TCP Variations

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TCP Variations

• Naveen Manicka
• CISC 856 Fall 2005
• Computer Information Sciences
• University of Delaware
• Nov 10, 2005

Most slides are borrowed from J. Leighton, B.
Forouzan, P. Amer., I. Aydin
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What Are TCP Variations?

• Implementations of TCP that use different
  algorithms to achieve end-to-end congestion
  control.
• Tahoe
• Reno
• NewReno
• Vegas
• SACK
• Rome
• Paris

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Evolution of TCP
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How Did TCP Cause Congestion? (Original Recipe
TCP)

• Poor Efficiency
• In telnet-like applications, TCP sends 1 byte of
  data with 4000 overhead.
• Sending too much, too soon
• Unnecessary retransmits
• Sending window too large
• Very little change in behavior due to congestion

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TCP Variation TCP Tahoe

• 1st improvement was TCP Tahoe (1988)
• Adjusts sending window as congestion increases or
  decreases (AIMD congestion avoidance
  slow-start)
• Improved retransmission policy (Fast Retransmit)
• Nagles algorithm
• Improved RTO calculation and back-off (Karns
  algorithm)

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Self-clocking or ACK Clock

• Maintain equilibrium of system
• Self-clocking systems tend to be very stable
  under a wide range of bandwidths and delays.
• The principal issue with self-clocking systems is
  getting them started.

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TCP Tahoe Window Control

• TCP sender maintains two new variables
• cwnd congestion window
• cwnd is inferred from the level of congestion in
  the network.
• ssthresh slow-start threshold
• ssthresh can be thought of as an estimate of the
  level below which congestion is not expected.
• send_win min (rwin, cwnd)

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Slow Start Phase (cwnd lt ssthresh)

• Initially
• cwnd 1MSS (Maximum Segment Size)
• ssthresh is very large.
• If no loss
• cwnd 1MSS (after each new ACK)
• (This gives exponential growth of cwnd)
• If loss (timeout)
• ssthresh max( flight size/2, 2MSS)
• cwnd 1MSS

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Congestion Avoidance Phase (cwnd gt ssthresh)

• If no loss
• increase cwnd at most 1MSS per RTT (additive
  increase)
• cwnd ( MSSMSS / cwnd ) on every ACK
  (approximation to increasing cwnd by 1MSS per
  RTT)
• If loss
• ssthresh max ( flight size/2, 2MSS )
  (multiplicative decrease)
• cwnd 1MSS.

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Slow Start Congestion Avoidance

• Initally
• - cwnd 1MSS
• - ssthresh very high (65535)
• If a new ACK comes
• - if cwnd lt ssthresh ? update cwnd according to
  slow start
• if cwnd gt ssthresh ? update cwnd according to
  congestion avoidance
• If cwnd ssthresh ? either
• If timeout (i.e. loss)
• - ssthresh flight size/2
• - cwnd 1MSS

(initial) ssthresh
cwnd
Loss, e.g. timeout
ssthresh
time
slow start in green
congestion avoidance in blue
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R
S
cwnd 1
Example Slow Start/Congestion Avoidance
cwnd 2
assume ssthresh 8MSS

cwnd 4
Eight TCP-PDUs
cnwd 8
ssthresh
Eight ACKs
nine TCP-PDUs
cwnd 9
nineACKs
ten TCP-PDUs
cwnd 10
ten ACKs
cwnd 11
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TCP Tahoes Retransmission Policy

• When a segment is lost, original TCP waits for an
  ACK thats not coming and eventually times-out.
• Often, many, if not all, of the segments sent
  after the lost segment arrive at the receiver.
• For each segment received, the receiver sends a
  duplicate ACK, notifying the sender that the
  receiver is waiting for the missing segment.
• TCP Tahoe interprets duplicate ACKs as an
  indication that a segment was lost.

