Computer%20Networks

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Computer Networks
Congestion Avoidance
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Recall

• TCP Sliding Window Operation

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Important Terms

• Maximum Segment Size (MSS)
• Window
• Retransmission
• Timer
• Delayed ACK
• Duplicate ACK

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Congestion Window

• Sender based flow control
• Sender uses min(cwnd,advertised window)
• Tranmission window
• Detect network condition and adjust
• Use timers, ACKs and network feedback

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Retransmitting Segments

• TCP starts timer after sending a segment
• If ACK returns, reset timer
• If no ACK, retransmit and increase timer (2x)
  after resetting

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Round Trip Time (RTT)

• TCP measures RTT and uses RTT to calculate timer
  values
• If ACKs returning quickly, timer values are kept
  low
• Retransmit quickly when throughput is high
• Allow time for ACK to return when throughput is
  low

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Slow Start

• Initialize cwnd to 1 MSS
• Increase cwnd by 1 MSS for every ACK
• Not very slow!

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Congestion Avoidance

• When packet loss occurs, TCP must slow its flow
  of traffic
• When retranmission timer expires, save value of
  transmission window ½ (ssthresh)
• If congestion occurred due to a timeout, set cwnd
  to 1 segment
• When data is ACK'd, increase cwnd via slow start
  up to ssthresh
• Thereafter increase cwnd by 1/cwnd for each ACK

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Congestion Avoidance Illustrated
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Fast Retransmit

• If gt 2 duplicate ACKs are received, a segment was
  probably lost
• Sender assumes enough time has elapsed for out of
  order segments to have arrived and assembled
• Retransmit segment without waiting for a
  retransmission timer to expire
• Enter Fast Recovery

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Fast Recovery

• Sender has to assume data is still flowing due to
  the reception of duplicate ACKs
• Loss was probably a rare or temporary event
• Do not go into slow start, just go into
  congestion avoidance mode

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Other mechanisms

• Selective Acknowledgements (SACK)
• TCP Pacing
• Packeteer
• RED
• ECN
• Network pricing