congestion management features control congestion once it occurs. LLQ and CBWFQ are advanced mechanisms that manage traffic on networks with time- sensitive voice and video traffic. WRED prevents congestion before it occurs by randomly dropping packets within low-priority queues. Traffic is conditioned using traffic policing (class-based policing) to drop (or mark) packets or traffic shaping on outbound traffic (class-based policing) to buffer packets. Link efficiency mechanisms, such as cRPT and LFI, improve the way that traffic is transmitted across low-speed links.