EE689 Lecture 4

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EE689 Lecture 4

• Review of Last Lecture
• UDP-based delivery
• TCP-modifications

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End-to-End Congestion

• UDP-based applications should do end-to-end
  congestion control
• Otherwise -- BW divided unfairly
• -- Loss of Goodput
• TCP-friendly -- enforce TCP-like control
• Hard to enforce TCP-friendly mechanism
• Multiple connections/IP-spoofing get around those
  mechanisms.

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UDP-based delivery

• Nice Applications will follow end-to-end
  congestion
• All of the flow control/congestion control
  reliability to be built into application.
• ALF - Application Level Framing - design
  principle - advocates application level of
  transport mechanisms.

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UDP-based delivery

• Packet-pair One mechanism for BW estimation
• Send two packets back-to-back, measure the delay
  between acks
• ACK Compression - Receiving timestamps
• Round robin scheduling /FIFO
• Minimum ack spacing indicates link BW

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Cyclic UDP

• Developed at UC Berkeley
• Notion of rounds -- data sent in a fixed size
  time units
• Retransmit data within the round
• Move on to next round if data not received within
  round
• Apply flow control within the round.

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Cyclic UDP

• Prioritize packets within round
• Order higher priority packets in front
• CUDP improves chances of delivery of higher
  priority packets -- retransmissions have higher
  priority
• NACK - indicate not received packets so far on
  each packet reception

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Random vs. Prioritized Loss
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Cyclic UDP

• Rounds allow timely delivery
• Rounds allow prioritization of current packets
• Application prioritizes within the round
• Uses delay and packet losses for determining
  available BW

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Estimating BW

• Loss rate 1 - bytes recvd/ bytes sent
• Expected loss rate X
• Expected delay D1
• If (L gt X) EstBW (1-L)LtBW/(1X) else if
  (meanDelay gt D1)
  EstBW (1X)LtBWD1/meanD
  elay else EstBW (1X)LtBW

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Cyclic UDP

• Adapts to Congestion
• Assumes all available BW can be used
• Not clear what happens to competing TCP
  applications
• Results indicate multiple CUDP flows share
  available BW
• Loss rate -- scalability ??

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Stored Media/Live Media

• Stored Media allows buffering of data, can
  tolerate startup delays
• More buffering -- more chances of overcoming
  congestion/smoother playout
• Buffering gt Longer latencies, not acceptable for
  interactive applications
• Vxtreme etc. exploit buffering

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CUDP Summary

• Possible to design adaptive flow control into UDP
  application
• Allows Timely Delivery and discard of expired
  packets, prioritization
• Shown to scale against CUDP apps.
• Not clear if more aggressive than TCP
• Not a multiplicative decrease response

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TCP Modifications

• If TCP congestion/flow control is so good, why
  not retain it?
• Easy to show that TCP-friendly
• Get rid of reliable/in-order delivery mechanisms
  that get in the way.
• HPF Heterogeneous Packet Flows

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HPF

• Allows marking packets high/low priority
• Provides In-order reliable delivery of high
  priority packets
• Allows low priority packets to be delivered when
  enough BW available
• If routers support priority, can drop low
  priority packets ahead of high priority packets

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HPF Layers

• Application Framing (AF) -- convert frames into
  packets, packets into frames
• Windowing, Reliability, Timing and Flow-control
  (WRTF) -- window management, flow control,
  reliability, deadlines
• Congestion Control (CC) -- congestion response,
  estimation of RTTs

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HPF Architecture
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HPF
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Another Example
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HPF Sender Interface
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HPF Receiver Interface
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HPF

• Separate the reliable delivery from windowing
  mechanisms.
• Advance rcv_next pointer based on Reliable
  packets
• Compute fraction of packets lost as opposed to
  based on sequence numbers -- a burst of losses
  doesnt lead to cwnd being set to 1.
• Multiplicative Decrease/Additive Increase

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HPF Congestion Control

• If ack_fraction gt ?, cwnd fi(cwnd), fi TCP
  slow start
• If ack_fraction lt ?, cwnd fd(cwnd), fd
  1/(1/cwnd 1 - ack_fraction)
• ? 1 - random loss probability

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HPF Performance
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HPF Priorities
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HPF - App. Adaptation
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Summary

• UDP-based delivery puts application in charge to
  do flow control, congestion response etc. -- ALF
  plus more
• Tedious for every application to implement all
  the basic mechanisms
• Separate ALF policies and implementation -HPF
  does this and follows TCP based congestion
  response