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Adapting TPC To Wireless Networks
Saâd Biaz
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What Is TCP?

• Transmission Control Protocol

Client
Server
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TCP

• Hides/adapts defects and limitations of the
  network sys.

Upper Layers
Upper Layers
Too big message
Transport Layer
Transport Layer
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Network System (may loose, duplicate, reorder)
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TCP Role

• Guarantees reliable in order stream
• Detects corrupted or lost packets
• Recovers corrupted or lost packets
• Detects and eliminate duplicate
• Reorders packets
• Avoids swamping slow receiver
• Adapts to network conditions

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Network As Black Box
Network
Packet Loss, Round trip time, or Throughput
Load (Rate)
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Challenge to Adapt to Network

• How to achieve a good shower with closed eyes ?
• Listen to the person under the shower
• Avoid to get your feet wet

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Real Situation
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Van Jacobson Congestion Control Algorithms

• Packet Loss is assumed to indicate congestion
• Sender load driven by packet losses

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Van Jacobson Congestion Control Algorithms

• Start at very low rate
• Increase quickly until a threshold (half the rate
  when you lost packets).
• Increase slowly after threshold
• If loss happens, drop sharply your sending rate.

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In Wireless Networks

• Packets may be lost due to reasons other than
  congestion
• Channel quality
• mobility
• Disconnection (tunnel)

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Situation in wireless
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Adapt TCP to Wireless Networks

• For congestion losses keep slowing down
  (sharply) rate
• For wireless losses keep rate or slightly slow
  down
• Necessity diagnose the cause of loss

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How to Distinguish Congestion Losses from
Wireless Losses?

• Monitor the round trip time (time between sending
  a packet and getting an acknowledgement)
• Monitor the throughput (amount of packets
  traversing the network per unit time)

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Why Is the Problem so hard?

• Because congestion losses appear to be as random
  as wireless losses

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De-randomize Congestion LossesBiased Queue
Management

• Packets of the same flow are marked with
  different discard priorities
• When congestion occurs drop FIRST packet with
  highest discard priority.

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Biased Queue Management
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How to Diagnose Losses?

• Suppose that 1 packet ot of 8 packets is marked
  red.
• In a set of packet losses, 1/8 should be red if
  losses are random (wireless)
• If a high proportion of red packets are lost.
  Then it is most likely congestion

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Results and Future Work

• Corruption or Detection
• evaluated with Pr. Nitin Vaidya (Appeared in June
  2005 IEEE/ACM Transactions on Networking)
• Continuous congestion measure
• Design how to respond congestion level (marking
  of lost packets)
• Establish a dual model TCP/BQM
• Optimize