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273 adap7ng over time alloca7on changes as ows start

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Unformatted text preview: Efficient means most capacity is used but there is no conges7on •  Fair means every sender gets a reasonable share the network 254 Bandwidth Alloca7on (2) •  Why is it hard? (Just split equally!) –  Number of senders and their offered load is constantly changing –  Senders may lack capacity in different parts of the network –  Network is distributed; no single party has an overall picture of its state 255 25 11/4/13 Bandwidth Alloca7on (3) •  Key observa7on: –  In an effec7ve solu7on, Transport and Network layers must work together •  Network layer witnesses conges7on –  Only it can provide direct feedback •  Transport layer causes conges7on –  Only it can reduce offered load 256 Bandwidth Alloca7on (4) •  Solu7on context: –  Senders adapt concurrently based on their own view of the network –  Design this adap7on so the network usage as a whole is efficient and fair –  Adap7on is con7nuous since offered loads con7nue to change over 7me 257 26 11/4/13 Introduc7on to Computer Networks Fairness of Bandwidth Alloca7on (§6.3.1) Computer Science & Engineering Topic •  What’s a “fair” bandwidth alloca7on? –  The max- min fair alloca7on 260 27 11/4/13 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 261 Efficiency vs. Fairness •  Cannot always have both! –  Example network with traffic Aà༎B, Bà༎C and Aà༎C –  How much traffic can we carry? A 1 B 1 C 262 28 11/4/13 Efficiency vs. Fairness (2) •  If we care about fairness: –  Give equal bandwidth to each flow –  Aà༎B: ½ unit, Bà༎C: ½, and Aà༎C, ½ –  Total traffic carried is 1 ½ units A 1 B 1 C 263 Efficiency vs. Fairness (3) • ...
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