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TCP Tahoes Fast Retransmit
S
R

1. Sender receives 3 dupACKS.
2. Sender infers that the segment is lost.
3. Sender re-sends the segment immediately!
4. Sender returns to slow-start.

ACK 1
cwnd 2
segment 2
segment 3
ACK 2
ACK 3
cwnd 4
segment 4
segment 5
segment 6
segment 7
ACK 3
3 duplicate ACKs
ACK 3
ACK 3
segment 4
fast-retransmit of segment 4
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TCP Tahoe Trace (with one dropped segment)
Lost segment
Fast Retransmit
RTT
Begin congestion avoidance
Begin slow-start
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Could Tahoe Do Better?

• Receipt of dupACKs tells the sender that the
  receiver is still getting new segments, i.e.
  there is still data flowing between sender and
  receiver
• Why does sender go back to slow start after fast
  retransmit?
• Why does sender let Ack clock die?

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TCP Variation TCP Reno

• 2nd Improvement was TCP Reno (1990)
• From Tahoe
• Nagles algorithm
• Improved RTO calculation and back-off
• AIMD congestion avoidance with slow-start
• Fast retransmit
• New to Reno
• Fast recovery

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Fast Recovery

• Concept
• After fast retransmit, reduce cwnd by half, and
  continue sending segments at this reduced level.
• Observations
• Receiver is still getting T-PDUs. There cant
  be overwhelming congestion.
• How does sender transmit T-PDUs on a dupACK?
  Need to use a trick - inflate cwnd.

cwnd
(initial) ssthresh
fast-retransmit
fast-retransmit
timeout
new ACK
new ACK
Time
Slow Start
Congestion Avoidance
inflating cwnd with dupACKs
deflating cwnd with a new ACK
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Fast Retransmit Fast Recovery

• After receiving 3 dupACKS
• Retransmit the lost segment.
• Set ssthresh flight size/2.
• Set ndupacks3 and cwndssthresh ndupacks. ---
  (inflating)
• In Reno send_win min ( rwnd, cwnd
  ndupacks ).
• If dupACK arrives
• cwnd 1MSS --- (inflating)
• Transmit new segment, if allowed.
• If new ACK arrives
• ndupacks 0
• cwnd initial ssthresh in (2) --- (deflating)
• Exit fast recovery.
• If RTO timer expires
• ndupacks 0
• Perform slow-start -- (ssthresh flight
  size/2, cwnd 1 MSS)

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TCP Reno Trace (with one dropped segment)
Lost segment
Fast Retransmit
Exit fast recovery
RTT
Begin congestion avoidance
Begin fast recovery
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TCP Tahoe Reno Trace (with one dropped segment)
Congestion Avoidance
Slow Start
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What if There are Multiple Losses in a Window?

• With two losses in a window, Reno will
  occasionally timeout.
• With three losses in a window, Reno will usually
  timeout.
• With four losses in a window, Reno is guaranteed
  to timeout!
• With three or more losses in a window, Tahoe
  typically out performs Reno!

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TCP Reno Trace (with two dropped segments)
Fast Retransmit 2
Fast Retransmit 1
Begin fast recovery 2
Begin fast recovery 1
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TCP Variation TCP NewReno

• 3rd Improvement was TCP NewReno (1995)
• From Tahoe
• Nagles algorithm
• Improved RTO calculation and back-off
• AIMD congestion avoidance with slow-start
• New to NewReno
• Fast retransmit modified fast recovery

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Modifications to Fast Recovery

• Partial ACKs An ACK that acknowledges some but
  not all the segments that were outstanding at the
  start of fast recovery. NewReno interprets this
  as an indication of multiple loss.
• If partial ACK received, re-transmit the next
  lost segment immediately and set ndupacks 0
  (deflate send_win).
• Sender remains in fast recovery until all data
  outstanding when fast recovery was initiated is
  ACKed. Additional dupACKs increase ndupacks.

