Re: [lp-wan] ACK compression

[Olivier Gimenez <[e-mail]>]

Pascal, Dominique, thank you for your comments

You are right, there is an issue with the algorithm I proposed. Please find following an updated proposition.

Also I think that my example was wrong and misleading: an uncompressed bitmap when penultimate tile is lost is not finishing by 0 but 1, and bitmap size is the number of fragments (2^N with N = FCN size). This is shown in  figure 29 of draft-ietf-lpwan-ipv6-static-context-hc-21


Olivier


For transmission, the Compressed Bitmap in the SCHC ACK message is
   defined by the following algorithm (see Figure 16 for a follow-along
   example):

   o  Build a temporary SCHC ACK message that contains the Header
      followed by the original Bitmap (see Section 8.2.2.3<https://tools.ietf.org/html/draft-ietf-lpwan-ipv6-static-context-hc-21#section-8.2.2.3> for a
      description of Bitmaps).

   o  Select following case regarding bitmap’s last two bits

   o  Case b’11

        x  Position scissors at the end of the Bitmap, after its last bit.

        x  While the bit on the left of the scissors is 1 and belongs to the
           Bitmap, keep moving left, then stop.  When this is done,

        x  While the scissors are not on an L2 Word boundary of the SCHC ACK
           message and there is a Bitmap bit on the right of the scissors,
          keep moving right, then stop.

        x  At this point, cut and drop off any bits to the right of the
           scissors
   o Case b’01
        x  Position scissors at the penultimate bit (after the last 0)

        x  While the bit on the left of the scissors is 0 and belongs to the
           Bitmap, keep moving left, then stop.  When this is done,

        x  If the scissors are on an L2 word boundary of the SCHC ACK
           Bitmap, move right until next L2 word boundary. When this is done,

        x  While the scissors are not on an L2 Word boundary of the SCHC ACK
           message and there is a Bitmap bit on the right of the scissors,
          keep moving right, then stop.


So the examples might be, for FCN size N = 4


Uncompressed bitmap 1:

   |---- SCHC ACK Header ----|--------      Bitmap     ------|

             |-- T --|-M-| 1 |   2^N                         |

   +--- ... -+- ... -+---+---+-------------------------------+

   | Rule ID |  DTag | W |C=0|1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1|

   +--- ... -+- ... -+---+---+-------------------------------+

           next L2 Word boundary ->|

compressed bitmap 1:

   |---- SCHC ACK Header ----|-----   Bitmap    ----|

             |-- T --|-M-| 1 |

   +--- ... -+- ... -+---+---+----------------------+

   | Rule ID |  DTag | W |C=0|1 0 1 0 0 0 0 0 0 0 0 |

   +--- ... -+- ... -+---+---+----------------------+

           next L2 Word boundary ->|



Uncompressed bitmap 2:

   |---- SCHC ACK Header ----|--------      Bitmap     ------|

             |-- T --|-M-| 1 |   2^N                         |

   +--- ... -+- ... -+---+---+-------------------------------+

   | Rule ID |  DTag | W |C=0|1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1|

   +--- ... -+- ... -+---+---+-------------------------------+

           next L2 Word boundary ->|

compressed bitmap 2:

   |---- SCHC ACK Header ----|Bitmap|

             |-- T --|-M-| 1 |

   +--- ... -+- ... -+---+---+------+

   | Rule ID |  DTag | W |C=0|1 0 0 |

   +--- ... -+- ... -+---+---+------+

           next L2 Word boundary -> |


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