Transport Layer for Mobile Ad Hoc Networks

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Transport Layer for Mobile Ad Hoc Networks

• Based on slides by Eric Law, UCR
• March 2005

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The Quest for the Transport Layer
Introduction

• Assess the state of the art of transport
  protocols
• Target environment mobile ad hoc nets (MANET)
• Which is the best TCP variant?
• Do we need a new transport protocol?
• The question is very timely

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Network Architecture at a Crossroads
Introduction

• The community recognizes the need for change
• Wireline-centric network design is obsolete
• New network environments have emerged
• Ad hoc, sensors, consumer-owned, delay-tolerant
• New networking technologies have emerged
• UWB, cooperative approaches, MIMO, directed
  antennas

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New Class of Networks
Introduction

• Tens of nodes, resource constrained, wireless
  links, line powered (computers)

• Tens of nodes, resource constrained, wireless
  links, line powered (embedded devices)

• Tens of nodes, resource constrained, wireless
  links, charged every day (PDAs)

• Hundreds of nodes, resource constrained,
  unreliable wireless links (Sensors)

• Thousands of nodes, highly resource constrained,
  highly unreliable wireless links, low duty cycle
  (smartdust)
• Tens - thousands of nodes, Nano-sensors

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A New Era Has Begun
Introduction
6
The Role of Networking is Central
Introduction
Embedded SensorApplications
WirelessNetworking
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Revisiting the Architecture
Introduction

• The vision
• Wireless as an integral part of the network
• Multiple wireless hops not just the last mile
• Pockets of wireless ad hoc connectivity
• A new protocol stack is required
• Is TCP/IP capable of delivering?

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Revisiting The Hourglass
Introduction
User Application
Application Protocol
Transport Protocol
Internet Protocol
Media Access Protocol
Media Sharing Principles
Physical System
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Problem Evaluate TCP

• Why does TCP perform poorly in MANETs?
• Developed for wire-line networks.
• Assume all losses are due to congestion.
• Many TCP variants have been proposed.
• How good are they? Are they sufficient?
• Are there any other alternatives?
• Are non-TCP protocols the solution?

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Our goal

• Identify the problems of TCP in MANETs.
• Evaluate various major TCP variants.
• 12 TCP variants, 7 improvement techniques
• Observations
• Most TCP variants are NOT sufficient.
• A new transport layer protocol is needed.

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Overview of Results

• The best TCP variants
• TCP-Westwood and TCP-Jersey seem the best.
• Both protocols estimate bandwidth more
  accurately.
• TCP mechanisms
• Feedback from intermediate nodes leads to big
  gains.
• The best non-TCP approaches
• Ad-hoc Transport Protocol (ATP) seems to address
  most issues
• Non-window based estimates achievable rate
  periodically
• Split-TCP promising new way of looking at
  transport layer
• Dynamically buffer packets mid-path
• Key Separation of congestion control from
  reliability.

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Roadmap

• Overview of TCP
• The problems of TCP over MANETs
• Overview of best transport protocols
• In depth
• Specific problems of TCP over MANETs
• Details of major TCP variants
• Discussion - other efforts
• Conclusion

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Overview of TCP concepts
Overview

• Conventional TCP Tahoe, Reno, New-Reno
• Sending rate is controlled by
• Congestion window (cwnd) limits the of packets
  in flight
• Slow-start threshold (ssthresh) when CA start
• Loss detection
• 3 duplicate ACKs (faster, more efficient)
• Retransmission timer expires (slower, less
  efficient)
• Overview of congestion control mechanisms
• Slow-start phase cwnd start from 1 and increase
  exponentially
• Congestion avoidance (CA) increase linearly
• Fast retransmit and fast recovery Trigger by 3
  duplicate ACKs

Slow-start
Congestion avoidance
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What is different in MANETs?
Overview

• Mobility
• Route stability and availability
• High bit error rate
• Packets can be lost due to noise
• Unpredictability/Variability
• Difficult to estimate time-out, RTT, bandwidth
• Contention packets compete for airtime
• Intra-flow and inter-flow contentions
• Long connections have poor performance
• More than 4 hops thruput drops dramatically

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Overview of the Best Protocols
Overview

• TCP-Westwood Casetti et. al.
• Estimate bandwidth to alleviate the effect of
  wireless errors.
• TCP-Jersey Xu et. al.
• Estimate bandwidth to alleviate the effect of
  wireless errors.
• Congestion warning assists the determination of
  packet loss due to wireless error from
  congestion.
• ATP Sundaresan et. al.
• Rate based transmission, periodic rate feedback,
  no timeout concept, reliability provided by SACK.
• Split-TCP Kopparty et. al.
• Separating congestion control from reliability.
• Dropped packets are recovered from the most
  recent proxy instead of the source.

