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W3C Gen2 payload compression

Ulf Bjorkengren, Sanjeev BA@Samsung

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Compression overview

  • Low processing load through context aware algorithm using lookup tables.
  • Implemented and tested on https://github.com/UlfBj/W3C_VehicleSignalInterfaceImpl (compress branch). Plan to push to upstream master.
  • Achieved compression 3 - 5 times (version 1), 4,5 - 7 times (in coming version 2).
  • Current design/impl is invokable on Websocket only, but can be extended to HTTP.
  • Design pattern can be extended with implementations that invoke other compression algos, e. g. gzip, LZW.
  • To become a Gen2 Note, or in Best Practises?

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Compression design, version 1

  • It is activated on Websocket sessions by setting the subprotocol to “gen2cx”, where x = [1..32] which sets the number of (MSB) UUID bytes used to replace a path.
  • A message is divided up into “tokens”, where a token is everything in between two quotes, or common JSON characters ({, }, [, ], :, ,,).
  • Tokens being common JSON characters are not compressed.
  • Tokens in between quotes are compared with a predefined list, and if a match is found the list index for the matching element is added to 128, stored in a character and inserted in the compressed message as a replacement for the token.
  • Tokens being paths are compared to a list of all paths in the VSS tree, and if matching it is replaced by the number of MSB bytes in the UUID corresponding to that path that were signalled in the subprotocol.

​

  • Tokens being timestamp values are replaced by six bytes:
  • Byte1: The two least significant digits of the year (2020 is replaced by 20)
  • Byte2: The month [1..12]
  • Byte3: The day[1..31]
  • Byte4: The hour[0..23]
  • Byte5: The minute[0..59]
  • Byte6: The second[0..59]
  • Currently subseconds are not handled.

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Compression example

  • WebSocket init:
    • new WebSocket("ws://" + hostIP + ":8080", "gen2c4");
  • Request:
    • {"action":"get", "path":"Vehicle/ADAS/ABS?$pathEQ*", "requestId":"235"}

​

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