The Role of PCE in the Evolution of Transport Protocols

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The Role of PCE in the Evolution of Transport
Protocols

• Pfldnet 2005, Lyon, France
• M. Y. Medy Sanadidi
• http//www.cs.ucla.edu/medy
• http//www.cs.ucla.edu/NRL/hpi/tcpw/

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Recent Issues in Transport Protocols

• Large Pipes Utilization
• Steady state
• Start-up
• Impact of Wireless Links
• Last-hop wireless
• Multihop contention networks
• Fairness for asymmetric flows
• Protocols Co-Existence
• New Paradigms
• Voice/Video
• Store-and-forward at Transport layer (e.g. PEPs,
  P2P/Overlays)

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Example Satellite/802.11 Networks
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Outline

• Path Characteristics Estimation (PCE)
• Prospects for Higher Efficiency
• Future of Friendly Co-Existence
• Addressing the New Paradigms
• Summary

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Path Characteristics Estimation (PCE)

• Characteristics of Interest
• Links capacity
• Path dynamic range, i.e. buffering capacity
• Cross traffic level, path-persistence,
  responsiveness
• Random loss
• Multihop wireless connectivity, contention, route
  diversity
• Participating Nodes
• Sources only
• Sources and Destinations
• Forwarding nodes (routers, base stations,
  multihop wireless nodes)

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Sharing a Link
interface queue
Buffer space
bottleneck
backlog
residual bandwidth
Flow2
bandwidth
2 flows, red one is non-responsive
Flow1
fair share ?
Propagation Time
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A Hierarchy of Characteristics

• Achieved rate
• Delay/Dynamic Range
• Packet loss

Flow Behavior

• Intensity
• Path persistence
• Elasticity

Cross Traffic Load

• Links capacities
• Propagation times
• Buffer space
• Errors

Architecture
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Path Capacity Estimation

• Path Capacity capacity of narrow link
• Pathrate rely on packet pair dispersion
  measurements followed by statistical processing
  of results
• CapProbe use dispersion measurements perform on
  line filtering of results based on end-to-end
  delay
• TcpProbe an adaptation of CapProbe into TCP with
  minimal sender side only changes

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CapProbe and TcpProbe
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Prospects for Higher Efficiency

• Steady State
• Congestion avoidance (FAST) stable at high
  throughput, co-existence ??, and random loss
  impact ??
• Scaling up congestion recovery (HSTCP, STCP)
  higher throughput, but fairness and stability ??
• Scaling up congestion recovery (BIC) improves on
  the above in fairness
• Forwarder Based (XCP) superb, when we are done
  with implementation issues
• PCE reliance (TCP Westwood, TCP Peach) Peach
  requires forwarder priority support, TCPW
  requires good estimation at high speeds

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Using PCE

• Tahoe/Reno/NewReno estimate
• Packet loss via Dup Acks
• RTT average and variance
• Maintain a pipe size (or bandwidth-delay product)
  estimate ssthresh
• Vegas/FAST
• Achieved Rate and its relation to the Expected
  Rate, or equivalently RTT and RTTmin, or Queuing
  delay
• HSTCP/STCP/BIC
• Use current window size (Expected Rate) in
  addition to all items above in Reno

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Using PCE (2)

• TCPW estimates
• Packet loss and type of loss
• Narrow link capacity, or Path capacity
• Achieved Rate
• Dynamic Range resulting from buffering space
• (RTTmax-RTTmin)
• XCP measures at forwarders the actual
• Links capacities
• Load intensity
• RTT (obtained from sources)

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Large Pipes Measurements Results
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Acceptable Long Term Efficiency
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Some Difference in Completion Times
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Co-Existence at Gbps Speed
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Random Loss Impact
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Effect of Random Loss
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TCPW Mining ACK Streams for PCE
Bottleneck
packets
Receiver
Sender
ACKs
Internet
measure

