Network QoS
This overview discusses end-to-end Quality of Service (QoS) in networking, emphasizing its dependence on per-hop QoS across various domains. Essential requirements include transmission and processing delays, as well as issues related to router congestion and packet loss. The classification, queuing, and scheduling processes are crucial for managing traffic effectively, while traffic shaping and policing methodologies maintain network performance. We also explore the edge-and-core model, balancing traffic classes, and technical challenges in achieving optimal QoS.
Network QoS
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Presentation Transcript
End-to-end QoS • The end-to-end QoS is dependent on the QoS provisioned by each of the domains • Requirements of end-to-end QoS: • Per-hop QoS • Routing and traffic engineering • Signaling and provisioning
Per-Hop Behavior • Each router is the smallest controllable convergence and divergence point for numerous unrelated flows of packets • Routers have buffer to handle the transient high rate of inflow of packets • End to end latency = transmission delay + processing delay at the routers • Delay introduced due to the router’s congestion-induced buffering is not predictable • Packets are lost when the router buffers are full
Classification, Queuing and Scheduling • QoS metrics: latency, jitter, packet loss • Issues: QoS characteristics of links, dynamics of queue utilization, and queue management within each router • Can be done through the CQS model: • Classification: arriving packets are classified • Queuing: multiple queues with different queue management and packet discard policies • Scheduling: mechanism to interleave packets from each queue
Edge-to-Edge Behavior • End-to-end service is formed from both edge-to-edge and per-hop behaviors • Network designers face a trade-off between the number of traffic classes carried by the networks and the number of traffic classes their router’s CQS architectures can handle • The existing destination-based shortest path routing algorithms in the Internet are not necessarily optimal for different classes of traffic across an arbitrary mesh of routers and links
Edge and Core? • The edge-and-core model allows core routers to leverage hardware implementations while leaving complex processing to edge routers • Edge routers can classify large number of traffic classes and the core routers will facilitate a limited number of traffic classes and could possibly employ behavioral aggregation • Edge router should smooth out the burstiness of traffic entering the network
Traffic Shaping • The focus of CQS architecture is the isolation of traffic in each queue from the burstiness in another queue. • Managing customer expectation through rate capping is essential • Placing an upper bound on the maximum bandwidth (on minimum inter-packet interval) available to a traffic class is known as “traffic shaping” • A shaping scheduler is configured to provide both a minimum service interval and a maximum service interval • A simple shaping scheduler: “leaky bucket”
Policing • Policing involves dropping of packets when too many packets arrive in too short an interval • Can be implemented through counters for each traffic class • Dropping of packets translate to the notification of congestion to the TCP sources • Policing protects the network by continuing to drop packets that exceed the allowable parameters
Marking and Reordering • Policing can be softened by marking the packets that exceed certain threshold • The marked packets would be treated as low priority compared to the unmarked packets • The queue management algorithms can begin dropping the marked packets before beginning to drop the unmarked ones • Marking improves the bandwidth utilization, but care should be taken to avoid packet reordering in the implementation.
