Proxy Caching Mechanism for Multimedia Playback Streams in the Internet

1
Proxy Caching Mechanism for Multimedia Playback
Streams in the Internet

• R. Rejaie, M. Handley, H. Yu, D. Estrin
• USC/ISI
• http//netweb.usc.edu/reza/
• WCW99 April 1, 1999

2
Motivation

• Rapid growth in deployment of realtime
  streams(audio/video) over the Internet
• Goals
• Maximize the quality of the delivered stream
• Minimize startup latency
• Low-latency VCR-functionality
• Minimize the load on the server the network

3
Outline

• An End-to-end Architecture
• Multimedia Proxy Caching
• Conclusion
• Future Directions

4
Streaming Applications in Best-effort
Networks (The Internet)

• End-to-end congestion control is crucial for
  stability, fairness high utilization
• Results in variable transmission rate
• Streaming applications require constant average
  consumption rate
• Streaming applications should be quality adaptive

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Quality Adaptation(QA)

• Buffering only absorb short-term variations
• Long-lived session could result in buffer
  overflow or underflow
• QA is complementary for buffering
• Adjust the quality(rate) with long-term
  variations
• Layered framework

BW(t)
Time
6
The End-to-end Architecture
Server
Client
Error Control
Quality Adaptation
Cong. Control
Acker
Playback Buffer
Internet
Buffer Manager
Buffer Manager
Transmission Buffer
Decoder
Archive
Adaptation Buffer
Data path
Control path
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Limitation

• Delivered quality is limited to the average
  bandwidth between the server and client
• Solutions
• Mirror servers
• Proxy caching

Client
Client
Client
ISP
Internet
Server
Quality(layer)
Time
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Multimedia Proxy Caching

• Assumptions
• Proxy can perform
• End-to-end congestion ctrl
• Quality Adaptation
• Goals of proxy caching
• Improve delivered quality
• Low-latency VCR-functions
• Natural benefits of caching

Client
Client
Client
Proxy
Internet
Server
9
Challenge

• Cached streams have variable quality
• Layered organization provides opportunity for
  adjusting quality

L
4
L
Quality (no. active layers)
3
L
2
L
1
L
0
Time
10
Issues

• Delivery procedure
• Relaying on a cache miss
• Pre-fetching on a cache hit
• Replacement algorithm
• Determining popularity
• Replacement pattern

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Cache Miss Scenario
Client
Client
Client

• Stream is located at the original server
• Playback from the server through the proxy
• Proxy relays and caches the stream
• No benefit in a miss scenario

Proxy
Internet
Server
12
Cache Hit Scenario
Client
Client
Client

• Playback from the proxy cache
• Lower latency
• May have better quality!
• Available bandwidth allows
• Lower quality playback
• Higher quality playback

Proxy
Internet
Server
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Lower quality playback

• Missing pieces of the active layers are
  pre-fetched on-demand
• Required pieces are identified by QA
• Results in smoothing

L
4
L
Quality (no. active layers)
3
L
2
L
1
L
0
Time
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Higher quality playback

• Pre-fetch higher layers on-demand
• Pre-fetched data is always cached
• Must pre-fetch a missing piece before its
  playback time
• Tradeoff

L
4
L
Quality (no. active layers)
3
L
2
L
1
L
0
Time
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Replacement Algorithm

• Goal converge the cache state to optimal
• Average quality of a cached stream depends on
• popularity
• average bandwidth between proxy and recent
  interested clients
• Variation in quality inversely depends on
• popularity

Client
Client
Client
Proxy
Internet
Server
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Popularity

• Number of hits during an interval
• Users level of interest (including
  VCR-functions)
• Potential value of a layer for quality adaptation
• Calculate whit on a per-layer basis
• Layered encoding guarantees monotonically
  decrease in popularity of layers

whit PlaybackTime(sec)/StreamLength(sec)
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Replacement Pattern

• Multi-valued replacement decision for multimedia
  object
• Coarse-grain flushing
• on a per-layer basis
• Fine-grain flushing
• on a per-segment basis

Cached segment
Fine-grain
Quality(Layer)
Coarse-grain
Time
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Conclusion

• End-to-end architecture for delivery of
  quality-adaptive multimedia streams
• Congestion control Quality adaptation
• Proxy caching mechanism for multimedia streams
• Pre-fetching
• Replacement algorithm
• State of the cache converges to the optimal

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Future Directions

• Extensive simulation(using VINT/ns)
• e.g. access pattern, the bandwidth distribution
• Exploring other replacement patterns
• Chunk-based popularity function

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Alternative Replacement Algorithm

• Goal to cache popular portion of each stream
• Keep track of per-chunk popularity
• Identify a victim chuck
• Apply the same replacement pattern within the
  victim chunk