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TCP NewReno Trace (with two dropped segments)
Fast retransmit of lost segment
Outstanding Data Ack
Partial Ack
Modified fast recovery
Exit fast recovery
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Tahoe, Reno NewReno Trace(with two dropped
segments)
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State Transitions for Tahoe, Reno New Reno
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Is There a Better Way?

• The only way Tahoe, Reno and NewReno can detect
  congestion is by creating congestion!
• They carefully probe for congestion by slowly
  increasing their sending rate.
• When they find (create), congestion, they cut
  sending rate at least in half!
• This slow advance and rapid retreat approach
  results in a saw-toothed sending rate and highly
  erratic throughput.
• What if TCP could detect congestion without
  causing congestion?

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TCP Variation TCP Vegas(True Congestion
Avoidance)

• Introduced by Brakmo and Peterson (1994)
• Three changes to TCP Reno
• Modified congestion avoidance
• Dont wait for a timeout, if actual throughput lt
  expected throughput decrease the congestion
  window. (AIAD!)
• Estimate of expected throughput,
• Texpected window size / smallest measured RTT
• New retransmission mechanism
• motivation what if sender never receives
  3-dupACKs (due to lost segments or window size is
  too small.)
• mechanism sender does retransmission after a
  dupACK received, if RTT estimate gt timeout.
• Modified slow start
• motivation sender tries finding correct window
  size without causing a loss.
• mechanism exponential cwnd growth only every
  other RTT.

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TCP Variation TCP Vegas

• Congestion Avoidance
• 2 thresholds a and ß, to control amount of extra
  data i.e Textra Texpected Tactual
• Textra lt a gt Window size increased by 1.
• a lt Textra lt ß gt No change in window size.
• Textra gt ß gt Window size decreased by 1.
• Avoids large oscillations like in other
  variations.
• More balanced throughput

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Vegas vs. NewReno
TCP NewReno throughput with simulated background
traffic
TCP Vegas throughput with simulated background
traffic
Source Brakmo and Peterson, TCP Vegas End to
End Congestion Avoidance on a Global Internet,
IEEE JSAC, Vol 13, No. 8, Oct. 1995, pp. 1465
1480
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What Variations Are Being Used?

• Experimental results obtained by testing 84394
  web servers (27914 classified)
• NewReno 76
• Tahoe 4 (w/o Fast Retransmit)
• Reno 15
• Other 1
• Tahoe 4

Source Medina, Allman, and Floyd, Measuring the
Evolution of Transport Protocols in the
Internet, May 2004
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TCP Today

• TCP is currently defined by
• IETF Stds. RFC793, RFC1122 (Tahoe w/o FR)
• IETF Proposed Stds.
• RFC1323 (Scaled windows timestamps)
• RFC2018, RFC2883, RFC3517 (SACK)
• RFC2581 (Reno)
• RFC2988 (RTO)
• RFC3168 (ECN)
• RFC3390 (Larger IW)
• IETF Exp. RFCs
• RFC2582 (NewReno)
• Many many more!

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• Questions ?

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Summary of TCP Behavior
TCP Variation Response to 3 dupACKs Response to Partial ACK of Fast Retransmission Response to full ACK of Fast Retransmission
Tahoe Do fast retransmit, enter slow start cwnd cwnd
Reno Do fast retransmit, enter fast recovery Exit fast recovery, deflate window, enter congestion avoidance Exit fast recovery, deflate window, enter congestion avoidance
NewReno Do fast retransmit, enter modified fast recovery Fast retransmit and deflate window remain in modified fast recovery Exit modified fast recovery, deflate window, enter congestion avoidance

• When entering slow start, if connection is new,
• ssthresh arbitrarily large value
• cwnd 1.
• else,
• ssthresh max(flight size/2, 2MSS)
• cwnd 1.
• In slow start cwnd on new ACK

• When entering either fast recovery or modified
  fast recovery,
• ssthresh max(flight size/2, 2MSS)
• cwnd ssthresh.
• In congestion avoidance
• cwnd 1MSS per RTT