{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Fairness 3 if we care about eciency maximize total

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: x ­min fair alloca7on 16 Recall •  We want a good bandwidth alloca7on to be fair and efficient –  Now we learn what fair means •  Caveat: in prac7ce, efficiency is more important than fairness 17 7 11/12/13 Efficiency vs. Fairness •  Cannot always have both! –  Example network with traffic AB, BC and AC –  How much traffic can we carry? A 1 B 1 C 18 Efficiency vs. Fairness (2) •  If we care about fairness: –  Give equal bandwidth to each flow –  AB: ½ unit, BC: ½, and AC, ½ –  Total traffic carried is 1 ½ units A 1 B 1 C 19 8 11/12/13 Efficiency vs. Fairness (3) •  If we care about efficiency: –  Maximize total traffic in network –  AB: 1 unit, BC: 1, and AC, 0 –  Total traffic rises to 2 units! A 1 B 1 C 20 The Slippery No7on of Fairness •  Why is “equal per flow” fair anyway? –  AC uses more network resources (two links) than AB or BC –  Host A sends two flows, B sends one •  Not produc7ve to seek exact fairness –  More important to avoid starva7on –  “Equal per flow” is good enough 21 9 11/12/13 Generalizing “Equal per Flow” •  Bo`leneck for a flow of traffic is the link that limits its bandwidth –  Where conges7on occurs for the flow –  For AC, link A–B is the bo`leneck A 1 B 10 C Bo`leneck 22 Generalizing “Equal per Flow” (2) •  Flows may have different bo`lenecks –  For AC, link A–B is the bo`leneck –  For BC, link B–C is the bo`leneck –  Can no longer divide links equally … A 1 B 10 C 23 10 11/12/13 Max ­Min Fairness •  Intui7vely, flows bo`lenecked on a link get an equal share of that link •  Max ­min fair alloca7on is one that: –  Increasing the rate of one flow will decrease the rate of a smaller flow –  This “maximizes the minimum” flow 24 Max ­Min Fairness (2) •  To find it given a network, imagine “pouring water into the network” 1.  Start with all flows at rate 0 2.  Increase the flows un7l there is a new bo`leneck in the network 3.  Hold fixed the rate of the flows that are bo`lenecked 4.  Go to step 2 for any remaining flows 25 11 11/12/13 Max ­Min Example •  Example: network with 4 flows, links equal bandwidth –  What is the max ­min fair alloca7on? 26 Max ­Min Example (2) •  When rate=1/3, flows B, C, and D bo`leneck R4—R5 –  Fix B, C, and D, con7nue to increase A Bo`leneck 27 12 11/12/13 Max ­Min Example (3) •  When rate=2/3, flow A bo`lenecks R2—R3. Done. Bo`leneck Bo`leneck 28 Max ­Min Example (4) •  End with A=2/3, B, C, D=1/3, and R2—R3, R4—R5 full –  Other links have extra capacity that can’t be used •  , linksxample: network with 4 flows, links equal bandwidth –  What is the max ­min fair alloca7on? 29 13 11/12/13 Adap7ng over Time •  Alloca7on changes as flows start and stop Time 30 Adap7ng over Time (2) Flow 1 slows when Flow 2 start...
View Full Document

{[ snackBarMessage ]}