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Why does TCP fail in MANETs?
Overview

• Specific problems are identified
• TCP misinterprets route failures as congestion
• TCP misinterprets wireless errors as congestion
• Intra-flow and inter-flow contention reduce
  throughput and fairness
• Delay spike causes TCP to invoke unnecessary
  retransmissions
• RTO too small ? unnecessary retransmissions.
• Inefficiency due to the loss of retransmitted
  packet
• When retransmitted packet is lost? timer expires
  ? performance drops

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Roadmap

• Overview of TCP
• The problems of TCP over MANETs
• Overview of best transport protocols
• In depth
• Specific problems of TCP over MANETs
• Details of major TCP variants
• Discussion - other efforts
• Conclusion

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Specific problems of TCP over MANETs

• TCP misinterprets route failures as congestion
• Effects Reduce sending rate
• Buffered packets (Data and ACKs) at intermediate
  nodes are dropped.
• Sender encounters timeout.
• Under prolonged disconnection, a series of
  timeouts may be encountered.

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Specific problems of TCP over MANETs

• TCP misinterprets wireless errors as congestion
• Effects Incorrect execution of congestion
  control ? Performance drops.
• Wireless channel is error-prone compared to
  wireline
• Fading, interference, noise

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Specific problems of TCP over MANETs

• Intra-flow and inter-flow contention
• Effects Increased delay, unpredictability, and
  unfairness.
• Inter-flow contention contention of nearby
  flows.
• Intra-flow contention between packets of the
  same flow (e.g. forward data and reverse ACKs).
• Wireline only packet on same link compete

Two nearby flows
Data stream
ACKs stream
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Specific problems of TCP over MANETs

• Delay spike causes TCP to invoke unnecessary
  retransmissions
• Effects Performance drops and many unnecessary
  retransmissions. Ludwig Katz
• Variability Spikes are not uncommon here
• Spikes throw off parameter estimation and tuning
• RTO, window size, slow-start threshold

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Specific problems of TCP over MANETs

• Inefficiency due to the loss of retransmitted
  packet
• Effects Performance drops significantly under
  high loss environment (e.g. MANETs).
• Losing a retransmitted packet hurts
• TCP can recover from one loss (fast
  retransmission)
• Wired networks packet loss rate is low.
• Here, high packet loss makes the problem
  significant

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Classification of Transport protocols

• TCP variants try to improve the performance by
  the following ways
• Estimating the available bandwidth
• Determining route failure and wireless error
• Reducing contention
• Detecting spurious retransmission
• Exploiting buffering capability
• New approaches Non TCP variants
• Use rate based instead of window based approach
• Enable dynamic buffering (split TCP)

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Estimating the available bandwidth
Details of some major TCP variants

• TCP-Vegas Brakmo et. al.
• Use difference expected rate - actual rate to
  regulate the sending rate.
• Pro Avoid congestion and typical loss at the end
  of the slow-start.
• Con Route changes invalidate rate calculation.
  Unfairness problems.
• TCP-Westwood Casetti et. al.
• Use the rate of return of ACKs to estimate
  bandwidth.
• Sender re-computes the cwnd and ssthresh upon any
  loss.
• Pro Robust to wireless errors.
• Con Route changes invalidate bandwidth
  estimation. Depends on the behavior of returned
  ACKs.
• TCP-Jersey Xu et. al.
• Use the rate of return of ACKs to estimate
  bandwidth.
• Intermediate nodes warn sender of congestion
• Pro Robust to wireless error
• ProAvoids congestion proactively
• Con Require assistance from intermediate nodes.
  Depends on the behavior of returned ACKs.

