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EE689 Lecture 4

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Title: EE689 Lecture 4 Author: Steven Woodward Last modified by: Steven Woodward Created Date: 1/26/1999 7:04:42 PM Document presentation format: On-screen Show – PowerPoint PPT presentation

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Title: EE689 Lecture 4


1
EE689 Lecture 4
  • Review of Last Lecture
  • UDP-based delivery
  • TCP-modifications

2
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.

3
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.

4
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

5
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.

6
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

7
Random vs. Prioritized Loss
8
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

9
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

10
(No Transcript)
11
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 ??

12
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

13
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

14
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

15
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

16
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

17
HPF Architecture
18
HPF
19
Another Example
20
HPF Sender Interface
21
HPF Receiver Interface
22
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

23
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

24
HPF Performance
25
HPF Priorities
26
HPF - App. Adaptation
27
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
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