复旦大学：《计算机网络 Computer Networking》课程电子教案（PPT课件讲稿）22 Queue Management and QoS

15-441 Computer Networking ecture 22- Queue Management and QoS

15-441 Computer Networking Lecture 22 – Queue Management and QoS

Congestion Control Review What is congestion control? What is the principle of TCP?

2 Congestion Control Review • What is congestion control? • What is the principle of TCP?

Traffic and resource Management Resources statistically shared >Demand, (t)> Resource(t) Overload causes congestion packet delayed or dropped application performance suffer Local vs. network wide Transient vs persistent Challenge high resource utilization high application performance O

3 Traffic and Resource Management • Resources statistically shared • Overload causes congestion • packet delayed or dropped • application performance suffer • Local vs. network wide • Transient vs. persistent • Challenge • high resource utilization • high application performance

Resource Management Approaches > Demand, (t)>Resource(t) Increase resources install new links faster routers capacity planning provisioning traffic engineering happen at longer timescale Reduce or delay demand Reactive approach: encourage everyone to reduce or delay demand Reservation approach some requests will be rejected by the network

4 Resource Management Approaches • Increase resources • install new links, faster routers • capacity planning, provisioning, traffic engineering • happen at longer timescale • Reduce or delay demand • Reactive approach: encourage everyone to reduce or delay demand • Reservation approach: some requests will be rejected by the network

Congestion Control in Today' s Internet End-system-only solution(TCP) TCP dynamically estimates network state packet loss signals congestion TCP reduces transmission rate in presence of congestion routers play little role TCP F k edpac Capacit Control Planning Control Time scale RTT(ms) Months

5 Congestion Control in Today’s Internet • End-system-only solution (TCP) • dynamically estimates network state • packet loss signals congestion • reduces transmission rate in presence of congestion • routers play little role TCP TCP TCP Control Time scale Months Capacity Planning RTT (ms) Feedback Control

More Ideas on Traffic Management Improve TCP Stay with end-point only architecture Enhance routers to help tcP Random Early Discard Enhance routers to control traffic Rate limiting Fair Queueing Provide Qos by limiting congestion

6 More Ideas on Traffic Management • Improve TCP • Stay with end-point only architecture • Enhance routers to help TCP • Random Early Discard • Enhance routers to control traffic • Rate limiting • Fair Queueing • Provide QoS by limiting congestion

Router mechanisms Buffer management: when and which packet to drop? Scheduling: which packet to transmit next? classifie flow 2 Scheduler Buffer management 7

7 Router Mechanisms • Buffer management: when and which packet to drop? • Scheduling: which packet to transmit next? 1 2 Scheduler flow 1 flow 2 flow n Classifier Buffer management

Typical Internet Queuing FIFO drop-tail Simplest choice Used widely in the Internet FIFo (first-in-first-ou Implies single class of traffic Drop-tail Arriving packets get dropped when queue is full regardless of flow or importance Important distinction FIFO: scheduling discipline Drop-tail: drop policy

8 Typical Internet Queuing • FIFO + drop-tail • Simplest choice • Used widely in the Internet • FIFO (first-in-first-out) • Implies single class of traffic • Drop-tail • Arriving packets get dropped when queue is full regardless of flow or importance • Important distinction: • FIFO: scheduling discipline • Drop-tail: drop policy

FIFO Drop-tail Problems Leaves responsibility of congestion control completely to the edges(e.g, TCP) Does not separate between different flows No policing: send more packets> get more service Synchronization end hosts react to same events

9 FIFO + Drop-tail Problems • Leaves responsibility of congestion control completely to the edges (e.g., TCP) • Does not separate between different flows • No policing: send more packets → get more service • Synchronization: end hosts react to same events

FIFO Drop-tail Problems Full queues Routers are forced to havehave large queues to maintain high utilizations TCP detects congestion from loss Forces network to have long standing queues in steady-state Lock-out problem Drop-tail routers treat bursty traffic poorly Traffic gets synchronized easily With old TCP, caused very low tput Can be very unfair in b/w between flows

10 FIFO + Drop-tail Problems • Full queues • Routers are forced to have have large queues to maintain high utilizations • TCP detects congestion from loss • Forces network to have long standing queues in steady-state • Lock-out problem • Drop-tail routers treat bursty traffic poorly • Traffic gets synchronized easily • With old TCP, caused very low tput • Can be very unfair in b/w between flows