Digital Fountains: Understanding the Ideal Data Transmission Paradigm and Related Advances, Slides of Business Administration

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Digital Fountains:

Applications and Related Issues

Goals

• Explain the digital fountain paradigm for network

communication.

• Examine related advances in coding.
• Summarize work on applications.
• Speculate on what comes next.

How Do We Build

a Digital Fountain?

• We can construct (approximate) digital

fountains using erasure codes.

• Including Reed-Solomon, Tornado, LT, fountain codes.
• Generally, we only come close to the ideal

of the paradigm.

• Streams not truly infinite; encoding or decoding times; coding overhead.

History

• Reed-Solomon codes
• Tornado Codes
• Luby Transform
• Rateless/Raptor codes

Applications

• Reliable multicast
• Parallel downloads
• Long-distance transmission (avoiding TCP)
• One-to-many TCP
• Content distribution on overlay networks
• Streaming video

Reliable Multicast

• Many potential problems when multicasting

to large audience.

• Feedback explosion of lost packets.
• Start time heterogeneity.
• Loss/bandwidth heterogeneity.
• A digital fountain solves these problems.
• Each user gets what they can, and stops when they have enough.

Point-to-Point Data Transmission

• TCP has problems over long-distance connections.
  • Packets must be acknowledged to increase sending window (packets in flight).
  • Long round-trip time leads to slow acks, bounding transmission window.
  • Any loss increases the problem.
• Using digital fountain + TCP-friendly congestion control can greatly speed up connections.
• Separates the “what you send” from “how much” you send. - Do not need to buffer for retransmission.

One-to-Many TCP

• Setting: Web server with popular files, may have many open connections serving same file. - Problem: has to have a separate buffer, state for each connection to handle retransmissions. - Limits number of connections per server.
• Instead, use a digital fountain to generate packets useful for all connections for that file.
• Separates the “what you send” from “how much” you send. - Do not need to buffer for retransmission.
• Keeps TCP semantics, congestion control.

Video Streaming

• For “near-real-time” video:
  • Latency issue.
• Solution: break into smaller blocks, and encode

over these blocks.

• Equal-size blocks.
• Blocks increases in size geometrically, for only logarithmically many blocks.
• Engineering to get right latency, ensure blocks

arrive on time for display.

Other Applications

• Other possible applications outside of

networking

• Storage systems
• Digital fountain codes for errors
• Others???

Patent Issues

• Several patents / patents pending on irregular LDPC codes, LT codes, Raptor codes by Digital Fountain, Inc.
• Supposition: the theory/practice of digital fountains was greatly developed by the company and its employees.
• Supposition: but this stifles external innovation.
  • Potential threat of being sued.
  • Potential lack of commercial outlet for research.
• Suggestion: need unpatented alternative that approximate a digital fountain. - There is work going on in this area, but more is needed to keep up with recent developments in rateless codes.

Perceived Complexity

• Digital fountains are now not that hard…
• …but networking people do not want to deal with developing codes.
• A research need:
  • A publicly available, easy to use, reasonably good black box digital fountain implementation that can be plugged in to research prototypes.
• Issue: patents.
  • Legal risk suggests such a black box would need to be based on unpatented codes.

Conclusions

• Digital fountain paradigm and enabling codes have significant potential. - Many proposed applications. - More to come.
• Applications helped push forward the technology.
  • Codes with better and better properties.
• Challenge in moving from a “technology” to use in the real-world. - A simple, easy-to-use implementation based on non- proprietary might spur research community. - Need more potential killer apps to spur business community.