Open%20Issues%20on%20TCP%20for%20Mobile%20Computing

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Open Issues on TCP for Mobile Computing
Ibrahim Matta Computer Science, Boston
University Vassilis Tsaoussidis Computer Science,
Northeastern University Technical Report,
2001 Presented by Brad Burres
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First things First

• SORRY!
• This paper was interesting, but it was long
• Apparently picking another paper based solely on
  the author wasnt a good idea!

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Agenda

• Introduction
• Review of TCP
• TCP problems in a wireless world
• Fixing TCP Error Detection
• TCP error recovery and performance enhancements
• TCP Strategy
• Conclusions

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Introduction

• Mobile networking is growing quickly
• Protocols were tuned the wired world
• TCP is the most prevalent example
• Packet loss meant congestion (or total link
  failure)
• The only performance metric was throughput
• Assuming fairness, stability, etc.
• TCP is insufficient for the wired/wireless world
  of today and tomorrow

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What does TCP need?

• TCP needs to not only detect an error, but to
  detect the nature of an error
• That is, detect the error characteristics of the
  link
• TCP needs have more flexible and robust recovery
  mechanisms
• Awareness of the wireless power constraint
  reduce a clients energy consumption

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Review of TCP

• This is review for all of us, but Im going to
  hit a few key points
• TCP was designed with a few key goals
• Reliable Transmission
• Reasonable Fairness between flows
• The ability to quickly adapt to fluctuations in
  available bandwidth
• Mechanisms for congestion avoidance and error
  recovery

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TCP Key Concepts

• TCP controls data transmissions through a sliding
  window
• Error control consists of error detection and
  then error recovery
• TCP detects errors by monitoring acknowledged
  data segments (if a timeout occurs, a packet was
  lost)
• Error recovery consists of two things
• The retransmission of lost data
• The reduction of the congestion window size
• AIMD Additive Increase Multiplicative Decrease

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TCP Flavors of Congestion Control

• TCP Tahoe basic TCP mechanisms (Slow Start) but
  added the idea of Fast Retransmit
• TCP Reno TCP Tahoe Fast Recovery
• Retransmit the lost segment, and continue (after
  reducing CWND by ½)
• TCP Vegas Totally different. Keep a RTT
  estimate, and use that to see if the channel is
  being utilized (and hence to resize the
  transmission window).

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A Picture is worth 1000 words
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TCP problems in a wireless world

• TCP is NOT capable of detecting the nature of an
  error. It only knows a drop occurred. (at least
  two general types of errors)
• Short Lived Errors
• Entering Slow Start or decreasing the
  transmission window causes missed opportunities
  to send error-free data and increases connection
  time.
• Persistent Errors
• Forced re-transmission over bad links leads to
  high energy consumption with minimal goodput gain
  
• TCP backs off too much when it should not and too
  little when it should, thereby hurting battery
  life

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Other TCP comments

• TCP error control is designed for congestion
  induced errors only
• ECN contributions are limited in wired/wireless
  networks
• Not receiving an ECN doesnt mean the packet
  wasnt dropped
• Congestion algorithms running on top of TCP (RED)
  could constrain the evolution of TCP
• If TCP evolves, it might make RED unfair

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Fixing TCP Error Detection

• The nature of an error calls for a distinctive
  recovery strategy
• Congestion same as wired, gradual
  retransmission
• Hand-Off suspend transmission for short amount
  of time, then send aggressively
• Fading reduce transmission until the channel is
  better
• Tsaoussidis and Badr propose TCP-Probing

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TCP-Probing

• When a packet is lost, enter a Probe Cycle
• In a Probe Cycle
• All data transmission is suspended
• A probe (small packet) is sent
• If Probe or Ack is lost, re-initiate Probe Cycle
• Otherwise, estimate RTT (and congestion)
• If Congested, enter Tahoe/Reno congestion
  response
• If not Congested, enter Immediate Recovery and
  being sending with the full window enabled

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WTCP (proxy)

• Add intelligence to the wireless base station
  (BS) and let it detect the nature of the error
• Essential two TCP connections between the
  Sender and BS, and between the BS and receiver
• BS buffers the data segment
• Re-transmit non-ACKd segments
• Subtract out WTCP residence time when ACKing
  segments back to the sender
• Data lost over the wireless link could adversely
  effect the Time Out value

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Error Recovery

• If nature of the error is known, be smart about
  the recovery
• CWND
• ACK strategy
• Timeout Mechanism
• Entering Slow Start
• Others.
• If its not a congestion based error, dont apply
  a congestion derived strategy.

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Congestion Window

• Freeze-TCP avoid a penalty during handoff
• Immediate Recovery if the failure is not due to
  congestion, dont adjust the CWND or enter Slow
  Start.
• Re-transmit aggressively when the channel
  recovers.
• Decouple the CWND from RTT. A lost or delayed
  ACK doesnt say anything about the forward path,
  only the backward path.

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ACK Strategy

• The ACK strategy sets the transmission rate
• Fast Retransmission improved hand-off
• After HO, send Duplicate ACKs.
• Using partial ACKs to re-transmit can lead to
  transmitting a non-lost packet.
• This is bad from the energy conservation
  perspective
• Implementing SACKs (selective ACKs) gives the
  sender more information

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Proxy Based Approach

• Proxy lies between the wired and wireless
  networks
• Tries to hide wireless losses from TCP
• ITCP Implement TCP up to the Application layer
  in the BS
• WTCP like ITCP, but up to the Transport layer.
• Snoop - Like WTCP, but up to the Link layer.
  Less buffering (no guarantee) of finding data
  locally.

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Proxy performing Local Recovery
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Increased Improvement by not increasing the
Time-Out Value
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TCP Strategy

• TCP needs to move towards the adaptive error
  control
• One way to achieve this is to implement the
  discussed TCP-Probing
• By determining the nature of the error and
  reacting in an appropriate manner, the total
  throughput will be increased

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Faster Task Completion with Probing
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Even Faster Completion with Longer Delays (100ms
extra)
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TCP for Handhelds

• TCP for wireless networks needs to make tradeoffs
  between the effort expended and the goodput
  achieved
• A good performance metric is goodput/overhead
  ratio because it shows the efficiency of a
  battery powered transfer
• In general, reducing the total time spent
  transferring data will reduce power consumed
• More research is needed in this area

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Probing Reduces Overhead
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Probing Reduces Overhead more with longer
propagation delays
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Conclusions

• Traditionally, protocols (namely TCP) did not
  account for wireless architecture
• Key issues for wireless networks
• error characteristics of the link
• performance metrics to evaluate efficiency
• TCP needs
• Adaptive Error Control to respond to each error
  appropriately
• Energy- and time-saving capabilities to support
  the mobile, battery powered nature of wireless
  devices

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Acknowledgements

• borrowed fast retransmit picture from
  http//www.ensc.sfu.ca/ljilja/cnl/presentations/w
  an/wireless_TCP/sld003.htm