Simulation based analysis of FAST TCP using OMNET

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Simulation based analysis of FAST TCP using
OMNET

• Umair ul Hassan

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Outline

• TCP Congestion Control
• Variations
• FAST TCP
• OMNET

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TCP Congestion Control

• Retransmission Timer Management
• Jacobsons Algorithm
• Exponential RTO Backoff
• Karns Algorthim
• Window Management

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Window Management

• Slow Start
• Increase cwnd by 1 for each ACK
• Exponential Increase
• Dynamic Window Sizing on Congestion
• Reset cwnd on packet loss and restart slow start
• Linear increase after threshold

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Window Management

• Fast Retransmit
• On receiving out-of-order segment send an ACK for
  last in-order segment.
• Sender should wait atleast 3 duplicate ACKs
  before retransmission
• FAST Recovery
• Set cwnd ssthreshold 3
• Increase by 1 on each duplicate ACK
• Set cwnd ssthreshold on ACK

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TCP Tahoe and Reno
Measure Tahoe Reno
Slow Start ? ?
Dynamic Window Sizing on Congestion ? ?
Fast Retransmit ? ?
Fast Recovery ?
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Problems with TCP Reno

• Packet Level
• Slow additive increase and Fast Multiplicative
  decrease
• Oscillations due to binary congestion singal
• Flow Level
• Very small loss probability required
• Instability because of oscillations

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TCP Vegas

• Delay Based Congestion Control
• New Retransmission Mechanism
• On duplicate ACK retransmit if RTT gt RTO, dont
  wait till 3 DACKs
• Congestion Aviodance
• Diff Expected Actual
• Modified Slow Start
• Alow exponential growth every other RTT

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Congestion Window Update

• TCP Vegas
• cwnd 1 if Difflta
• Cwnd cwnd 1 if Diffgtß
• cwnd otherwise
• TCP FAST
• cwnd ?(a Diff) if
  Difflta
• Cwnd cwnd ?(Diff - a) if Diffgta
• cwnd otherwise

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Implementation

• Per ACK

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Implementation

• Per RTT

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Problems

• Fairness when used with other variations
• Priority Queues

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OMNET

• Discrete Event Simulation System
• NED language
• Atomic Modules
• Compound Modules
• Message based communication

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INET Framework

• Internet Protocols (IP, TCP, UDP, PPP, ARP)
• TCPBaseAlg
• TCPReno and TCPTahoe
• TCPFast

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