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;...
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CS716 Advanced Computer Networks
By Dr. Amir Qayyum
Trang 2Lecture No. 34
Trang 3– Address global scale
– Endtoend issues and common protocols
• Today’s topic
– Congestion control
Trang 6Congestion Control
• Reading: Peterson and Davie, Ch. 6
• Basics: problem, terminology, approaches, metrics
Trang 7– Buffers in a queue
Trang 8• Resource allocation: prevent
congestion from occurring
Trang 9Model of Network
• Packetswitched internetwork (or network)
• Connectionless flows (logical
channels/connections) between
hosts
Trang 12Finite Buffering Capacity per Output
1x6 Switch
x a
Standard checkin lines Customer Service
you are turned away at door
angry mob eagerly awaiting opportunity to address underpaid customer service
representative
Trang 13Congestion Control Taxonomy
Trang 14• Need to track more state
• Certainly can’t track state for all sources
Trang 15Congestion 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
Trang 17• 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 )
Trang 19Congestion Control Metrics
• How do we evaluate solutions ?
– Effectiveness (shown as power in the figure below)
– fairness