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Determining Route Failure and Wireless Error
Details of some major TCP variants

• Explicit Link Failure Notification (ELFN)
  Holland Vaidya
• Upstream node of the failed link notifies the
  sender of the failure.
• Disable timers, stop all data transmission and
  send probing to detect restored route.
• Pro Avoid execution of congestion control upon
  route failures.
• Con Old TCP states (cwnd and timer) are used
  after route restoration.
• Ad-hoc TCP (ATCP) Liu Singh
• Hide errors from TCP sender (timeout and the 3rd
  DUPACKs).
• Employ ECN and route failure notification to
  assist TCPs decision.
• Pro Does not modify TCP itself.
• Con Require assistance from intermediate nodes.
• ADTCP Fu et. al.
• Use multiple metrics to determine the network
  behavior
• Congestion, Channel error, Route change,
  Disconnection states.
• Pro Does not rely on intermediate node feedback.
• Con Thresholds used to determine network states
  must be carefully defined.

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Reducing Contention
Details of some major TCP variants
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2
3
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6
7

• Adaptive Congestion Window Limit (CWL) Chen et.
  al.
• Upper bound of bandwidth-delay product of a chain
  is kN.
• 1/8 lt k lt 1/4, N is the round-trip hop-count x
  packet size
• From experiment, k approximately 1/5 of the
  round-trip hop-count.
• Pro Adapt the maximum cwnd to avoid excessive
  contention.
• Con Can only apply in chain of nodes. Does not
  account for the nearby flows.
• Link RED (LRED) and Adaptive Pacing (AP) Fu et.
  al.
• Intermediate nodes mark packets when
• the average of retires of current transmission
  gt threshold.
• Sender reduces sending rate.
• Nodes back-off an additional time if they start
  to markpackets
• Pro Avoid congestion and reduce contention.
• Con Require modification at link-layer on each
  node.
• TCP Adaptive Delayed Acknowledgement (TCP-ADA)
  Singh Kankipati
• Send one ACK for a window of data.
• Pro Reduce the number of ACKs and thus reduce
  contention.
• Con Increase burstiness of the forward
  transmissions.

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Detecting Spurious Retransmission
Details of some major TCP variants

• TCP-Eifel Ludwig Katz
• Use timestamp option of TCP to solve
  retransmission ambiguity.
• Sender can determine whether the received ACK is
  from original transmission or retransmission.
• After retransmission, if next ACK is from
  original transmission ? spurious!
• Restore the cwnd and ssthresh if spurious
  retransmission is detected.
• Pro Robust to sudden delay spike.
• Con Require the use of timestamp option or
  modification of TCP header.

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Details of some major TCP variants
Exploiting Buffering Capability

• Buffering capability and Sequence information
  (TCP-BuS)Kim et.al.
• Use explicit route failure notification to detect
  route failure.
• When route failure occurs, intermediate nodes
  buffer the pending packets and TCP sender doubles
  the retransmission timeout (RTO) value.
• Avoid timeouts and unnecessary retransmissions.
• Pro Reduce the number of timeout events ? reduce
  the number of retransmissions.
• Con Require assistance from intermediate nodes.
  Special routing protocol is used.

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Non-TCP based approaches
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A Rate-based Transport Layer Protocol

• Ad-hoc Transport Protocol (ATP) Sundaresan et.
  al.
• Feedback from intermediate nodes
• path failure, queueing delay, periodic feedback
  on rate
• Rate based transmission
• Entirely rate-controlled. (no window concept)
• Evenly distribute transmissions over time.
  (reduce burstniess)
• Decoupling of congestion control and reliability
• Does not require the arrival of ACKs to clock out
  segment.
• Does not employ cumulative ACKs but solely relies
  on periodic SACK (with 20 SACK blocks) to
  identify losses.
• Pro 1) Estimate rate accurately. 2) Reduce
  traffic on the reverse path. 3) Recover more than
  one lost segment at a time.
• Con 1) Incompatibility problem. 2) Require the
  assistance from the intermediate nodes. 3)
  Fastest possible time to detect and recover
  packet lost is 1 second.

