Explicit Loss Notification to Improve TCP Performance over Wireless Channels

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Explicit Loss Notification to Improve TCP
Performance over Wireless Channels

• Gergo Buchholcz¹, Thomas Ziegler², Tien Van Do¹
• ¹Department of Telecommunications, Budapest
  University of Technology and Economics
• ²Telecommunications Research Center Vienna (ftw.)

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Outline

• Motivation
• Overview of the proposal
• The ELN proposal
• Simulation results
• Conclusion

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Motivation

• TCP was developed for wired environments
• TCP in wireless environments
• Lossy wireless links
• TCP performs poorly when corruption occurs
• No distinction between corruption and congestion
• Reduces sending rate, timeouts and slow start
• Correct behaviour
• Resend copies of lost packets
• Keep sending rate the same

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Improvement proposals (1)

• Two major types of improvement ideas
• Split connection approach
• Splits the connection into a wired and a wireless
  part
• The base stations need to be modified
• Packet processing overhead at the base stations
• Violates TCP end-to-end schemantics
• Improves both upload and download performance

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Improvement proposals (2)

• End-to-End proposals
• additional information about the data flow
  available for the receiver may help the sender to
  improve the flow control mechanism
• only the communicating peers need to be modified
• easily applicable
• TCP-SACK
• TCP-WESTWOOD
• Explicit loss notification (ELN)

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Improvement proposals (3)

• Explicit Loss Notification
• additional information the receivers MAC layer
  is able to detect packet corruption at the
  wireless link
• receiver informs the sender about packet
  corruptions
• the sender may avoid unnecessary window reductions

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Explicit Loss Notification (1)

• Prerequisite
• the sender and the sequence number of the lost
  packet must be identified in order to return loss
  notifications
• retrieve sender address and sequence number from
  the corrupted packet
• additional TCP header option field to protect the
  integrity of the TCP header with a checksum
• since payload is much longer than the header bit
  errors are likely to appear in the payload,
  leaving the header intact

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Explicit loss notification (2)

• Applicability
• End-to-end proposal
• Only the peers need to modified
• Comparatiely easy to apply
• Receiver monitors the incoming packages only on
  the last wireless link
• Only the download direction can be optimized
• Best to apply in networks containing only one
  wireless link between the mobile node and the
  base station

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Explicit Loss Notification (3)TCP-ELN receiver

• Uses acknowledgements to carry loss information
• The sequence number of the corrupted packet is
  returned in a new TCP header option field
• An acknowledgement can store a maximum of 9
  additional sequence number but only 6 are stored
• Acknowledgements are generated when intact TCP
  packets are received
• When generating a new ACK the latest notification
  is excluded while the last is included
• No additional acknowledgements

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Explicit Loss Notification (4)TCP-ELN sender

• Flow control mechanism
• Reported lost packets are resent immediately
• in corruption free case it behaves exactly as
  TCP-NewReno
• handles the loss notifications with a new
  recovery state (wireless recovery)

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Explicit Loss Notification (5)TCP-ELN sender

• Decision about the new recovery state
• Loss information is stored at the sender
• a duplicate ACK is a sign of packet loss or
  packet reordering
• After the 3rd DupACK in a row if there is a
  refering loss notification the sender enters the
  wireless recovery state
• When a partial ACK is received, the sender stays
  in the wireless recovery state providing that
  refering loss information is stored
• if there is no loss notification, it behaves as
  TCP-NewReno (enters fast ret/fast rec. after the
  3rd DupACK)

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Explicit Loss Notification (6)TCP-ELN sender

• Wireless recovery may invoke fast recovery if
  congestion occurs
• Keep the sending rate constant during wireless
  recovery

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Simulation results (1) environment

• Topology

• NS-2.27
• 802.11 MAC interface
• FTP traffic
• Throughput
• 1 session from the server to each mobile node
• Error models
• Uniform
• Markov
• 3 models 3 different user speed

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Simulation results (2)

• FTP throughput of TCP-ELN versus TCP-NewReno over
  
• uniform error model

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Simulation results (3)

• Improvement of TCP-ELN versus TCP-NewReno over
• uniform error models

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Simulation results (4)

• FTP throughput and relative improvement of
  TCP-ELN versus TCP-NewReno over markov error
  models

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Conclusion

• TCP-ELN improves TCPs performance on wireless
  networks
• based on the idea of explicit loss notification
• differentiation between packet losses caused by
  congestion and corruption
• High performance improvement in wide range of
  environments

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Future work

• Investigate corrupted packet handling
• Integrate TCP-ELN with wireless MAC layers (UMTS,
  802.11)
• Combine with other orthogonal proposals
• TCP-Westwood
• Related to multilayer modeling (interaction
  between loss models for the air interface and
  models for TCP/IP traffic, validation /comparison
  with real measurements)

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Thank You!