Chapter 6 Flow Control 1 Flow Control n

Chapter 6 Flow Control 1

Flow Control n Flow control n n Major objectives n n A mechanism to throttle traffic input into the network in order to reduce congestion Keep delay small Maintain fairness(priorities) Prevent buffer overflows Window flow control n Permit scheme(token scheme) n n Focused on network and transport layer Window size=number of outstanding packets 2

6. 2. 1 End-to-End window n What is the best window size? n n Depends on propagation delay Depends on the number of links on the paths 3

End-to-End window Window size W=3 X origin Ack dest d 4

End-to-End window n n 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) 5

End-to-End window 1 2 3 3 2 32 3 origin dest d 6

End-to-End window n n 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 } 7

End-to-End window d: should be measured in packet tx time 1/X W/d WX Packet length = 1 Kbits d 8

End-to-End window n For low-speed channel n Cap =50 k bits/sec X= 0. 02 S=20 ms 9

End-to-End window n For high-speed channel n n Fiber optical channel Cap =3 G bits/sec 10

End-to-End window n n Suppose there are n active flow control processes in the network with window size W 1…Wn The total number of packets in the network is 11

End-to-End window n By Little’s formula n Average number in system = (total arrival rate) * (average delay in system) There is a max value corresponding the channel cap. 12

End-to-End window n To reduce T n n Reduce window size for each process(window flow control) 13

6. 5. 1 Joint Routing ad Flow control n Major objective of flow control are n n Reduce delay Maintain fairness 14

Joint Routing ad Flow control n Flow control throttles input to the network n For each O-D pair, say w , there is a flow requirement Consider O-D pair 1(OD pair 1 3) 2 1 3 4 15

Joint Routing ad Flow control n Recall routing 16

Joint Routing ad Flow control n 這裡改為 類似要max flow 17

Joint Routing ad Flow control n Joint routing and flow control problem is equivalent to a routing problem n Let yw = - rw = overflow traffic requirement 18

Joint Routing ad Flow control 19

Joint Routing ad Flow control n Optimality conditions(necessary and sufficient) 20

Joint Routing ad Flow control n Example origin dest r 21

Joint Routing ad Flow control n No flow control: 22

Joint Routing ad Flow control n Flow control 23

Joint Routing ad Flow control Accepted load r Point where flow control becomes active 24

6. 5. 2 Max-Min flow control(Fairness) n Maximize the minimum allocation to any one user n n Allocate proportionally to traffic requirement Allocate proportionally to cost 25

Max-Min flow control(Fairness) n Using the Max-Min criteria 26

Max-Min flow control(Fairness) 27

Max-Min flow control(Fairness) n n n Reduce delay Maintain fairness Criterion n n Max the min allocation to the users Describe a procedure to achieve this 28

Max-Min flow control(Fairness) n 1 Ex : (p 451) 4 1 4 5 5 2 3 29

Max-Min flow control(Fairness) O-D 1 (1 3 5) 1/3 2/3 x O-D 2 (2 3) 1/3 x O-D 3 (2 3) 1/3 x O-D 4 (4 5) 1/3 2/3 O-D 5 (2 3 5) 1/3 1 x x 1/3 x 30

Max-Min flow control(Fairness) n Homework III n n Due 1/3 共四題 n n 5. 25 6. 4 6. 6 31