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Split-TCP a new approach

• Split-TCP work done at UCR Kopparty et. al.
• Setup proxies along the connection ? many short
  TCP connections.
• Congestion control and reliability are separated.
• Proxies buffer packets from the previous proxy or
  the source.
• Any dropped packets are recovered from the most
  recent proxy but not from the source.
• Pro Enhance parallelism. Reduce bandwidth
  consumption on retransmission.
• Con Optimal frequency of proxy placement is not
  clear.

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Split TCP in more detail
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What is our idea Split-TCP

• Split long TCP connection into shorter segments
• Small segments can be more adaptive to
  conditions
• Proxies glue segments together.
• Proxies buffer packets and becomes responsible
  for the delivery of the packet.
• INTUITION separate reliability from congestion
  control
• the former is end-to-end but the latter is not!

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TCP Proxies Functionality

• Proxies send an Local ACK (LACK) to the previous
  proxy or the source upon the receipt of a TCP
  packet.
• Thus, the flow of the packet now occurs in
  stages.
• End to end ack to ensure end to end reliability.

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Improving Throughput

• When link failures occur in one segment, TCP
  data may still be transmitted on other segments.
• Thus the throughput improves.

A
X
Y
P
S
Z
B
C
D
Link Failure
Data transfer continues in spite of failure
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Improving Fairness

• Proxies alleviate the unfair advantage that
  shorter connections have over longer connections.
• Now longer connections are split into shorter
  segments.
• The throughput of longer connections however
  cannot equal that of shorter connections due to
  interactions between segments.
• Packets cannot be sent and received at the proxy
  at the same time adjacent TCP segments have to
  transport data in stages.

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Alleviating The 802.11 Capture Effect
D1
S1
P1
P2
S2
P3
D2

• Intuition Localize capture effect
• Instead of the capturing the whole path TCP
  connection, only the region spanned by a segment
  is captured
• Thus, it is possible to have two TCP sessions in
  the vicinity of each other S1-P1 segment does
  not bother P3 - D2 segment

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1.Throughput Improvement

• Mobile Ad Hoc Network with 50 nodes in 1 Km X 1
  Km area 3 TCP connections
• Speed uniform between 0 and 10 m/s random
  waypoint model.

• At a time between 50 and 60 seconds a link
  failure occurs.
• Notice that after this, if proxies are used
  performance is better.
• Fair share for all three connections in spite of
  failure.
• If no proxies are used the share of channel of
  the connection with failed link taken by other
  two connections.
• Elimination of intra-segment contention on
  connection with failed link improves performance

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2. Fairness Improvement

• Multiple TCP connections with varying lengths in
  terms of hop count.
• Longer connections achieve lower throughput than
  shorter ones.
• Introduction of proxies improves throughput
• For a connection of length 16 hops, the
  throughput improves from around 22 Kbps to 27
  Kbps.
• Improvement in fairness

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3. Alleviating Capture Effect

• Two connections are active in the vicinity of
  each other.
• Both are heavily loaded.
• Connection 2 begins slightly later than
  Connection 1.
• If TCP is used without proxies, Connection 2 has
  a very low share of the throughput until
  Connection 1 is done sending its data.
• TCP with proxies alleviates this effect fairer
  share of bandwidth is available to Connection 2.

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Conclusion of Overview

• We identify the factors that degrade the
  performance of TCP over MANETs.
• We identify the most promising TCP variants
• TCP-Westwood, TCP-Jersey
• Current TCP variants do not seem sufficient
• Promising transport protocols emerge
• Split-TCP, ATP

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TCP VariantsAdditional Information
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TCP-Vegas
Bandwidth Estimation

• Rate-based congestion control
• diff expected rate actual rate
• If diff lt a, Vegas increases cwnd linearly
• If diff gt b, Vegas decreases cwnd linearly
• If alt diff lt b, Vegas keeps cwnd unchanged
• Modified slow-start
• Allows cwnd to grow exponentially only in every
  other RTT
• If diff gt c, Vegas switches from slow-start to
  congestion avoidance
• New retransmission
• Reads and records transmission time.
• When DUPACK arrives, checks if it is expired.
• Retransmits without waiting for third DUPACK.

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TCP-Westwood - I
Bandwidth Estimation

• Bandwidth estimation
• Measure the rate of return of ACKs.
• Adaptive to variation of inter-arrival of ACKs.
• Exponentially goes to zero upon prolonged absence
  of ACKs.
• Faster recovery
• Compute cwnd and ssthresh using the estimated
  bandwidth upon any loss event.

