10Congestion Control_2010

2011-6-8 ISE-UJN(c) Zhenxiang Chen, University of Jinan 1
Congestion Control--Background Control--Background
Two sides of the same coin pre-allocate resources so that to avoid congestion precontrol congestion if (and when) is occurs
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Destiuter
Destination 2
Source 3 ISE-UJN(c) Zhenxiang Chen, University of Jinan
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Queuing Technology
Originally TCP assumed FIFO (First-In(First-InFirstFirst-Out) queuing
Network Congestion Control
Advanced Network Architecture For Postgraduates Chen Zhenxiang and Yang Bo. School of Information Science and Engineering. University of Jinan
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Dest
2011-6-8
ISE-UJN(c) Zhenxiang Chen, University of Jinan
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TCP Slow Start
Rate Timeouts
halved t
Exponential “slow start” Slow start in operation until it reaches half of previous cwnd.
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ISE-UJN(c) Zhenxiang Chen, University of Jinan
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Slow Start Threshold (ssthresh)
2011-6-8 ISE-UJN(c) Zhenxiang Chen, University of Jinan 6
Additive Increase/Multiplicative Decrease
TCP is modified to have no more than(only) the minimum of the congestion window and the advertised window bytes of unacknowledged data. MaxWindow = MIN(CongestionWindow, AdvertisedWindow) MIN(CongestionWindow, AdvertisedWindow) EffectiveWindow = MaxWindow - (LastByteSent - LastByteAcked). LastByteAcked). TCP source sets the CongestionWindow based on the level of congestion it perceives to exist in the network. This involves decreasing the congestion window when the level of congestion goes up, and increasing the congestion window when the level of congestion goes down. The mechanism is commonly called AIMD--additive increase / AIMD-multiplicative decrease. “cwnd” follows additive increase/multiplicative decrease (AIMD) (线性加 cwnd” AIMD) (线性加 乘性减－慢增快减） 乘性减－慢增快减） On receipt of Ack: Ack: cwnd += 1 ISE-UJN(c) cwnd *= University On packet loss (timeout): Zhenxiang Chen, 0.5
Router 1.5-Mbps T1 link Source 2
Destination
Two points of implementation hosts at the edges of the network (transport protocol) routers inside the network (queuing discipline) Underlying service model bestbest-effort (Now) multiple QoS (qualities of service ) (Now and later)
does not discriminate between traffic sources
Fair Queuing (FQ)
Explicitly segregates traffic based on flows Ensures no flow captures more than its share of capacity Variation: weighted fair queuing (WFQ) (WFQ)
2011-6-8
Throughput/delay
Optimal load
Load
ISE-UJN(c) Zhenxiang Chen, University of Jinan
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Congestion Control Framework
Keeps network operating at full capacity, but minimizes packet loss maximize “goodput” goodput” Two cases:
t
2011-6-8
ISE-UJN(c) Zhenxiang Chen, University of Jinan
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TCP AIMD: Slow Start (慢启)
Idea: Begin with CongestionWindow (cwnd) = 1 packet; For each ACK received (one RTT), increase the cwnd by 1; Results in cwnd pattern of: 1, 2, 4, 8, 16, 32, …… 32, Takes time proportional to log2W to reach a window of W ( may be longer if ACK delayed ). Src
of Jinan 7
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AIMD: Additive Increase
Src
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Dest The cwnd is set to 1 MSS (Maximum Segment Size) for a slow start. Actually, TCP uses bytes, not segments to count: When ACK is received: cwnd += MSS * (MSS/cwnd) (in Bytes)
self-interference: self-interference: if stream > link capacity
typical for residential access links
drop own packets
mutual interference: multiple streams competing for bottleneck interference: bandwidth
loss as congestion indicator unfairness rude streams push aside polite ones
Source 1 Router
Two common approaches: rate-based: rate-based: control rate of traffic window-based: window-based: limit number of unacknowledged packets-packets-window size controls rate
2011-6-8 ISE-UJN(c) Zhenxiang Chen, University of Jinan 8
AIMD: Multiplicative Decrease Leads to the TCP “ sawtooth ”
Rate
Timeouts
halved
Could take a long time to get started!
2011-6-8 Flow 1 Flow 2 Round-robin service Flow 3
Flow 4
ISE-UJN(c) Zhenxiang Chen, University of Jinan
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Section 1:
TCP Congestion Control
TCP uses a round-trip delay estimate for its adaptive roundwindowing scheme to transmit data reliably over an unreliable network with time varying bandwidth. In other words, send at the rate of the slowest component: network or receiver. receiver. TCP uses a CongestionWindow (cwnd) in the sender side to cwnd) do congestion avoidance. The congestion window indicates the maximum amount of data that can be sent out on a connection without being acknowledged. It is congestion control's counterpart to flow control's AdvertisedWindow. AdvertisedWindow. The AdvertisedWindow is sent by the receiving side of the connection, How about the CongestionWindow(cwnd)? CongestionWindow(cwnd)?