TCP Performance Issues over Wireless Links

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TCP Performance Issues over Wireless Links

• Sirisha Medidi

2
Outline

• Wireless link characteristics
• Performance problems of the TCP/IP protocol suite
  over wireless links
• Proposals examine their benefits limitations.

3
Introduction

• Characteristics of Wireless systems
• high error rates similar to traditional wireless
  systems (satellite terrestrial microwave)
• low physical layer propagation delay similar to
  wired systems.
• IP over wireless issues
• environmental conditions terrestrial
  obstructions and reflections lead to high
  unpredictable error rates.
• Cellular systems suffer from long communication
  pauses whenever mobile devices move between
  adjacent cells.

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Wireless systems generic characteristics

• The delivery delay for a link layer frame
  consists of
• transmission delay (frame size divided by the
  link speed),
• propagation delay ( the time the signal takes to
  cross the link), and
• processing delay at the sender and receiver.

sender
receiver
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Characteristics of wireless systems

• Wireless link
• have propagation delays similar to wired links
• have propagation delay much lower than those of
  satellite links.
• Suffer from high error rates due to external
  interferences.
• Wireless links affected by atmospheric conditions
  multi-path fading obstructions).
• Wireless links suffer from multi-path fading
  (furniture people).
• Mobility changes error characteristics of link
  error behavior varies faster unpredictably than
  satellite links.

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Characteristics of wireless systems

• Application specific private switched circuit
  service, or a shared best effort connectionless
  service.
• Link layer encapsulates IP datagrams into link
  frames to support TCP/IP
• isolates higher layers from low-level details
• Voice telephony typical random frame losses of
  1-2
• No audible speech degradation

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Characteristics of wireless systems

• Internet congestion error control is delegated
  to higher (end-to-end) layers (avoids recovery
  overhead on all applications).
• Good for reliable wired links.
• Error-prone wireless links
• local link-layer recovery can be faster more
  adaptable to link characteristics.

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TCP fundamentals

• TCP provides
• transparent segmentation reassembly of user
  data
• handles flow congestion control
• TCP packets are cumulatively ackd as they arrive
  in sequence.
• Out-of-sequence packets cause duplicate acks.
• Loss detection by sender
• when multiple duplicate acks (usually 3) arrive ?
  next packet was lost
• IP may reorder datagrams ? TCP cannot assume that
  gaps in packet sequence signify losses.

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TCP fundamentals

• Idle session or lost ACKs
• TCP detects losses using time outs
• Update retransmission timers based on a weighted
  average of previous round-trip time (RTT)
  measurements.
• Congestion TCP assumes that all losses indicate
  congestion.
• Upon loss detection, TCP reduces its transmission
  rate besides re-transmitting the lost packet.
• Next it gradually increases its transmission rate
  to gently probe the network capacity.

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TCP congestion

• TCP maintains a congestion window, which is in
  transit without causing congestion.
• New packets are only sent if allowed by both this
  an estimate of the number of packets that can
  window and the receivers advertised window.
• The congestion window starts at one packet, with
  new acks causing it to be incremented by one,
  thus doubling after each RTT.
• This is the slow start phase (exponential
  increase).

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TCP Congestion

• In Figure slow start stops after four RTTs when a
  loss is detected by a timeout.
• A slow start threshold is then set to half the
  value of the congestion window, the congestion
  window is reset to one packet, and the lost
  packet is retransmitted.
• Slow start is repeated until the threshold is
  reached after three RTTs, allowing routers to
  drain their queues.
• Subsequently, the congestion window is
  incremented by one packet per RTT. This is the
  congestion avoidance phase (linear increase).

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TCP Congestion
18 16 14 12 10 8 6 4 2 0
Timeout
Congestion Window (packets)
0 2 4 6 8 10 12
14 16
Time (RTT)
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TCP Congestion

• TCP-Reno
• losses detected by duplicate ACKs indicate
  subsequent packets have been received
• TCP retransmits the lost packet, halves the
  congestion window restarts with the congestion
  avoidance phase.
• Multiple losses
• reduce the slow start threshold
• cause slower congestion avoidance phase to take
  over immediately
• lead to large throughput degradations

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TCP Performance

• TCP assumes all losses are due to congestion
  problematic over wireless links.
• In shared medium WLANs, forward TCP traffic
  (data) contends with reverse traffic (acks).