• Rely on PCE ( e.g. capacity, achieved rate,
  dynamic range) to determine an Eligible Rate
  Estimate (ERE)
• ERE is used to size the congestion window after a
  packet loss

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TCPW BE (2001)

• BE Sampling
• With Saverio Mascolo (P. Bari) and Claudio
  Casetti (P. Torino)
• Packet pair
• a noisy estimate of achieved rate/capacity
• Provides throughput boost under random loss,
  overestimates under congestion
• Efficient but not friendly

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TCPW RE (2002)

• RE Sampling
• Packet train
• Fair estimate under congestion, underestimates
  under random loss
• Used in TCPW RE and inTCP Westwood (S. Mascolo)
  
• Friendly

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Adaptive Estimation in TCPW

• TCPW CRB ERE ? BE if random loss, else ERE? RE
• TCPW ABSE ERE ? RE ltX lt BE by continuously
  adapting the bandwidth sample width to congestion
  level
• TCPW Astart use ERE to help short lived flows
• TCPW BBE ERE ? u C (1-u) RE,
• where u is a congestion measure taking into
  account path dynamic range

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TCPW CRB (2002)

• Combined Rate and Bandwidth
• Binary adaptive
• Congestion measure Expected Rate/Achieved Rate
• Clarified Efficiency/Friendliness tradeoff

over a threshold ?
ssthresh, cwnd BE x RTTmin
Congestionmeasure
Packet Loss Detected
Ssthresh, cwnd RE x RTTmin
under a threshold ?
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TCPW ABSE (2002)
Under Congestion
Under No Congestion

• Adaptive Bandwidth Share Estimation
• Adapt the sample interval Tk according to
  congestion level
• Congestion measure, similar to Vegas
• Tk ranges from one interACK interval to
  current RTT
• Better Efficiency/Friendliness profile than CRB

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Helping Short Lived Connections

• Approaches
• Cached ssthresh
• Larger initial window
• PCE based Hoes TCPW Astart
• Negotiation Quick-Start
• No problems here for XCP!

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TCPW Astart (2003)

• Take advantage of ERE
• Adaptively and repeatedly reset ssthresh ? ERE
  until sender window reaches estimated pipe size,
  or encounters packet loss

• Includes multiple mini exponential increase,
  and mini linear increase phases
• cwnd grows slower as it approaches BDP
• Connection converges faster to its pipe size with
  less buffer overflow, since it adapts to pipe
  size and transient loading

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Astart First 20 Seconds Throughput

• RTT 100ms, Buffer BDP

• Good scaling with capacity and propagation time
• Robust to buffer size variation

Bottleneck capacity 40 Mbps, Buffer BDP
RTT 100ms, Bottleneck 40 Mbps
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TCPW BBE (Work in Progress)

• With H. Shimonishi (NEC, Tokyo)
• Buffer and Bandwidth Estimation
• Estimates Capacity using TcpProbe (much more
  accurate than BE!!)
• Higher efficiency at higher random loss rates
  (e.g. 5-10)
• Estimates Dynamic Range (related to buffer size)
• Improves TCPW control as a function of congestion
  
• The result is higher efficiency and robust
  friendliness even at small buffers!

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TCPW BBE Algorithms (ICC 2005)

• Dynamic Range estimate
• Dmax RTTcong loss - RTTmin
• Current Delay Distance
• D RTT RTTmin

• Eligible Rate estimate
• ERE u C (1-u) RE
• Note u0 if D and Dmax are small

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Opportunistic Friendliness of TCPW-BBE
If Reno under-performuse all the opportunity
provided without hurting co-existing Reno flows
TCP-Reno Sender
Receiver
RTT 40msec
0.001 loss
10M-1Gbps
Receiver
TCPW-BBE Sender
If Reno performsachieve similar to Reno
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The Future of Friendly Co-Existence

