Subject: Re: [rohc] SigComp User Guide, deflate bytecode

From: Herve Siegrist <[email-masked]>
Date: Fri, 01 Apr 2005 09:37:40 +0200

Well, actually, I would think that these extra bytes are actually
record marking bytes, but not SigComp record marking: they are the
bytes generated by the ZLIB library when you use it for partial
compression, i.e.: call its deflate method with the flush argument set
to Z_SYNC_FLUSH to compress a partial block.

This lets your compressor compress successive messages while keeping
the compression history of the previous messages, hence making the
compression of further messages more efficient. This mode of operation
is needed to implement dynamic compression as per RFC 3321 when using
ZLIB to compress the data.

>From the SigComp standpoint, these ZLIB record marking bytes can then
safely be suppressed from the compressed data as the decompression
bytecode does not need them...

Herve.

Hi,

I
hope I'm not too late but I just saw a posting dated from 14 Mar 2005 (see below). After reading
the User Guide, I thought the extra "ff ff" may have been placed
there for the sake of record marking in a stream-based application.
Does this make sense ?

Jean-Pierre

Hi Herve,

> While I'm at it, just one more little question on that topic:

> what are the extra "0000 00ff ff00" bytes added at the end of

> the sample message provided in the draft? They do not seem to

> relate to actual compressed data, and the sample message

> decompresses just as well without them...

You're right, it does.

They don't break anything by being there because although the sentinel
of 7

bits of zero is not necessary (as the draft says) it just happens that
in

this message the total number_of_bits_needed mod 8 = 1 so we do have
the 7

bit sentinel to make the message byte aligned.  Any bytes beyond it are

ignored.

I'm not sure why the bytes are there and they're not needed, so I'll
remove

them.

Thanks for pointing it out.

Best regards,

Abbie

> Herve Siegrist.

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