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Outline

• Review proposals for solving performance problems
  of TCP/IP protocol suite when employed in
  wireless links.
• I TCP, Snoop TCP

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TCP Performance Enhancements

• Poor performance of TCP over wireless links is
  mostly due to mistaking wireless losses for
  congestion.
• In CC systems, packets may be delayed or even
  lost.
• Recovery from these losses should be initiated
  right after handoff completion, without waiting
  for a time out.
• TCP can achieve this by receiving appropriate
  signals from lower layers.
• Alternately, TCP can exploit mobility hints from
  lower layers to heuristically distinguish losses
  due to handoffs.

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TCP Performance Enhancements

• Due to losses, TCP can avoid halving the slow
  start threshold during recovery, skipping the
  congestion avoidance phase.
• Alternate approach
• choke TCP senders during handoffs, by
  transparently closing the receivers advertised
  window.
• sender then freezes all pending timers and starts
  periodically probing the receivers window.
  (Shrinking the advertised window violates TCP
  guidelines).

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TCP Performance Enhancements

• I-TCP
• Since end-to-end retransmissions are slow, TCP
  connections may be split using as pivot points
  routers connected to both wireless and wired
  links.
• End-to-end connections are split into separate
  TCP sessions for the wired and wireless parts of
  the path.
• Another protocol optimized for error recovery may
  be substituted over the wireless links.

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TCP Performance Enhancements

• I-TCP
• Split schemes violate end-to-end TCP semantics,
  since acks may reach the sender before data
  packets reach their destination.
• To preserve TCP semantics, acks must be delayed
  (this reduces throughput).
• Pivot points face significant overhead, since
  packets undergo TCP processing twice and
  considerable per-connection state is maintained
  there.

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TCP Performance Enhancements

• Snoop - TCP
• Error-recovery is performed at the IP-level.
• Snoop tracks TCP data and acks by maintaining
  state for each TCP connection traversing a pivot
  point.
• Snoop catches un-ackd TCP packets uses loss
  indications conveyed by duplicate acks, plus
  local timers, to transparently retransmit lost
  data.
• Hides duplicate acks indicating wireless losses
  from TCP sender to prevent redundant TCP recovery.

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TCP Performance Enhancements

• Snoop - TCP
• It exploits the info present in TCP packets to
  avoid link-layer control overhead.
• It outperforms split TCP schemes without
  violating TCP semantics.
• It avoids conflicting local TCP retransmission
  by suppressing duplicate TCP acks.

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TCP Performance Enhancements

• Snoop TCP (features)
• TCP receiver must located right after the pivot
  point.
• If a wireless host is sending data to a remote
  receiver, TCP acks are returned too late for
  efficient recovery, may even signify congestion
  losses.
• Then, explicit loss notification (ELN) is needed
  to distinguish between congestion and wireless
  losses.
• If Snoop agent detects a non-congestion-related
  loss
• it sets an ELN bit in TCP headers propagates
  it to the receiver echoed back to the sender.

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TCP Performance Enhancements

• Snoop TCP (features)
• TCP receiver must located right after the pivot
  point.
• If a wireless host is sending data to a remote
  receiver, TCP acks are returned too late for
  efficient recovery, may even signify congestion
  losses.
• Then, explicit loss notification (ELN) is needed
  to distinguish between congestion and wireless
  losses.
• If Snoop agent detects a non-congestion-related
  loss
• it sets an ELN bit in TCP headers propagates
  it to the receiver echoed back to the sender.

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TCP Performance Enhancements

• Snoop TCP (features)
• It uses queue length information to heuristically
  distinguish congestion from wireless errors.
• When ELN notification is received, TCP sender
  retransmits the lost packet without invoking
  congestion control.
• A lost packet can only be retransmitted after an
  RTT has elapsed, when an ack with ELN bit set is
  returned.