LTTCP: EndtoEnd Framework to Improve TCP Performance over Networks with Lossy Channels

1
LT-TCP End-to-End Framework to Improve TCP
Performance over Networks with Lossy Channels

• Omesh Tickoo, Vijay Subramanian, Shiv
  Kalyanaraman
• (Rensselaer Polytechnic Institute)
• K. K. Ramakrishnan (ATT)

2
Overall Motivation

• TCP response to errors and congestion is the
  same
• drop the window, and thus reduce load on the
  network
• In the worst case, timeout when particular
  sequence of packets get lost (retransmits, entire
  window)
• TCP was designed for congestion, loss rate in the
  1-2 max. range.
• TCP suffers significant timeout penalties with
  erasure rates gt 5.
• Wireless channels becoming more pervasive
• With mesh networks (infrastructure or community)
  it is likely that more than the last hop will be
  wireless.
• Wireless links
• individual links can experience loss that can be
  high (even 10-15) in transient situations, until
  power and link rate adjustments kick in
• interference can also result in high loss rates.
• E.g., ad-hoc networks, Mesh networks.

3
Approach

• Tools available to us
• Method of getting congestion indication that is
  separate from packet loss due to errors Explicit
  Congestion Notification (ECN)
• Use error recovery methods beyond retransmission
  and timeouts to overcome packet loss, so that
  TCPs performance is retained.
• Use FEC on an end-end basis
• Dynamic knowledge of the loss information can be
  exploited by the end-system.
• Track short term loss rates.
• Protect data by using FEC proactively and
  reactively.
• FEC can work in a coordinated fashion with TCPs
  window mechanisms to optimize the usage of FEC
  within a window (which is not available at the
  link level).

4
Goals

• We pose the following questions..
• Dynamic Range
• Can we extend the dynamic range of TCP into high
  loss regimes?
• Can TCP perform close to the theoretical capacity
  achievable under high loss rates?
• Congestion Response
• How should TCP respond to notifications due to
  congestion..
• but not respond to packet erasures that do not
  signal congestion?
• Mix of Reliability Mechanisms
• What mechanisms should be used to extend the
  operating point of TCP into loss rates from 0 -
  50 packet loss rate?
• How can Forward Error Correction (FEC) help?
• How should the FEC be split between sending it
  proactively (insuring the data in anticipation of
  loss) and reactively (sending FEC in response to
  a loss)?
• Timeout Avoidance
• Timeouts Useful as a fall-back mechanism but
  wasteful otherwise especially under high loss
  rates.
• How can we add mechanisms to minimize timeouts?

5
Available Capacity
RECEIVER
SENDER
X
X
Loss Feedback Through Acknowledgements
X
Packet Erasure
6
Building Blocks

• ECN-Only We infer congestion solely from ECN
  markings. Window is cut in response to
• ECN signals which means that hosts/routers have
  to be ECN-capable.
• Timeouts The response to a timeout is the same
  as before.
• Window Granulation and Adaptive MSS We ensure
  that the window always has at least G segments at
  all times.
• Window size in bytes initially is the same as
  normal SACK TCP.
• Initial segment size is small to accommodate G
  segments.
• Packet size is continually so that we have at
  least G segments. Once we have G segments, packet
  size increases with window size.
• Loss Estimation The receiver continually tracks
  the loss rate and provides a running estimate of
  perceived loss back to the TCP sender through
  ACKs. An adaptive EWMA approach to estimating
  loss is used.

7
Building Blocks

• Proactive FEC TCP sender sends data in blocks
  where the block contains K data segments and R
  FEC packets. The amount of FEC protection (K) is
  determined by the current loss estimate.
• Proactive FEC based upon estimate of per-window
  loss rate (Adaptive)
• Reactive FEC Upon receipt of 1 or 2 dupacks,
  Reactive FEC packets are sent based on the
  following criteria.
• Number of Proactive FEC packets already sent.
• Number of holes still left in the decoding block.
• Loss rate currently estimated.
• Reactive FEC to complement retransmissions

