TCP-LP: A Distributed Algorithm for Low Priority Data Transfer

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TCP-LP A Distributed Algorithm for Low Priority
Data Transfer
Aleksandar Kuzmanovic Edward W. Knightly
Rice Networks Group http//www.ece.rice.edu/networ
ks
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Motivation

• Traditional view of service differentiation
• High priority real-time service
• Best-effort everything else
• Whats missing?
• Low-priority (receiving only excess bandwidth)
• Lower than best-effort!
• Non-interactive apps, bulk download
• Speeds up best-effort service
• Inference of available bandwidth for resource
  selection
• Routers could achieve via a low (strict) priority
  queue
• Objective realize low-priority via end-point
  control
• Premise routers will not help

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Applications for Low Priority Service

• LP vs. rate-limiting
• P2P file sharing
• Often rate limited
• Isolation vs. sharing
• LP vs. fair-share
• Bulk downloads
• Improve my other
• applications
• Data-base replication
• across the Internet

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Problem Formulation Design Objectives

• Low-priority service objectives
• Utilize the excess/available capacity
• What no other flows are using
• TCP-transparency (non-intrusiveness)
• Inter-LP flow fairness (fair-share of the
  available bandwidth)

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Origins of the Available Bandwidth

• Why is excess bandwidth available when TCP is
  greedy?
• TCP is imperfect
• Cross-traffic burstiness
• Delayed ACKs due to reverse traffic frees up
  available bandwidth
• Short-lived flows
• Majority of traffic consists of short-lived flows
  (web browsing)
• Bandwidth gaps between short lived-flows

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Illustration of TCP Transparency

• LP flow utilizes only
• excess bandwidth
• Does not reduce the throughput of TCP flows

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How Is This Different from TCP?

• In presence of TCP
• cross-traffic
• TCP achieves fairness
• LP achieves
• TCP-transparency

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Fairness Among LP Flows

• Inter-LP-fairness is
• essential for
• simultaneous
• file transfers
• estimates of available
• bandwidth

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TCP-LPA Congestion Control Protocol

• Key concepts
• Early congestion indication
• One-way delay thresholds
• Modified congestion avoidance policy
• Less aggressive than TCP
• Implication Sender-side modification of TCP
• incrementally deployable and easy to
  implement

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Early Congestion Indication

• For transparency, TCP-LP must know of congestion
  before TCP
• Idealized objective buffer threshold
• indication
• Endpoint inference one-way delay threshold
• RFC1323
• Source - destination time stamping
• Synchronized clocks not needed
• Eliminates bias due to reverse traffic

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TCP-LP Congestion Avoidance

• Objectives LP-flow fairness and TCP transparency
• LP-flow fairness
• AIMD with early congestion indication
• Transparency
• Early congestion indication
• Inference phase goals
• Infer the cross-traffic
• Improve dynamic properties
• MD not conservative enough

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TCP-LP Timeline Illustration
- Send 1 pkt/RTT - Ensure available x bandwidth
gt 0
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TCP-LP Timeline Illustration

• - AI phase
• - CWND/2 upon __early congestion xxindication
• - Inference phase

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TCP-LP Timeline Illustration

• 2nd CI gt CWND1
• - Inference phase

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TCP-LP Timeline Illustration
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Low-Aggregation Regime

• Hypothesis TCP cannot attain 1.5 Mb/s throughput
  due to reverse cross-traffic
• How much capacity remains and can TCP-LP utilize
  it?

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TCP-LP in Action

• TCP alone 745.5 Kb/s
• TCP vs. 739.5 Kb/s
• TCP-LP 109.5 Kb/s

TCP-LP is invisible to TCP traffic!
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High-Aggregation Regime with Short-Lived Flows

• Bulk FTP flow using TCP-LP vs. TCP
• Explore delay improvement to web traffic
• Explore throughput penalty to FTP/TCP-LP flow

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TCP Background Bulk Data Transfer

• Web response times
• are normalized

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TCP-LP Background Bulk Data Transfer

• Web response times improved
• 3-5 times
• FTP throughput TCP 58.2
• TCP-LP 55.1

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Conclusions

• TCP-LP adds a new service to the Internet
• General low priority service (compared to
  best-effort)
• TCP-LP is easy to deploy and use
• Sender side modification of TCP without changes
  to routers
• TCP-LP is attractive for many applications ftp,
  web updates, overlay networks, P2P
• Significant benefits for best effort traffic,
  minimal throughput loss for bulk flows
• http//www.ece.rice.edu/networks/TCP-LP