CADPCPTP in a nutshell

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CADPC/PTP in a nutshell
Michael Welzlhttp//www.welzl.at,
michael.welzl_at_uibk.ac.at Distributed and Parallel
Systems Group Institute of Computer
Science University of Innsbruck
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Outline

• Problem identification
• The PTP signaling protocol
• The CADPC congestion control mechanism
• Simulation results
• Conclusion

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Some problems with TCP(-like) CC

• Special links (becoming common!)
• noisy (wireless) links
• long fat pipes (large bandwidthdelay product)
• Stability issues
• Fluctuations lead to regular packet drops
  reduced throughput ? problematic for streaming
  media
• Stability depends on delay, capacity, load, AQM
• Rate hard to control / trace / predict
• Load based charging difficult
• Main reason binary congestion notification (E)CN
• when it occurs, its (almost) too late

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The CADPC/PTP Solution

• Totally different CC model
• only rely on rare explicit bandwidth (traffic)
  signaling
• Assumptions
• extra forward signalling for CC good idea (?
  common belief)
• router support
• mechanism must clearly outperform TCP to justify
  (even a little!) additional traffic
• timeouts necessary for loss of signalling
  packetsrate should not depend on round trip time

... yes it does -)
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The Performance Transparency Protocol (PTP)

• Basic idea query routers for performance related
  information
• designed to be as lightweight as possible
• Stateless simple ? scalable!
• all calculations _at_ end nodes
• Only every 2nd router needed for full
  functionality
• PTP Available Bandwidth Determination
• packet queries for
• nominal bandwidth (ifSpeed) address traffic
  counter (if(In/Out)Octets) timestamp)
• 2 consecutive such packets table of traffic
  interval at all routers at receiver
• receiver calculates available bandwidth at
  bottleneck, informs sender
• Designed for flexibility - two modes
• Forward Packet Stamping, Direct Reply (not for
  available bandwidth (byte counters))

No problems w/ wireless links!
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Forward Packet Stamping how it works
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Forward Packet Stamping how it works
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Forward Packet Stamping how it works
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CADPC a new CC mechanism

• Congestion Avoidance with Distributed
  Proportional Controlfully distributed
  convergence to max-min fairness
• each source increases/decreases the rate
  depending on its capacity share
• Only depends on old rate, smoothness factor and
  traffic
• does not depend on RTT
• Feedback packets can be delayed ? scalable
• reasonable choice 4 x RTT
• Control realises logistic growth
• Asymptotically stable in simplified fluid model
  with synchronous RTTs
• Smooth convergence to a steady rate

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Some simulation results

• Many more can be found in
• Michael Welzl, Scalable Performance Signalling
  and Congestion Avoidance, Kluwer Academic
  Publishers 2003.

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CADPC vs. TCP
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Smoothness
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Startup enhancement
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Heterogeneous RTTs Robustness
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CADPC vs. TCP-friendly CC. mechanisms
Throughput
Loss
Avg. Queue Length
Fairness
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CADPC vs. 3 TCP(ECN) flavors
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CADPC Advantages

• high utilisation
• close to zero loss
• small bottleneck queue length
• very smooth rate
• fully distributed precise max-min-fairness
• rapid response to bandwidth changes (e.g. from
  routing)
• provable asymptotic stability (synchronous RTTs,
  fluid model)

some say its impossible )
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CADPC Advantages /2

• Useful for asymmetric links
• Useful for noisy (wireless) links long fat
  pipes
• Useful for QoS and load-based charging

Disadvantages

• Requires router support
• Requires traffic isolation because
• not tcp-friendly
• slowly responsive bad results with web traffic

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Related work

• XCPclosely related main difference
• CADPC/PTP explicit extra signaling packets (e.g.
  only 1 probe message every 4 RTTs, no TCP-like
  ACKs for every packet)
• XCP routers examine every (payload) packet,
  TCP-like ACKs for every packet necessary
• FAST TCP
• no explicit help from routers utilizes delay
• good idea, but less efficient than utilizing
  traffic measurements (like ECN reaction almost
  too late)
• patent protected
• Scalable TCP, Highspeed TCP
• different increase decrease behavior
• less efficient than above variants

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Conclusion

• Separate signaling protocol RTT-independence
  high efficiency make CADPC/PTP applicable in
  various domains
• Possibilities
• control of traffic aggregates (signaling between
  edge routers)
• control of microflows within a DiffServ class ?
  scalable fine-grain QoS
• etc. !
• This is where projects come into play !!!

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The End ...

• Publications
• CADPCPTP ns code
• PTP Linux code (router kernel patch end system
  implementation)
• Future updates

http//www.welzl.at/ptp