Question #679313 on rohc changed:
https://answers.launchpad.net/rohc/+question/679313
Status: Open => Answered
Didier Barvaux proposed the following answer:
Hello,
OK, I now understand your question.
The objective of the LSB algorithm is to transit only a fraction of the
field (the Least Significant Bits of the field) instead of all the bits
of the field. The LSB algorithm makes possible to transit some LSB but
in a robust way. The LSB algorithm shall be robust to a few packet
loss/damage, packet reordering, or decompression failures.
The LSB robustness uses a "reference window" of values for that. The LSB
algorithm computes the minimum number of bits that the compressor must transmit
to be 100% sure that the remote decompressor is able to correctly decode the
value from:
* the LSB received from the compressor
* any decoded value from the reference window.
If the decompressor did not decode correctly any value from the
reference window, the LSB robustness is not ensured anymore. That means
that a reference window of 4 values protects from a loss/damage of up to
3 consecutive packets.
So, the ROHC compressor performs the following actions for every packet field
that is encoded with the LSB algorithm:
a/ the compressorrecords the last N encoded values of that field in the
reference window.
b/ when a new packet is compressed:
* for every value in the reference window of N values, the compressor
computes the number of LSB required to transmit the new value.
* the compressor keeps the greatest number of those N numbers of
LSB. This is the k variable.
c/ once all k are computed for all fields, the compressor chooses a ROHC packet
format that is able to transmit the k LSB of all fields. For example, the seq_1
packet type for the TCP profile is able to transmit 16 LSBs of the sequence
number, so the compressor may choose the seq_1 packet if k <= 16. If not, the
compressor chooses another packet type.
I hope that the explanation is clear enough ;-)
Regards,
Didier
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