Queuing. Congestion Control and Resource Allocation. Resource Allocation Evaluation Criteria. Resource allocation Drop disciplines Queuing disciplines

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1 Resource allocation Drop disciplines Queuing disciplines Queuing 1 Congestion Control and Resource Allocation Handle congestion if and when it happens TCP Congestion Control Allocate resources to avoid congestion Decide when to say no and to whom Partly implemented in routers and switches Partly implemented in transport protocols Mainly concerns bandwidth and buffer space in routers and switches 2 Resource Allocation Evaluation Criteria Throughput Delay Power = throughput / delay Not a perfect metric Goal: A stable system without congestion collapses How: Maximize power through load control Hard to do! 3 1

2 Resource Allocation Evaluation Criteria Fair resource allocation Hard to define 4 Queuing and Drop Disciplines The queuing algorithm Allocates both bandwidth and buffer space Directly affects the latency experienced by a packet Defines a scheduling discipline The drop policy Determines which packets get dropped 5 Drop Disciplines Tail drop RED Random Early Detection 6 2

3 Queuing Disciplines FIFO First-In-First-Out Priority Queuing BRFQ - Bit-Round Fair Queuing WFQ Weighted Fair Queuing Leaky Bucket More like admission control 7 Queuing Disciplines - FIFO Traditionally FIFO has been (and is) used Drawbacks of FIFO No way to differentiate between flows Flows of larger packets get better service Greater average delay per packet for flows with shorter packets Greedy TCP connections can crowd out more altruistic connections 8 Queuing Disciplines Priority Queuing Multiple FIFO queues one for each priority class Classify packets into the correct queue Transmit packets from the highest priority class that has a nonempty queue Drawback Low-priority queues can get starved out 9 3

4 Queuing Discipline Fair Queuing Multiple queues at each output port One queue for each flow (source) The queues are served in round-robin fashion Fig 11.9 Pros: The scheme is fair in that each busy flow gets to send exactly one packet per cycle No advantage being greedy It is a form of load balancing Cons: Short packets are penalized 10 Queuing Discipline Bit-round Fair Queuing To overcome the unfairness in FQ Each busy flow gets to transmit one bit per cycle This approach is known as processor sharing (PS) Impractical to implement Bit-round Fair Queing (BRFQ) emulates PS Compute virtual starting and finishing times on the fly Whenever a packet finishes transmission, the next packet sent is the one with the smallest virtual finishing time Fig Queuing Discipline Bit-round Fair Queuing BRFQ fairly allocates the available capacity among all active flows through a node But BRFQ is not able to provide different amounts of the capacity to different flows 12 4

5 Queuing Discipline General Processor Sharing General Processor Sharing (GPS) Each flow is assigned a weight that determines how many bits are transmitted from that queue during each round Provides a means of responding to different service request Provides a way of guaranteeing that delays for a wellbehaved flow do not exceed some bound Like PS, GPS is impractical to implement 13 Queuing Discipline Weighted Fair Queuing Weighted Fair Queing (WFQ) emulates GPS Just like BRFQ Compute virtual starting and finishing times on the fly Whenever a packet finishes transmission, the next packet sent is the one with the smallest virtual finishing time Fig Packet size = 1, link speed = 1, guaranteed rate for connection 1 = 0.5, guaranteed rate for the other 10 connections = 0.05 Same advantages as GPS and practical to implement 14 Queuing Discipline Leaky Bucket More like admission control Regulate the rate a flow is allowed to inject packets into the network Average rate Peak rate r tokens/time unit Bucket Burst size holds up to b tokens Packets Token Remove To network wait token 15 5

6 Queuing Disciplines - Summary Drop disciplines Tail drop RED Random Early Detection Queuing disciplines FIFO First-In-First-Out Priority Queuing BRFQ - Bit-Round Fair Queuing WFQ Weighted Fair Queuing Leaky Bucket 16 6

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