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CS144An Introduction to Computer Networks
Flow Control, Congestion Control and the size of Router Buffers
Section
Nick McKeownProfessor of Electrical Engineering and Computer Science, Stanford University
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Outline
Flow ControlCongestion ControlThe size of Router Buffers
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Sliding WindowWindow Size
Outstanding Datasent, but un-ack’d
Data OK to send
Data not OK to send yet
Data ACK’d
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Flow Control
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userOS
Resequence, ACK, etc
RcvBuffer
Inside Destination
Arriving packets
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Dynamics of flow control
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Animation at:http://media.pearsoncmg.com/aw/aw_kurose_network_4/applets/flow/FlowControl.htm
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Outline
Flow ControlCongestion ControlThe size of Router Buffers
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TCP Sliding Window
ACK
Window Size
Round-trip time
(1) R x RTT > Window size
ACK
Window Size
Round-trip time
(2) R x RTT = Window size
ACK
Window Size
ACK ACK
A
B
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“Bag of packets”
If there is a single flow, TCP is probing to find out how big the “bag” is so it can fill it.
In general, a TCP flow is trying to figure out how much room there is in the “bag” for its flow.
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TCP Congestion ControlTCP varies the number of outstanding packets in the
network by varying the window size:
Window size = min{Advertised window, Congestion Window}
Receiver Transmitter (“cwnd”)
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AIMDAdditive Increase, Multiplicative Decrease
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Leads to the AIMD “sawtooth”
t
cwnd
halved
“Drops”
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Dynamics of an AIMD flow
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Animation at:http://guido.appenzeller.net/anims/
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Outline
Flow ControlCongestion ControlThe size of Router Buffers
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The size of a router buffer
CB
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Rule-of-Thumb
Buffer size = 2T x C, where:2T = RTTmin = 2(propagation delay + packetization delay)
C = capacity of outgoing line.
Example: 10Gb/s interface, with 2T = 250ms300Mbytes of buffering.Read and write new packet every 32ns.
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The Story
)(logO2
2 )2()1( Wn
CTCT
)(logO
22 )2()1( W
n
CTCT
10,000 20# packetsat 10Gb/s 1,000,000
(1) Assume: Large number of desynchronized flows; 100% utilization(2) Assume: Large number of desynchronized flows; <100% utilization
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Time evolution of a single TCP flow through a router. Buffer is < 2T*C
Time Evolution of a Single TCP FlowTime evolution of a single TCP flow through a router. Buffer is 2T*C
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Buffer size = 2T x C
Interval magnifiedon next slide
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When sender pauses, buffer drains
one RTTDrop
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Origin of rule-of-thumbBefore and after reducing window size, the sending rate of the
TCP sender is the same
Inserting the rate equation we get
The RTT is part transmission delay T and part queueing delay B/C . We know that after reducing the window, the queueing delay is zero.
newold RR
new
new
old
old
RTT
W
RTT
W
T
W
CBT
W oldold
2
2/
/2
BCT 2
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Rule-of-thumb
Rule-of-thumb makes sense for one flowTypical backbone link has > 20,000 flowsDoes the rule-of-thumb still hold?
Answer: - If flows are perfectly synchronized, then Yes.- If flows are desynchronized then No.
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The Story
)(logO2
2 )2()1( Wn
CTCT
)(logO
22 )2()1( W
n
CTCT
10,000 20# packetsat 10Gb/s 1,000,000
(1) Assume: Large number of desynchronized flows; 100% utilization(2) Assume: Large number of desynchronized flows; <100% utilization
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Synchronized Flows
Aggregate window has same dynamicsTherefore buffer occupancy has same dynamicsRule-of-thumb still holds.
2maxW
t
max
2
W
maxW
maxW
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ProbabilityDistribution
Buffer Size
W
Many TCP Flows
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2T C
n
Simulation
Required Buffer Size - Simulations
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Level3 WAN Experiment
High link utilization for two weeksBuffer sizes on three parallel links:
- 190ms (190K packets)- 10ms (10K packets)- 1ms (1k packets)
Note: This slide had a typo that caused some confusion (for me!) during class.
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Drop vs. LoadBuffer = 190ms, 10ms
0
0.04
0.08
0.12
0.16
0.2
0 20 40 60 80 100
Load (%)
Pa
ck
et
Dro
p R
ate
(%
)
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Drop vs. LoadBuffer = 1ms
0
0.04
0.08
0.12
0.16
0.2
0 20 40 60 80 100
Load (%)
Pa
ck
et
Dro
p R
ate
(%
)
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The Story
)(logO2
2 )2()1( Wn
CTCT
)(logO
22 )2()1( W
n
CTCT
10,000 20# packetsat 10Gb/s 1,000,000
(1) Assume: Large number of desynchronized flows; 100% utilization(2) Assume: Large number of desynchronized flows; <100% utilization
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Buffer SizeT
hro
ug
hpu
t
10,000
100%
1,000,00010Gb/s WAN
Number of packets
On-chip buffersSmaller designLower power
tW
indo
w S
ize
Buffer
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Buffer SizeT
hro
ug
hpu
t
log(W)
10,000~50
~ 90%100%
1,000,00010Gb/s WAN
Number of packets
20 pkts
Integrated all-optical buffer [UCSB 2008]
On-chip buffersSmaller designLower power
3232
Consequences?
10-50 packets on a chip
