CUBIC - the slide for typical TCP protocol

1. CUBIC: A New TCP-
Friendly High-Speed TCP
Variant
S Ha, I Rhee, L Xu - ACM SIGOPS Operating Systems Review, 2008
Presenter: Deawoo Kim
LANADA. KAIST
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2. TCP Congestion control
• TCP seeks to
• Achieve high utilization
• Without congestion
• Source calculates cwnd from indication of network
congestion
• Congestion indications
• Losses
• Delay
• Marks
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3. Window Flow Control
• W packets transmitted per RTT
• Source rate =
• If W too small, then rate « capacity
• If W too big, then rate > capacity => congestion
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4. TCP Reno (Jacobson 1990)
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5. CUBIC Congestion Control
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6. Wikipedia: CUBIC TCP
• CUBIC is an implementation of TCP with an
optimized congestion control algorithm for
high bandwidth networks with high latency
1. … the window is a cubic function of time since the
last congestion event.
Scalable, stable
2. CUBIC does not rely on the receipt of ACKs to
increase the window size. CUBIC's window size is
dependent only on the last congestion event.
• Standard TCP, flows with very short RTTs will receive ACKs
faster and therefore have their congestion windows grow
faster than other flows with longer RTTs. CUBIC allows for
more fairness between flows since the window growth is
independent of RTT.
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7. Bandwidth and Delay Product (BDP)
• Network feature – BW, delay product (BDP)
• Total number of packet needed in flight while keeping the BW
fully utilized
• BDP(capacity in bits) = path bandwidth (bps) * round-trip
time(s)
• Internet evolves to include many very high speed and
long distance network paths
• These networks are characterized by large BDP
Source Destination
Delay (RTT)
Bandwidth
Fully-utilized
Under-utilized
BDP
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8. Under-utilization in Large BDP (example)
• Network environment
• BW : 10 Gbps
• RTT : 100ms
• Packet : 1250 bytes
• BDP of the path : 100,000 packets
• TCP grow its window from mid-point of BDP (50,000
packets)
• After 50,000 RTT, fully-utilized (100,000packets)
(5,000 sec, 1.4hour)
• If flow finish before 1.4 hour then under-utilization
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9. Legacy TCP
• 1. Multiplicative decrease
• 2. Growth of CWND depends on RTT
Time
cwnd
CA
TD
ssthresh
TD TD TD
Wm
CA CA CA
Large RTT
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10. Related work
• STCP (Scalable TCP)
• Recovery time from loss events be constant regardless of the
window size -> scalable
• HSTCP (HighSpeed TCP)
• 기본적으로 AIMD
• Window size가 특정 threshold 넘으면,
increase factor 증가, decrease factor 감소
• TCP-Vegas
• δ < 𝛼 : path is not congested -> increase sending rate
• δ > 𝛽 : congestion -> reduce sending rate
• δ : difference btw expected throughput and actual throughput
• 𝛼 : low threshold
• 𝛽 : upper threshold
• HTCP
• Growth function is function of elapsed time (indep. RTT)
• Decrease factor is adjusted to be proportional to the queue size
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11. Abstract
• CUBIC
• Congestion control protocol for TCP
• Default TCP algorithm in Linux
• Modify linear window growth function -> cubic function
• Goal
1. Improve the scalability of TCP over fast and long distance
networks
2. Equitable BW allocation among flows with different RTT
3. Stable : keep the CW much longer at the saturation point
4. TCP friendliness
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12. BIC-TCP
• Window growth function : simply a logarithmic concave
function
• Stable and Scalable
• How does it work?
• Binary search
• Window grows to the mid-point of max and min
• Additive increasing
• Max probing
• Feature
• Far from the saturation point : aggressive increase CWND
• Close to saturation point : slowly increase CWND
• Overshoot occurs very small
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13. BIC-TCP Algorithm
Packet loss
event
Wmax
Wmin = β*Wmax
midpoint = (Wmin + Wmax)/2
Wmin – midpoint > Smax
Wmin + SmaxWmin
midpoint = (Wmin + Wmax)/2
Wmin + Smax
Wmin
Additive Increase
midpoint = (Wmin + Wmax)/2
Wmin + Smin
Wmin
(Wmin – midpoint) < Smin
Binary Search
Wmax + Smin
Wmax + Smin
Wmax +Smax
Slow Start
Wmax + 2Smax
Wmax + 3Smax
Additive Inc.
Max Probing
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14. BIC-TCP
• Additive increasing
• Jumping to the midpoint can be too much increase within one
RTT
• Binary search algorithm
• Stable
• Max probing
• Find the new saturation point if it has increased
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15. New Window Growth Function
• BIC-TCP
• 장점
• Scalability in high speed networks
• Stability with low window oscillations
• 단점
• Growth function can be too aggressive for TCP for short RTT or
low speed networks (RTT-fairness)
• Difference phase (binary search, max probing, 𝑆 𝑚𝑎𝑥, 𝑆 𝑚𝑖𝑛)
complexity
• New window growth function 필요
• BIC-TCP 장점 취함
• Simple
• TCP friendliness
• RTT-fairness
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16. Introduction
• CUBIC
• Next version of BIC-TCP
• Window growth -> cubic function
• cubic function – Incidental, out of convenience
• In mathematics, a cubic function is a function of the
form
• The window growth function of CUBIC
• 𝐶 : CUBIC parameter
• 𝑡 : elapsed time from the last window reduction
• 𝐾 : time period take to 𝑊𝑚𝑎𝑥
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17. TCP CUBIC
Packet loss
event
Time
Steady State Behavior
Max Probing
Wmax
Around Wmax, window growth almost
becomes zeroFast growth upon
reduction
Cubic starts probing for
more Bandwidth
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18. RTT-fairness
• Window growth depends only on the real time(congestion
epoch) between two consecutive congestion events
• Independent of RTT
BIC-TCPCUBIC
Large RTT
Small RTT
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19. TCP-friendliness
• Short RTT – window growth rate slower than TCP
standards
• Current wnd in CUBIC < window size of standard TCP
at time 𝑡
• Set cwnd as Wtcp(t) (Standard TCP)
• Improve convergence speed of CUBIC
• New flow join -> existing flow give up BW
• 𝑊𝐿𝑎𝑠𝑡 𝑚𝑎𝑥 > 𝑐𝑤𝑛𝑑 -> reducing 𝑊𝑚𝑎𝑥 further
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Fast Convergence