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TCP-Westwood - II

• Main idea Bandwidth estimation
• Sender monitors ACKs to estimate the bandwidth
  available to the connection.
• Two pieces of information are used
• ACK reception rate.
• Information the ACKs conveys (amount of data
  delivered).
• Bandwidth calculation
• The sampled bandwidth (bk) at time tk is
• where dk is amount of data delivered and ?k is
  the time difference between the recent and the
  last reception of ACK.
• Discrete-time filter (Tustin approximation). The
  filtered estimate of the available bandwidth at
  time tk is
• where
  and 1/t is the cutoff frequency of the
  filter.
• The weight ak are made to depend on Dk to
  counteract the effect of non-deterministic
  inter-arrival times.

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TCP-Jersey
Bandwidth Estimation

• Available Bandwidth Estimation
• Time-sliding window estimator
• Congestion Warning (CW)
• Mark all packets if the average queue length
  exceeds a threshold.
• DUPACK with CW mark ? congestion
• DUPACK without CW mark ? wireless error
• Adjust cwnd and ssthresh if receives (DUP)ACK
  with CW mark
• Explicit retransmit
• If DUPACK without CW mark, retransmits with cwnd
  unchanged

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Explicit Link Failure Notification (ELFN)
Route Failure and Wireless Error Determination

• Upstream node of the failed link sends a host
  unreachable ICMP message to the sender.
• Sender disables retransmission timers and enters
  a standby mode.
• Periodic probing to detect restored route.
• Restores retransmission timers and continues
  transmissions when ACK is received.

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Link RED (LRED) and Adaptive Pacing (AP)
Contention Reduction

• Link RED (LRED)
• Maintain an average of retries of recent packet
  transmission.
• If exceeds the minimum threshold minth, LRED
  marks packets with probability depending on the
  average of retries value.
• TCP will then reduces sending rate.
• Adaptive Pacing (AP)
• Distribute traffic in a more balanced way.
• Let some nodes wait an extra amount of back-off
  period.
• Use in conjunction with LRED.
• When LRED starts to mark packets, AP increases
  the back-off time of the pending transmission.

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Congestion Window Limit (CWL)
Contention Reduction

• The maximum spatial reuse of wireless channel is
  1/4 of the chain.
• When 1 transmits, 2 and 3 cannot transmit, but 4
  can.
• Assume perfect scheduling and no contention.
• TCP achieves best throughput when cwnd is
  approximately 1/5 of the round-trip hop-count
  (RTHC).
• Adaptively adjust the maximum cwnd to ensure the
  spatial reuse is not exceeded.

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Other Transport Layer Efforts

• Can FAST TCP and XCP work well over MANETs?
• Do not seem suitable for MANETs.
• Basic idea React faster to change.
• Fast TCP Jin et. al.
• Determine equilibrium by queuing delay and loss
  information.
• cwnd far away from equilibrium? ? Rapid (Large)
  change.
• cwnd approach equilibrium? ? Small change.
• XCP Katabi et. al.
• Explicit congestion signaling.
• Intermediate nodes estimate spare bandwidth and
  generate feedback to the sender.
• Neither protocol can deal with mobility.
• Mobility and route changes will throw off
  calculations.

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Ad-hoc TCP (ATCP)
Route Failure and Wireless Error Determination

• Normal state
• Connection initialization. TCP works normally.
• Loss state
• RTO almost expired or ATCP receives the 3rd
  DUPACK.
• TCP in persist mode (no congestion control).
• ATCP retransmits lost segment.
• Congested state
• When ECN is received. TCP works normally.
• Disconnected state
• When Destination Unreachable is received.
• TCP in persist mode.
• Send probe packets to detect re-connection.

