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Chapter 6 Flow Control
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Flow Control Flow control Major objectives Window flow control
A mechanism to throttle traffic input into the network in order to reduce congestion Major objectives Keep delay small Maintain fairness(priorities) Prevent buffer overflows Window flow control Permit scheme(token scheme) Focused on network and transport layer Window size=number of outstanding packets
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6.2.1 End-to-End window What is the best window size?
Depends on propagation delay Depends on the number of links on the paths
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End-to-End window origin dest X d Ack Window size W=3
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End-to-End window X = length of packet/cap. Of channel = tx time
dW X The source can transmit at full speed and flow control is not active Full speed = cap. of channel = 1/X(packets/sec)
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End-to-End window origin dest d 1 2 3
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End-to-End window If d>W X, then the source can only transmit at a rate of W/d (packets/sec) The max. rate of transmission r = min { 1/X , W/d }
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End-to-End window d: should be measured in packet tx time 1/X W/d WX d
Packet length = 1 Kbits W/d 1/X d WX
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End-to-End window For low-speed channel
Cap =50 k bits/sec X= 0.02 S=20ms
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End-to-End window For high-speed channel Fiber optical channel
Cap =3 G bits/sec
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End-to-End window Suppose there are n active flow control processes in the network with window size W1…Wn The total number of packets in the network is
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End-to-End window By Little’s formula
Average number in system = (total arrival rate) * (average delay in system) There is a max value corresponding the channel cap.
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End-to-End window To reduce T Reduce n
Reduce window size for each process(window flow control)
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6.5.1 Joint Routing ad Flow control
Major objective of flow control are Reduce delay Maintain fairness
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Joint Routing ad Flow control
Flow control throttles input to the network For each O-D pair, say w , there is a flow requirement 1 4 3 2 Consider O-D pair1(OD pair 13)
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Joint Routing ad Flow control
Recall routing
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Joint Routing ad Flow control
這裡改為 類似要max flow
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Joint Routing ad Flow control
Joint routing and flow control problem is equivalent to a routing problem Let yw = - rw = overflow traffic requirement
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Joint Routing ad Flow control
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Joint Routing ad Flow control
Optimality conditions(necessary and sufficient)
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Joint Routing ad Flow control
Example origin dest r
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Joint Routing ad Flow control
No flow control:
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Joint Routing ad Flow control
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Joint Routing ad Flow control
Accepted load r Point where flow control becomes active
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6.5.2 Max-Min flow control(Fairness)
Maximize the minimum allocation to any one user Allocate proportionally to traffic requirement Allocate proportionally to cost
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Max-Min flow control(Fairness)
Using the Max-Min criteria
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Max-Min flow control(Fairness)
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Max-Min flow control(Fairness)
Reduce delay Maintain fairness Criterion Max the min allocation to the users Describe a procedure to achieve this
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Max-Min flow control(Fairness)
2 1 3 4 5 Ex : (p451)
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Max-Min flow control(Fairness)
O-D1 (135) 1/3 2/3 x O-D2 (23) 1/3 x 1/3 x O-D3 O-D4 (45) 2/3 1 x O-D5 (235)
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Max-Min flow control(Fairness)
Homework III Due 1/3 共四題 5.2 5.25 6.4 6.6
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