8
Proactive and Reactive FEC in Action..
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Block Behavior Per-Block Loss Estimator for P-FEC
Packet Erasure Rate EWMA Estimator E
?Elatest (1-?)E
Estimation is done at receiver and fed-back to
the sender
10
Loss Tracking at Sender
Sender can quickly and accurately track the loss
rate based on feedback from the receiver.
Packet Error Rate
(Time)
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Reed-Solomon FEC RS(N,K)
RS(N,K)
FEC (N-K)
Block Size (N)
Data K
Recovery possible if we receive at least K
packets out of N
12
Timout Cause 1 Burst Errors Large MSS
5
4
Window
3
4
3
2
1
2
Transmission Loss
1
X
X
X
X
Complete Window Lost!
13
Window Granulation Reduces the Risk of Losing the
Complete Window
7
6
5
Window
7
6
5
4
3
2
1
4
3
Transmission Loss
2
X
X
X
X
1
2
3
8
ACK Stream
6
5
4
3
Rexmins
14
Timout Cause 2 Insufficient Dupacks gt SACK
not triggered
6
5
Window
4
6
5
4
3
2
1
3
Transmission Loss
2
X
X
X
1
2
ACK Stream
3
3
DUPACK-1
Timeout because of insufficient dupacks
15
Proactive FEC
P-FEC
P-FEC
4
Window
P-FEC
P-FEC
4
3
2
1
3
2
Transmission Loss
X
X
1
Receiver FEC Decoder



P-FEC
P-FEC
2
1
Recover data packets
16
Timeout Cause 3 Loss of Retransmissions
6
Window
3
2
ACK Stream
2
2
2
DUPACK3
DUPACK1
DUPACK2
2
Retransmission
X
Transmission Loss
ReXMITS ESPECIALLY vulnerable!
17
Reactive FEC Complements Rexmits
6
5
Transmission Loss
Window
4
6
5
4
3
2
1
3
2
X
X
1
2
ACK Stream
6
5
4
DUPACK3
DUPACK1
DUPACK2
Selective Acknowledgements
R-FEC
R-FEC



R-FEC
R-FEC
4
1
Receiver FEC Decoder
4
3
2
1
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Putting it Together.
Application Data
MSS Adaptation
Granulated Window Size
Window
P-FEC
(n,k)
Window Size
Loss Estimation
Data
FEC Computation
Loss Estimate
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Simulation Configuration
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Performance Results
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Contribution of Components (20 PER case (Single
Source)

• LT-TCP is able to
• reduce timeouts drastically
• keep the queue non-empty maximizing throughput
  and capacity utilization.
• minimize use of FEC to level needed

22
Comparison w/ Link Layer FEC, HARQ
LL FEC FEC based upon average PER HARQ 10 FEC
ARQ persistence 3 LT-TCP end-to-end
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Summary

• TCP performance over wireless with residual
  erasure rates 0-50 (short- or long-term).
• E2E FEC
• Granulation ensures better flow of ACKs
  especially in small window regime.
• Adaptive FEC (proactive and reactive) can protect
  critical packets appropriately
• Adaptive gt No overhead when there is no loss.
• ECN used to distinguish congestion from loss.
• Near-optimal performance for wide range from low
  to high loss rates.
• Future Work
• Optimal division of reliability functions between
  PHY,MAC, E2E
• Study of interaction between LT-TCP and
  link-layer schemes.

24
Thanks!
Researchers Omesh Tickoo tickoo_at_rpi.edu Vijay
Subramanian subrav_at_rpi.edu Shiv Kalyanaraman
shivkuma_at_rpi.edu K.K. Ramakrishnan,
kkrama_at_research.att.com
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Building Block Behavior Adaptive MSS (Window
Granulation)

• Adaptive MSS behavior.
• Congestion window (in segments) kept above G 10
• MSS increases when CWND grows,
• MSS shrinks when CWND shrinks to maintain G

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Shortened Reed Solomon FEC (per-Window)
RS(N,K)
RS(N,K)
0
0
z
Zeros (Z)
0
0
0
0
Reactive FEC (R)
K d z
Block Size (N)
Proactive FEC (F)
Window (W)
Data D
d
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Performance Results..
Drop in Performance from 40 to 50
LT-TCP (Single Source)

• At 50 error rate, timeouts increase drastically
  because..
• Few Proactive FEC packets received.
• Proactive FEC cannot counter variation in error
  patterns.
• Reactive FEC is insufficient in this case to
  avoid timeouts.
• This effect can be mitigated by increasing FEC
  protection.

28
Changes w.r.t. submitted paper

• FEC is now done on a block-by-block basis.
• Proactive protection is determined solely by the
  loss estimate. (no arbitrary constants)
• Reactive FEC packets may be wasted if they belong
  to the wrong block.
• Conditions under which Reactive FEC packets are
  sent are restricted (discussed earlier).
• Window granulation is done using the following
  rule
• Send as big a packet as possible while
  maintaining granularity
• Throughput and goodput are measured at the
  receiver for better accuracy.
• On partial dupacks, we make sure that
  retransmission are not duplicated. We send new
  TCP data instead.
• Loss tracking is now done whenever we receive an
  ACK.
• Loss estimation at receiver has changed to
  accommodate block-by-block decoding.