TCP in MANETs with Cross Layer Approaches

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TCP in MANETs with Cross Layer Approaches

• Ikjun Yeom

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Problems in MANETs

• Data-link layer
• hidden terminal problem
• high bit error rate
• Network layer
• dynamic on-demand routing may be unstable
• Transport layer
• congestion control algorithm of TCP may pump too
  much data in burst

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Negative Interactions
Congestion Control in TCP
arbitrarily long delay
On Demand Routing
network overloading
high loss rate
CSMA/CA MAC
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Cross-layer Approaches for TCP

• Using routing information
• Collaborations with MAC protocol
• TCP for stable routing

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Performance Measure
ti duration of a flow connected via i hops Ti
throughput over a linear chain using i hops
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Performance Measure
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Mobility Induced Behaviors

• some mobility patterns yield very low throughput

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Mobility Induced Behaviors

• anomaly throughput increases when speed is
  increased.

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Impact of Caching
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ELFN

• Explicit Link Failure Notification
• Objective
• To avoid pumping packets to failed links, which
  ends up to burst loss and timeouts.
• Procedure
• Entering a standby mode upon receiving an ELFN.
• Periodically sending probe packets to see if the
  link is recovered.
• Resuming its transmission upon receiving an ACK

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Performance of ELFN
Without ELFN
With ELFN
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Impact of Probe Interval
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Collaboration with MAC Layer

• Problem
• the sending rate of a TCP sender is adjusted
  responding to packet losses.
• loss behavior in wireless multi-hop networks is
  quite different with in wired networks.

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Claim

• TCPs optimal throughput is achieved when its
  window size is h/4.

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Optimal Throughput
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Loss Behavior
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Improving TCP with L-RED
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Performance Evaluation
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CWL for MANETs

• Setting Congestion Window Limit (CWL) to avoid
  sending packets which are undelivered eventually.
• Theorem In a MANET, bandwidth-delay product of
  a path cannot exceed N, where N is the round-trip
  hop count of the path.

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Proof
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Proof
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CWL in IEEE 802.11 MAC

• Corollary In a IEEE 802.11 MANET, the upper
  bound of bw-delay product of a chain is kN, where
  N is the round-trip hop-count of the path, and
  1/8 lt k lt 1/4 is a reduction factor due to
  transmission interference at the MAC layer.

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Validation of CWL
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Stability of Multihop Routing
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Impact of TCP Window Mech.
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Fractional Window Increment

• Motivation
• one packet per a RTT is too much for a small
  bandwidth network.
• FEW scheme

upon receiving an ACK cw ?/cw where 0 lt ?
lt 1
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Results