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Point is a possible alloca7on host 1 1 congested fair

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Unformatted text preview: s Flow 1 speeds up when Flow 2 stops Flow 3 limit is elsewhere Time 31 14 11/12/13 Introduc7on to Computer Networks Addi7ve Increase Mul7plica7ve Decrease (AIMD) (§6.3.2) Computer Science & Engineering Recall •  Want to allocate capacity to senders –  Network layer provides feedback –  Transport layer adjusts offered load –  A good alloca7on is efficient and fair •  How should we perform the alloca7on? –  Several different possibili7es … 34 15 11/12/13 Bandwidth Alloca7on Models •  Open loop versus closed loop –  Open: reserve bandwidth before use –  Closed: use feedback to adjust rates •  Host versus Network support –  Who sets/enforces alloca7ons? •  Window versus Rate based –  How is alloca7on expressed? TCP is a closed loop, host ­driven, and window ­based 35 Addi7ve Increase Mul7plica7ve Decrease •  AIMD is a control law hosts can use to reach a good alloca7on –  Hosts addi7vely increase rate while network is not congested –  Hosts mul7plica7vely decrease rate when conges7on occurs –  Used by TCP •  Let’s explore the AIMD game … 37 16 11/12/13 AIMD Game •  Hosts 1 and 2 share a bo`leneck –  But do not talk to each other directly •  Router provides binary feedback –  Tells hosts if network is congested Host 1 Host 2 1 Bo`leneck 1 Router 1 Rest of Network 38 AIMD Game (2) •  Each point is a possible alloca7on Host 1 1 Congested Fair Op7mal Alloca7on Efficient 0 1 Host 2 39 17 11/12/13 AIMD Game (3) •  AI and MD move the alloca7on Host 1 1 Congested Fair, y=x Addi7ve Increase Op7mal Alloca7on Mul7plica7ve Decrease Efficient, x+y=1 0 1 Host 2 40 AIMD Game (4) •  Play the game! Host 1 1 Congested Fair A star7ng point Efficient 0 1 Host 2 41 18 11/12/13 AIMD Game (5) •  Always converge to good alloca7on! Host 1 1 Congested Fair A star7ng point Efficient 0 1 Host 2 42 AIMD Sawtooth •  Produces a “sawtooth” pa`ern over 7me for rate of each host –  This is the TCP sawtooth (later) Host 1 or Mul7plica7ve Addi7ve 2’s Rate Decrease Increase Time 43 19 11/12/13 AIMD Proper7es •  Converges to an alloca7on that is efficient and fair when hosts run it –  Holds for more general topologies •  Other increase/decrease control laws do not! (Try MIAD, MIMD, AIAD) •  Requires only binary feedback from the network 44 Feedback Signals •  Several possible signals, with different pros/cons –  We’ll look at classic TCP that uses packet loss as a signal Signal Example Protocol Pros / Cons Packet loss TCP NewReno Cubic TCP (Linux) Hard to get wrong Hear about congestion late Packet delay Compound...
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