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End-to-End Arguments in System Design

End-to-End Arguments in System Design. CSCI 634, Fall 2010. What ’ s e2e argument?. Function placement among the modules of a distributed system Functions placed at low levels may be redundant or worthless Placing functions at low levels are justified only as performance enhancements .

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End-to-End Arguments in System Design

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  1. End-to-End Arguments in System Design CSCI 634, Fall 2010

  2. What’s e2e argument? • Function placement among the modules of a distributed system • Functions placed at low levels may be redundant or worthless • Placing functions at low levels are justified only as performance enhancements E2E argument: against low-level function implementation

  3. Rationale supported by e2e • Moving a function upward in a layered system closer to the application that uses the function • The function can completely and correctly be implemented only with the knowledge and help of the application • Incomplete version at low level may be useful as a performance enhancement

  4. Communication systems • Reliable data transmission • Encryption • Duplicate message detection • Message sequence • Many more …

  5. Reliable file transfer • Objective: correctly moving a file from host A to host B • Five threats (low in probability): • Hardware faults in disk • Software bugs of the file system, file transfer program and data delivery systems • Hardware errors in buffering and copying • Errors during data delivery • Hosts may crash

  6. Solution • End-to-End check and retry • Each file is stored with checksum • File is delivered with checksum • Compare the received checksum with the recalculated one • A real story happened at MIT • Should low levels play no part in reliability?

  7. Performance aspect • The probability of a file being corrupted exponentially increases with its length • Retransmit the whole file is expensive, but retransmit a corrupted packet is cheap • A reliable data delivery at low levels can significantly improve application performance

  8. Reliability at low-level • No need for “perfect” reliability • E2E checksum is still needed no matter how reliable at the low levels • Placing functions at low-level should be carefully designed

  9. It may cost more • Some high-level applications may not need the function, but have to pay for it anyway • Low levels may not have as much information as the higher levels to do the job

  10. Comments • E2E argument is not an absolute rule, but rather a guideline for system design • We must carefully identify the end to which the argument should be applied

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