principles of congestion control
DESCRIPTION
Principles of congestion control. Contents. Causes and cost of congestion Three examples How to handle congestion End-to-end Network-assisted TCP congestion control ATM ABR congestion control Summary. What is congestion?. Overflowing of routers buffers Packet retransmission - PowerPoint PPT PresentationTRANSCRIPT
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ContentsCauses and cost of congestion
Three examples How to handle congestion
End-to-end Network-assisted
TCP congestion controlATM ABR congestion controlSummary
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What is congestion?Overflowing of routers buffersPacket retransmission
Delay the problem?How to treat problem?
Mechanisms to throttle senders
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Causes and cost Example 1
2 senders Router with infinite buffer spaceNo retransmissionSending time infinteLink capacity (R)
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•Large queuing delays
Causes and cost
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Causes and costExample 2
2 sendersRouter with finite buffer spaceRetransmissionOffered loadLink capacity (R)
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Causes and cost•Retransmission due to buffer overflow•Retrassmission of unneeded copies
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Causes and costExample 3
4 senders4 routers with finite buffer spaceMultihop pathsLink capacity (R)Offered load
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Causes and cost•Throughput goes to zero in the the limit of heavy traffic•Wasted use of upstream links capacity
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How to handle congestionEnd-to-end congestion control
No explicit support from networkIP layer provides no feedbackObserved network behavior, ex. packet loss or
delayTCP use end-to-end control
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TCP congestion controlAdditive-increase, multiple-decrease (AIMD)Slow start (exponential)Congestion avoidance (linear)Fast recovery
Timeout Tripple duplicat ACK’s
TCP Tahoe TCP Reno (Newer versions of TCP)
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How to handle congestion Network-assisted
Explicit feedbackDirect feedbackFeedback via receiverLow impact on trafficATM (Asynchronous Transfer Mode)ABR (Available Bit-Rate)XCP protocol
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ATM ABR congestion control ATM takes a VC approach
ATM is a kind of architectureTrack behavior of individual senders
ABR takes advantage of spare bandwidthResource management cells (RM cells)RM cells convey congestion-related
informationDirect feedback or feedback via receiver
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ABRThree mechanisms
Explicit forward congestion indication (EFCI) bit
Congestion indication and No increase (CI and NI) bit
Expclicit rate (ER) field
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SummaryWhy do we need to handle congestion?
Minimize delaysMake traffic more effectiveMaximize use of bandwidth
How is congestion problem solved?End-to-end congestion controlNetwork-assisted congestion control
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Questions?