RRTCP: A ReorderingRobust TCP with DSACK

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RR-TCP A Reordering-Robust TCP with DSACK

• Ming Zhang (Princeton)
• Brad Karp (Intel Research/Carnegie Mellon)
• Sally Floyd (ICSI)
• Larry Peterson (Princeton)

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Motivation

• TCP performs badly under packet reordering
• In todays Internet, reordering is considered
  maladaptive
• Route oscillation Paxson96
• Router software errors Paxson97
• Striping packets across multiple links
  Bennett99
• Satellite links Ward95
• The above are not the main reasons that TCP
  should be reordering-robust
• Beneficial systems cannot be deployed because of
  reordering
• Multi-path routing
• Parallel packet forwarding path Chen01
• Goal improve TCPs robustness on paths that
  reorder packets, without increasing
  aggressiveness under congestion.

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Outline

• Motivation
• False fast retransmit and dupthresh
• Measuring reordering length and DSACK-FA
• Cost of false fast retransmit and timeout
• Using costs to adapt dupthresh to maximize
  throughput
• Experimental evaluation
• Conclusion and future work

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Problem False Fast Retransmit

• TCP detects loss with duplicate ACKs (dupacks)
• TCP enters fast retransmit when number of dupacks
  reaches some theshold dupthresh
• dupthresh is 3 per Jacobson89
• If a packets position is perturbed by more than
  3 packets, sender misinterprets reordering as
  loss and enters false fast retransmit
• False fast retransmit causes congestion window
  (cwnd) to be halved unnecessarily
• DSACK can help identify false fast retransmits by
  reporting duplicate data packets to sender

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Increasing dupthresh has risks

• Setting dupthresh greater than max reordering
  length can avoid false fast retransmit
• Risks of increasing dupthresh include
• Generate one-second-minimum timeouts, not enough
  ACKs return after real loss to trigger fast
  retransmit
• Increased end-to-end delay for dropped packets,
  longer for enough dupacks to return
• Delayed response to congestion, window reduction
  delayed until enough dupacks arrive
• The scheme for adapting dupthresh must balance
  these opposing goals
• Avoid both false fast retransmit (too small a
  dupthresh) and timeout (too large a dupthresh)

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Measuring Reordering Length

• When packet 1 is delayed, one dupack will be
  generated by receiver for each packet 2, 3, 4, 5
  that arrives before 1
• Packet 1 will create a hole in SACK senders
  scoreboard
• The returning cumulative ACK (C) or SACK (S) for
  packet 1 will close the hole in the scoreboard
• Packet 1s reordering length is the difference
  between the greatest SACKed/ACKed packet number,
  5, and 1

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Measuring Reordering Length Distribution

• Sender stores timestamped samples of reordering
  length in a reordering histogram
• Sample expires after a configurable interval
• Histogram summarizes distribution of reordering
  for any persistent reordering process
• Histogram stores up to n reordering events, each
  of which requires a timestamp (4 bytes) and a
  pointer (4 bytes)
• The reordering length measurement scans the same
  number of scoreboard entries as standard SACK

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DSACK-FA Avoiding False Fast Retransmit

• To avoid false fast retransmit for X (FA ratio)
  of reorderings, set dupthresh to the X value in
  the reordering length cumulative distribution
• Histogram maps FA ratio to dupthresh value
• Fixed FA ratio DSACK-FA (false fast retransmit
  avoidance)
• Can we adapt FA ratio dynamically?
• Central idea choose FA ratio to optimize
  bandwidth of a connection

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Outline

• Motivation
• False fast retransmit and dupthresh
• Measuring reordering length and DSACK-FA
• Cost of false fast retransmit and timeout
• Using costs to adapt dupthresh to maximize
  throughput
• Experimental evaluation
• Conclusion and future work

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Cost False Fast Retransmit

• False fast retransmit has opportunity cost in
  needlessly missed packet transmissions
• cwnd is reduced by half until sender learns
  retransmit is false
• To recover from false fast retransmit when DSACK
  returns, sender restores previous window value
  Floyd99, Ludwig00
• How many packets could we have transmitted but
  didn't?
• The cost depends on the duration D of wrongly
  reduced window
• For fixed cwnd W and

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Cost Timeout

• After a real loss, timeouts have two main costs
  beyond fast retransmit cost
• The idle period after sender cannot send any new
  packets, but before retransmission timer expires
• Slow start, during which cwnd grows from one to
  half the previous cwnd
• Extend limited transmit to send kcwnd additional
  dupack clocked packets
• For fixed cwnd W, C(timeout) C(idle) C(ss)
  where,

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DACK-TA Adapting FA ratio to maximize throughput

• Limited transmit can also introduce an
  opportunity cost, C(limited transmit), in idle
  time.
• Let S be the fundamental step by which we adapt
  FA ratio (0.01 in this work)
• Upon every false fast retransmit, increase FA
  ratio by S
• Upon every timeout, decrease FA ratio by
• Upon every limited-transmit-induced idle period,
  decrease FA ratio by
• Algorithm name DSACK-TA (timeout avoidance)

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Experimental Evaluation

• Use NS-2 network simulator
• Extended to delay or drop packets according to
  various distributions
• One long-lived TCP flow lasting 1000s
• Initial FA ratio is 90ile sampled
• Limited transmit bound is 1cwnd
• Reordering length sample life time is 80s
• Compare 4 TCP variants SACK, DSACK-R, DSACK-FA
  and DSACK-TA

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False Fast Retransmit Avoidance

• Link delay 50ms, delay time 0, 50ms, normal
  distribution

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False Fast Retransmit Ratio
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Timeout Avoidance Timeout Ratio

• Link delay 100ms, delay time 0, 400ms, uniform
  distribution
• Loss rate 0.6 and delay rate 1.4

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Timeout Avoidance Fast Retransmit Ratio
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Timeout Avoidance - Throughput
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Loss and Reordering

• link delay 50ms, 5 delayed pkt, mean delay 25ms,
  normal

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Related Work

• Ludwig00 Use TCP timestamp option to identify
  and recover from false fast retransmit, but no
  dupthresh adaptation
• Blanton01 Increase dupthresh to avoid false
  fast retransmit, but no explicit timeout avoidance

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Conclusion

• RR-TCP improves TCPs robustness under reordering
• Use reordering histogram to adapt dupthresh to
  avoid false fast retransmit
• Balance false fast retransmit, timeout and
  limited transmit according to their relative
  costs
• Not more aggressive than SACK under congestion,
  only more slowly responsive Bansal01
• Demonstrate the relative importance of loss and
  reordering to throughput, reordering matters most
  under low loss rate
• Future work
• Receiver-side RR-TCP, store reordering state and
  dupthresh at clients
• Share reordering state among short-lived flows
• Study RR-TCP together with multi-path routing
• URL http//www.icir.org/bkarp/RR-TCP

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Multi-path routing

• Link delay 50 ms, 50 packets delayed