20. CUBIC window growth function
• Congestion avoidance basic
• Compute the window growth rate during next RTT period
• Set 𝑊(𝑡 + 𝑅𝑇𝑇) as target congestion window
• 3 modes
• TCP-friendly region
• Concave region
• Convex region
• Multiplicative decrease of cwnd factor of 𝛽
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21. TCP Cubic Advantages
• Good RTT fairness
• Growth dominated by t, competing flows have same t after
synchronized packet loss
• Real-time dependent
• Similar to BIC but linear increases are time dependent
• Does not depend on ACK’s like TCP/ Reno
• Scalability and stability
• Cubic increases window to Wmax quickly and keeps it there longer
• High utilization in large BDP
• Scalability
• Independent RTT
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22. TCP Cubic Drawbacks
• The speed to react
• It can be sluggish to find the new saturation point if the
saturation point has increased far beyond the last one
• Slow Convergence
• Flows with higher cwnd are more aggressive initially
• Prolonged unfairness between flows
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23. CUBIC action
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24. Fairness to
standard TCP
• Standard TCP performs well
• Small BDP
• Short RTT
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25. Thank you
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26. BIC Algorithm
while (cwnd != Wmax){
If ((Wmin – midpoint) > Smax)
cwnd = cwnd + Smax
else
If ((Wmin – midpoint) < Smin)
cwnd = Wmax
else
cwnd = midpoint
If (no packet loss)
Wmin = cwnd
else
Wmin = β*cwnd
Wmax = cwnd
midpoint = (Wmax + Wmin)/2
}
(Additive Increase)
(Binary Search)
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while (cwnd >= Wmax){
If (cwnd < Wmax + Smax)
cwnd = cwnd + Smin
else
cwnd = cwnd + Smax
If (packet loss)
Wmin = β*cwnd
Wmax = cwnd
}
Max Probing

27. CUBIC window growth function
• Window growth function of CUBIC is a cubic function
• CUBIC use both concave and convex for window size
• Details of CUBIC
• Loss event
• Register 𝑊𝑚𝑎𝑥 loss 발생한 지점
• Multiplicative decrease of cwnd factor of 𝛽 (decrease constant)
• Start to increase the window using concave profile of the cubic
function
• Plateau at 𝑊𝑚𝑎𝑥, concave growth continue until the window
size becomes 𝑊𝑚𝑎𝑥
• Cubic function turn into convex profile (max probing)
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28. Slow Start
Congestion Avoidance
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29. Fast Retransmit
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30. TCP Reno (Jacobson 1988)
Fast Recovery
• Motivation: prevent ‘pipe’ from emptying after fast retransmit
• Idea: each dupACKrepresents a packet having left the pipe
• Enter CA after 3 dupACKs(Fast Retransmit/Fast Recovery)
• Set ssthresh←cwnd/2
• Set cwnd←ssthresh
• Enter SS after timeout
• Set ssthresh←cwnd/2
• Set cwnd←1
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