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ADTCP
Route Failure and Wireless Error Determination

• Classification of network states
• Congestion(CONG), Channel error(CHERR), Route
  change(RTCHG), Disconnection(DISC)
• Multiple metrics
• Inter-packet delay difference (IDD)
• Short-term throughout (STT)
• Packet Out-of-order delivery Ratio (POR)
• Packet Loss Ratio (PLR)
• Identifying network states

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Detecting Spurious Retransmission
Details of some major TCP variants

• TCP-Eifel Ludwig Katz
• Use timestamp option of TCP to solve
  retransmission ambiguity.
• Sender can determine whether the received ACK is
  from original transmission or retransmission.
• After retransmission, if next ACK is from
  original transmission ? spurious!
• Restore the cwnd and ssthresh if spurious
  retransmission is detected.
• Pro Robust to sudden delay spike.
• Con Require the use of timestamp option or
  modification of TCP header.
• Forward RTO-Recovery (F-RTO) Sarolahti et. al.
• If the first ACK after retransmission advances
  the window, send two new segments.
• If the next ACK still advances the window ?
  retransmission is likely to be spurious. (It
  should be a DUPACK generated by the new
  transmitted segments)
• Pro Does not require the use of timestamp
  option.
• Con Only detect spurious retransmission
  triggered by timeout.

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TCP Adaptive Delayed Acknowledgement (TCP-ADA)
Contention Reduction

• Reduce of ACKs ? reduce contention on forward
  data transmissions.
• Maximizing the number of packet received before
  an ACK is sent, K, increases TCP throughout.
• K is equal to a full window of packets.
• Receiver estimate the average inter-arrival time
  of data packets.
• Wait wait factor x average inter-arrival time
  before sending the corresponding cumulative ACK.
• If the waiting time is completed, sends ACK.

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ACK Congestion Control (ACC) ACK Filtering (AF)
Asymmetry Alleviation

• ACK Congestion Control (ACC)
• Reduce frequency of ACKs.
• Use RED to mark ECN bit of packets.
• Packet with ECN bit set
• Sender reduces the sending rate
• Receiver increases multiplicatively the
  delayed-ACK factor, d
• For each subsequent RTT with no packet with ECN
  bit set
• Receiver decrease linearly the delayed-ACK
  factor, d
• ACK Filtering (AF)
• ACKs are cumulative.
• Traverse the queue to remove some (or all) of the
  ACKs of the same flow.

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Specific problems of TCP over MANETs

• Delayed retransmissions due to the use of
  coarse-grained timer Brakmo et. al.
• Timer
• EffectsInaccuracy in the calculation of the RTO
  ? performance drops.
• RTT and mean variance estimates are computed
  using coarse-grained timer (around 500ms).
• This granularity influence also how often TCP
  checks for an timeout event.
• Calculated RTO is gt actual RTO.
• Why is it bad for MANETs?
• TCP heavily relies on this inefficient timeout
  mechanism to detect losses.
• Why?
• cwnd is usually small.
• Not enough DUPACKs to trigger the fast retransmit.

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So Far, So Good Next Steps

• Split-TCP a new way to look at transport layer
• Separate congestion control from reliability
• Use control theory to analyze the problem
• Use directed antennas to improve TCP thruput
• Focusing the beam, minimizes interference
• Challenge co-develop an appropriate MAC protocol
• Exploit other physical layer capabilities
• CDMA, cooperative diversity

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Exciting Projects at UCR

• Exploit Spatial Diversity in MANETs
• MIMO systems multiple element antennas
• Cooperative diversity virtual MIMO
• Develop secure wireless networks
• Detecting misbehavior and intrusions
• Develop a complete comprehensive architecture
• Develop UWB-based networks
• Design a CDMA-based MAC protocol
• Develop a routing protocol

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Alleviating Asymmetry
Details of some major TCP variants

• ACK Congestion Control (ACC)
• Use Random Early Detection (RED) algorithm to
  mark packets.
• When sender receives packets with mark ? reduce
  the sending rate.
• When receiver receives packets with mark ?
  multiplicatively increase the delay-ACK factor,
  d.
• If receiver receives no mark for a RTT ? linearly
  decrease the factor d.
• Pro Reduce congestion on the constrained reverse
  path.
• Con Require RED to be implemented in each node.
  Increase burstiness of the forward data flow.
• ACK Filtering (AF)
• Take the advantage of the fact that ACKs are
  cumulative.
• Remove some (or all) the ACKs buffered in the
  queue that belong to the same flow.
• Pro Reduce congestion on the constrained reverse
  path.
• Con Special queue management is needed. Increase
  burstiness of the forward data flow.