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Advanced Computer Networks: Lecture 34. This lecture will cover the following: queuing discipline; reacting to congestion; avoiding congestion; TCP congestion control; congestion control basics; model of network; state maintained in routers; finite buffering capacity per output;...
CS716 Advanced Computer Networks By Dr. Amir Qayyum 1 Lecture No. 34 Where we are now … • Understand how to – – – – – – – Build a network on one physical medium Connect networks together (with switches) Implement a reliable byte stream on Internet Implement a UDP/TCP connection/channel Address network heterogeneity Address global scale Endtoend issues and common protocols • Today’s topic – Congestion control Congestion Control Outline – Queuing Discipline – Reacting to Congestion – Avoiding Congestion Air Travel • Planning a vacation for your semester break ? • Try a trip to scenic Northern Areas • Its a mere 3 hops from VUIslamabad Islamabad Airport Flight was overbooked Peshawer Sakardu VU Congestion Control • Reading: Peterson and Davie, Ch. 6 • Basics: problem, terminology, approaches, metrics • Solutions – Routerbased: queuing disciplines – Hostbased: TCP congestion control • Congestion avoidance – DECbit – RED gateways • Quality of service Congestion Control Basics • Problem – Demand for network resources can grow beyond the resources available – Want to provide “fair” amount to each user • Examples – Bandwidth between Islamabad and Peshawer – Bandwidth in a network link – Buffers in a queue Range of Solutions • Congestion control: cure congestion when it happens • Congestion avoidance: stay away from congestion • Resource allocation: prevent congestion from occurring Model of Network • Packetswitched internetwork (or network) • Connectionless flows (logical channels/connections) between hosts State Maintained in Routers • Hard state: state used to support routing • Soft state: – Information about flows – Helps control congestion – Not necessary for correct routing 10 Congestion Control Basics • Router role • Controls forwarding and dropping policies • Can send feedback to source • Host role • Monitors network conditions • Adjusts accordingly • Routing vs congestion • Effective adaptive routing schemes can sometimes help congestion • But not always 11 Finite Buffering Capacity per Output Standard checkin lines Customer Service x a angry mob eagerly awaiting opportunity to address underpaid customer service representative 1x6 Switch you are turned away at door 12 Congestion Control Taxonomy 13 Congestion Control Taxonomy • Why is explicit perflow congestion feedback from routers rarely used in practice? – Too many sources to track • Millions of flows may fan in to one router • Can’t send feedback to all of them – Adds complexity to router • Need to track more state • Certainly can’t track state for all sources 14 Congestion Control Taxonomy – Wastes bandwidth: network already congested, not the time to generate more traffic – Can’t force the sources (hosts) to use feedback • Sol: use stochastic methods to select fairly between flows, or classify flows into categories at edge routers 15 Why Consider Rate Control ? • Buffer space (window size) is an intrinsic physical quantity • Can provide rate control with window control • Only need estimate of RTT • Answer: – Want rate control when granularity of averaging must be smaller than RTT 16 Criticism of Resource Allocation • Example: you have 10 Gbps total bandwidth • Case 1: reserve whatever you want – Users’ line of thought: On average, I don’t need much bandwidth, but when my personal web crawler goes to work, I need at least 100 Mbps, so I’ll reserve that much – Result: 100 users consume all bandwidth, all others get 0 • Case 2: fair/equitable reservations – 35,000 students + 5,000 faculty and staff – Each user gets 250 kbps, almost 5x a modem! (watch that crawler crawl ) 17 Back to Air Travel Analogy • Daily Islamabad to Peshawer flight, ~200 seats • Case 1: reserve whatever you want – 200 of us get seats. I’m Gold are you ? • Case 2: fair/equitable reservations – 2,000 possible customers – 0.099 seats per customer per flight – Disclaimer: the passenger assumes all risks and damages related to unsuccessful reassembly in Peshawer !!! 18 Congestion Control Metrics • How do we evaluate solutions ? – Effectiveness (shown as power in the figure below) – fairness 19 Congestion Control Metrics: What’s Fair ? • Give flow A 1/3 of the link bandwidth ? – Globally “fair” – A uses 2 links at 1/3 bandwidth – B and C use 1 link at 2/3 bandwidth • Give flow A 1/2 of the link bandwidth ? – Locally “fair” (fair on each link) – Also globally “fair”: maximizes minimum flow 20 Power Measure for a Sliding Window • Bottleneck link with capacity C • Roundtrip time RTT 21 .. .Lecture? ?No.? ?34 Where we are now … • Understand how to – – – – – – – Build a network on one physical medium