A%20Stochastic%20Model%20of%20TCP%20Reno%20Congestion%20Avoidance%20and%20Control

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A Stochastic Model of TCP Reno Congestion
Avoidance and Control

• Jitendra Padhye
• Victor FiroiuiDon Towsley
• Presented by Group 2

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Agenda

• Introduction
• Goals
• Issues Discussed
• Significance of the paper/Relation to Coursework
• Stochastic Model
• Previous Work - Whats different ?
• Issues not Addressed
• Related Work
• Questions/Comments

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Group 2 - Members

• Bijendra Vishal
• Domin Lee
• Guoqiang Shu
• Hannafi Tjan
• Lei Chai
• Jilani Sarmad Syed
• Juan Torres
• Rohit Shenai
• Wissam El Riachy
• Yaoyao Gu

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Tasks/Responsibilities

• Task 1
• Report
• What issues does the paper address ?
• Task 2
• What is the significance of the paper ?
• How is the paper related to the course work?
• Task 3
• Solutions to the Issues mentioned
• Task 4
• How does the paper differ from its previous work?
• Task 5
• What related issues do the paper not address
• Task 6
• Presentation

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Introduction TCP Reno

• Most modern TCPs are Reno
• Reno refined four key mechanisms of TCP
• slow start
• Congestion Avoidance
• Fast Retransmit
• Fast Recovery

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Goals

• Validate the previous model
• Analyze the performance of bulk transfer TCP
  flows using more precise, stochastic analysis
• Compare the results from the approximate
  (previous) model and new and more precise model
• Provide both an approximate formula and more
  accurate stochastic model for the steady state
  throughput of a bulk transfer TCP flow

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Issues Discussed

• Use of a finite state Markov Chain
• For more precise and stochastic analysis
• Probability transition matrix
• Comparisons
• To validate the approximate (previous) model
• Send rate
• Throughputs

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Significance of this paper

• Addresses achievements and weaknesses of previous
  work in the area.
• Develops a stochastic approach to the problem to
  validate a previous and less complicated model.
• Extends the knowledge of the topic by developing
  a random (real) model for TCP congestion
  avoidance.

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Relation to Course Work ????

• Paper uses many of the TCP characteristics
  learned in class, (i.e. window size increment
  when no losses present).
• Connection oriented nature of TCP (concept
  learned in class) is clearly seen because a round
  finishes when one or more ACKs are received, and
  repeated versions of ACKs (at the sender) tell  
  that a package or more was lost (retransmission
  needed).

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Impact of Packet Losses
Packet and ACK transmissions preceding a loss
indication
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Comparison of Throughputs (Cont)
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Analysis of TD period
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Stochastic Model

• To model the behavior of TCP in terms of rounds,
  the following random variables are defined
• Wi is window size for round i, i 0,8
• Ci allows to model the increment of window by
  one every two rounds during no-loss period.
• Li is the number of packets lost in the (i-1)st
  round, i 0,. 8

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Stochastic Model (Continued)

• Ti denotes whether the connection is in a
  timeout state in round i, i0,. 8
• Ri denotes the duration of round i, i 0,..
  8RTT denotes the RoundTrip TimeTo denotes the
  base timeout value
• Ni denotes the number of packets in round i, i
  0, 8
• Mi denotes the number of packets transmitted
  in round i, i 0, 8

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Stochastic Model (Continued)

• The sequence of random variables
• is finite state Markov chain with probability
  transition matrix
• Set S is defined as the set consisting of the
  states of irreducible sub-chains and over which
  the following probability could be defined

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Conditional Expectations

• Let and
  denote two successive states in the sequence
• Let
• conditional expectations is defined as

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Steady State Send Rate (SR)

• For t gt0, Nt Number of packets transmitted in
  the interval 0, tMt Number of packets
  received
• The long term steady state send rate of a
  TCP connection is defined as

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Steady State Throughput

• The long term steady state throughout of a TCP
  connection is defined as

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No Packets are lost
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One or more packets are lost in a round
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One or more packets are lost in a short round
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Exponential BackOff
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Previous Work - Whats different ?

• A simple approximate model was proposed earlier
• SIGCOMM 98
• Predict steady SR as a function of loss rate and
  RTT
• This paper uses more precise Markov-Chain model
• Result matches the approximation of previous
  paper
• Verifies the previous results
• Difference from other works (e.g. Ott)
• Model as many as 6 backoff (26 TO)
• Do not model slow start phase
• Do not model fast retransmit

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Comparison of Send Rates

• Approximate model proposed in 9

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Comparison of Send Rates
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Comparison of Throughputs
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Related Issues not Addressed

• Bulk transfer is analyzed
• Short lived flows like HTTP or Real time traffic
  may behave differently and may not be captured
  the model which is presented in the paper.
• Only stochastic analysis is done that has the
  following shortcomings
• Transition probabilities from one state to
  another is assumed to be constant which in real
  traffic may vary over time.
• Transition probabilities are determined by
  present state, not by history
• Other characteristic of TCP like fast recovery
  and non delayed ACKs are modeled.

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Related Works

• Approaches to analyze TCP connection
• Deterministic analysis of steady state
• Stochastic analysis of the steady state
• This paper
• Fluid queuing model
• Algebraic computation
• Refined models of losses

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Questions/Comments