• Defining Friendliness
• TCP Friendliness
• Achieve throughput equal to that of TCP Reno
  under some conditions (RTT, packet loss rate)
• Problematic if Reno under-perform e.g. under
  random losses
• Opportunistic Friendliness
• If Reno performs, achieve similar to Reno
• If Reno under-perform use all the opportunity
  provided without hurting co-existing Reno flows

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Evaluating a New Proposed ProtocolThe
Efficiency/Friendliness Profile

• Each point in the graph is obtained as follows
• N legacy flows gt
• legacy throughput tR1
• total utilization U1
• N/2 legacy, N/2 proposed flows gt
• legacy throughput tR2
• Total utilization U2
• Efficiency Improvement
• E U2 / U1
• Friendliness
• F tR2 / tR1

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E/F Profiles of TCPW BE, CRB and ABSE
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E/F Profile of Vegas
1.5
Vegas vs. NewReno (RED)
1.4
1.3
Utilization Ratio G (Efficiency)
N8
N16
1.2
N4
N24
N2
1.1
1
0.4
0.6
0.8
1
1.2
1.4
Throughput Ratio L (Friendliness)
Vegas uses fixed targeted queue length gt varying
friendliness depending on number of connections!
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Addressing New Paradigms

• Audio/Video Streaming
• Increasing portion of the total traffic with
  distinct requirements
• Multihop Wireless
• Difficult fundamental issues
• Store-and-forward at the Transport Layer
• Revisit early problems and new opportunities

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Continuous Media Transport

• Requirements
• Minimum bandwidth
• Upper bound on delay
• Lower reliability requirements than in FTP
• Adaptive streaming objectives
• Delivered quality
• Congestion control
• Support for adaptive coding

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Addressing Continuous Media Issues

• Issues with the standard protocols
• UDP no congestion or error control
• TCP AIMD behavior undesirable due to fluctuation
  in rate, and consequently delay, and intolerance
  to random loss
• DCCP provides an excellent framework, recommends
  TFRC as one possible protocol, but allows for
  alternatives
• TFRC is equation based, rate-equivalent to Reno,
  with smoother delivery suitable for streaming
• SCTP enables multiple streams with different
  congestion control mechanisms, among other
  features

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Streaming Over Wireless

• Under random loss, Reno and its rate-equivalent
  TFRC, will both under-perform
• Approaches, some with loss discrimination, have
  been proposed
• TFRC Wireless
• Combination of loss discrimination schemes,
• Multi-TFRC
• Multiple TFRC connections until link is congested
• VTP
• Rate estimation and loss discrimination

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Performance Comparison
Efficiency in presence of errors5 error rate,
single connection
Rate adaptation5 error rate, single
connectionwith on/off CBR cross traffic
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TCP over Multihop Wireless

• Packet losses due to
• Contention due to hidden terminals
• Varying channel quality
• Route collapse
• Buffer overflow ??
• Solution approaches
• Neighborhood RED
• Delayed ACK extension
• Sizing the TCP window for contention reduction

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Store Forward at the Transport Layer

• Overlays/P2P tunneling through TCP connections
• PEPs breaking ETE path into concatenated TCP
  connections, e.g. satellites
• New(?) Requirements
• Buffer management and priority schemes for better
  ETE application protocol performance
• TCP Receiver advertised window role
• Related item Prioritized TCP for QOS at the
  Transport layer (TCP-LP, TCPW-LP)

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Summary

• Excellent progress by many approaches for scaling
  efficiency with pipe size
• Focus on PCE techniques is promising, e.g. TCPW
  provides
• Scalable efficiency
• Robustness to random loss
• Tunable opportunistic friendliness
• Streaming, multihop wireless, and forwarding at
  the Transport layer to receive attention and make
  good progress

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Steady State Characteristics (TCPW RE)
For small loss rate, TCPW has much larger
window than NewReno. More scalable!
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Fairness (TCPW RE)
For small loss rate, TCPW is more fair than
NewReno