Subject: [rohc] CRC calculation example in ROHC implementer's guide

From: Remi Pelland <[email-masked]>
Date: Thu, 24 Jul 2003 18:22:30 -0400

Carsten,

I am not very familiar with CRC calculations and I have a question about the
algorithm you provided in the ROHC implementer's guide.  Actually, I want to
make sure I get it straight :

a) Assume the data on which the CRC is to be calculated is located somewhere
in memory;
b) The CRC calculation starts with the LSB of the first byte and bits are
processed sequentially until the MSB of the first byte is reached and
processed;
c) Then, it processes the LSB of the second byte and progresses until it
reaches the MSB of the second byte;
d) Bits are processed from LSB to MSB until the last byte is reached and
processed.  The last bit processed is thus the MSB of the last byte.

Do I have the proper understanding?

I have not seen anything in RFC-3095 saying that for each byte, the LSB is
the first bit that is shifted in the calculation register.  Instead, I was
under the impression that the data over which the CRC is calculated would
simply be treated as a stream of bits in memory, starting with the MSB of
the first byte and progressing linearly until the LSB of the last byte.

Was this method chosen so bits are processed in the order they are sent out
on the network? (Assuming all link layers send the LSB first.)

Thanks,
Rémi.

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