1 dude, where is my packet?. nus.soc.cs5248 ooi wei tsang 2 overview characteristics of the internet...

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Dude, Where is My Packet?

NUS.SOC.CS5248OOI WEI TSANG

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Overview

Characteristics of the InternetGeneral techniquesError recovery for audioEffect of loss on MPEGError recovery for MPEG

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Loss Characteristics of The Internet

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Characteristics of Internet60-70% of paths do not show

any loss

Those with loss have an average of 4.5 – 6% packet loss

[Paxson97] End-to-end Internet packet dynamics

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Packet Loss Pattern

1000

30

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Characteristic of InternetBursts of loss are typically short

(2-3 consecutively loss packets)

Long burst do occur

Burst may occur periodically

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Wireless Link

Loss rate measured in my office 10 ~ 20%

up to 50% reported!

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General Error Recovery Techniques

Perkins, Hodson and HardmanIEEE Network Magazine 1998

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Retransmission

1 2 3 4

1 2 4

3

3

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Redundant Data

1 2 3 4

1 2 4

2

2 3

3 4 5

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2 4

Error Concealment

1 2 3 4

1

12

Retransmission

for audio

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Retransmission is Bad

Need scalable feedbackNACK consumes bandwidthMay need to retransmit

frequentlyRetransmission maybe useless

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Retransmission is GoodOnly retransmit when neededExact recovery

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Scalable RetransmissionOn packet loss

T = random(0, RTT)wait for Tmulticast NACK

On receiving NACK from otherssuppress own NACK

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Retransmit when

group size is small loss rate is low large latency acceptable

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Redundant Data

for audio

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Parity FEC

1011001

1000010

0001001

XOR 0010010

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Parity FEC

1011001

1000010

0001001

0010010

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Parity FEC Ordering

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Parity FEC Ordering

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Parity FEC Ordering

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Parity FEC Ordering

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Parity FEC Ordering

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Reed-Solomon Code

RS(n,k)

RS

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Reed-Solomon Code

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Media Specific FEC

1 2 3 4

1 2 4

2

2 3

3 4 5

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Interleaving

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Pro and Cons

Media Independent FECOverhead for redundant dataExact recoveryCould be computationally costly

Media Specific FECApproximate recovery only

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Error Concealment

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Effect of Loss on AudioSpeech

Human ears can interpolateLoss up to length of phoneme can

still be tolerable

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Insertion-based RepairSpliceSilence SubstitutionNoise SubstitutionRepetition

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Other Repair Methods

InterpolationRegeneration

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Pro and Cons

Could be computationally costlyApproximate recovery onlyDoes not work well for long burst

of packet loss

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Colin’s Recommendations

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Non-Interactive Apps

InterleavingFECRetransmission for unicast only

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Interactive ApplicationsMedia Specific FEC

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Error Concealment

Repeat

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Effects of Packet Loss on MPEGJill Boyce and Robert GaglianelloACM Multimedia 1998

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Measurement-based StudyNeed to understand the problem

before proposing solution

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Data Gathering MethodFrom: NYC13 , Austin21, London18

To: Holmdel, NJ

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Data Gathering MethodVideo:

Two 5-mins MPEG30 fps384 kbps and 1 MbpsIBBPBBPBBPBBPBBQSIF 176x112 and SIF 352x240One row per slice

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Average Packet Loss

0

2

4

6

8

10

12

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Packet Loss Rate

(% )

New York Texas London

1Mbps384kbps

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Packet Loss vs Time

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How to Fill Packets

B 1 frame 1 packetP 1 slice 1 packetI 1 slice 1 packet

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Packet Size Distributions

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Packet Size vs Loss Rate

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Lesson

1 slice per packet even if larger than MTU

If smaller packets means higher loss rate: fill packet until MTU

Else one slice per MTU

Alt: Change size of slice

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Frame Effected by Errors

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Correlation of Packet Loss

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Overview of Error Recovery for Video

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Methods

RetransmissionError ConcealmentFECLimiting Error Propagation

Reference Frame Selection

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Reference Frame Selection

I P B B P

X

I

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Methods

RetransmissionError ConcealmentFECLimiting Error Propagation

Reference Frame SelectionChanging Temporal Pattern

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Changing Temporal Patterns

Injong RheeSIGCOMM ‘98

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Basic Idea

“Better Late Than Never!”, orLate packet is still useful

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MPEG Frame Pattern

I B B P B B P

I P B B P B B

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H.261 Error Propagation

I P P P P P PX X X X X X

I P P P P P PX X X X

retransmission

loss

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H.261 Frame Pattern

I P P P P P PX

loss

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I P P P P P P

H.261 Frame Pattern

X

loss

X X

retransmission

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PTDD

Periodic Temporal Dependency Distance

Large PTDDLater DeadlinesLonger Error PropagationsLess Temporal Dependencies

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QAL

I P P P

I P P P Base Layer

EnhancementLayer

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QAL

I P P P

I P P P Base Layer + FEC

EnhancementLayer

X

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QAL + PTDD

I P P P

I P P P Base Layer

EnhancementLayer

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SR-RTP

Feamster and BalakrishnanPV’02

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Overview

An analytical model for packet loss

SR-RTPPost Processing

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Analytic Model

p : prob. packet loss f : observed frame rate : frame drop rateSi : mean num. of packets in

frames of type iP(i) : prob. frame type is iP(F) : prob. frame is useless

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p : prob. packet loss f : observed frame rate : frame drop rateSi : mean num. of packets in frames of type iP(i) : prob. frame type is iP(F) : prob. frame is useless

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Is it accurate?

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SR-RTP

Extensions to RTP for selective reliability

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SR-RTP

0 Length

ADU Sequence Number

ADU Length

ADU Offset

Priority Layer Number

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Feamster & Balakrishnan’s ADU1 ADU = 1 Frame

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Loss Detection

LEN: 50ADU: 0ADU Size: 100ADU Off: 0

LEN: 20ADU: 0ADU Size: 100ADU Off: 80

LEN: 50ADU: 1ADU Size: 150ADU Off: 40

LEN: 50ADU: 3ADU Size: 50ADU Off: 0

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Retransmission RequestBased on priorityExample:

I-Frame: highest priorityP-Frame: higher priority if closer

to prev IB-Frame: no retransmition

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Error Concealment for VideoRepeat pixels from previous

frameInterpolate pixels from

neighbouring regionInterpolate motion vectors from

previous frame

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Inferred MV for P Frame

I P

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Inferred MV for I Frame

P P I

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Evaluation

Peak Signal to Noise Ratio (PSNR)

2552 over average square of differences in pixel values

Calculated in dB (10 log 10)

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Performance of SR-RTP

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Today’s Summary

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How to recover packet lossRetransmissionFECError Concealment

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How to limit the damage of error InterleavingKey frame selectionChange reference frame