Chapter 6 Flow Control

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

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

End-to-End window origin dest X d Ack Window size W=3

End-to-End window X = length of packet/cap. Of channel = tx time dW  X The source can transmit at full speed and flow control is not active Full speed = cap. of channel = 1/X(packets/sec)

End-to-End window origin dest d 1 2 3

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 }

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

End-to-End window For low-speed channel Cap =50 k bits/sec  X= 0.02 S=20ms

End-to-End window For high-speed channel Fiber optical channel Cap =3 G bits/sec

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

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.

End-to-End window To reduce T Reduce n Reduce window size for each process(window flow control)

6.5.1 Joint Routing ad Flow control Major objective of flow control are Reduce delay Maintain fairness

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 13)

Joint Routing ad Flow control Recall routing

Joint Routing ad Flow control 這裡改為 類似要max flow

Joint Routing ad Flow control Joint routing and flow control problem is equivalent to a routing problem Let yw = - rw = overflow traffic requirement

Joint Routing ad Flow control

Joint Routing ad Flow control Optimality conditions(necessary and sufficient)

Joint Routing ad Flow control Example origin dest r

Joint Routing ad Flow control No flow control:

Joint Routing ad Flow control

Joint Routing ad Flow control Accepted load r Point where flow control becomes active

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

Max-Min flow control(Fairness) Using the Max-Min criteria

Max-Min flow control(Fairness)

Max-Min flow control(Fairness) Reduce delay Maintain fairness Criterion Max the min allocation to the users Describe a procedure to achieve this

Max-Min flow control(Fairness) 2 1 3 4 5 Ex : (p451)

Max-Min flow control(Fairness) O-D1 (135) 1/3 2/3 x O-D2 (23) 1/3 x 1/3 x O-D3 O-D4 (45) 2/3 1 x O-D5 (235)

Max-Min flow control(Fairness) Homework III Due 1/3 共四題 5.2 5.25 6.4 6.6