1. SRP A Multimedia Protocol
Introduce ourselves
Purpose of SRP
Stream multimedia over a network.
Ken French
George Oprica
Mike Piecuch
Advisor: Mark Claypool
11/9/2018

2. Multimedia Characteristics
Loss
Latency
Undesirable for multimedia
2 key issues that SRP is designed to address
Loss – Message lost in the network transmission of the stream
Causes gaps in data that user can see
Latency - Time from sender to receiver
Causes delay in transmission
Example: Satellite feed on TV
Variance disrupts timing of playback
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3. Multimedia Over TCP Guaranteed delivery (No loss) Flow Control
Unbounded delay
Commonly used
Guarantees no overall loss
automatically retransmits for losses
flow control
exponential back-off
slow retransmission rate in heavy traffic
Cost is unbounded delay
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4. Multimedia Over UDP Best effort delivery (minimal latency)
Unbounded loss
No guarantee
Small latency
High loss
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5. Multimedia Over SRP Selective Retransmission Protocol
Combination of TCP & UDP
Retransmits a percentage of lost packets
Application layer client/server protocol
To remedy problems of multimedia transmission
SRP - Selective Retransmission Protocol
Balance between Latency of TCP and Loss of UDP
Retransmits percentage of lost packets
Application layer
Client/Server
Client requests retransmissions based on current loss/latency statistics
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6. SRP Implementation Algorithms Equal Loss Latency (ELL)
Optimum Quality (OQ)
Decision Algorithm
Retransmit
Give up
ELL
Equalize loss and latency OQ
Minimize loss and latency
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7. Testing Approach UDP blaster Token bucket router 11/9/2018
Network Topology
UDP Blaster
Random traffic over network
Token bucket router
Modified
Depth controls latency
UDP blaster
Token bucket router
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8. Sample Data 11/9/2018 Horizontal blue line is per packet latency
y-axis Left hand scale (milliseconds)
x-axis is packet number received
Spikes are retransmissions
Rising pink line is lost packets
y-axis Right hand scale (packets)
Rises every time there is a packet loss
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9. Low Loss, Low Latency 11/9/2018 Low Traffic 3% Loss 50ms Latency
Average Latency
Y-axis
Average Loss
X-axis
Scales are normalized based on desired loss and latency
250ms latency
10% loss
Arc is range of acceptable qualities
Closer to the origin, the better the quality
Origin is perfect quality
TCP
exponential back-off
UDP
some loss
low latency OQ
has no loss
ELL
did not perform as well as oq
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10. High Loss, High Latency 11/9/2018 High Traffic conditions 15% loss
275ms RTT
TCP
off the chart
due to exponential back-off
UDP
Highest loss
Lowest latency
ELL
better than UDP OQ
within the acceptable quality range
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11. Conclusions SRP is a good balance Great potential Additional tuning
SRP is good balance of
High loss of UDP
High latency of TCP
Great potential for
Controlling multimedia streams
Tuning can
increase performance of SRP in extreme network conditions
tuning different parameters
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12. Future Work Flow control Loss detection Additional algorithms
in the idea of TCP’s exponential back-off
Loss detection
how long to wait for packet?
Additional algorithms
Decision algorithms can be customized
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