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2004.5.18. Last Class This Class Chapter 6.3. ~ 6.4. TCP congestion control. When congestion happens …. ssthresh = ½ of cwnd In case of TCP Tahoe, cwnd = 1 When timeout happened/triple ACKs arrived Start from minimum sending rate (slow start) In case of TCP Reno, cwnd = ssthresh

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2004 5 18
2004.5.18.
  • Last Class
  • This Class
    • Chapter 6.3. ~ 6.4.
    • TCP congestion control
when congestion happens
When congestion happens …
  • ssthresh = ½ of cwnd
  • In case of TCP Tahoe, cwnd = 1
    • When timeout happened/triple ACKs arrived
    • Start from minimum sending rate (slow start)
  • In case of TCP Reno, cwnd = ssthresh
    • When triple duplicate ACKs arrive
    • Sending rate decreased only by half (fast recovery)
    • When timeout happened, same as in Tahoe.
problems with tcp reno
Problems with TCP Reno
  • Triple ACKs often fails to be triggered due to either
    • Losses in burst
    • Small sending window
  • Timeout needs unnecessarily long delay.
  • Congestion control in Reno
    • Need to create packet losses to find the available bandwidth of the connection
    • Continually congesting the network
    • Creating losses for other connections sharing the link.
    • Oscillations
tcp vegas
TCP Vegas
  • Ideas
    • Detect congestion before losses occur
      • Compare RTT between two ACKs
      • If delay has increased, retransmit before a third ACK
    • Lower the rate when this imminent packet loss is detected
      • Estimate the thruput by cwnd and RTT
      • Increase/decrease cwnd by 1
how would you compare tahoe reno and vegas
How would you compare Tahoe, Reno, and Vegas?
  • Which town offers best skiing?
  • Highest probability to win a jackpot?
  • All employ AIMD
    • Distributed – no coordinationbetween connections needed?
    • Efficient – desired load level close to total?
    • Fairness – connections split the share equally?
    • Responsive to and smooth in equilibrium?
congestion avoidance
Congestion Avoidance
  • Instead of reacting to congestion, try to predict when congestion is about to happen and reduce sending rates
  • DECbit
    • router sets congestion bit to notify users of impending congestion
  • Random Early Detection
  • Source-based Congestion Avoidance
random early drop red
Random Early Drop (RED)
  • Main idea
    • Instead of waiting for the queue to get full to start dropping, it drops arriving packets with some drop probability whenver queue length exceeds some threshold.
slide8

MaxT

MinT

Drop Probability

1

MaxP

AvgQLen

MinT

MaxT

Drop with

some p

Drop all

slide9
AvgQLen = (1-α) AvgQLen + α SampleLen

Queue Length

Instantaneous

Average

Time

drop probability of red
Drop Probability of RED

TempP = MaxP ·(AvgQLen – MinT)/(MaxT-MinT)

P = TempP/(1-count · TempP)

count = # of pkts not dropped

while MinT < AvgQLen < MaxT

source based congestion avoidance
Source-Based Congestion Avoidance
  • Key idea
    • Figure out that some router’s queue is building up
    • Monitor RTT for increase
source based congestion avoidance mechanisms
Source-Based Congestion Avoidance Mechanisms
  • If CurrRTT > (minRTT+maxRTT)/2

Then decrease cwnd by 1/8

  • If (CurrW – OldW) x (CurrRTT – OldRTT) > 0

Then decrease cwnd by 1/8

Else increase cwnd by 1/8

  • Per RTT, increase/decrease cwnd by 1 pkt

Compare thruput with previous one

  • TCP Vegas
when congestion happens1
When congestion happens …
  • ssthresh = ½ of cwnd
  • In case of TCP Tahoe, cwnd = 1
    • When timeout happened/triple ACKs arrived
    • Start from minimum sending rate (slow start)
  • In case of TCP Reno, cwnd = ssthresh
    • When triple duplicate ACKs arrive
    • Sending rate decreased only by half (fast recovery)
    • When timeout happened, same as in Tahoe.
problems with tcp reno1
Problems with TCP Reno
  • Triple ACKs often fails to be triggered due to either
    • Losses in burst
    • Small sending window
  • Timeout needs unnecessarily long delay.
  • Congestion control in Reno
    • Need to create packet losses to find the available bandwidth of the connection
    • Continually congesting the network
    • Creating losses for other connections sharing the link.
    • Oscillations
tcp vegas1
TCP Vegas
  • Ideas
    • Detect congestion before losses occur
      • Compare RTT between two ACKs
      • If delay has increased, retransmit before a third ACK
    • Lower the rate when this imminent packet loss is detected
      • Estimate the thruput by cwnd and RTT
      • Increase/decrease cwnd by 1
quality of service
Quality of Service
  • Real-time applications
    • need more than best-effort
    • require some form of QoS guarantee
    • intolerant of loss/delay
voip timing charts
VoIP Timing Charts

Sue speaks

encoded and packetized

at receiver

played out

qos support approaches
QoS Support Approaches
  • IntServ’s RSVP
  • DiffServ
    • EF/AF
  • ATM
    • CBR
    • VBR-rt, VBR-nrt
    • ABR
    • UBR
  • Equation-based congestion control
next class
Next Class
  • HW #8 due
  • End-to-End data (Chap. 7)
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