Applications: A Selective Text-Based vs. Multimedia Retransmission - - PDF document

1 A Selective Retransmission Protocol for Multimedia on the Internet

Mike Piecuch , Ken French, George Oprica and Mark Claypool

Computer Science Department Worcester Polytechnic Institute

Proceedings of SPIE Multimedia, Systems and Applications Conference Boston, November 2000

Applications: Text-Based vs. Multimedia

• Text

– Strict loss constraints – Minimal timing constraints

• Multimedia

– Forgiving to loss – Requires timing constraints

Protocols: TCP vs. UDP

• TCP

– No loss – Retransmits all lost messages – Potentially large latency

• UDP

– Potentially unbounded loss – Does no retransmission – Minimal latency

• Neither is what you want!

Our Solution: A Selective Retransmission Protocol

• Balances the extremes of TCP and UDP
• Tradeoff between loss and latency
• Retransmits a percentage of lost packets

– If end-to-end delay is large, may accept loss – If end-to-end delay is small, may always request retransmission – If loss rate is very high, may request retransmission – How to decide?

Groupwork

• Measure of loss
• Measure of latency
• Packet is lost
• … Do you request retransmission?
• Consider:

– Quiet WAN, interactive audio – LAN, broadcast video – Lossy MAN, interactive audio

Decision Algorithms

Increasing Loss

Increasing Latency

(Kleinrock, 1992)

(Request Retransmission) (Give up)

2

Decision Algorithms

Increasing Loss

Increasing Latency

(Kleinrock, 1992)

(Request Retransmission) (Give up)

Policies

• OQ
• ELL

Acceptable Quality

Approach

• Implement SRP and “application”
• Setup “WAN” test-bed
• Run “application” over

– TCP

• No loss
• Low latency

– UDP

• Medium loss
• Medium latency

– SRP

• High loss
• High latency
• Measure “Quality”
• Analyze Results

Implementation of SRP

• Application layer client/server protocol

– No “kernel hacking” (yet) – Built on top of UDP

• Measure loss and latency

– Use to decide when to request retransmission

• Decision algorithm modular

– Equal Loss Latency (ELL) – Optimum Quality (OQ)

Sample SRP Session

Data Block Time Client Server

Request retransmission (Sequence Numbers)

Sample SRP Session

Data Block Time Client Server

Do not request retransmission

Experiments

• UDP traffic generator
• Token bucket router to control loss and latency
• Audio session 8000 bytes/sec

– Sample rate 160ms, packet size 1280

Client Traffic Generator

10BaseT network 10Base2 network

Traffic Generator Router Server

3

Sample Data

50 100 150 200 250 1 51 101 151 201 251 301 351 401 451 501 551 601 Packet Number Latency (ms) 5 10 15 20 25 30 35 40 45 Lost Packets

Low Loss, Low Latency

3% Loss , 50 ms Round Trip Time 0.2 0.4 0.6 0.8 1 1.2 1.4 0.5 1 1.5 Loss (normalized) Latency (normalized) SRP - ELL S R P

• O

Q UDP TCP

(Kleinrock, 1992)

High Loss, High Latency

15% Loss , 275 ms Round Trip Time 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0.5 1 1.5 2 Loss (normalized) 10% Latency (normalized) 250ms SRP - ELL SRP - OQ UDP TCP

Conclusions

• TCP and UDP provide extremes

– Not what Multimedia wants

• SRP can provide a balance
• Tuning of SRP depends upon

– Application – Measure of “quality” – Measurement of network (loss, RTT)

Future Work? Future Work

• Repair (FEC)
• Congestion control
• Loss detection (timeout)
• Additional decision algorithms
• Multicast