|  Chapter Goals This Chapter discusses how you can guarantee content delivery by using the following Quality of Service (QoS) technologies: |
Cisco Quality of Service (QoS) ensures that routers give appropriate network services to real-time applications that are sensitive to delay, jitter, packet loss, and bandwidth. Your applications may be sensitive to any one of the following network-related constraints: Packet Delay Congestion causes delay due to an overload of traffic on the network. Routers that store packets in queues for excess durations prior to forwarding them to the network may cause packets to delay. Jitter Jitter refers to a variation in packet delay, resulting in differing packet inter-arrival times or out-of-sequence packets or both. Packet Loss Packet loss results from network errors due to congestion or other faults. You learned in Chapter 2, "Exploring the Network Layers," how TCP hosts automatically detect and retransmit lost packets. Note Delay and jitter may cause out-of-sequence packets, which are a major problem when UDP is used as the transport. Applications that rely on TCP for transport deal with out-of-order packet delivery by buffering and reordering TCP segments as they arrive. TCP also can deal with packet loss by resending missing packets.
Bandwidth Applications often require fixed bandwidth in order to deliver content, such as streaming audio-video.
The quality of real-time applications invariably suffers from delay, jitter, packet loss, and insufficient bandwidth. The basis for QoS stems from the need to address these common network issues. QoS is divided into the following components: Classification and Marking Packet forwarding priorities for different groups of applications. Congestion Management Based on packet forwarding priorities, switch packets with various queuing mechanism and apply shaping or policing. Congestion management also dictates the type of traffic that can be delayed. This traffic can either withstand some delay or is the least important type of traffic, making it acceptable for dropping. Congestion Avoidance Based on class, you can configure the router to randomly drop TCP segments periodically before congestion occurs. You can also configure bandwidth shaping and policing to ensure that your bulky applications do not flood your links. Additionally, Policy Signaling can be used to allocate bandwidth and QoS for traffic flows prior to sending the application traffic.
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